RENAL NA,K-ATPASE AND OSMOREGULATION IN THE CRAYFISH, PROCAMBARUS-CLARKII

The freshwater crayfish, Procambarus clarkii, is a strong hyperosmoreg ulator in fresh water (FW) and 25% sea water (25% SW), and it weakly h yperosmoregulates in 50 and 75% SW. Procambarus clarkii produces a dil ute urine in FW and 25% SW, but the urine becomes progressively more c oncentrated in 50% SW and is nearly isosmotic with the haemolymph in 7 5% SW. Antennal gland Na,K-ATPase ESA was highest in crayfish acclimat ed in FW and decreased significantly in animals acclimated for two wee ks or two months in 50 and 75% SW. It is hypothesized: (a) that only a portion of the antennal gland Na,K-ATPase powers renal salt reabsorpt ion and, thus, the production of dilute urine in P. clarkii, and (b) t hat P. clarkii may maintain high ''basal'' levels of antennal gland Na ,K-ATPase to power other, non-osmoregulatory transport functions such as organic acid secretion into the urine and sugar and amino acid reab sorption from it. The time course of decreased antennal gland ESA afte r abrupt transfer of crayfish from FW to 50% SW is consistent with enz yme degradation.