When the stenohaline catfish Heteropneustes fossilis was transferred from fresh water (FW) to 30% seawater (SW), the Na(+)/K(+)-ATPase activity significantly increased in the kidney, while in gills it remained more or less constant. A reverse pattern was observed for succinic dehydrogenase (SDH) activity inasmuch as it significantly increased in gills and remained unchanged in the kidney. Plasma osmolality significantly increased within 3 days of transfer to 30% SW and remained significantly higher throughout the duration of experiment. These results suggest that catfish gills may not be able to reverse their function from salt uptake in FW to salt excretion at higher salinity, and that the elimination of monovalent as well as divalent ions is performed by the kidney but not the gills. The significant decline in plasma cortisol (F) levels following transfer to higher salinity may not be due to reduced production but rather to an enhanced utilization and clearance rate, a conclusion supported by the fact that exogenous administration of cortisol acetate (FA) resulted in significant increases in branchial and renal Na(+)/K(+)-ATPase in FW and 30% SW. FA also improved the plasma osmotic regulatory ability of the catfish, possibly due to a change in branchial function from salt-absorption to salt excretion, as was evident from a significant increase in branchial Na(+)/K(+)-ATPase activity in the fish in 30% SW pretreated with FA for 5 days. Consistently higher levels of plasma thyroxine (T4) following transfer to higher salinity suggest the involvement of this hormone at higher salinity.

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Transfer of the stenohaline catfish, Heteropneustes fossilis from tap water (TW) to deionized water (DW) resulted in an increase in the glomerular filtration rate, urine volume and osmolar and free water clearance. In a closed system, where the DW was renewed only once a day, no change in the plasma osmolality was evident for up to 14 days. When DW was renewed four times a day for 25 days, a significant reduction in the plasma osmolality was observed within 24h. When the fish were transferred back to TW, plasma osmolality increased to normal freshwater level within 24h. These observations suggest the existence of highly efficient branchial mechanisms for active uptake of salts from an exceedingly dilute ambient medium. The fact that prolactin-secreting cells as well as corticotrophs in the pituitary of the fish in DW were highly stimulated suggests the involvement of the hormones in the adaptive responses of the catfish to DW.

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

Cortisol is a major hormone directly associated with stress in fish and is a reliable physiological indicator of primary stress response in fish, whereas glucose and osmolality are the indicators of secondary stress response in fish. This study explored the stress levels in the catfish, Clarias batrachus (Magur) by measuring the cortisol, glucose, and osmolality levels in plasma by exposing the fish to three different kinds of interventions namely, non-anaesthetized, anaesthetized, and stressed. No statistically significant changes were reported in the plasma cortisol, plasma glucose, and plasma osmolality levels when the blood samples were collected after the three interventions. These results indicated that Clarias batrachus is a sturdy fish, which can withstand routine laboratory handling, and that the blood samples can be collected without anaesthetization.