1 We studied the relation of nitric oxide-mediated relaxation of smooth muscle to changes in membrane potential of cells in the proximal colon of rats. 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 (ij.ps) were recorded from all circular muscle cells tested, but rarely from the longitudinal muscle cells. 5 The ij.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 ij.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' nitro-L-arginine (10 JLM) inhibited the relaxations of both muscles, but did not affect the ij.ps. 8 Exogenously added nitric oxide (0.1-101AM) induced relaxations of both muscles concentrationdependently, 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 ij.ps of the cell membrane.

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