Nitric oxide inhibits arginine-vasotocin-induced increase of water osmotic permeability in frog urinary bladder

The present study addressed the question of whether nitric oxide (NO) participates in regulation of osmotic water permeability in the urinary bladder of the frog Rana temporaria L. Experiments were carried out on isolated, paired hemi-bladders filled with amphibian Ringer solution diluted 1:10 with distilled water. Sodium nitroprusside (SNP, 125–250 µM), an NO donor, markedly attenuated the increase of osmotic water flow elicited by arginine-vasotocin (AVT) (AVT 10−10 M: 2.20±0.26; AVT plus 200 µM SNP: 1.21±0.15 µl/min cm2, n=20, P<0.001). This effect of SNP was apparent only in the presence of 50 µM zaprinast, an inhibitor of the cGMP-specific phosphodiesterase-5 (PDE5). In the presence of zaprinast, SNP elevated cGMP production significantly both in control and AVT-stimulated urinary bladders, but had no effect on the level of cAMP (AVT 5×10−10 M: 7.6±0.6; AVT plus SNP 200 µM: 7.5±0.4 pmol/mg protein, n=8, N.S.). 1H-[1,2,4]-oxadiazole-[4,3-a]-quinoxalin-1-one (ODQ, 25–100 µM), an inhibitor of soluble guanylate cyclase, enhanced the AVT-induced water flow, decreased the SNP-stimulated increase of cGMP in the bladder tissue and almost abolished the inhibitory effect of SNP on the AVT-induced hydroosmotic response. 8-(p-Chlorophenylthio)-cGMP (8-pCPT-cGMP, 25 or 50 µM), a membrane-permeable cGMP analogue specific for cGMP-dependent protein kinase (PKG), inhibited, whereas 2 µM KT-5823, an inhibitor of PKG, significantly stimulated the increase of water flow induced by AVT. The inhibitory effect of SNP on AVT-induced water flow was almost completely reversed by KT-5823, but not by 50–100 µM erythro-9-[2-hydroxy-3-nonyl]adenine (EHNA), an inhibitor of cGMP-activated PDE2. Immunohistochemistry of urinary bladder slices with antibodies against different types of NO synthase (NOS) revealed a positive immunostaining for neuronal NOS (nNOS) in the mucosal epithelium. These results suggest that in the frog urinary bladder endogenous NO is involved in regulation of water osmotic permeability. NO inhibits the AVT-induced increase of water flow at least partly by activation of PKG, which interferes with the hydroosmotic effect of AVT probably at (a) post-cAMP step(s).