Nonadrenergic modulation by clonidine of the cosecretion of catecholamines and enkephalins in adrenal chromaffin cells

Abstract

Cultured bovine chromaffin cells cosecrete catecholamines and enkephalins following cholinergic nicotinic stimulation. Initial reports on the inhibitory effect of clonidine on catecholamine secretion raised the possibility of a modulation of chromaffin cell function through a presynaptic adrenergic mechanism. The purpose of this work was to investigate the pharmacological characteristics of this inhibitory effect of clonidine on the cosecretion of catecholamines and enkephalins in 4-day-old cultured chromaffin cells. We observed that clonidine completely inhibits nicotine-stimulated secretion of both leucine-enkephalin and catecholamines with an IC₅₀ of 34 μM. Treatment of chromaffin cells for 3 days with 100 nM reserpine leads to a 67% increase in nicotine-stimulated secretion of leucine-enkephalin without any effect on the IC₅₀ of clonidine. In reserpine-treated chromaffin cells, norepinephrine (100 μM) inhibits only by 27% nicotine-stimulated secretion of leucine-enkephalin with an IC₅₀ of 50 μM. Neither the alpha₂-adrenergic antagonist yohimbine nor the alpha₁-adrenergic antagonist prazosin could fully reverse the inhibitory effect of clonidine on leucine-enkephalin secretion at 10 nM. These results tend to rule out the role of alpha-adrenergic receptors in the mediation of clonidine inhibition of cosecretion in chromaffin cells.