POSSIBLE INVOLVEMENT OF DIFFERENT GTP-BINDING PROTEINS IN NORADRENALINE-STIMULATED AND THROMBIN-STIMULATED RELEASE OF ARACHIDONIC-ACID IN RABBIT PLATELETS

Noradrenaline (NA) stimulated the release of arachidonic acid (AA) from the [3H]AA-labelled rabbit platelets via α2-adrenergic receptors, since the effect of NA was inhibited by yohimbine. The stimulatory effect of NA in digitonin-permeabilized platelets was completely dependent on the simultaneous presence of GTP and Ca2+. The NA- and thrombin-stimulated releases of AA were markedly decreased by the prior ADP-ribosylation of the permeabilized platelets with pertussis toxin. Antiserum directed against the pig brain G(o) (a GTP-binding protein of unknown function), recognizing both α39 and β35,36 subunits, but not α41, of pig brain, reacted with 41 kDa and 40 kDa bands, with not one of 39 kDa, in rabbit platelet membranes. Anti-G(o) antiserum inhibited guanosine 5'-[γ-thio]triphosphate-, AlF4--, NA- and thrombin-stimulated AA releases in the membranes. Although the effect of thrombin was inhibited by low concentrations of anti-G(o) antiserum, high concentrations of the antiserum were needed for inhibition of the NA effect. Antiserum directed against the pig brain G(i) (inhibitory G-protein), recognizing both α41 and β35,36 subunits, but not α39, of pig brain, reacted with the 41 kDa band in platelets. Anti-G(i) antiserum inhibited only the effect of NA. Reconstitution of the platelet membranes ADP-ribosylated by pertussis toxin with G(o), not G(i), purified from pig brain restored the thrombin-stimulated release of AA. In contrast, reconstitution of those membranes with G(i), not G(o), restored the NA-stimulated release of AA. These results indicate that different GTP-binding proteins, G(i)- and G(o)-like proteins, may be involved in the mechanism of signal transduction from α2-adrenergic receptors and thrombin receptors to phospholipase A2 in rabbit platelets.