Insulin Directly Reduces Platelet Sensitivity to Aggregating Agents: Studies In Vitro and In Vivo

The aim of this study was to investigate the influence of insulin on platelet function, both in vitro and in vivo. For the in vitro investigation, we evaluated whether insulin affects platelet function at a physiological hormone concentration by incubating the platelet-rich plasma (PRP) of fasting subjects with human regular insulin at the final concentration of 40 μU/ml for 30 min; we observed a significant reduction of platelet sensitivity to all the aggregating agents employed, i.e., ADP, platelet-activating factor (PAF), epinephrine, collagen, and Na⁺ arachidonate. To investigate whether the insulin effect on platelets is dose dependent, we incubated the PRP of fasting subjects with different concentrations of human regular insulin (40, 80, 120, and 160 μU/ml) for 5 min, and we observed that the insulin-induced reduction of platelet sensitivity to aggregating agents is a dose-dependent phenomenon. Furthermore, the comparison between the platelet responses after 5 and 30 min of incubation with insulin showed that the insulin effect on platelet aggregation is time dependent. The lack of specificity of its inhibiting activity suggests that insulin does not interfere with the initial binding of each aggregating agent at specific sites but does influence a common step of platelet aggregation. Our study rules out the possibility that insulin reduces platelet-function–modifying intraplatelet cAMP levels or thromboxane A₂ production, because this hormone decreases the platelet concentrations of cAMP–a phenomenon that, per se, promotes platelet aggregation–and does not modify collagen or Na⁺ arachidonate–induced platelet production of thromboxane A₂, measured by radioimmunoassay of its stable-metabolite thromboxane B₂. Insulin seems to help in modifying platelet membrane properties, as has already been shown for erythrocytes. The in vivo investigation comprised three studies of the influence of insulin on platelet function in male volunteers: 1) a euglycemic-hyperinsulinemic (40-μU/ml) clamp for 90 min followed by 60 min of euglycemia; 2) a euglycemic-hyperinsulinemic (160-μU/ml) clamp for 30 min followed by 60 min of euglycemia; and 3) an intravenous bolus of human regular insulin (3.84 U/m²). Throughout the three studies, we serially measured platelet sensitivity to ADP, PAF, epinephrine, collagen, and Na⁺ arachidonate. We observed that insulin in vivo and at the physiologic concentration of 40 μU/ml reduced platelet aggregation. For some aggregating agents, we demonstrated a dose and time dependence of the insulin effect. The latter was reversed after the insulin infusion. When insulin was administered as an intravenous bolus and platelet aggregation was studied before the appearance of hypoglycemia, we observed that insulin influence on platelets can be detected after only 10 min. In conclusion, this study suggests that insulin may have a role in the physiological modulation of platelet function and that the long-term insulin deficiency might account for the enhanced platelet aggregability frequently observed in diabetic patients.