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Strength of Contraction can be Modulated in Cardiac Muscle

FIGURE 9 Positive staircase effect. As the frequency of stimulation increases (first arrow), the tension produced by each contraction increases. Several contractions at the new rate occur before the new steady state is reached. When the frequency is returned back to its starting level (second arrow), the tension produced by each contraction gradually returns to the initial level.

FIGURE 9 Positive staircase effect. As the frequency of stimulation increases (first arrow), the tension produced by each contraction increases. Several contractions at the new rate occur before the new steady state is reached. When the frequency is returned back to its starting level (second arrow), the tension produced by each contraction gradually returns to the initial level.

contractility. The model of excitation-contraction coupling illustrated in Fig. 8 can explain the positive staircase phenomenon. Increasing the heart rate increases the number of action potentials per unit of time and, thus, the rate of Ca2+ influx from the extracellular compartment. At the same time, it shortens the duration of diastole when the exchanger removes Ca2+. As a result, more cytosolic Ca + is pumped into the SR than is removed by the Na+-Ca2+ exchange mechanism, causing more Ca2+ to be available for release from the SR with each beat. Because the new equilibrium will take several beats to be reached, the increased contractility is seen to increase stepwise.

Catecholamines

When the catecholamines norepinephrine and epinephrine secreted by the sympathetic nerve endings and by the adrenal medulla bind to the heart's firadrenergic receptors, they exert a profound effect on the cardiac its contractile state. Receptor binding, as shown in Fig. 10, causes adenylyl cyclase to make cyclic adenosine monophosphate (cAMP), which in turn activates cAMP-dependent kinase (PKA). Catecholamines exert their effect by influencing both of the sarcolemmal and SR Ca2+ fluxes. PKA phosphorylates the L-type calcium channels, which causes more Ca2+ to enter the cell with each action potential. If more Ca2+ enters the cell, more will be available to be pumped into the SR, causing more to be available for release with subsequent action potentials (the same basic argument was used to explain the positive staircase effect discussed earlier). The second action of catecholamines is to increase the activity of SERCA. This is accomplished when PKA phosphorylates a protein called phospholamban on the SR. If the activity of SERCA is increased, a greater portion of the cytosolic Ca2+ will be pumped into the SR, making a greater amount available for release by a subsequent action potential. Catecholamines also tend to shorten phase 2 and thus shorten the duration of systole.

Inhibition of the Sodium Pump by Cardiac Glycosides

Cardiac glycosides such as digitalis were traditionally used to treat congestive heart failure. The rationale for their use as a therapeutic agent at first may seem questionable, because at the molecular level cardiac glycosides block the Na+-K+ pump in all of the body's

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