The Cardiac Sodium—Calcium Exchange System
The strength of contraction of cardiac muscle is controlled in part by regulating the amount of Ca2+ that enters and leaves the myocardial cells with each beat. This involves a very complex interplay between Ca2+ channels and pumps located in both the plasma membrane of the heart cell (sarcolemma) and intracellular organelles, particularly the sarcoplasmic reticulum (SR). There are two principal sources for the Ca2+ that activates contraction: the extracellular medium and Ca2+ stored within the SR. The rate of delivery of Ca2+ to the myofilaments from each source is determined by the activity of various Ca2+ channels located within the SR membrane and the sarcolemma. The distribution of Ca2+ between the two compartments is determined primarily by the relative activities of Ca2+ pumps located within the same membrane systems. One such pump is the Ca-ATPase of the SR membrane, an enzyme that couples the hydrolysis of ATP to the accumulation of Ca2+ within the lumen of the SR. In effect, the SR Ca-ATPase “competes” for Ca2+ with two different types of Ca2+ pumps in the sarcolemma: a Ca-ATPase (which is a different molecular entity than the SR Ca-ATPase) and the Na—Ca exchange system.
KeywordsSarcoplasmic Reticulum Sarcoplasmic Reticulum Membrane Exchange Carrier Sarcolemmal Vesicle Initial Rate Measurement
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