The mechanism of catecholamine release from the adrenal medulla and the role of calcium in stimulus—secretion coupling

Abstract

Although evidence has existed for many years that the sympathetic nerves innervating the adrenal medulla are cholinergic, it is only quite recently that a number of observations have been made which together provide a possible explanation of how acetylcholine, the chemical mediator, stimulates the chromaffin cells to secrete the catecholamines adrenaline and noradrenaline. The recent evidence has come from studies on perfused adrenal glands showing that the stimulant effect of acetylcholine involves some calcium-dependent process (Douglas & Rubin, 1961). Thus acetylcholine was found to be without effect on catecholamine release when calcium was omitted from the perfusion fluid, and its efficacy in releasing catecholamines was directly related to the extracellular calcium concentration over a wide range. Moreover, calcium itself, in conditions known to increase the permeability of cell membranes, proved to be an adequate stimulus for catecholamine secretion. These findings, considered along with the known ability of acetylcholine to increase membrane permeability at some other sites in the body where it also acts as a chemical transmitter, led us to suggest that acetylcholine evokes adrenal medullary secretion through some action on the chromaffin cell membranes leading to an increased uptake or influx of calcium ions, and that calcium ions provide the immediate stimulus for the release of catecholamines. Support for this idea has come from more recent studies showing that the rate of 45Ca uptake by the adrenal medulla is indeed increased by ACh (Douglas & Poisner, 1961, 1962.The present experiments have been carried out to examine further the role of calcium and other inorganic ions in the process we have termed 'stimulus-secretion coupling' at the adrenal medulla.

The Part Played by Calcium in Determining the Response to Stimulation of Sympathetic Postganglionic Fibres

“…The part played by calcium in the discharge of catechol amines from the adrenal medulla in response to acetylcholine has recently been demonstrated by Douglas & Rubin (1961, 1963. They perfused the adrenal gland in the cat with Locke solution, and measured the output of catechol amines in the adrenal vein, injecting acetylcholine to provoke a secretion.…”

The Effect of Amethocaine on Acetylcholine‐induced Depolarization and Catecholamine Secretion in the Adrenal Chromaffin Cell

“…Although older, indirect, experiments have offered conflicting testimony on the electrical excitability of medullary tissue (Rosenblueth & Cannon, 1934;Sgrosso, 1935;Hermann, Jourdan, Morin & Vial, 1936 ;Cannon & Rosenblueth, 1937), we have never detected any spike generation in isolated chromaffin cells when we have passed depolarizing current across the membrane (unpublished observations). The fact that chromaffin cells yield vigorous secretory responses to acetylcholine in sodium-free environments (Douglas & Rubin, 1963) certainly demonstrates that no sodium-dependent mechanism is necessary for the response to acetylcholine.…”

“…This is fresh evidence that neither sodium entry nor the attendant depolarization are tightly coupled to the secretory response. Previously this view has rested on evidence of quite a different sort based partly on the observation that acetylcholine retains its stimulant activity in sodium-free media (Douglas & Rubin, 1963) when its depolarizing effect is profoundly reduced (Douglas et al, 1967b) and partly on the observation that excess potassium, which depolarizes chromaffin cells (Douglas et al, 1967b) does not evoke secretion provided calcium is absent from the extracellular environment (Douglas & Rubin, 1961). The second, and contrasting, finding that amethocaine does block the depolarization that is attributable to inward movement of calcium ions in response to acetylcholine, clearly supports the view that calcium entry is intimately related to the secretory response.…”

Tumor promoters potentiate the adrenomedullary secretion induced by acetylcholine and excess K possibly by stimulating phorbol ester receptors.

“…On the other hand, the responses to histamine (0.5 mM) or Ca (0.5 mM) reintroduction during exposure to a medium lacking divalent cation (DOUGLAS and RUBIN, 1963;SORIMACHI and NISHIMURA, 1984) were little affected by TPA (55±11 % in case of histamine, N=3, and 52±17% of the preceding controls in case of Ca reintroduction, N= 3). These results raise the possibility that TPA somehow activates voltage-dependent Ca channels as does dihydropyridine BAY-K-8644 (GARCIA et al, 1984).…”