Ca2+ enters the cardiac cell during the plateau phase of the action potential via the L-type Ca2+ channel. (The channel tightly binds the Ca2+ channel blocking agents of the dihydropyridine family). The sarcolemmal Na+/Ca2+ exchanger transports 3 Na+/Ca2+. The energy for this transport is indirectly derived from ATP hydrolysis via the sodium gradient that is established by the Na+/K+ pump. It is the major Ca2+ extrusion mechanism of the cardiac myocyte and may contribute significantly to myocardial relaxation. The sarcolemma also houses the Na+/K+-ATPase that transports 3 Na+ out and 2 K+ into the cell per molecule of ATP, and thus moves out one net charge per cycle. This is the key transporter that sets up the sarcolemmal ionic gradients for Na, K, and Ca, and consequently allows ion channels to function. The Na+ and K+ channels in the sarcolemma are involved in action potential generation. Fig.H10 (at the end of the chapter) illustrates the channels and the ion movements in the sarcolemma.
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