Gill Na+-K+ ATPase, carbonic anhydrase activities and plasma osmotic and ionic variations during smoltification of Atlantic salmon in the north of Portugal

Abstract

Gill Na+-K+ ATPase and carbonic anhydrase activities were measured, on a fortnightly basis, from February to July, in 0+ age Atlantic salmon (Salmo solar), hatched and reared in a freshwater experimental station in Covas, northern Portugal. Plasma osmolarity and ionic composition were also measured. Gill Na+-K+ ATPase activity increased slowly until April (15–19 Μmoles Pi mg prot−1 h−1). From April to late May there was a great increase in activity (19–32 Μmoles Pi mg prot−1 h−1) followed by a sharp decline in June (15 Μmoles Pi mg prot−1 h−1). In contrast, carbonic anhydrase activity decreased significantly from early April to early June (170-70 Μmoles p-nitrophenol mg prot−1 h−1) and increased in late June, suggesting the existence of a compensatory mechanism for the changes in Na+-K+ ATPase activity. Plasma osmolarity and sodium concentration showed lower levels during the period of high ATPase activity. On the other hand, plasma calcium concentrations showed an increase during the same period (3.47–5.98 mm1−1 of plasma). A transitory decrease in osmolarity and plasma sodium and chlorine concentrations occurred in March, prior to the surge in Na+-K+ ATPase activity, suggesting that the physiological changes, characteristic of parr-smolt transformation can be a consequence of this loss of freshwater osmoregulatory capacity.