Calcium-dependent exocytosis in bovine adrenal medullary cells with leaky plasma membranes

Abstract

SECRETION from nerve terminals and many other cell types occurs by exocytosis, a process in which the contents of small intracellular vesicles are released after fusion of the vesicle membrane with the plasma membrane of the cell. Exocytosis generally requires calcium ions^1,2, but little is known about the site at which these Ca ions act. The electrophysiological experiments of Katz and Miledi³ strongly point to an intracellular site, a conclusion that is supported both by the observation that transmitter release is increased after microinjection of Ca into the presynaptic terminal of the squid giant synapse⁴ and the finding that the Ca ionophore A23187 promotes secretion from a variety of cells⁵. Further progress has been severely hampered by lack of preparations in which this presumed intracellular site is freely accessible to experimental investigation. We now present evidence that the plasma membrane of medullary cells from the bovine adrenal gland can be rendered permeable to small molecular weight substances without blocking Ca-dependent exocytosis. This preparation has been used to examine the dependence of exocytosis on Ca and Mg ions.