SMOOTH-MUSCLE CELLS FROM GUINEA-PIG STOMACH POSSESS HIGH-AFFINITY GALANIN RECEPTORS THAT MEDIATE RELAXATION

Galanin-like immunoactivity occurs in nerves and plexi in muscle layers throughout gastrointestinal tract including the stomach. Galanin can affect gastric emptying and contraction or relaxation of gastric muscle in different species. The aim of this study was to investigate the direct effect of galanin on dispersed gastric smooth muscle cells and to characterize any galanin receptors that mediated any effect. Dispersed gastric smooth muscle cells were prepared from guinea pig stomach by collagenase digestion. Porcine galanin (p-galanin; 1 mu M) did not stimulate contraction when present alone; however, p-galanin (1 mu M) inhibited carbachol-induced contraction with a half-maximal effect at 7 nM. p-Galanin (1 mu M) increased cellular adenosine 3',5'-cyclic monophosphate (cAMP) content by 10 s and caused a maximal increase of 80% over basal. I-125-galanin (porcine) bound to dispersed cells in a time- and temperature-dependent manner. Binding was saturable, reversible, and specific. Binding of I-125-galanin was inhibited almost equally by porcine and rat galanin (K-i = 6-8 nM) but was not inhibited by the galanin-associated peptide [preprogalanin-(108-123)]. The fragment galanin-(1-16) was equally potent to rat galanin; however, the fragment galanin-(9-29) was 56-fold less potent (K-i = 370 nM). Computer analysis demonstrated there were two binding sites for p-galanin on gastric smooth muscle cells, a high-affinity site (K-d = 2.6 nM) with low capacity (B-max = 175 fmol/mg protein) and a low-affinity site (Kd = 150 nM) with large capacity (B-max = 3,611 fmol/mg protein). I-125-galanin binding to gastric smooth muscle cell membrane was inhibited by 5'-guanylyl imidodiphosphate [Gpp(NH)p] in a dose-dependent manner(IC50 = 75 nM). Computer analysis demonstrated that with Gpp(NH)p the high-affinity binding site was lost. These results demonstrate that gastric smooth muscle cells possess high-affinity galanin receptors, occupation of which causes cAMP generation and muscle relaxation. The NH2-terminus of galanin determines the receptor affinity that is regulated by a guanine nucleotide-binding protein.