Hydrostatic pressure alters the time course of GTP

The effects of hydrostatic pressure on the receptor-stimulated exchange of guanosine triphosphate (GTP) for guanosine diphosphate (GDP) on the a subunit of G proteins were studied in two congeneric marine teleost fishes that differ in their depths of distribution. The poorly hydrolyzable GTP analog [35S]guanosine 5'-[gamma-thio]triphosphate ([35S]GTP[S]) was used to monitor the modulation of signal transduction by the A1 adenosine receptor agonist N6-R-(phenylisopropyl)adenosine (R-PIA) in brain membranes of the scorpaenids Sebastolobus alascanus and S. altivelis. The maximal binding (Bmax) and dissociation constant (K(d)) values, determined from equilibrium binding isotherms at atmospheric pressure (5 degrees C), were similar in the two species. The Bmax values for these species are much lower than literature values for mammalian brain tissue (25 degrees C); however, the K(d) values of the teleost and mammalian G proteins are similar. The EC50 values for the A1 adenosine receptor agonist R-PIA were similar in the two species. Hydrostatic pressure of 204 atm altered the binding of [35S]GTP[S]; basal [35S]GTP[S] binding decreased 25%. The A1 adenosine receptor agonist R-PIA and the muscarinic cholinergic receptor agonist carbamyl choline stimulated [35S]GTP[S] binding at 1 and 204 atm. At atmospheric pressure the half-time (t1/2) of [35S]GTP[S] binding differed between the two species. The GTP[S] on rate (k(on)) is larger in the shallower-living S. alascanus. Increased hydrostatic pressure altered the time course, decreasing the t1/2 in both species. The pressures that elicit this change in the time course differ between the species. However, interpolating over the range of in situ pressures the species experience, the values are similar in the two species. The guanyl nucleotide binding properties of the G protein a subunits appear to be conserved at the environmental temperatures and pressures the species experience.