The overall goals of this research are to understand first, the function and regulation of chromaffin cell Ca channels and second, how Ca channels and catecholamine secretion are related and modulated in different chromaffin cells. The chromaffin cells of the adrenal medulla secrete catecholamines and several neuropeptide hormones in response to neural stimulation. Secretion is a Ca2+-dependent process that is triggered by the entry of Ca2+ through voltage-gated Ca channels in the plasma membrane. Thus, understanding the properties of Ca channels is essential to understanding the mechanism and regulation of catecholamine secretion. We have found that bovine chromaffin cells contain three distinct types of Ca channel, including a novel class known as faciliation Ca channels. The latter resemble """"""""L-type"""""""" Ca channels found in other cells, in that they have a single channel conductance of 27 pS and are sensitive to dihydropyridines. These channels can be recruited by repetitive depolarizations in the physiological range, by depolarization to very positive potentials or by neurotransmitters that activate adenylyl cyclase and elevate intracellular cAMP by a mechanism involving PKA. Our working hypothesis is that these channels may be the underlying regulatory system in the """"""""fight or flight"""""""" response. These cells also have two other types of Ca channels, both of which have a 14 pS conductance. One type is responsible for the """"""""non classical"""""""" N-type Ca current, while the other carries a P-type Ca current component. We propose to: a) investigate the presence of these channels in epinephrine - and norepinephrine-containing bovine chromaffin cells as well as the feline chromaffin cells; b) study Ca channel modulation by different compounds co-stored and co-released during the exocytotic process and c) study the secretory response in the three cell types. In particular, we will try to evaluate to what extent each type of Ca channel participates in catecholamine secretion. Comparing secretion data from three different cell types with possibly different ratios of the same types of Ca channels will allow us to understand the role of such channels in the regulation of secretion. By simultaneously studying Ca currents (using patch-clamp electrophysiology techniques) and catecholamine secretion (using capacitance measurements), we hope to uncover the relationship between these two critical parameters. These studies will expand our understanding of Ca channels in general and their role in catecholamine secretion and by extension in synaptic neurotransmitter release.
