Regulation of cardiac muscle contraction Graded contractions Effect

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Regulation of cardiac muscle contraction • Graded contractions • Effect of cardiac muscle stretching

Regulation of cardiac muscle contraction • Graded contractions • Effect of cardiac muscle stretching • Channel activity during action potentials – In myocardial contractile cells – In autorhythmic pacemakers

Graded contraction • The amount of force varies with the number of cross-bridges formed

Graded contraction • The amount of force varies with the number of cross-bridges formed • Low Ca++ few cross-bridges • High Ca++ more cross-bridges

The effect of epinephrine and norepinephrine of contraction • NE and E bind to

The effect of epinephrine and norepinephrine of contraction • NE and E bind to beta 1 receptors on contractile myocardial cells • The beta 1 receptor is coupled to a G protein • Cyclic AMP is formed

The effect of epinephrine and norepinephrine of contraction • cyclic AMP is formed •

The effect of epinephrine and norepinephrine of contraction • cyclic AMP is formed • 1. Voltage gated Ca++ channels are phosphorylated stay open longer more intracellular Ca++ stronger contractions • 2. A regulatory protein, phospholamban, is phosphorylated increased activity on SR Ca++ ATPase contractions shorten duration

Effect of phospholamban on Ca++ release • NE and E activity increase phospholamban activity

Effect of phospholamban on Ca++ release • NE and E activity increase phospholamban activity increase Ca++ ATPase activity on SR more Ca++ is sequestered into the SR more Ca++ is available for Ca++ release during stimulation stronger force of contraction

Effect of NE and E on contraction • Stronger, more frequent contractions

Effect of NE and E on contraction • Stronger, more frequent contractions

When myocardial cells elongate • The amount of Ca++ entering the myocardial cells may

When myocardial cells elongate • The amount of Ca++ entering the myocardial cells may increase the force of contraction increases

Myocardial contractile cell action potentials • Resting potential is stable -90 m. V •

Myocardial contractile cell action potentials • Resting potential is stable -90 m. V • Wave of depolarization through gap junctions • Voltage gated Na+ channels open • Voltage gated K+ channels open • Slow voltage gated Ca++ channels open and K+ channels close • Ca++ channels close and K+ channels open

Long action potential • Myocardial cell refractory period and contraction end simultaneously

Long action potential • Myocardial cell refractory period and contraction end simultaneously

Action potentials in myocardial autorhythmic cells • The channels: – If channels allow passage

Action potentials in myocardial autorhythmic cells • The channels: – If channels allow passage of Na+ and K+ – Ca++ channels

Action potentials in myocardial autorhythmic cells • Unstable resting membrane potential • Pacemaker potential

Action potentials in myocardial autorhythmic cells • Unstable resting membrane potential • Pacemaker potential • At a membrane potential of -60 m. V Na+ enters through the If channels • mb depolarizes • Ca++ channels open • Ca++ channels close • K+ leaves

Modulation of autorhythmic cells • NE (sympathetic) and E (adrenal hormone) • Autorhythmic cells

Modulation of autorhythmic cells • NE (sympathetic) and E (adrenal hormone) • Autorhythmic cells have beta 1 receptors • Cyclic AMP levels increase • Properties of If and Ca++ channels altered • More rapid Na+ and Ca++ entry • Rapid action potential • Rapid contractions

Modulation of autorhythmic cells • Parasympathetic, acetyl choline • Muscarinic receptors • K+ channels

Modulation of autorhythmic cells • Parasympathetic, acetyl choline • Muscarinic receptors • K+ channels open mb hyperpolarizes cell less excitable • Ca++ channel less likely to open slower depolarization cell is less excitable

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