Bradykinin, angiotensin II and a mascarnic agonist, acetyl-B-methacholine (methacholine) were all found to elict catecholamine release from cultured bovine adrenal chromaffin cells. Bradykinin was the most potent of these secretagogues and methacholine the weakest, with angiotenin II intermediate in efficacy. All three secretagogues were much less effective than nicotinic stimulation. The three secretagogues all produced a rise in cytoplasmic free calcium concentration ([Ca2+]i), measured with the fluorescent indicator fura2, which was partially independent of external calcium. In the case of bradykinin the full rise in ([Ca2+]i) may involve a component of calcium entry in addition to release of calcium from an internal store. Secretion was also found to be partially independent of external calcium. The different efficacies of the three secretagogues in elicting secretion were correlated with the rise in ([Ca2+]i) produced. The differeing efficacies of the three secretagogues may be due to the extent of release of calcium from an intracellular store which itself is less effective in eliciting secretion than a rise in [Ca2+]i following calcium entry due to nicotine. Bradykinin also stimulates calcium entry, and this may increase the efficacy of the initial rise in [Ca2+]i. Treatment with pertussis toxin resulted in an enhancement of secretion in response to all of the secretagogues.
Abbreviations ([Ca2+]i), cytoplasmic free calcium concentration; EGTA, ethylene glycol bis (β-amino ethyl ether)-N,N,N,N,-tetraacetic acid; Hepes, 4-(2-hydroxy ethyl)-1-piperazinethanesulphonic acid; TPA, 12-O-tetradecanoylphorbol-13-acetate; DAG, diacyl glycerol; IP3, inositol-1,4,5-trisphosphate; PIP2, phosphatidylinositol-4,5-bisphosphate