Electrical aspects of excitation-contraction coupling.

Control of contraction in heart muscle is an interesting question partly because it is so complex. Contraction can vary by as much as an order of magnitude under the influence of drugs, or by different patterns of stimulation. Membrane depolarization is the initial determinant of contraction. It appears to be initiated by a membrane Ca current that causes graded release of Ca from a cellular store. This cellular store is quite labile; it is increased by entry of Ca by the transmembrane current and is decreased by a membrane Ca pump. Recovery of contraction requires recovery of the store into a releasable form and recovery of the membrane "trigger" current. A plausible scheme for contraction can be proposed without a direct role of membrane depolarization in Ca release, but a direct effect cannot yet be dismissed. Key future studies will require direct monitoring of the sarcoplasmic reticulum transmembrane potential and cytoplasmic Ca, and definition of the factors controlling the size of the Ca store.