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The Journal of Experimental Biology 206, 1503-1510 (2003)
doi: 10.1242/jeb.00284

Regulation of Na⁺/K⁺-ATPase activity by nitric oxide in the kidney and gill of the brown trout (Salmo trutta)

Christian K. Tipsmark and Steffen S. Madsen^*

Institute of Biology, University of Southern Denmark, Odense University, Campusvej 55, DK-5230 Odense M, Denmark

^* Author for correspondence (e-mail: steffen{at}biology.sdu.dk)

Accepted 10 February 2003

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.

Key words: brown trout, Salmo trutta, kidney, gill, Na⁺/K⁺-ATPase, nitric oxide, SNP, sodium nitroprusside, cyclic GMP.

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