Mechanisms of Galanin-induced Contraction of Isolated Rat Gastric Fundus Strips†

Abstract

Galanin (Gal) constricts rat gastric fundus by acting on receptors located in the cell membrane. We compared the role of intracellular Ca2+ release with extracellular Ca2+ influx in Gal-stimulated contraction of isolated gastric smooth muscle strips. We also tested if phospholipase C (PLC) or protein kinase C (PKC) participate in the signal transduction cascade. Concentration-contraction curves were constructed non-cumulatively in the presence of atropine, hexamethonium, guanethidine and tetrodotoxin. The half-maximum effective concentration (EC50) of Gal was 21.62nM and Hill's coefficient was 1.02. The effects of Gal were decreased by diltiazem, Ca2+-deficiency in the buffer, Ca2+ removal from the extracellular medium or quercetin. Depletion of intracellular Ca2+-stores, ryanodine and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester diminished the contractile effect of Gal concentration-dependently. Trifluoroperazine and phosphatidylinositol-specific phospholipase C (PI-PLC) inhibitors, neomycin and U-73122, attenuated the gastric fundus response to Gal, whereas phosphatidylcholine-specific phospholipase C (PC-PLC) and phospholipase D (PLD) blockers, D609 and propranolol, were ineffective. The inhibitors of PKC or myosin light chain kinase, calphostin C, chelerythrine, ML-7 and ML-9, lowered the myogenic activity of Gal. Our data confirmed that the stimulation of Gal receptors in gastric fundus is coupled to Ca2+ influx through voltage-dependent channels and intracellular Ca2+ release from ryanodine- and inositol 1,4,5-triphosphate-sensitive stores. Enzymes such as PI-PLC and PKC, but not PC-PLC or PLD, play a role in the signal transduction cascade. Calmodulin and myosin light chain kinase lie downstream of the increases in intracellular Ca2+ concentration evoked by Gal.