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 subunits of the guanine nucleotide binding protein pool, was stimulated by the A~ adenosine receptor agonists N6(R-phenylisopropyl)adenosine and N*-cyclopentyladenosine, and the muscarinic cholinergic agonist carbamyl choline. Pertussis toxin-catalyzed ADP-ribosylation of the membranes for 2 h at 5 ~ prior to the GTPase assay decreased the basal GTPase activity 30-40% and abolished N 6 (R-phenylisopropyl) adenosine stimulation of GTP hydrolysis. Basal high-affinity hydrolysis of GTP, measured at 0.3 gmol.14GTP, 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 Vm~x values are increased 11 and 15.9% in S. alascanus and S. altivelis, respectively. The apparent volume changes associated with the decreased K~ of GTP and the increased Vm~x ranged from -7.0 to -9.9 ml. mol "1. Increased pressure markedly decreased the efficacy of N 6 (R-phenylisopropyl) adenosine, N6-cylcopentyladenosine and carbamyl choline in stimulating GTPase activity. The effects of increased hydrostatic pressure on transmembrane sig-Abbreviations: [3H] DPCPX, 3H cyclopentyl-1, 3-dipropylxanthine; AppNHp, 5'-adenylylimidodiphosphate; cpm, counts per minute; CPA, N6-cyclopentyladenosine; EDTA, ethylenediaminetetra acetic acid; EGTA, ethyleneglycol-bis ([3-aminoethylether) N, N, N', N'-tetra-acetic acid; G protein, guanine nucleotide binding protein; Gi, inhibitory G protein; Go, other G protein, common in brain membranes; G~, stimulatory G protein; GTPase, guanosine triphosphatase; K~, inhibition constant; Km, Michaelis constarit; pK~, -log of the dissociation constant; R-PIA, N 6 (R-phenylisopropyl) adenosine; TRIS, tris[hydroxymethyl]aminomethane; V ..... maximal velocity; [7-32p]GTP, [7-32p] guanosine 5'-triphosphate (tetra (triethylammonium) salt); Correspondence to: J. F. Siebenaller nal transduction by the A 1 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.