Nitric oxide (NO) may serve as a non-adrenergic, non-cholinergic (NANC) neurotransmitter released from enteric inhibitory nerves in the gastrointestinal tract. We tested whether guanosine 3':5'-cyclic monophosphate (cyclic GMP) may serve as a second messenger in transducing the NO signal into inhibitory junction potentials (i.j.ps) and relaxation in the canine proximal colon. 2. The membrane permeable analogue of cyclic GMP, 8-bromo cyclic GMP (8-Br-cyclic GMP) mimicked the effects of NO by hyperpolarizing cells near the myenteric border of the circular muscle layer and shortening slow waves in cells near the submucosal surface of the circular muscle layer. 8-Br-cGMP also inhibited spontaneous phasic contractions. 3. The specific cyclic GMP phosphodiesterase inhibitor, M&B 22948, hyperpolarized cells near the myenteric border and prolonged the duration of i.j.ps. M&B 22948 also inhibited phasic contractile activity. 4. Methylene blue failed to reduce significantly the amplitude and duration of i.j.ps and had variable effects on contractions. 5. Cyclic GMP levels were assayed in unstimulated muscles and in muscles exposed to exogenous NO and electrical field stimulation. Both stimuli hyperpolarized membrane potential, inhibited contractions, and elevated cyclic GMP levels. 6. Treatment of muscles with L-NG-nitroarginine methyl ester (L-NAME) increased spontaneous contractile activity and lowered cyclic GMP levels. The inhibitory effect of M&B 22948 on contractions was greatly reduced after muscles were treated with L-NAME. 7. These data support the concept that the effects of NANC nerve stimulation and NO (which may be one of the enteric inhibitory transmitters) may be mediated by cyclic GMP.
