a. The heart: 4 chambers and 4 sets of valves

• 2 Atria
• 2 Ventricles
• 2 AV valves
• 2 Semilunar valves

b. The systemic circulatory system

• Arteries
• Arterioles

c. The pulmonary circulatory system

• Veins
• Venules

Via a series of conduits, arteries and capillary beds, to provide nutrients to all organs and via a series of capillary beds and veins to provide a means to remove the effluents of metabolism from these same organs.

As a muscular pump the heart is required to eject into the circulatory system a volume of blood appropriate for the metabolic activity of the organs.

a. Of the large arteries

• They serve as conducting vessels - transfer blood to the periphery
• The elastic properties of the vessel wall promote flow through the periphery during diastole as well as systole

b. Of the small arteries

• Relative to the size of the lumen of these vessels, they possess an abundant amount of smooth muscle
• They control the steady delivery of blood to the capillary beds
• They control blood pressure via the degree of contraction of their smooth muscle in the vessel wall

The veins are responsible for the return of blood to the heart. The veins hold 70% of the blood volume. Contraction of the smooth muscle in the venous circulation increases blood return to the heart. Venodilation reduces the return of blood to the heart.

The pulmonary arterial system receives the output from the right heart, deoxygenated blood, and delivers it to the lungs for carbon dioxide removal and oxygen loading of blood.

The pulmonary venous tree returns the oxygenated blood to the left heart for propulsion to the rest of the body.

The pulmonary arteries, unlike the other arteries of the body, carry deoxygenated blood. The pulmonary veins, unlike the other veins of the body, carry oxygenated blood. The pulmonary arteries respond to hypoxia with contraction. The systemic arteries respond to hypoxia with vasodilation.

The right ventricle ejects the same volume of blood that the left ventricle ejects. The pulmonary arterial system is a low-pressure system, unlike the high pressures encountered in the systemic arterial system. The pulmonary arterial walls contain less smooth muscle than the systemic arteries. The reduced amount of tone that can be generated in the pulmonary arterial system is responsible for the lower pressures in the pulmonary arterial system. This arterial tone is also known as arterial resistance. Hence the pulmonary arterial resistance is much less than the systemic arterial resistance.

The capillary beds are the site of diffusion of gases, nutrients, and waste products within each organ.

Starling forces are the "forces" that are involved with the movement of fluid across the permeable membranes of the capillary beds. Within the capillary, the capillary hydrostatic pressure "forces" fluid out of the capillary and into the interstitium; the interstitial hydrostatic pressure "forces" fluid to leave the interstitium and enter the capillary; the capillary plasma colloid osmotic pressure (also called plasma oncotic pressure) "draws" fluid into the capillary; and the interstitial fluid colloid osmotic pressure (also called oncotic pressure) "draws" fluid into the interstitium. The net movement of fluid across the capillary membrane is affected by both the magnitude of the imbalance between the hydrostatic and osmotic forces and the permeability of the capillary membrane to water. Kf is known as the permeability coefficient.

Thus the net movement of water across the capillary membrane can be expressed via the equation: Kf[(HPc-HPif)-(COPc-COPif)].

a. Pericardium: Limits cardiac distention with cardiac filling. This effect is greatest on the thin walled chambers such as the atria and the right ventricle.

b. Right Atrium: It serves as a storage reservoir for blood returning to the heart via the cranial and caudal vena cavas.

c. Right Ventricle: It must receive all the blood presented to it via the right atrium and expel this blood to the lungs for gas exchange. Since the pulmonary arterial tree is a rather low-pressure system (as compared to the systemic arteries), the right ventricle develops into a rather thin walled chamber (as compared to the left ventricle).

d. Left Atrium: It serves as a storage reservoir for blood returning from the pulmonary veins.

e. Left Ventricle: It must receive all the blood presented to it via the left atrium and expel this blood through the systemic arterial tree to the organs of the body. Since the systemic arterial tree is a rather high-pressure system (as compared to the pulmonary arteries), the left ventricle develops into a rather thick walled chamber (as compared to the right ventricle).

f. The A-V valves: They function to promote the flow of blood (unidirectional flow only) from the atria to the ventricles during diastole. Flow occurs across these valves when the pressure in the ventricles falls below that of the atria. Flow across the A-V valves is characterized by three phases: rapid filling phase at the onset of diastole (responsible for most of the filling of the ventricle), diastasis (minimal flow occurs at this time), and atrial contraction at the end of diastole.

g. Semilunar valves: They function to promote the flow of blood from the ventricles into the arterial trees during systole. Flow occurs across these valves when the pressure in the ventricles exceeds that present in the arterial tree. Flow across these valves is uni-modal (unlike the bimodal nature of flow across the AV valves).

Disease/malfunction can occur due to abnormalities within the heart:

Edema is excessive fluid accumulation in interstitial spaces.

Edema can develop in any organ or collect as free fluid in body cavities (such as ascites, pleural effusion, pericardial effusion)

Edema occurs because of either:

• Elevated hydrostatic pressure usually due to heart failure (see Starlings Forces) or over-hydration
• Reduced plasma colloid osmotic pressure (hypoproteinemia) which itself can be due to:
• Excessive protein loss from the kidneys or bowel
• Reduced production from the liver
• Increased capillary membrane permeability

To prevent the accumulation of excessive interstitial fluid especially in the lung. In the absence of lymphatic obstruction lymph flow can increase 20 to 50 times to remove excess fluid accumulation.

Lymph fluid is carried into the thorax and enters the subclavian veins to flow into the cranial vena cava and right atrium.

These vessels continuously remove interstitial fluid and prevent its excessive accumulation.

It forms as a result of Starling Forces that promote a net efflux of fluid from the vascular compartment into the interstitial spaces. Plasma protein is also deposited into the interstitial spaces by pinocytosis. This fluid and protein accumulation is removed continuously by lymphatics.

Into the lymphatic vessels

Out of the lymphatic vessels

What is diffusion?

What is pinocytosis?

What is ultrafiltration?

Hemodynamic factors:

Blood flow = perfusion pressure / vascular resistance

Perfusion pressure = mean arterial pressure - mean venous pressure

Autoregulatory factors (local mechanisms): due to local mechanisms within the tissue. These intrinsic mechanisms predominate over the extrinsic mechanisms for control of blood flow to critical organs (heart, brain, and working skeletal muscle)

• Increased metabolic rate causes increased tissue metabolites (especially CO2, adenosine, lactic acid, and K) and decreased O2
• Increased metabolism causes local arteriole vasodilation with increased blood flow to the tissue
• As the increased blood flow removes these metabolic products the stimuli for increased flow is removed so a reduction in blood flow back to resting state occurs.
• Another form of metabolic response to tissue perfusion is known as reactive hyperemia, which refers to a temporary increase in blood flow to tissue after a period of restricted flow to this tissue. This occurs due to the accumulation of metabolites as described for active hyperemia.

Extrinsic control factors (nerves/hormones): these mechanisms predominate over the intrinsic mechanism to control blood flow to non-critical organs such as the kidneys, skin, splanchnic, and resting skeletal muscles.

• Sympathetic stimulation:Alpha1 and alpha2 adrenergic stimulation
• Angiotensin II receptor stimulation
• Endothelin receptor stimulation
• Arginine vasopressin stimulation

• Parasympathetic stimulation (vagal): Muscarinic (M3) receptors cause vasodilation via NO
• Sympathetic stimulation:
• Beta2 stimulation: causes vasodilation of skeletal muscle arterioles
• Natriuretic peptides
• Adenosine
• Prostacyclin
• Endothelium derived relaxin factor
• Bradykinin

Baroreceptors are stretch receptors in the carotid sinus and aortic arch that maintain BP within a normal range and respond to changes in BP to normalize it.

Cardiopulmonary (stretch) receptors: These receptors located in the left atrium, right atrium, pulmonary arteries, and ventricular endocardium, are activated by increased volume. They send signals to the brain to inhibit sympathetic outflow and increase vagal activity to reduce arterial vasomotor tone and decrease BP.

The converse is also true.

Syncope refers to a sudden and transient loss of consciousness due to the temporary loss of cerebral perfusion. The metabolism of the brain, unlike other organs, is exclusively dependent on perfusion. In contrast to skeletal muscle, for example, storage of high-energy phosphate in the brain is limited, and energy supply depends largely on the oxidation of glucose extracted from the blood. Thus, cessation of cerebral blood flow causes a loss of consciousness within about 10 seconds.

Natriuretic peptides are a family of ring shaped vasoactive hormones.

• Antagonists to the renin-angiotensin-aldosterone system
• ANP - Promotes potent natriuresis, diuresis, vasodilation (arterial and venous dilation), suppression of the renin-angiotensin-aldosterone axis, reduces sympathetic tone, lowers the activation threshold of vagal afferents, inhibits secretion of vasopressin, antimitogenic, inhibits growth of fibroblasts and retarding collagen deposition
• BNP has cardiovascular effects similar to ANP
• CNP is a more potent dilator of veins than the other two.
• Diuresis occurs due to:
  • Increase in glomerular filtration pressure (due to constriction of glomerular efferent arterioles and dilation of afferent arterioles)
  • Direct tubular action - antagonizes vasopressin thus inhibits water transport in the cortical collecting ducts
  • Inhibit angiotensin II - stimulated aldosterone secretion
• C-type NP inhibits aldosterone secretion but has little effect on arterial pressure or salt and water excretion
• Urodilatin - the unique renal ANP (see below)
• Stimulates diuresis and natriuresis at doses lower than doses of ANP
• More resistant to endopeptidase inactivation

• Atrial natriuretic peptide
• Discovered by de Bold in 1981 from Kingston Ontario
• Stored in secretory granules in the atria as a 126 amino acid pro hormone (proANP)
• This is cleaved, by a membrane-bound endonuclease [Corin] in association with exocytosis, into 2 fragments
  • An N-terminal fragment called proANP1-98 or NT-proANP
  • The major biologically active hormone called C-terminal peptide ANP 99-126 (or ANP1-28)
  • These fragments are produced in equal amounts
• The ANP gene is very actively expressed in the fetal and neonatal ventricle. Soon after birth, the ANP expression in the ventricles decreases to very low levels, but it can be re-induced by increased ventricular load.
• Release of ANP from the atrial myocardium - occurs in response to
  • Atrial stretch due to volume overload
  • Hypoxia
  • Endothelin 1
  • Catecholamines
  • Angiotensin
  • Arginine vasopressin
  • Prostaglandins
  • Glucocorticoids
  • Thyroid hormones
• The ANP gene is also expressed in the kidney where a 32 amino acid peptide is produced called urodilatin - a local regulator of sodium and water handling in the kidney.
• ANP is elevated in congestive heart failure

The components are:

Wiggers Diagram shows what is occurring in the atria and ventricles (with respect to volume and pressure) during each phase.

Identify the events known as isovolumetric contraction and relaxation in terms of:

The pressure-volume loop - be able to draw and label it.

So-called failure may be due to two causes:

In both cases BP falls.

All disorders that result in a fall in BP are addressed by "so-called compensation" on the part of the circulatory system as if the cause for this drop in BP was due to blood loss and reduced preload and the development of shock.

The response involves the following:

• An increase in HR - which increases CO
• An increase in contractility - which increases SV
• An increase in arterial vasomotor tone (systemic arterial resistance) - which helps bring BP back to normal
• An increase in venomotor tone - which increases preload (the fundamental abnormality)
• Activation of renin and thus RAAS

• The production of angiotensin II which causes:
• An increase in arterial vasomotor tone (systemic arterial resistance) - which helps bring BP back to normal
• An increase in venomotor tone - which increases preload (the fundamental abnormality)
• Activates the Arginine Vasopressin (Antidiuretic) System that causes:
• An increase in arterial vasomotor tone (systemic arterial resistance) - which helps bring BP back to normal
• An increase in water retention at the level of the kidney - which increases preload (the fundamental abnormality)
• The production of aldosterone which causes:
  • Water retention at the level of the kidney - which increases preload (the fundamental abnormality)

The response the circulatory system introduces, for a true case of shock, is intended to support the circulatory system for a time period measured in hours.

The so-called shock response was never intended to support a circulatory system in need of support for a time period measured from days to years.

In the setting of heart failure that results in a fall in blood pressure, the “Shock Response” is activated for a period of days to years.

How is sustained activation of the sympathetic nervous system detrimental?

How is sustained activation of the RAAS detrimental?

• The increase in arterial vasomotor tone (systemic arterial resistance) causes an increase in afterload which reduces SV and increases MVO2
• The increase in venomotor tone - which increases preload promoting further pulmonary edema, pleural effusion, ascites, and organ edema
• Myocardial necrosis
• Activates the Arginine Vasopressin (Antidiuretic) System that causes:
• An increase in arterial vasomotor tone (systemic arterial resistance) causes an increase in afterload which reduces SV and increases MVO2
• An increase in water retention at the level of the kidney - which increases preload promoting further pulmonary edema, pleural effusion, ascites, and organ edema

These are the factors that affect the ability of the heart to contract effectively and they include:

Involved in several of these factors is the concept of myocardial oxygen demand (MVO2).

This refers to the amount of oxygen required "demanded" of the heart to contract

The following factors increase the myocardial oxygen requirement to contract:

• An increase in heart rate.
• An increase in contractility.
• An increase in afterload.
• An increase in wall stress

Note the opposite changes in any of these factors reduces MVO2

Wall stress refers to the tension applied to a cross sectional area of muscle and the units are force per unit area. Laplace's Law is used to describe wall stress:

Wall stress = (pressure x radius) divided by (2 x wall thickness).

SV is the volume of blood ejected from the heart with each contraction. Under normal conditions only about 50% of the volume of blood present in the heart is ejected with each heart beat.

CO is a commonly used measure of the performance of the heart.

CO = Heart Rate x Stroke Volume.

CO is sometimes indexed to body weight and is called cardiac index (CI = CO/Body Weight [kg])

An increase in HR increases CO and CI

However an increase in HR:

• increases MVO2
• reduces the time for ventricular filling (preload)
• reduces the time for coronary perfusion

Under normal physiologic conditions heart rate is under autonomic control.

• Cephalic stimulation - excitement, boredom, sleeping, physical activity
• Respiration: Hering-Brewer reflex. HR increases with inspiration and decreases with expiration. This rhythm is a sinus arrhythmia. During inspiration the reflex is stimulated to inhibit the vagal center resulting in a relative increase in sympathetic activity.
• Baroreceptor activity: Involved in the minute to minute control of blood pressure which involves altering the tone on the arterial tree and altering heart rate. Any increase in BP activates the baroreceptors which stimulates the vasomotor center with increased vagal stimulation decreasing HR, decreasing CO, and returning BP to normal. Baroreceptors (or high pressure receptors) are located in the carotid sinus and aortic arch. These receptors respond to stretch and not pressure (thus mechanoreceptors). Activation of these receptors sends inhibitory impulses to the vasomotor center in the medulla of the brain via the vagus and glossopharyngeal nerves. The vasomotor center sends sympathetic nerve traffic to the body. Activation of the baroreceptor reflex inhibits sympathetic outflow and increases vagal tone. Reduced activation of these receptors, as with hypotension, increases sympathetic outflow (and thus HR) and inhibits vagal tone.
• Other reflexes involved in HR control are:
• Bainbridge Reflex: Increased left atrial pressure (due to increased volume) causes an increase in HR.
  • Mechanoreceptors located at the junction of the right atrium and caval veins or at the junctions of the pulmonary veins and the left atrium.
  • Volume expansion causes a tachycardia. Result is sometimes the opposite.
  • It serves as a counterbalance to the baroreceptor reflex.
  • Causes withdrawal of parasympathetic tone.

Under abnormal/diseased conditions:

• A number of disorders can induce either excessive heart rates (tachyarrhythmias) or excessively slow heart rates (bradyarrhythmias)

Baroreceptor activity also affects the tone of arteries and veins and the contractility of the heart via activation or de-activation of the sympathetic and vagal output.

Baroreceptor activity affects HR

It may manifest as either:

Preload refers to the volume of blood present in the heart at the end of diastole (before the onset of contraction).

Preload is measured as either the pressure in the ventricle at end diastole or the volume of blood in the ventricle at end diastole. Note that pressure and volume are intricately related

Preload alters cardiac performance by way of the Frank Starling Law of the heart.

This law states that as preload is increased the contractility of the heart is increased to increase stroke volume.

The mechanism for the action of the Frank Starling Law is as follows: the increased stretch of cardiac fibers at the onset of systole induces an increase in contractility by way of increasing the sensitivity of Tn-C for the existing amount of cytosolic Ca.

Preload = volume of blood returning to the heart + the volume of blood left over from the last contraction (recall that approximately 50% of the blood in the ventricle is ejected during each contraction).

Factors that increase preload:

• Venoconstriction
• Increased blood volume
• Reduced ejection of blood from the ventricle due to reduced contractility so that more blood is left over at the end of the last contraction.
• Increased blood volume due to valvular insufficiencies.

Factors that reduce preload:

• Venodilation
• Blood loss with reduced circulating blood volume
• An increase in the volume of blood ejected at the time of the last contraction

If preload is too high? We usually observe signs of congestion.

• If the ventricle is the left ventricle with the elevated preload, elevated pressures will develop in the left atrium, pulmonary veins, and pulmonary capillary bed and pulmonary arteries, right ventricle, right atrium, vena cavae, and veins that drain into the vena cavae. When the flux of fluid from the vasculature exceeds the ability of the lymphatics to accommodate then fluid accumulates.
• The elevated hydrostatic pressure in the pulmonary capillary bed will promote the efflux of fluid into the pulmonary interstitium with pulmonary edema developing.
• The elevated pressures in the right atrium will promote the collection of fluid in the pleural space (pleural effusion), in the abdominal cavity (ascites or abdominal effusion), or collection of fluid in the other organs such as the skin (subcutaneous edema) or edema of other organs.
• If the ventricle is the right ventricle with the elevated preload, elevated pressures will develop in the right ventricle, right atrium, vena cavae, and veins that drain into the vena cavae.
• The elevated pressures in the right atrium will promote the collection of fluid in the pleural space (pleural effusion), in the abdominal cavity (ascites or abdominal effusion), or collection of fluid in the other organs such as the skin (subcutaneous edema) or edema of other organs.

If preload is too low? We observe signs of reduced blood volume or perfusion.

• If the ventricle is the left ventricle with the reduced preload, hypotension will develop and reduced organ perfusion will occur with signs related to the organs involved.
• If the ventricle is the right ventricle with the reduced preload, pulmonary artery hypotension will develop with reduced filling of the left heart and reduced organ perfusion will occur with signs related to the organs involved.

Afterload refers to the resistance the ventricle encounters as it tries to eject blood. Afterload is only conceptual and so it cannot be directly measured

Afterload is increased by:

• An increase in ventricular volume
• An increase in arterial vasomotor tone (arterial vascular resistance)
• A decrease in ventricular wall thickness

Afterload is decreased by:

• A decrease in ventricular volume
• A decrease in arterial vasomotor tone (arterial vascular resistance)
• An increase in ventricular wall thickness

Since arterial vasomotor tone is a strong component of afterload, blood pressure (BP) is frequently used as a surrogate for afterload, although a weak surrogate.

BP = CO x Arterial resistance

Factors that influence systolic blood pressure:

• Stroke Volume, Stiffness of the arterial tree, Arterial resistance

Factors that influence diastolic blood pressure:

• Duration of diastole, Elasticity in the arterial tree, Semilunar valve insufficiency, Arterial resistance

Pulse pressure refers to the difference between systolic BP and diastolic BP

BP cannot be directly assessed by physical examination. Palpation of the arterial pulse provides very indirect information as to BP but is usually inadequate.

Afterload cannot be directly measured. However systemic arterial resistance can be measured but not without some difficulty? BP is a common surrogate used to assess afterload

The effect of an increase in afterload is dependant on the inherent strength of the heart:

• Very short term increases in afterload actually increase cardiac output.
• More sustained increases in afterload result in a mild reduction in cardiac performance.

• A moderate reduction in cardiac performance occurs with an increase in afterload.

• A profound reduction in stroke volume occurs with an increase in afterload.

The factors that determine arterial resistance are explained in Poiseuille's Law:

• Change in pressure across the vessel
• The radius of the vessel raised to the 4th power

• The length of the vessel
• Viscosity of blood (which is related to the # of red cells and protein content of the blood).

If afterload is too high:

• Any disorder that causes a reduction in cardiac output will be associated with a "compensatory" increase in arterial vasomotor tone (systemic vascular resistance). Thus all cases of heart failure are associated with an increase in afterload.

If afterload is too low:

• Signs of a low BP may be observed

Thus low BP may be observed with both an increase and decrease in afterload.

An increase in sympathetic tone increases afterload by increasing arterial resistance and increasing cardiac output via an increase in HR, preload (due to vasoconstriction) and Frank Starling's Law, and an increase in contractility. Reducing sympathetic tone will have the opposite effect.

An increase in vagal tone will generally have the same effect as a decrease in sympathetic tone. A decrease in vagal tone will generally have the same effect as an increase in sympathetic tone.

Refers to the inherent strength of the heart muscle - referred to as inotropy.

Factors that increase contractility:

• Increased Beta adrenergic stimulation
• Increase preload
• Reduced vagal tone
• Positive inotropic agents

Factors that reduce contractility:

• Reduced sympathetic stimulation
• Reduced preload
• Increased vagal tone
• Negative inotropic agents

An increase in contractility results in:

• An increase in stroke volume
• A reduction in preload
• An increase in MVO2

A fall in contractility will have the opposite effects

If contractility is too high:

• This condition will likely go undetected.

If contractility is too low:

• The resultant fall in BP will manifest as signs of hypotension
• Fluid accumulation in the organs that drain into the weak ventricle

Distensibility refers to the ease of ventricular filling during diastole (ability to stretch).

Lusitropy refers to the ability of the ventricle to distend/relax and fill

Distensibility is reduced by:

• Reduced sympathetic tone
• Increased wall thickness
• Increased collagen, scarring, or cellular infiltration within the ventricular wall

Distensibility is increased by the opposite effects.

Measurement of distensibility involves determining the instantaneous changes in pressure and volume within the chamber, which is invasive and difficult.

Distensibility is not routinely measured.

Indirect measures of the filling properties of the heart are obtained by echocardiography.

The effect of a reduction in distensibility is identical to the effect of an increase in preload. Thus, for the same volume of the heart at the end of diastole there will be an increase in pressure in the ventricle.

If distensibility is reduced:

• The clinical signs will be those of excessive preload with a reduction in SV

If distensibility is excessive:

• This is a situation not encountered in clinical practice.
• Disease is characterized by a fall in distensibility

Beta-adrenergic receptor activation increases the ability of the ventricle to relax

Synergy of contraction refers to the normal harmonious, co-ordinated and efficient contraction of all regions of the heart yielding optimal ejection of fluid. Abnormalities of synergy of contraction are referred to as dyssynergy of contraction.

The mal-coordinated contraction of all regions of the heart result in reduced SV.

This disorder is the result of abnormal ventricular activation as with premature ventricular contractions.

Therapy is directed as for that appropriate to treat ventricular dysrhythmias.

The sympathetic nervous system typically promotes ventricular dysrhythmias

It provides for the moment to moment control of the heart and the circulatory system.

Activation of the sympathetic nervous system:

• Increases heart rate
• Increases the speed of conduction
• Decreases the refractory period
• Increases contractility
• Increases the ability of the heart to relax and fill

Activation of the parasympathetic system:

• Decreases heart rate
• Decreases contractility
• Reduces the speed of conduction
• Increases the refractory period
• Inhibits the release of and therefore effect of sympathetic action at the level of the heart

For the SNS it is epinephrine and norepinephrine

• The source of Epinephrine is mainly from the secretion of the adrenal gland
• The source of Norepinephrine is mainly from the nerve terminals and the adrenal gland

For the PNS it is acetylcholine

The catecholamines act on specific receptors called adrenergic receptors that are located on the cell surface of the target organ.

• Alpha 1: mainly located on blood vessels; stimulation causes vasoconstriction (arterial vasoconstriction or venoconstriction). They are mainly located post-synaptically.
• Alpha 2: mainly located pre-synaptically on the nerve terminal and activation inhibits the release of norepinephrine. Thus they reduce the effect of Alpha 1 activity (negative feedback on norepinephrine). However, distal to the synaptic cleft alpha 2 receptors promote vasoconstriction. Vasoconstriction is the dominant effect.
• Beta 1: mainly located on the heart; stimulation causes increase in heart rate, contractility, relaxation, etc
• Beta 2: mainly located on the arteries (mainly coronary and skeletal muscle); stimulation causes vasodilation; no specific innervation to these receptors thus stimulated by circulating catecholamines.

• M2: located on the heart; stimulation causes decreased heart rate, contractility, etc. Acetylcholine acts on the M2 receptor.
• M3: located on the arterial tree; stimulation causes vasodilation. Nitric oxide acts via the M3 receptor activation.

Activation of a beta receptor by epinephrine or norepinephrine results in activation of the cytosolic messenger cAMP which promotes the opening of the Ca channel during the action potential

• PKA phosphorylates the Ca channel protein (L type) enhancing Ca entry into the cell during Phase 2 of the action potential. This starts the process of Ca induced Ca release and contraction.
• PKA also phosphorylates phospholamban, releasing the inhibition of SERCA, resulting in Ca uptake by the SR and induction/promotion of diastole and relaxation.

Beta2 receptors on the vascular smooth muscle wall are activated mainly by epinephrine.

• As with Beta receptors on the heart, Beta2 vascular receptors result in the release of cytosolic cAMP.
• Cyclic AMP inhibits the myosin light chain kinase, a protein that enables actin-myosin interaction, hence Beta 2 stimulation promotes vasodilation.

• The activated M2 receptor activates a G protein (Gi), which binds to adenylate cyclase and inhibits its activation. This results in less cAMP being produced and thus less Ca influx into the cell at the time of the next action potential.
• The activated Gi protein activates a K channel (K_ACH) that promotes the loss of K and causes resting membrane potential to become more negative (hyperpolarize) at the end of Phase 3 of the action potential of the SA node thus slowing the heart rate (Phase 4 takes longer to reach threshold and fire).
• In addition to inhibiting adenylate cyclase, M2 activation may activate a guanylate cyclase enzyme system to increase the level of cGMP in the cell, which reduces the flow of Ca across the L type Ca channel during Phase 2 of the action potential thus mediating a reduction in contractility.
• M2 receptors are also located on the pre-synaptic nerve terminal and function to inhibit the release of norepinephrine from the nerve terminal thus reducing the activity of the sympathetic system.

• Cyclic GMP may also inhibit Ca entry into the smooth muscle cell thus promoting vasodilation

Beta receptor activation is used as a model to describe G protein coupled receptor activation

Beta receptor (G protein coupled receptor [GPCR]) is activated by epinephrine or norepinephrine

• The GPCR also called the 7 transmembrane receptor
• Conformational change to heterotrimeric Gs protein
  • Gby subunits are separated
  • Ga binds to receptor-ligand complex causing a second messenger response
    • 4 functional subtypes of Ga are described from 16 heterotrimeric Gs proteins:
      • Ga_i - this subunit is involved in M2 receptor activation
      • Ga_s - this subunit is involved in beta adrenergic receptor activation
      • Ga_q - this subunit is involved in alpha adrenergic, angiotensin II receptor activation
      • Ga₁₂ - role of this subunit is unknown
• Promotes the production of cAMP

Activation of GPCR by its ligand initiates the process of receptor desensitization (the GPCR is phosphorylated)

• This process arrests G protein signaling
• Requires/involves two families of proteins:
  1. G protein-coupled receptor serine/threonine kinases (GRKs) {also known as B-ark [Beta agonist receptor kinase]}
     • There are 7 GRKs
     • GRK2 and GRK3 are most involved in Beta GPCR regulation
       • They are not membrane proteins but cytosolic and must be translocated to the membrane to work
         • This translocation requires G protein activation which liberates Gby dimers that attract the GRK2 and GRK3
         • GRK2 and GRK3 must bind to Gby dimmers for phosphorylation of the receptor
  2. Arrestins
     • There are 4
     • Arrestin 2 and 3 (formerly Beta arrestin 1 and 2)
     • Action of Arrestin 2 and 3:
       • Bind to the phosphorylated receptor that blocks further G protein-initiated signaling through a steric mechanism
       • Also increase the rate of degradation of cAMP by recruiting phosphodiesterases (PDs) to the receptor thus placing the PDs in close proximity to the sites of generation of cAMP.
         • Thus beta arrestins act to desensitize cAMP formation and PKA activation by both impeding the rate of its synthesis and enhance the rate of its degradation.
       • Also induces the process of internalization
         • They bring activated receptors to calthrin-coated pits for endocytosis, a process required for receptor recycling and degradation.
         • They bind to other proteins that assist receptor internalization
     • Ubiquitination of arrestin
       • Ubiquitination involves the attachment of ubiquitin (a 76 aa residue containing protein)
       • Previously thought to tag proteins for proteasomal destruction (kiss of death)
       • Ubiquitination is now believed to be an important process such that the attachment of ubiquitin mediates novel outcomes including protein trafficking and signal transduction
         • Beta arrestins are ubiquitinated by Mdm2, an E3 ubiquitin ligase
         • This is required for the endocytic functions
       • The beta arrestin must be interacting with a stimulated receptor to be ubiquitinated.
       • Ubiquitination is not required for receptor internalization but is required for targeting the internalized receptors to lysosomes for degradation.
       • Signaling leads to various cell survival and anti-apoptotic effects, some forms of chemotaxis, effects on cardiac contractility
       • Cellular signal transduction involves highly coordinated cascades of events. The number of possible downstream targets for any given member of a signaling network is vast, and to maintain integrity and specificity of signaling, cells employ molecular scaffolds. These are large chaperone complexes that hold together specific members of a signaling network to give them preferential access to one another, thus ensuring the fidelity of a particular signaling response. Thus in addition to their classic roles in desensitization and internalization, beta arrestins can also act as signaling scaffolds for many pathways and in particular those of the MAPKs (mitogen-activated protein kinases).
         • The downstream effectors of MAPKs control many cellular functions, including cell cycle progression, transcriptional regulation, and apoptosis.
         • ERKs are a specific MAPK
       • Beta arrestins are involved in nuclear function - transcriptional regulation
       • Beta arrestins may be activated without activation of a G protein receptor

• GRK2 and GRK3 can also reduce Gq-coupled receptor activity (alpha receptor IP3 system)
  • GRK2 and GRK3 bind to activated Gaq subunit and sequester it preventing its coupling to downstream effectors
• Binding of GPCR to ligand causes a conformational change - this enables binding of GRKs which results in phosphorylation of residues on the intracellular loops and carboxyl terminus of GPCR
• Binding of GRK 2 and 3 to GPCR, without receptor phosphorylation, may also be sufficient to suppress signaling
• Phosphorylation of the GPCR residues promotes the high-affinity binding of the arrestin family of proteins (there are 4) to the GPCR which prohibits further coupling to G proteins. Can cause an 80% reduction of receptor signaling.

Phosphorylation of GPCR can also occur by PKA and PKC and c-Src causing receptor desensitization.

• This is a feedback mechanism whereby the agonist-stimulated GPCR generates a second messenger (like cAMP) that activates a kinase (like PKA) that decreases the activity of the receptor (the GPCR) and ultimately attenuates the production of the second messenger.

Regulation of GRKs

• Other kinases phosphorylate GRKs modulating their activity
  • Site of phosphorylation affects GRKs activity
    • Site is dependant on the type of kinase
• Basal state GRK2 exists in a phosphorylated state in the cytosol that is the inactive form
  • Phosphorylated by Erk1/2 (extracellular signal-regulated kinases)
    • Phosphorylates GRK2 at Ser670
      • Causes a marked reduction in activity of GRK2
      • Causes a marked decrease in ability to bind to Gby subunits
    • Erk1/2 are activated by Erk1/2 mitogen-activated protein kinase/s
  • Agonist-occupied GPCR and Gby subunits promote GRK2 and Erk1 rapid association
• PKA and PKC enhances GRK2 activity
• The non receptor tyrosine kinase c-Src phosphorylates GRK2
  • This phosphorylation is dependent on the ability of beta-arrestin to bind to c-Src and recruit c-Src to the GPCR
  • Tyrosine phosphorylation affects GRK2 activity in two ways
    • Rapidly and transiently increases GRK2 activity
      • Enhances phosphorylation of GPCR and desensitization
    • Promotes degradation of GRK2 by ubiquitin/proteosome pathway
      • Reduce GRK2 protein levels

The scheme:

• GPCR activation → splits the heterotrimeric G protein separating off the Ga subunit
  • Phosphorylated GRK2 is dephosphorylated by a phosphatase → dephosphorylated GRK2 (active) is recruited to the plasma membrane → GRK2 binds to receptor and Gby subunits → GRK2 binds Erk1 or 2 → phosphorylates GRK2 on Ser670 → deactivates GRK2 → promotes release of GRK2 from plasma membrane and back into the cytosol in its inhibited ("off") phosphorylated state.
  • Phosphorylated GRK2 promotes the interaction of beta arrestins with the phosphorylated receptor leading in turn to desensitization of further G protein signaling by steric exclusion by the beta arrestin.

Other Regulation of GPCR activity

• Activated GPCR activates a second messenger:
  1. cAMP → activates PKA
  2. Diacylglycerol → activates PKC
  3. IP3 → activates calcium/calmodulin-activated kinases to mobilize Ca
• Two of these kinases (PKA and PKC) directly phosphorylate GPCR (desensitize it)
• PKA augments GRK2 activity
  • Increases its affinity for Gby dimer
    • This promotes recruiting GRK2 to the plasma membrane and to complex with the activated receptor substrate
• PKC augments GRK2 activity
  • Increases activity of GRK2 by promoting its translocation to the plasma membrane

The action potential results in an influx of Ca into the cell.

• Ca enters the cell via the long type Ca channels during the plateau phase of the action potential (Phase 2).
• The entry of Ca into the cell is the first step toward contraction of the cell.
• The entry of Ca into the cell does so because/as a result of the electrical excitation of the cell (depolarization).

For relaxation to occur a number of events must take place:

• The removal of Ca from binding sites of Tn-C to enable:
• Tn-I, Tn-T and tropomyosin to take up their inhibitor roles hiding action binding sites to prevent the interaction of actin and myosin heads

• A transmembrane protein in the SR called SERCA (Sarcoendoplasmic Reticulum Ca ATPase) is a Ca pump that moves Ca into the SR.
• At rest the SERCA pump is inhibited by the protein phospholamban, thus Ca movement into the SR is impaired. When phospholamban is phosphorylated by protein kinase A, the inhibition of SERCA is removed, allowing it to remove Ca from the cytoplasm.

Some cardiac disorders are associated with a reduction in contractility. In these circumstances we attempt to enhance contractility.

Any process that increases the influx of Ca into the cell results in more Ca induced Ca release from the SR.

Normally the Ca concentration in the cardiac cytosol during systole is such that the contractile sites are half activated. Thus the heart has considerable contractile reserve which can be used by increasing the Ca occupancy of the Tn-C binding sites.

Beta adrenergic stimulation is the common method of increasing cytosolic Ca by increasing the flow of Ca across the Ca channel (L type) into the cell. This is mediated by increasing cytosolic cAMP.

Inhibiting the degradation of cAMP (the second messenger of beta stimulation) also increases cytosolic Ca. Phosphodiesterase degrades cytosolic cAMP. Phosphodiesterase inhibitors thus increase cAMP.

Blocking the Na/K-ATPase pump increases cytosolic Ca. As Na accumulates in the cell in the face of an inhibited Na/K pump, Na is extruded from the cell by way of the Na/Ca exchange mechanism and Ca accumulates in the cell. Digoxin functions by blocking the Na/K ATPase pump.

Independent of increasing cytosolic Ca, contractility can be enhanced by increasing the affinity of Tn-C for Ca. Calcium sensitizers function this way (e.g. Pimobendan).

The thin actin fibers slide between the thicker myosin fibers as a result of repetitive movements of the myosin heads. The sliding of the actin filament results in the Z lines approaching each other. The linkage between the myosin head and the actin filament is the crossbridge. The crossbridge cycle is the repetitive attachment and detachment of myosin heads to and from actin filaments.

ATP is required for the process of attachment and detachment. The terminal phosphate bond of ATP is split off by myosin ATPase releasing energy for attachment.

Crossbridging is inhibited at rest by both the avid binding of Tn-T and tropomyosin that promotes the positioning of the tropomyosin molecule which is twisted in such a way blocking the interaction of the myosin heads with actin.

The binding of Ca to the Tn-C results in:

• binding of activated Tn-C with Tn-I is strengthened, Tn-I moves, and this weakens the interaction between Tn-T and tropomyosin causing the tropomyosin to be repositioned on the thin filament and
• the repositioning of the tropomyosin molecule exposes more binding sites on the actin molecule to myosin heads.

A number of proteins are involved in contraction including:

• Myosin: The thick filament contains the myosin heads that bind to actin.
• Actin: The thin filament consist of 2 actin units that are intertwined in a helical pattern, both being carried on a heavier tropomyosin molecule that functions as a backbone. The actin units contain sites that bind to the myosin heads and contain the regulatory proteins.

• Tropomyosin: Part of the thin filament acts as a structural backbone to the actin helix. At rest tropomyosin is structurally positioned to obstruct the interaction of myosin heads with actin binding sites.
• Troponin-I (Tn-I): Part of the troponin complex and it is positioned on the tropomyosin protein. I stands for inhibitor. When Tn-I is not bound tightly to Tn-C (tight binding occurs as a result of Ca activation), Tn-I promotes a tight interaction between Tn-T and tropomyosin that enables the position of tropomyosin to restrict the interaction between actin and the myosin heads. When Ca binds to Tn-C, activated Tn-C binds tightly to Tn-I. Tn-I now moves to a new position on the thin filament that causes a weakening of the interaction between Tn-T and tropomyosin. This promotes a conformational change (repositioning of tropomyosin on the thin filament) that exposes more actin binding sites to myosin heads.
• Troponin-C (Tn-C): Part of the troponin complex and it is positioned on the tropomyosin protein. C stands for calcium. Tn-C is activated by Ca binding (the Ca released by the SR). This starts the contraction process (cross bridge cycling). Tn-C, thereby once activated by Ca, binds to Tn-I. Tn-I now moves to a new position on the thin filament causing a weakening of the interaction between Tn-T and tropomyosin. Tropomyosin now moves to expose more actin binding sites to cross bridge with myosin heads.
• Troponin-T (Tn-T): Part of the troponin complex and it is positioned on the tropomyosin protein. T stands for tropomyosin binding. Tn-T links the whole troponin complex to tropomyosin. When Tn-T is tightly bound to tropomyosin, tropomyosin takes up a position on the thin filament that blocks most of the actin sites that could bind to myosin heads.

• Titin: Consists of 2 segments, an anchoring segment and an elastic segment. It is a very large molecule. Mutations in the titin gene have been incriminated in familial dilated cardiomyopathy in the Doberman Pinscher.
• It has 2 major roles:
• It tethers the myosin molecule to the Z line.
• The folded elastic segment is important to allow the myocardium to regain its original shape in diastole by stretching during systole.

a. Systole: refers to the contraction phase of the cardiac cycle.

Begins at the onset of AV valve closure (onset of isovolumetric contraction) and continues to the closure of the semilunar valves (second heart sound).

Begins with the closure of the semilunar valves (onset of ventricular relaxation) to the end of ventricular filling (closure of the AV valves).

d. Chronotropy: refers to the ability to increase heart rate.

e. Lusitropy: refers to the ability of the heart to relax.

f. Dromotropy: refers to the speed of conduction.

• It is the command center for the coordination of all electrical activity
• The cells of the SA node have the property of automaticity
• These cells are under the influence of the autonomic nervous system.
• The sympathetic system (Beta adrenergic stimulation) increases the firing rate of the SA node.
• The parasympathetic system (Acetylcholine stimulation) decreases the firing rate of the SA node.

• A short segment of "electrical highway" connecting the AV node and the Bundle Branches.

• Left and right bundle branches deliver the electrical current from the bundle of HIS to the left and right ventricle respectively.

• The bundle branches terminate in the myocardial tissue along the endocardium in purkinje fibers.

Based on the distribution of the purkinje fibers, animals can be divided into two groups:

Consists of one negative pole and one positive pole.

It provides "one line of sight", in a direct line from the negative pole to positive pole of that lead, to "view" the wave of depolarization as it passes into that line of sight.

It is a graphic representation of the change in the cellular membrane potential as the cell depolarizes and repolarizes as the wave of excitation passes over the cell.

• In skeletal muscle: very short; order of milliseconds
• In myocardial muscle: very long; order of 100's of milliseconds

• In skeletal muscle: shorter than the length of the action potential
• In myocardial muscle: lasts almost the length of the action potential

• In skeletal muscle: repetitive stimulation can induce tetany
• In myocardial muscle: repetitive stimulation cannot induce tetany. Tetany for the heart would cause death. The heart needs a pause between each contraction to allow for filling. The long duration of the action potential and refractory period allows the contraction of the heart to end before the next excitation can induce a second contraction. Hence between two repetitive beats filling of the heart can occur.

The refractory period (RP) represents a time period after the onset of phase 0 of the action potential during which another stimulus, no matter how strong, fails to induce another depolarization within that cell.

The RP prevents a cell from being depolarized at an extremely high rate. For myocardial muscle cells in prevents the heart muscle from experiencing tetany.

For all cardiac cells except the SA and AV nodal cells, the RP ends during Phase 3 of the action potential. For cells of the SA and AV node, the RP ends after the end of Phase 3 (during phase 4) of the action potential.

The RP ends during the first half of the T wave. It should be finished by the middle of the T wave.

a. Identify the members of each group:

• Automatic cells are the cells of the SA node, some cells in the leaflets of the AV valves, some cells around the coronary sinus, cells of the distal AV node, cells of the HIS, Bundle Branches and Purkinje system.

b. What determines automaticity?

• Automaticity is determined by the ability of cells to spontaneously depolarize during Phase 4 of the action potential.
• Cells that are not normally automatic may gain automaticity a result of disease or electrolyte disturbance. Thus these cells with normally a flat Phase 4 develop a Phase 4 that gradually depolarizes.

c. What affects automaticity?

• Disease can induce automaticity in cells that are normally devoid of this property.
• The autonomic nervous system can alter automaticity:
• The sympathetic nervous system increases automaticity.
• The parasympathetic nervous system reduces automaticity.

In the conduction system of the heart:

• Tachycardia
• Bradycardia

In the valve system of the heart:

• Valvular stenosis
• Valvular insufficiency

Inadequate Preload:

• Blood loss
• Peripheral vasodilation
• Pericardial effusion
• Pulmonary artery hypertension

In the heart muscle:

• Weakened heart muscle (loss of contractility)
• Taurine deficiency
• Tachycardia induced
• Idiopathic
• Impaired distensibility/relaxation
• Concentric hypertrophy
• Myocardial infiltrate

Infection of the heart:

• Bacterial, viral, fungal, rickettsial, protozoal, heartworm

Shunting:

• Left-to-right: volume overload via the recirculation system
• Right-to-left: unoxygenated blood gains access to the systemic circulation