Mechanism of the Positive Inotropic Effect. With each cardiac myocyte depolarization, Na+ and Ca2+ ions shiftinto the intracellular space (Figure 28–5). Ca2+ thatenters the cell via the L- type Ca2+ channel during depolarization triggers the release of stored intracellular Ca2+from the sarcoplasmic reticulum via the ryanodinereceptor (RyR). This Ca2+-induced Ca2+ release increasesthe level of cytosolic Ca2+ available for interaction withmyocyte contractile proteins, ultimately increasingmyocardial contraction force. During myocyte repolarization and relaxation, cellular Ca2+ is re- sequestered bythe sarcoplasmic reticular Ca2+-ATPase and is removed
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