Excitationcontraction Coupling Muscle

Stimulation of the cardiac cell initiates the process of excitation, which has been related to ion fluxes through the cell membrane. Depolarization of the tissue in the atria of the heart is mediated by two inwardly directed ionic currents. When the cardiac cell potential reaches its threshold, ion channels in the membrane are opened, and Na+ enters the cell through ion channels. These channels give rise to the fast sodium current that is responsible for the rapidly rising phase, phase 0, of the ventricular action potential (Fig. 19.4). The second current is caused by the slow activation of an L-type Ca2+ ion channel that allows the movement of Ca2+ into the cell. This "slow channel" contributes to the maintenance of the plateau phase (phase 2) of the cardiac action potential. We now understand that the Ca2+ that enters with the action potential initiates a second and larger release of Ca2+ from the sarcoplasmic reticulum in the cell. This secondary release of Ca2+ is sufficient to initiate the contractile process of cardiac muscle.

Figure 19.4 • Diagrammatic representation of the membrane action potential, as recorded from a Purkinje fiber, and an electrogram recorded from an isolated ventricular fiber. The membrane resting potential is 90 mV relative to the exterior of the fiber. At the point of depolarization, there is a rapid change (phase 0) to a more positive value. Phases 0 to 4 indicate the phases of depolarization and repolarization. Note that phases 0 and 3 of the membrane action potential correspond in time to the inscription of the QRS and T waves, respectively, of the local electrogram.

Figure 19.4 • Diagrammatic representation of the membrane action potential, as recorded from a Purkinje fiber, and an electrogram recorded from an isolated ventricular fiber. The membrane resting potential is 90 mV relative to the exterior of the fiber. At the point of depolarization, there is a rapid change (phase 0) to a more positive value. Phases 0 to 4 indicate the phases of depolarization and repolarization. Note that phases 0 and 3 of the membrane action potential correspond in time to the inscription of the QRS and T waves, respectively, of the local electrogram.

Contraction of cardiac and other muscle occurs from a reaction between actin and myosin. In contrast to smooth vascular muscle, the contractile process in cardiac muscle involves a complex of proteins (troponins I, C, and T and tropomyosin) attached to myosin, which modulates the interaction between actin and myosin. Free Ca2+ ions bind to troponin C, uncovering binding sites on the actin molecule and allowing interaction with myosin, causing contraction of the muscle. The schematic diagram in Figure 19.5 shows the sequence of events.10 Contraction of vascular smooth muscle, like that of cardiac muscle, is regulated by the concentration of cytoplasmic Ca2+ ions. The mechanism by which the contraction is effected, however, includes a calcium- and calmodulin-dependent kinase as opposed to a Ca2+-sensitive troponin-tropomyosin complex (Fig. 19.2). The activating effect depends on a different type of reaction. The elevated free cytosolic Ca2+ in vascular smooth muscle cells binds to a high-affinity binding protein, calmodulin.

"j cell membrane

Ca2+ (released from sarcoplasmic recticulum)

Troponin C Ca2+ Complex 1

Actin + Myosin Interaction

Muscle Contraction

Figure 19.5 • Sequence of events showing excitation-contraction coupling in cardiac muscle.

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