GALANIN-INDUCED RELAXATION IN GASTRIC SMOOTH-MUSCLE CELLS IS MEDIATEDBY CYCLIC-AMP

Galanin has numerous effects on gastrointestinal motility in different species; however, its cellular basis of action in mediating these eff ects is unclear. Dispersed gastric smooth muscle cells have been shown to possess high-affinity galanin receptors that increase cAMP and cau se relaxation. Recent studies show some smooth muscle relaxants such a s VIP cause relaxation by both cAMP-dependent and -independent mechani sms. It is unknown if galanin's cellular basis of relaxation is simila r or different from that of VIP. To investigate galanin's relaxant eff ect and compare it to VIP's effect, dispersed smooth muscle cells from guinea pig stomach were prepared by collagenase digestion. The mean l ength in resting cells was 110 +/- 2 mu m and, with carbachol treatmen t, contracted to 89 +/- 2 mu m. VIP and galanin alone had no effect on cell length, but each caused a dose-dependent inhibition of carbachol -induced contraction and bath had an EC(50) of 37 nM. Galanin(1 mu M) and VIP(1 mu M) increased cellular cAMP from 118 +/- 10 pmol/10(6) cel ls in control to 212 +/- 14 and 214 +/- 12 pmol/10(6) cells, respectiv ely. The protein kinase A inhibitor, Rp-cAMPS, at 100 mu M, completely inhibited the relaxant effect of an EC(50) concentration of galanin ( 3 nM), but only inhibited that by VIP by 80% (p < 0.05). Adding the ni tric oxide inhibitor, L-NNA (N-G-nitro-L-arginine), at 100 mu M did no t alter the length of resting cells or inhibit carbachol-induced contr action. However, L-NNA (100 mu M) decreased VIP-induced relaxation by 45%, whereas it had no effect on galanin-induced relaxation. To determ ine the ability of each peptide to activate nitric oxide, the incorpor ation of [H-3]arginine into [H-3]citrulline was determined. Galanin (1 mu M) did not cause nitric oxide generation whereas VIP (1 mu M incre ased nitric oxide generation above the control by 97 +/- 14% (p < 0.01 ). These results demonstrated that with galanin, in contrast to VIP, n itric oxide is not involved in its ability to cause gastric smooth mus cle cell relaxation. The relaxant action of galanin can be accounted f or completely by its ability to activate protein kinase A and therefor e resembles recent results with beta-adrenergic agents.