Insulin stimulates a novel GTPase activity in human platelets

Area of Science:

• Biochemistry
• Cellular signaling
• Endocrinology

Background:

• Insulin mediates numerous cellular functions through complex signaling pathways.
• Guanine nucleotide-binding proteins (G-proteins) are key regulators of cellular signaling.
• The precise mechanisms of insulin action at the membrane level are not fully elucidated.

Purpose of the Study:

• To investigate the effect of insulin on GTPase activity in human platelet membranes.
• To characterize the novel GTPase activity stimulated by insulin.
• To explore the potential involvement of G-proteins in insulin-mediated GTPase activation.

Main Methods:

• Assay of high-affinity GTPase activity in isolated human platelet membranes.
• Determination of kinetic parameters (Ka, Km) for insulin-stimulated GTPase.
• Inhibition studies using cholera toxin and pertussis toxin.
• Mixed ligand experiments to assess additivity with other G-protein-coupled receptor agonists.

Main Results:

• Insulin significantly stimulated high-affinity GTPase activity (62%) in human platelet membranes.
• Half-maximal stimulation (Ka) was observed at 3.1 nM insulin, with a Km for GTP of 0.6 microM.
• Cholera toxin, but not pertussis toxin, inhibited insulin-stimulated GTPase activity.
• Insulin's effect was additive to stimulation by prostaglandin E1 (Gs), adrenaline (Gi), and vasopressin (Gp).

Conclusions:

• Insulin activates a novel, high-affinity GTPase in human platelet membranes.
• This GTPase activity is distinct from Gs, Gi, and Gp.
• The findings suggest the involvement of a putative G-protein, Gins, in mediating insulin's actions.