Bradykinin stimulation of bovine adrenal chromaffin cells.

Cultured bovine adrenal chromaffin cells provide a useful model of stimulus secretion coupling and respond to cholinergic agonists by secreting catecholamines. Work in this thesis concentrates on the responses to a non-cholinergic agonist, bradykinin. Bradykinin as shown to stimulate a two phase, dose dependent increase in catecholamine release which is mediated by a receptor of the B2 subtype. Calcium entry is shown to be required for release to occur but studies with various calcium channel blockers suggest that, in contrast to the response to potassium, a non-voltage sensitive calcium channel is involved. Other possible alternatives are discussed. As bradykinin stimulated an increase in inositol phosphate production, I attempted to measure the production of the other product of phospholipase C action on inositol phospholipids, diacylglycerol, in order to evaluate its possible role in the release response. This was attempted using both mass measurement, by the diacylglycerol kinase assay, and lipid labelling techniques. No increases in diacylglycerol in response to bradykinin were observed, even in the presence of inhibitors of diacylglycerol breakdown, which were able to increase basal diacylglycerol levels when added alone. These inhibitors, along with TPA, were used to evaluate the possible mechanism of action of protein kinase C in chromaffin cells, eg. feedback regulation or stimulation of release mechanisms. Failure to detect rises in diacylglycerol in response to bradykinin led to the final section of this work which looks at the production of one of the metabolic products of diacylglycerol breakdown, phosphatidic acid. Bradykinin is shown to stimulate a rapid, dose dependent increase in phosphatidic acid in chromaffin cells, which is, partially independent of extracellular calcium, independent of protein kinase C activation, and may be G-protein mediated. Studies of the route of formation of the phosphatidic acid show that phospholipase D is not involved and that inositol phospholipids or phosphatidylcholine are unlikely to be the main substrates for a phospholipase C mediated route, leaving the possibility of phospholipase C action on an alternative phospholipid. Finally the possible role of this production of phosphatidic acid in the chromaffin cell is discussed.