Effect of thyroxine on Na+/K+-ATPase and SDH activities in gills and kidney during osmotic adjustments of Heteropneustes fossilis at higher salinity

Abstract

Aim: The aim of the present study was to investigate the facilitatory role of thyroxine during adaptation process of catfish, Heteropneustes fossilis in higher salinity following its exogenous administration by monitoring changes in plasma osmolality, branchial and renal sodium-potassium dependent adenosine triphosphatase and succinic dehydrogenase enzyme activity profiles and study the effect of direct transfer of fish to higher salinities on plasma thyroxine levels. Methodology: Catfish were directly transferred from tap water to 30 and 35% sea water and plasma thyroxine profiles were analysed. Fish were injected with thyroxine at a dose of 2 and 5 µg g-1 b.wt. daily for five days and then transferred to tap water and 30% sea water. Fish were sampled after day 3 and day 6, post-transfer and plasma osmolality was measured, and the enzyme activities were determined in gills and kidney. Results: Higher levels of plasma thyroxine were observed after transfer of fish from tap water to higher salinities. Treatment of fish with thyroxine at higher dose (5 µg g-1 b.wt.) showed a significant increase in plasma osmolality in tap water (p<0.05) while the enzymes in gills were found to be significantly higher both in tap water (p<0.05; p<0.01for Na+/K+-ATPase and p<0.001 for SDH) and 30% sea water (p<0.001for Na+/K+-ATPase and p<0.01; p<0.001 for SDH). No significant changes were observed in any of the parameters analysed after exogenous administration of thyroxine at lower dose (2 µg g-1 b.wt.). Interpretation: Thyroxine affects the osmotic adjustment of fish following transfer to higher salinities and its exogenous administration at at a dose of 5 µg g-1 b.wt. may improve the hypoosmoregulatory ability. Key words: Hormonal control, Osmoregulation, Teleost, Thyroid hormones