Platelet activating factor and U44069 stimulate a GTPase activity in human platelets which is distinct from the guanine nucleotide regulatory proteins, Ns and Ni.

Platelet-activating factor (PAF, 2-acetyl-1-alkyl-sn-glycero-3-phosphocholine) and the stable thromboxane-receptor agonist U44069 (9 alpha, 11 beta-epoxymethanoprostaglandin H2) stimulated GTPase activity in platelet membranes in a dose-dependent fashion, yielding Ka values of 12 nM and 27 nM respectively. The degree of GTPase activation elicited by these agents was found to be additive with the GTPase activation due to either the stimulatory (Ns) or inhibitory (Ni) guanine nucleotide regulatory proteins when activated by prostaglandin E1 and adrenaline (+propranolol) respectively. Treatment of membranes with either cholera or pertussis toxins, which inhibited markedly the receptor-mediated stimulation of the GTPase activities of Ns and Ni respectively, had no or only a small effect, respectively, on the GTPase activity stimulated by PAF and U44069. It is suggested that PAF and U44069, which stimulate inositol phospholipid metabolism in platelets, exert actions through a guanine nucleotide regulatory protein which is distinct from Ns and Ni.