BRIEF REVIEWS Calcium Release from the Sarcoplasmic Reticulum

Calcium that induces contraction by interacting with the contractile proteins of muscle cells may be derived from different sources: extracellular space including the basement membrane, sarcoplasmic reticulum (SR) and, under certain circumstances, mitochondria." Among these sources of Ca for excitation-contraction coupling the contribution of the SR varies between different types of muscles: cardiac, skeletal, and smooth.' ~ An unresolved question concerns the nature of these variations: Are they only quantitative or also qualitative, consisting of fundamentally different mechanisms for the release of Ca from the SR? This review tries to answer that question. The greatest attention is placed on the mechanism of Ca release from the SR of cardiac muscle and this is compared to release in skeletal muscle. Not much space is devoted to smooth muscle, simply because little is known of the mechanisms of Ca release from the SR in this tissue. Studies concerning the sources of Ca other than the SR" and the mechanisms of Ca transport by the SR which are not directly related to the release processes will not be discussed. The first event in excitation-contraction coupling is the action potential. It is generally admitted that some Ca crosses the surface membrane of the cardiac cell during the action potential, whereas little or no Ca crosses the surface membrane of skeletal muscle cells." Two processes have been proposed as possible links between the action potential of the sarcolemma and the release of Ca from the SR: the Ca-induced release of Ca and the "depolarization-induced" release of Ca: Ca-Induced Release of Ca. According to this hypothesis, the small amount of Ca that crosses the sarcolemma during the action potential is insufficient by itself to activate the myofilaments but induces a release of Ca from the SR sufficient for activation." "Depolarization-Induced" Release of Ca. According to this hypothesis, the action potential traveling in the transverse tubules induces a modification of the SR membrane that increases its permeability to Ca and thereby results in a release of Ca from the SR." However, it is