Abstract
To investigate, the effects of hydrostatic pressure on transmembrane signaling in cold-adapted marine fishes, we examined the high-affinity GTPase activity in two congeneric marine fishes, Sebastolobus alascanus and S. altivelis. In brain membranes there are two GTPase activities, one with a low Km and one with a high Km for GTP. The high-affinity GTPase activity, characteristic of the α subunits of the guanine nucleotide binding protein pool, was stimulated by the A1 adenosine receptor agonists N6(R-phenylisopropyl)adenosine and N6-cyclopentyladenosine, and the muscarinic cholinergic agonist carbamyl choline. Pertussis toxin-catalyzed ADP-ribosylation of the membranes for 2 h at 5°C prior to the GTPase assay decreased the basal GTPase activity 30-40% and abolished N6 (R-phenylisopropyl)adenosine stimulation of GTP hydrolysis. Basal high-affinity hydrolysis of GTP, measured at 0.3 μmol·1-1GTP, was stimulated 22% in both species by 340 atm pressure. At 340 atm pressure, the apparent Km of GTP is decreased approximately 10% in each of the species, and the Vmax values are increased 11 and 15.9% in S. alascanus and S. altivelis, respectively. The apparent volume changes associated with the decreased Km of GTP and the increased Vmax ranged from-7.0 to-9.9 ml·mol-1. Increased pressure markedly decreased the efficacy of N6 (R-phenylisopropyl) adenosine, N6-cylcopentyladenosine and carbamyl choline in stimulating GTPase activity. The effects of increased hydrostatic pressure on transmembrane signal transduction by the A1 adenosine receptor-inhibitory guanine nucleotide binding protein-adenylyl cyclase system may stem, at least in part, from pressure-increased GTP hydrolysis and the concomitant termination of inhibitory signal transduction.