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We investigated the effects of nitric oxide (NO) donors, S-nitroso-L-cysteine (Cys-NO) and 3-morpholinosydnonimine hydrochloride (SIN-1), on the contractile and electrical activity of the circular muscle of guinea pig gastric antrum by using intracellular microelectrode technique. The gastric antral circular muscle showed spontaneous phasic contraction and slow wave of membrane potential. Cys-NO (0.001∼10 μM) and SIN-1 (0.01∼100 μM) reduced not only the tonic and phasic contraction but also the amplitude of slow wave in a concentration dependent manner. NO donors were more potent to inhibit phasic contraction than to do slow wave. These inhibitory effects of NO donors were mimicked by the membrane permeable guanosine-3´,5´-cyclic monophosphate (cGMP) analogue, 8-bromo-cyclic GMP (8-br-cGMP, 10∼300 μM). The inhibitory effects of SIN-1 and Cys-NO were antagonized by the guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 10 μM ). These results suggest that the inhibitory effects of NO donors on the mechanical and electrical activity is mainly mediated by cGMP pathway.
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