SALINITY-INDUCED CHANGES IN BRANCHIAL Na/K- ATPase ACTIVITY AND TRANSEPITHELIAL POTENTIAL DIFFERENCE IN THE BRINE SHRIMP ARTEMIA SAUNA

Silver staining of the adult brine shrimp, Artemia salina, revealed that only the metepipodites of the phyllopodia were significantly permeable to chloride and/or silver ions. The metepipodites stained in a reticulated pattern, possibly indicating areas in the cuticle over cells specialized for chloride secretion. Crude homogenates of metepipodites had very high Na/K-ATPase enzyme specific activity (ESA) which increased in proportion to the salinity of the external medium and, thus, in proportion to the need for outward salt transport in these strongly hypoosmoregulating animals. Metepipodite ESA as a percentage of whole-body ESA increased from 7.6% in 50% sea water (SW) to 25.0% in 400% SW. Gut and maxillary gland also had high Na/K-ATPase ESAs, implicating these organs in osmoregulatory processes as well. The time courses of increases in phyllopodial and gut ESAs in brine shrimps transferred from 100 % SW to 400 % SW are consistent with the induction of new Na/K-ATPase; 4-7 days was required for significant increases to occur. Haemolymph ion analyses and transepithelial potential differences, measured in brine shrimp acclimated in all the SW media, indicate that chloride is actively transported out of the brine shrimp while sodium is very close to electrochemical equilibrium across the body wall. Thus, the metepipodites of the brine shrimp appear to possess cells with many functional similarities to the teleost branchial chloride cells.