Calcium at the sarcolemma.

Experiments in mammalian ventricle in which calcium (Ca) is removed from and replaced to arterial perfusate indicate that a component of Ca important in contractile control is bound to rapidly exchangeable cellular sites. These sites appear to be in rapid equilibrium with extracellular Ca and are replenished, beat-to-beat, from the extracellular source. The binding sites appear to be anionic phospholipids of the sarcolemma. They are visualized to represent a store of sarcolemmal Ca which, upon membrane excitation, supplies the systems responsible for transsarcolemmal movement--the Ca channel and the Na-Ca exchanger. The amount of Ca bound to sarcolemma is an important factor in control of the amount of Ca which enters the cell via the channel or exchanger. Conversely, there is evidence that suggests that the activity of (at least) the exchanger determines, in part, the quantity of Ca bound. Increased activity of the Na-Ca exchanger is associated with increased Ca-binding to sarcolemmal sites.