The functions of two species of calcium channel in cardiac muscle excitation-contraction coupling.

The contractile state of cardiac muscle cell is determined by the level of calcium in the cytosol. Each action potential produces a transient elevation of cytosolic calcium. The interaction of calcium ions with the contractile elements of the cell initiates a series of events which culminate in cell shortening. The process by which cell excitation is coupled to contraction involves the function of two distinct species of calcium-selective membrane ion channel; the L-type, dihydropyridine-sensitive calcium channel of the cell surface membrane or sarcolemma and the ryanodine-sensitive calcium-release channel of the sarcoplasmic reticulum intracellular membrane network. The aim of this article is to provide information on the properties of these channels and to discuss the mechanisms involved in the coordination of their function in the coupling of cardiac muscle cell excitation to contraction.