Galanin-induced relaxation in gastric smooth muscle cells is mediated by cyclic AMP

Abstract

Galanin has numerous effects on gastrointestinal motility in different species; however, its cellular basis of action in mediating these effects is unclear. Dispersed gastric smooth muscle cells have been shown to possess high-affinity galanin receptors that increase cAMP and cause relaxation. Recent studies show some smooth muscle relaxants such as VIP cause relaxation by both cAMP-dependent and -independent mechanisms. It is unknown if galanin's cellular basis of relaxation is similar or different from that of VIP. To investigate galanin's relaxant effect and compare it to VIP's effect, dispersed smooth muscle cells from guinea pig stomach were prepared by collagenase digestion. The mean length in resting cells was 110 ± 2 μm and, with carbachol treatment, contracted to 89 ± 2 μm. VIP and galanin alone had no effect on cell length, but each caused a dose-dependent inhibition of carbachol-induced contraction and both had an EC 50 of 3–7 n M. Galanin (1 μM) and VIP (1 μM) increased cellular cAMP from 118 ± 10 pmol/10 6 cells in control to 212 ± 14 and 214 ± 12 pmol/10 6 cells, respectively. The protein kinase A inhibitor, Rp-cAMPS, at 100 μ M, completely inhibited the relaxant effect of an EC 50 concentration of galanin (3 n M), but only inhibited that by VIP by 80% ( p < 0.05). Adding the nitric oxide inhibitor, l-NNA ( N G - nitro- l-arginine ), at 100 μ M did not alter the length of resting cells or inhibit carbachol-induced contraction. However, l-NNA (100 μM) decreased VIP-induced relaxation by 45%, whereas it had no effect on galanin-induced relaxation. To determine the ability of each peptide to activate nitric oxide, the incorporation of [ 3H]arginine into [ 3H]citrulline was determined. Galanin (1 μM) did not cause nitric oxide generation whereas VIP (1 μM) increased nitric oxide generation above the control by 97 ± 14% ( p < 0.01). These results demonstrated that with galanin, in contrast to VIP, nitric oxide is not involved in its ability to cause gastric smooth muscle cell relaxation. The relaxant action of galanin can be accounted for completely by its ability to activate protein kinase A and therefore resembles recent results with β-adrenergic agents.