Physiological aspects of exocytosis in chromaffin cells of the adrenal medulla

The adrenal medulla is composed principally of groups of adrenergic and nor adrenergic chromaffin cells, with minor populations of small intensely fluo rescent cells and ganglionic neurones. Different molecular stimuli evoke di stinct secretory events in the gland, involving the release of either adren aline or noradrenaline together with various neuroactive peptides. The natu re of the secretory response can be controlled at a central level or regula ted locally within the gland. Specific innervation patterns to the differen t types of chromaffin cell have been implicated in central regulatory mecha nisms, while several explanations for regulating secretion locally have bee n proposed. The differential distribution of various types of receptors bet ween cell phenotypes, such as muscarinic or nicotinic acetylcholine recepto rs, histamine receptors, angiotensin receptors and different classes of opi ate receptors between the two principal chromaffin cell populations could b e involved in local control. In addition exocytosis parameters could be mod ulated differently in adrenergic and noradrenergic cells by phenotype-speci fic mechanisms, possibly involving molecules like Growth Associated Protein -43, Synaptosomal Associated Protein-25 isoforms or the pll annexin subunit . The distribution of the various types of calcium channels is also known t o vary between chromaffin cell subtypes. This short review examines possibl e ways in which specific innervation patterns in the adrenal gland could be programmed and discusses exocytosis mechanisms that could differ between c hromaffin cell phenotypes. Data reviewed here suggest that the adrenal medu lla should no longer be viewed as a homogeneous entity but as consisting of an ensemble of individual cell subpopulations each with a distinct secreto ry response that could in pari reflect its local history.