Nitric oxide inhibits contraction of isolated pig ureteral smooth muscle

Purpose: To investigate the L-arginine/nitric oxide (NO) pathway in the pig isolated ureter. Materials and Methods: Functional inhibitory effects mediated by NO were assessed and correlated with cyclic nucleotide levels. Nitric oxide synthase (NOS) activity was measured by monitoring the conversion of [H-3]-arginine to [H-3]-citrulline. Immunohistochemical studies were performed. Results: The NO-donor SIN-1 reduced in a concentration-dependent manner the frequency of contractions, whereas NO completely interrupted the contractile activity. In precontracted strips exposed to SIN-1 or NO, there were 6- and 12-fold increases of the cyclic GMP levels in comparison with control preparations. Activity of NOS was moderate. Overall innervation of the ureter was sparse, and there were few NOS-immunoreactive nerves. Conclusion: Although few NOS-containing nerves were found, pathways regulating the cyclic GMP levels of pig ureteral smooth muscle were demonstrated. Such pathways may be important targets for drugs producing relaxation of the mammalian ureter.