Calcium movement into cells is mediated by several mechanisms. Albrecht Fleckenstein showed that the calcium channels can be selectively blocked by a class of agents. He called these agents calcium antagonists. Calcium movement into the cells is mediated by several mechanisms. Calcium antagonists act at the plasma membrane to inhibit calcium entry into cells by blocking voltage-dependent calcium channels.
Calcium ions play an important role in the contraction of cardiac, skeletal, and smooth muscle. Myoplasmic calcium depends on calcium entry into the cell. Calcium binds to the regulatory protein troponin, removing the inhibitory action of tropomyosin. In the presence of adenosine triphosphate this allows the interaction between myosin and actin with consequent contraction of the muscle cell.
There are at least three different types of calcium channels designated as L, T, and N types. The L-type channels, once activated, remain for a long period of time and have a large calcium-carrying capacity. The T chan¬nels have a brief opening time and N channels have characteristics that are neither of the L nor T type. Only the L-type channels are sensitive to the action of calcium antagonists. The effect of the calcium antagonists is to restrict calcium entry, and over a given period of time fewer calcium ions are available for participation in intracellular events such as muscle contraction and neuro-nal activity. Thus some have labeled these compounds calcium channel blockers, calcium channel antagonists, calcium entry blockers, and slow calcium blockers.

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