Inositol 1,4,5-Triphosphate-induced Granule Secretion in Platelets

Phosphoinositide hydrolysis in platelets stimulated by thrombin is thought to be regulated by a pertussis toxin-sensitive guanine nucleotide binding protein (G protein) referred to as Gp. The present studies examine the role of GP in platelet responses to the thromboxane A2 analogue U46619 and in the pathway by which the phosphoinositide hydrolysis product inositol 1,4,5triphosphate (IP3) causes secretion. In permeabilized platelets, U46619 caused phosphatidic acid formation and secretion, which were abolished by the G protein inhibitor, guanosine 5'-O(2thiophosphate) (GDP,6S). Unlike thrombin, however, U46619induced phosphoinositide hydrolysis was unaffected by pertussis toxin, and U46619 was unable to inhibit the I32PIADP ribosylation of the 42-kD pertussis toxin substrate in platelets. IP3induced secretion, which is known to depend upon intracellular Ca release and subsequent arachidonic acid metabolism, was also inhibited by GDPj#S, as was Ca-induced secretion. These observations suggest (a) that platelet thromboxane A2 (TxA2) receptors are coupled to a toxin-resistant form of Gp distinct from the one that is coupled to thrombin receptors, and (b) that TxA2-stimulated phosphoinositide hydrolysis may serve as a feedback mechanism by which stimuli for arachidonic acid release, such as IP3 and Ca, amplify responses to agonists.