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A quantitative analysis of the influence of external calcium and magnesium concentrations on acetylcholine-induced adrenaline release in rat adrenal gland perfused in an antidromic or orthodromic direction.

The role of Ca in stimulus-secretion coupling has been analysed 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;5ƒÊm).A quantitative relation was found between the amount of adrenaline release by ACh and[Ca]o over a range 0.5-4mM 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 1mM to 20mM,but the responses in the presence of 20mM Mg were reduced to about one-half of those observed with 1mM Mg. The relation between the reciprocal of[Ca]o and the reciprocal of AChinduced adrenaline release in the presence of 1 or 20mM Mg corresponds to that of noncompetitive inhibition in a kinetic scheme of MichaelisMenten.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,however,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. Acetylcholine(ACh)liberated by the splanchnic nerve is the physiological Received for publication March 14,1977 石川一志,原 田悦守,菅 野富夫