Adrenaline and noradrenaline, hormones secreted by adrenal chromaffin cells directly into the circulatory system, have profound pharmacological effects on many different organ systems in the body including the heart, gut, central nervous system, immune system etc. Secretion is governed by Ca2+ entering chromaffin cells through voltage-dependent calcium channels. Acetylcholine, released by splanchnic nerve terminals in response to electrical activity, depolarizes the chromaffin cells thereby activating the calcium channels. Calcium channels play a dual role in chromaffin cells. First, they open in response to membrane depolarization and allow Ca2+ ions to flow down their electrochemical gradient; this influx carries depolarizing charge contributing to the cells electrical characteristics. Second, calcium channels act as transducers changing the signal encoded in the membrane potential into a chemical signal; changes in intracellular Ca2+ act as a second messenger that can modulate a wide variety of Ca2+-sensitive cellular processes. Our preliminary results show that there are three types of calcium channels present in chromaffin cells that are different than calcium channels found in other organs. What are the physiological roles of the various calcium channels of chromaffin cells? What are their electrical and pharmacological properties? Which calcium channel(s) governs the release of catecholamines? Can more than one channel play a role in secretion? Results also showed that there was not a good correlation between the Ca2+ ions entering the cells via calcium channels and the intracellular calcium-transients. Why don't calcium currents correspond to the calcium transients? What is the significance for catecholamine release? To answer some of these questions the applicant will perform the following studies: (1) Characterize both biophysically and pharmacologically the different bovine chromaffin cell's calcium channels. These results will be compared to feline chromaffin cells which may be dissimilar. (2) Study calcium channel modulation produced by adrenaline, noradrenaline and dopamine. The applicant will focus on single-channel mechanisms, effects of second messengers and GTP binding proteins. (3) Study the factors that control intracellular calcium homeostasis. (4) Determine which calcium channel(s) takes part in catecholamine secretion and the intracellular calcium buffering systems that may also regulate release.
