Some aspects of osmoregulation in a stenohaline freshwater catfish, Heteropneustes fossilis (Bloch), in different salinities

Abstract

Transfer of the catfish, Heteropneustes fossilis, to 10% sea water (101 mosmol l−1) or to 0·4% NaCl (140 mosmol l−1) does not evoke any change in plasma osmolarity from the normal freshwater values. There is, however, a reduction in urine flow rate (UFR) and increase in urine osmolarity without any change in the rate of osmolar clearance. In isosmotic (25% sea water or 0·7% NaCl) and in hyperosmotic (30% sea water or 1·1% NaCl) media there is a significant increase in plasma osmolarity accompanied by marked reduction in glomerular filtration rate (GFR), UFR and free water clearance. The results suggest that the catfish cannot effectively osmoregulate in isosmotic or hyperosmotic media and that the inability of the renal tubules to increase reabsorption of water and to reduce free water clearance may account for the restricted range of salinity tolerance of this catfish. Also, in the hyperosmotic media, plasma levels of cortisol are lowered while in the proximal pars distalis the corticotrophs appear active, suggesting increased utilization and clearance of cortisol. Prolactin‐secreting cells, however, are degranulated and chromophobic in catfish maintained in hyperosmotic environment.