SUMMARY
In teleost fish, successful osmoregulation involves controlled ion transport mechanisms in kidney and gill epithelia. In this study, the effect of nitric oxide (NO) on Na+/K+-ATPase was investigated in vitro in these two tissues in brown trout (Salmo trutta)acclimated to freshwater. The NO donor sodium nitroprusside (SNP) inhibited in situ Na+/K+-ATPase activity, measured as ouabain-sensitive Rb+ uptake, in both samples of kidney and gill tissue and in isolated gill cells. The effect was dose-dependent in both tissues, with a maximal observed inhibition of approximately 40–50% (1 mmol l –1 SNP). The time-course of inhibition revealed a maximum effect with 10 min pre-incubation. The effect of SNP was reproduced with another NO donor, papa-nonoate (NOC-15; 200 μmol l–1), and was prevented by the NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO; 1 mmol l–1). To further investigate the mechanism of the NO effect,whole-tissue Na+ and K+ levels were analysed. In kidney,SNP (1 mmol l–1) led to an increase in tissue Na+levels and a decrease in K+ levels in a 3:2 ratio. In gill tissue,no change in either ion was observed. These observations indicate that the effect on Na+/K+-ATPase is direct rather than due to a decrease in intracellular Na+, its rate-limiting substrate. SNP elevated the level of cyclic GMP (cGMP) in both kidney and gill tissue. Dibutyryl cyclic GMP (db-cGMP; 1 mmol l–1) also inhibited Na+/K+-ATPase activity in both tissues. Hence, a possible mechanism may involve the cGMP-activated kinase, even though other mechanisms cannot be excluded.
