Excitationcontraction Coupling Is Accomplished By Calcium Ions

Cardiac muscle cells, like skeletal muscle cells, have an extensive SR (see Fig. 3). Excitation-contraction coupling in cardiac muscle cells is similar to that in skeletal muscle cells. Specifically, an action potential travels down the T tubules and causes the release of Ca2+ from the SR, which in turn activates the contractile machinery. Three pools of Ca2+ are important to the cardiac muscle cell, as shown in Fig. 8: (1) in the extracellular fluid, (2) in the SR, and (3) in the cytoplasm. Only the cytoplasmic pool is available to bind with the troponin-binding sites and initiate contraction. Consider the consequences of an action potential. Ca2+ entry through the sarcolemma increases the concentration of Ca2+ in the cytoplasm. Because the amount of Ca2+ entering is relatively small, it accounts for only a fraction of the activation of the contractile proteins and, hence, is indicated by a dashed line in Fig. 8. In skeletal muscle, action potentials on the T tubules electrically stimulate the SR to release calcium. That is not the case in cardiac muscle however. Rather the small amount of Ca2+ entering the cell through the L-type calcium channels actually triggers the release of the sequestered Ca2+

Ca2 Exchanger
FIGURE 8 Simplified model of calcium handling in cardiac muscle cells. EX, Na+-Ca2+ exchanger. See text for details.
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