NONADRENERGIC, NONCHOLINERGIC RELAXATION MEDIATED BY NITRIC-OXIDE WITH CONCOMITANT CHANGE IN CA2+ LEVEL IN RECTAL CIRCULAR MUSCLE OF RATS

The mediators of nonadrenergic, noncholinergic (NANC) relaxation of th e circular muscle of rat rectum were examined in vitro in the circular muscle of rat rectum, N-G-nitro-L-arginine (L-NOARG) at 10 mu M did n ot affect electrical field stimulation-induced relaxation but at 100 m u M it inhibited electrical field stimulation-induced relaxation by ab out 75% and 1-mM L-arginine reversed the inhibition. Exogenous nitric oxide (NO) (1-10 mu M) concentration dependently relaxed the circular muscle. Electrical field stimulation increased the cyclic GMP content of the circular muscle to about twice its resting level. L-NOARG, even at 10 mu M, completely inhibited the electrical field stimulation-ind uced elevation of cyclic GMP content. However, L-arginine at 1 mM did not reverse the inhibition in cyclic GMP content. Inhibitory junction potentials (i.j.ps) induced by electrical field stimulation in the cir cular muscle cells were not affected by L-NOARG, 100 mu M Apamin (less than or equal to 1 mu M) did not affect the electrical field stimulat ion-induced relaxation, but almost completely inhibited electrical fie ld stimulation-induced i.j.ps. NO (0.3-10 mu M) induced relaxation of the circular muscle with a concomitant decrease in intracellular Ca2level ([Ca2+](i)). Abundant immunoreactivity of NO synthase was found in the circular muscle layer, in addition to myenteric and submucosal plexus. The results suggest that NO induces NANC relaxation with a con comitant change in [Ca2+](i) in the circular muscle of rat rectum. How ever, the involvement of changes in cyclic GMP level and in membrane p otentials in the mechanism was not shown in the present experimental c onditions. (C) 1998 Elsevier Science B.V. All rights reserved.