Insulin stimulates a novel GTPase activity in human platelets

Abstract

Insulin stimulated the activity of a high-affinity GTPase activity in human platelet membranes some 62% over that of the basal activity. Half-maximal stimulation (K _a) was achieved with 3.1 nM insulin. The K _m for GTP of the insulin-stimulated GTPase was 0.6 μM GTP. Treatment of isolated platelet membranes with cholera toxin, but not pertussis toxin, blocked insulin's ability to stimulate GTPase activity. Cholera toxin acted as a more potent inhibitor of the insulin-stimulated GTPase activity than that of the GTPase activity of the stimulatory guanine nucleotide regulatory protein, G_s, as monitored by stimulation using prostaglandin E₁ (PGE₁). Mixed ligand experiments showed that insulin stimulated GTPase activity in an additive fashion to GTPase activity stimulated by PGE₁, due to G_s; by adrenaline (+ propranolol), due to the inhibitory guanine nucleotide regulatory protein, G_i and by vasopressin, which stimulates the putative ‘G_p’, a G-protein suggested to control the stimulation of inositol phospholipid metabolism. Insulin thus appears to stimulate a novel high-affinity GTPase activity in human platelet membranes. This may reflect the functioning of the putative G_ins, a guanine nucleotide regulatory protein which has been suggested to mediate certain of insulin's actions on target tissues.