To determine the time course of increased branchial Na, K-ATPase ESA, 100% SW-acclimated crabs were abruptly transferred to 50% SW and Na, K-ATPase ESA of the sixth gill and hemolymph osmotic pressures were measured for 14 d (Fig. 5). Although hemolymph osmotic pressure decreased rapidly and was significantly lower than control values after only 12 h, branchial Na, K-ATPase ESA did
not increase significantly from control values until 3 d after transfer. This time course is consistent with both rapid modulation of existing Na, K-ATPase activity in branchial
ion transport cells in the short term and a longer-term increase in the amount of enzyme present in the cells.