IV. Receptors on gastric smooth muscle cells. IV.1. Somatostatin (SS) receptor subtypes on gastric smooth muscle cells. Gastric smooth muscle cells are shown to possess high affinity SS receptors. Smooth muscle cells rapidly degraded SS-14, however with appropriate protein inhibitors present and the use of a peptidase resistant ligand the pharmacology and stoichiometry of the SS receptors were explored. The receptor exists in 2 affinity states which are regulated by a G-protein. In collaboration with Dr. D.H. Coy (Tulane University) using SS receptor subtype specific analogues, the action of SS on these cells was shown to be mediated by the SS-TR-3 subtype. IV.2. Gastric smooth muscle cells possess two classes of endothelin (ET)receptors, however, only one alters contraction. Gastric smooth muscle cells were shown to possess both ET/A and ET/B receptors, however in contrast to the pancreatic cells, they were not regulated by agents that activate PLC. Only occupation of the ET/A receptor altered contractile behavior. IV.3. Gastric smooth muscle cells possess high affinity galanin receptors which mediate relaxation by activating adenylate cyclase. Galanin was shown to have a direct effect on gastric smooth muscle cells causing relaxation. By binding studies, high affinity galanin receptors were characterized, occupation of which by agonists activated adenylate cyclase. Structure-function studies showed the N-terminus of galanin determines receptor affinity and that this affinity is regulated by guanine nucleotide binding proteins. IV.4. Galanin receptor subtypes mediating relaxation. In collaboration with Dr. D.H. Coy (Tulane University) 13 NH2, COOH and mid-molecule galanin fragments were investigating for their abilities to function as galanin receptor agonists or antagonists. It is concluded that gastric smooth muscle cells possess a unique galanin receptor subtype, and these results suggest 3 not 2 subtypes of galanin receptors likely exist. V.5. Mechanism of galanin-induced relaxation. Recently, GI smooth muscle cell relaxants have been shown in some cases to both activate adenylate cyclase and NO synthetase. In isolated guinea pig gastric smooth muscle cells both VIP and galanin caused equipotent relaxation. Using NO synthetase and PKA inhibitors our results demonstrate galanin's relaxant effect is mediated only by cAMP whereas VIP both activates adenylate cyclase and nitric oxide synthetase.
