Clinical Note

As will be seen later, reducing the amount of calcium released into the cytosol during an action potential reduces the force of the resulting contraction. Attenuation of the mechanical activity of the heart is indicated in certain diseases such as high blood pressure and myocardial ischemia. Calcium channel blocking drugs such as verapamil allow less calcium to enter the cell during the plateau phase and, hence, over time, there is a net loss of releasable calcium from the intracellular pools.

within the SR. The trigger calcium acts at ryanodine receptors located on the SR. These are actually calcium-gated calcium channels that open in the presence of cytosolic calcium. The name derives from the original observation that they bind the toxin ryanodine, which blocks these release channels in the SR and hence contraction. The importance of this trigger calcium is evidenced by the fact that heart muscle will not contract when the influx of Ca2+ across the sarcolemma is blocked, even though adequate stores of Ca2+ are still present in the SR.

RELAXATION IS ACCOMPLISHED BY REMOVING Ca2+ FROM THE CYTOSOL

In cardiac muscle, as in skeletal muscle, there is a Ca2+ pump in the SR membranes termed the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA). SERCA (Fig. 8) removes Ca2+ from the contractile pool and pumps it into the SR. When enough Ca2+ is removed from the cytosol, the muscle relaxes. With each action potential some additional Ca2+ moves into the cell during phase 2. After a period of time, the intracellular Ca2+ concentration, if left unchecked, would be the same as that outside. Clearly, a mechanism is needed to remove Ca2+ from the cell. The Na+-Ca2+ exchange system in the sarcolemma is primarily responsible for removing calcium from the cytosol. The exchanger, which is not a pump, derives its energy from the Na+-K+ pump. The exchanger will pass three Na+ ions in one direction for one Ca2+ ion in the other direction, but, being an exchanger rather than a pump, will do so only along a favorable energy gradient. Because the Na+-K+ pump maintains a strong transmembrane gradient for sodium, any free Ca2+ in the cytosol will be favorably exchanged for three Na+ ions in the extracellular fluid. There are also true calcium pumps in the sarcolemma, but they account for only a small percentage of the calcium flux. At a steady state, the exact same amount of Ca2+ that entered the cell is removed with each beat. In this system the sarcolemmal exchanger and the SERCA compete for cytosolic Ca2+

during diastole. The amount of Ca2+ available for release in the SR is therefore dependent on the net outcome of this competition.

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