A QUANTITATIVE ANALYSIS OF THE INFLUENCE OF EXTERNAL CALCIUM AND MAGNESIUM CONCENTRATIONS ON ACETYLCHOLINEINDUCED ADRENALINE RELEASE IN RAT ADRENAL GLAND PERFUSED IN AN ANTIDROMIC OR ORTHODROMIC DIRECTION

Abstract

The role of Ca in stimulus-secretion coupling was analyzed in an isolated adrenal gland of the rat perfused in the reverse direction through the adrenolumbar vein. The omission of [Ca]o virtually abolished the release of adrenaline in response to continuous stimulation with acetylcholine (ACh; 5 .mu.M). A quantitative relation was found between the amount of adrenaline release by ACh and [Ca]o over a range of 0.5-4 mM in the perfusion fluid; the amount was a saturable function of [Ca]o. A similar relation was also observed when [Mg]o in the perfusion fluid was increased from 1 mM-20 mM, but the responses in the presence of 20 mM Mg were reduced to about 1/2 of those observed with 1 mM Mg. The relation between the reciprocal of [Ca]o and the reciprocal of ACh-induced adrenaline release in the presence of 1 or 20 mM Mg corresponds to that of noncompetitive inhibition in a kinetic scheme of Michaelis-Menten. The relation is in harmony with the view that carrier-mediated Ca influx is the major contributor to the stimulus-secretion coupling process in the adrenal chromaffin cell. The present results may also be explained by another mechanism; a fixed pore mechanism may be involved in the facilitated diffusion of Ca. The relation obtained by antidromic perfusion was not reproduced in preparations in situ perfused in the orthodromic direction. Possible explanation of this discrepancy is discussed.