Na+-K+-ATPase and Na+/Ca2+ exchange activities in gills of hyperregulating Carcinus maenas

Na+-K+-ATPase and Na+/Ca2+ exchange activities were studied in gills of Carcinus maenas in seawater(SW)and after transfer to dilute seawater (DSW). Carcinus hyperregulates its hemolymph osmolarity through active uptake of Na+, Cl-, and Ca2+. In DSW total Na+-K+-ATPase activity in posterior gills quadrupled; Na+/Ca2+ exchange specific activity was unaffected, and total activity increased 1.67-fold. Short-circuit current (I-sc) in voltage-clamped posterior gill hemilamellae was - 181 mu A/cm(2) in SW and -290 mu A/cm(2) in DSW and up to 90% ouabain sensitive; conductivity was similar in SW or DSW (42 and 46 mS/cm(2), respectively) and representative of a leaky epithelium. The new steady state of hemolymph osmolarity 24 h after DSW transfer was preceded, already 3 h after transfer, by increased Na+-K+-ATPase but not Na+/Ca2+ exchange activity. Western blot analysis indicated that the amount of Na+-K+-ATPase protein had increased 2.1-fold in crabs acclimated 3 wiz to DSW; however, 24 h after DSW transfer no difference in the amount of Na+-K+-ATPase-protein was observed. After DSW transfer branchial cAMP content decreased. A negative correlation between branchial Na+-K+-ATPase activity and cAMP content points to rapid regulation of Na+;Ki;ATPase through cAMP-dependent protein kinase A activity. Ca2+ transport may depend on the high-capacity Na+/Ca2+ exchanger coupled to the versatile sodium pump.