Abstract
The effects of hydrostatic pressure on branchial NaK‐ATPase and Mg‐ATPase activities were studied in two species of marine teleost fish: the relatively shallow water Scorpaena guttata, collected from a depth of 20 m, and the deeper living Anoplopoma fimbria, collected from a depth of 310 m. Specific activities in gill microsomes, measured at 18^o^C, were similar in both species, with the NaK‐ATPase usually comprising about 30–40% of the total ATPase activity. NaK‐ATPase activity of S. guttata was activated by low hydrostatic pressure (68 atm) but activity was inhibited at pressures above 136 atm. The NaK‐ATPase of A. fimbria showed a similar activation at low hydrostatic pressure but was less sensitive to a pressure of 340 atm than the __enzyme__from S. guttata. At a pressure of 476 atm, the NaK‐ATPase activity of both species was inhibited to the same extent. The Mg‐ATPase of both species was activated by pressure (maximal activation occurred at 136 atm), but in contrast to the NaK‐ATPase, remained activated at all pressures tested. Solubilization of the enzymes from gill microsomes of A. fimbria with Lubrol WX did not significantly alter the pressure responses. Inhibition of NaK‐ATPase at high pressure was reversible upon release of pressure. Inhibition of branchial NaK‐ATPase in both species occurred at pressures higher than the fish normally experience in their environment.