It has been known for a number of years that calcium ions play a crucial role in excitation-contraction (e-c) coupling (Sandow, 1952). The majority of the calcium required for this process is derived, at least in vertebrate striated muscle fibres, from discrete intracellular stores located at sites within the cell: the terminal cysternae (tc)/junctional SR of the sarcoplasmic reticulum (SR) (Fig. 1 a). These storage sites not only form a compartment that is distinct from the sarcoplasm of the fibre, but they are also closely associated with the contractile elements, the myofibrils. The SR release sites are activated following the spread of electrical activity (Huxley and Taylor, 1958) along the transverse (T) tubular system (Eisenberg and Gage, 1967; Adrian et al. 1969a, b; Peachey, 1973) from the surface membrane (Bm).

Many cells release preformed material contained in secretory granules by exocytosis. Exocytosis is a specialized means of secretion in which the granules fuse with the plasma membrane and thereby discharge their contents through the fusion pores. This mechanism mediates, for example, the formation of the fertilization envelope in eggs, the release of neurotransmitters and neuropeptides by neurons, the release of a variety of enzymes and mediators by mast cells and granulocytes or the secretion of hormones by endocrine cells. Classical methods for investigating exocytosis usually measure release of secreted material.