Influence of the ionic environment on the membrane potential of adrenal chromaffin cells and on the depolarizing effect of acetycholine

Abstract

Intracellular recordings were made from chromaffin cells isolated from adrenal medullae of gerbils to examine the effects, on membrane potential, of changes in the ionic environment that are known, to influence the rate of catecholamine secretion. Depolarization in response to acetylcholine fell linearly with the logarithm of the extracellular Na concentration over the range 154/3 m[image] and reached a value, in Na/free media, of about 30% of the control value. The depolarizing effect of acetylcholine in Na/free media increased linearly with the logarithm of the extracellular Ca concentration over the range 1-117 m[image]. Depolarization in response to acetylcholine involves inward movement of both Na and Ca ions. Depolarization was also observed in response to the secretagogues, excess K and Ba, both in Na/rich and Na/free media. The effect of Ba was antagonized by Ca, and it is suggested that these 2 cations interact at the level of the plasma membrane. Depolarization does not appear to be tightly coupled to secretion, for acetylcholine or excess K still depolarized the chromaffin cells when the environment was Ca/free or contained an excess of Mg, conditions that inhibit secretion. Furthermore, although acetylcholine had some depolarizing effect in Na/free media, the level to which the membrane potential fell was not below the control ''resting'' potential since the cells in Na/free medium were hyperpolarized; yet, secretory responses are augmented in such conditions. Depolarization in response to acetylcholine may be no more than the electrical sign of increased permeability to ions such as Na and Ca, and that depolarization is not, in itself, a key event in stimulus/secretion coupling. The view that movement of Ca into the chromaffin cells on exposure to acetylcholine is responsible for evoking secretion is favored.