Plasmalemmal calcium in cardiac excitation-contraction coupling.

1. Mammalian heart muscle is extremely sensitive to the external calcium concentration. It reacts to alterations of the external calcium concentration with an immediate adaptation of contractile force. 2. In mammalian heart there is a network of large transverse tubules throughout the cell. These structures are regularly arranged at the level of the sarcomeric Z- and I-lines and increase the cell surface by a factor of ten. 3. Experimental evidence favours the assumption that the plasmalemma could be the site of a loosely bound superficial Ca fraction which becomes ionized upon depolarization and is again bound upon repolarization of the cardiac cell membrane. 4. A mechanism is discussed which bases the excitation-contraction coupling process on a physicochemical interaction of calcium with membrane phospholipids. The degree of interaction is thought to be governed by the transmembrane electric field, the induced dipole moment of membrane constituents, and proton activity within the membrane.