Blunted inhibition by insulin of agonist-stimulated calcium, pH and aggregatory responses in platelets from hypertensive patients.

INTRODUCTION

Insulin may influence platelet function by modulating platelet responsiveness to vasoactive agonists. In essential hypertension platelet hyperactivity might be related to altered modulation by insulin. The present study examined the effects of physiological concentrations of insulin on agonist-stimulated platelet responses [free calcium concentration, intracellular pH (pHi) and thrombin-induced aggregation] in platelets from 30 normotensive subjects and 53 untreated essential hypertensive patients.

METHODS

Platelet free calcium concentration and pHi were measured spectrofluorometrically using specific fluorescent dyes [Fura-2/acetoxymethyl ester and 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein/acetoxymethyl ester, respectively] in unstimulated and in angiotensin II (Ang II; 1 nmol/l)- and endothelin-1 (1 nmol/l)-stimulated platelets that had been pre-exposed to insulin (70 microU/ml). Aggregatory responses were measured turbidometrically in platelets stimulated by thrombin (0.05 U/ml), alone or in combination with angiotensin II or endothelin-1 in the absence and presence of insulin pre-incubation.

RESULTS

Blood pressure and serum glucose and insulin levels were significantly elevated in the hypertensive patients. Basal and agonist-stimulated free calcium concentration, pHi and aggregation were significantly higher in the hypertensive than in the normotensive group. In platelets pre-exposed to insulin for 5 min, agonist-induced responses were significantly reduced (by 40-60%) in the normotensive group but not in the hypertensive group. Serum insulin was positively correlated with Ang II-stimulated cytosolic free calcium concentration (r = 0.47, P < 0.01) and pHi (r = 0.26, P = 0.047) in platelets pre-exposed to insulin.

CONCLUSIONS

This study demonstrates that insulin inhibits Ang II- and endothelin-1-stimulated platelet free calcium concentration and pHi, and thrombin-induced aggregation. In hypertension these insulin-related inhibitory effects are blunted. Attenuated inhibition of agonist-stimulated platelet responses by insulin could be a manifestation of peripheral insulin resistance which may contribute to platelet hyperactivity in essential hypertension.