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

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.

The hydroosmotic response of frog urinary bladder to serosal hypertonicity is dependent on adenylate cyclase for its maintenance and affected by [Cl−]o changes

The role of adenylate cyclase (AC) in the maintenance of the hydroosmotic response to serosal hypertonicity (SH) in anuran urinary bladder is disputed. In this study, norepinephrine (NE) significantly reversed the hydroosmotic response of Rana temporaria bladders in hypertonic medium (330 mosmol/kgH2O). The reversal was inhibited by yohimbine but was unaffected by prazosin and propranolol, indicating that NE action was mediated via α2-adrenergic receptors. Preincubation of bladders with indomethacin did not interfere with the inhibitory action of NE, contraindicating a role for prostaglandins. The SH hydroosmotic response was abolished in the presence of 5- n-ethyl- N-isopropyl amiloride (EIPA), but the antidiuretic hormone (ADH) hydroosmotic response was not. EIPA inhibits Na+/H+, known to be activated by cell shrinkage. An investigation of the anionic requirement of the SH hydroosmotic response revealed that replacement of bath Cl− with the nonpermeable anion gluconate reversibly abolished this response. In contrast, the hydroosmotic response to ADH was unaffected by Cl− removal; however, when Cl− was absent, it was no longer augmented in hypertonic bath. The SH response was inhibited by the Cl− channel blocker 5-nitro-2-(3-phenylpropylamino)benzoate but not by the Na/K/2Cl inhibitor bumetanide. Our results show that not only the onset but also the maintenance of the SH hydroosmotic response is dependent on AC activity and does not differ in this respect to the ADH hydroosmotic response. The effect of modifying extracellular Cl− concentration, suggests that this anion, possibly functionally linked with Na+/H+ activity, may be involved in invoking the SH hydroosmotic response in anuran urinary bladder.