FRESHWATER ADAPTATION IN THE EURYHALINE TELEOST, CHELON-LABROSUS .1. EFFECTS OF ADAPTATION, PROLACTIN, CORTISOL AND ACTINOMYCIN-D ON PLASMA OSMOTIC BALANCE AND (NA+-K+)ATPASE IN GILL AND KIDNEY

Microsomal (Na+K+)ATPase activities in gills and kidneys of thick-lipped mullet, Chelon labrosus, which had been gradually acclimated to fresh water were studied using initial velocity measurements. In mullet transferred to fresh water, the branchial ATPase activity rises gradually during a 2-week period (long-term fresh water adaptation: L-TFW). The renal ATPase activity rises rapidly after the transfer to fresh water (FW). In the L-TFW mullet and in seawater (SW) mullet, cortisol injections cause no, or only slight, stimulation of renal or branchial ATPase activity; on the other hand, cortisol causes a marked decrease in ATPase activity in mullet adapted to FW for only a short period of time (S-TFW). Only branchial ATPase, and not kidney ATPase, is inhibited by prolactin, and this only in S-TFW and SW mullet. Actinomycin D does not inhibit renal ATPase activity, but the response of branchial ATPase to this antibiotic depends on the environment; SW-acclimated mullet are sensitive to actinomycin D but the longer mullet are acclimated to FW the more insensitive they become. These findings may indicate that more than one functional form of gill (Na++K+)ATPase exists, and that these vary in activity with external salinity. © 1979.