NITRIC OXIDE-MEDIATED INHIBITORY RESPONSE OF RAT PROXIMAL COLON - INDEPENDENCE FROM CHANGES IN MEMBRANE-POTENTIAL

1 We studied the relation of nitric oxide-mediated relaxation of smooth muscle 2 The resting membrane potential and electrical field stimulation (EFS)-induced junction potentials were recorded from the circular and longitudinal muscle cells. 3 Localized distension with a small balloon caused relaxation of the circular muscle on the anal side of the distended region (descending relaxation). Relaxation of the longitudinal muscle was also induced by EFS. 4 Inhibitory junction potentials (i.j.ps) were recorded from all circular muscle cells tested, but rarely from the longitudinal muscle cells. 5 The i.j.ps were recorded only in the presence of atropine but relaxations of both muscles were induced even in the absence of atropine. 6 Apamin (100 nM) completely abolished the i.j.ps recorded in both circular and longitudinal muscle cells, but had no significant effect on the relaxations of either. 7 In contrast to apamin, N-G nitro-L-arginine (10 mu M) inhibited the relaxations of both muscles, but did not affect the i.j.ps. 8 Exogenously added nitric oxide (0.1-10 mu M) induced relaxations of both muscles concentration-dependently, but did not affect the membrane potentials at these concentrations. 9 These data strongly suggest that nitric oxide-mediated relaxation of rat proximal colon is not associated with the i.j.ps of the cell membrane.