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WATER AND ELECTROLYTE HOMEOSTASIS

Prostaglandin E₂ inhibits vasotocin-induced osmotic water permeability in the frog urinary bladder by EP₁-receptor-mediated activation of NO/cGMP pathway

¹Laboratory of Renal Physiology, I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry of the Russian Academy of Sciences, St. Petersburg, Russia; and ²Institute of Clinical Biochemistry and Pathobiochemistry, University of Würzburg, Würzburg, Germany

Submitted 17 November 2006 ; accepted in final form 9 March 2007

PGE₂ is a well-known inhibitor of the antidiuretic hormone-induced increase of osmotic water permeability (OWP) in different osmoregulatory epithelia; however, the mechanisms underlying this effect of PGE₂ are not completely understood. Here, we report that, in the frog Rana temporaria urinary bladder, EP₁-receptor-mediated inhibition of arginine-vasotocin (AVT)-induced OWP by PGE₂ is attributed to increased generation of nitric oxide (NO) in epithelial cells. It was shown that the inhibitory effect of 17-phenyl-trinor-PGE₂ (17-ph-PGE₂), an EP₁ agonist, on AVT-induced OWP was significantly reduced in the presence of 7-nitroindazole (7-NI), a neuronal NO synthase (nNOS) inhibitor. NO synthase (NOS) activity in both lysed and intact epithelial cells measured as a rate of conversion of L-[³H]arginine to L-[³H]citrulline was Ca²⁺ dependent and inhibited by 7-NI. PGE₂ and 17-ph-PGE₂, but not M&B-28767 (EP₃ agonist) or butaprost (EP₂ agonist), stimulated NOS activity in epithelial cells. The above effect of PGE₂ was abolished in the presence of SC-19220, an EP₁ antagonist. 7-NI reduced the stimulatory effect of 17-ph-PGE₂ on NOS activity. 17-ph-PGE₂ increased intracellular Ca²⁺ concentration and cGMP in epithelial cells. Western blot analysis revealed an nNOS expression in epithelial cells. These results show that the inhibitory effect of PGE₂ on AVT-induced OWP in the frog urinary bladder is based at least partly on EP₁-receptor-mediated activation of the NO/cGMP pathway, suggesting a novel cross talk between AVT, PGE₂, and nNOS that may be important in the regulation of water transport.

amphibian urinary bladder; nitric oxide; neuronal nitric oxide synthase