Hyperreactivity of platelets from spontaneously hypertensive rats. Role of external magnesium

Abstract

Thrombin-induced serotonin secretion from age-matched spontaneously hypertensive rats (SHR) and control Wistar-Kyoto rats (WKY) was compared in the presence of different Ca2+ and Mg2+ concentrations. Platelets from SHR were more reactive than those of WKY, and the difference was more marked in 11-week-old than in younger rats. The responses to three concentrations of extracellular Ca2+ and one extracellular Mg2+ concentration of 10-3 M were compared. A high external Ca2+ concentration (2 x 10-3 M) increased secretion in platelets of both strains without suppressing the difference between them. Platelets from SHR were most sensitive than those from WKY to a low external Ca2+ concentration (2 x 10-6 M). Platelet secretion which is independent of external Ca2+ concentration was higher in platelets from SHR than in those from WKY. External Mg2+ exerted an inhibitory effect on serotonin secretion in both types of platelets, but platelets from SHR were less sensitive to Mg2+ than were those from WKY. This inhibitory effect appeared to be complex. It could be observed in the absence of external Ca2+, and in this case, the difference in reactivity between platelets SHR and WKY depended on the external Mg2+ concentration (up to 2 x 10-3 M). Furthermore, a Mg2+-induced antagonism of the stimulatory effect of external Ca2+ concentration appeared at higher concentrations of extracellular Mg2+ and was more potent in platelets from WKY than in those from SHR. These results suggest that the hyperreactivity of platelets from SHR is related to an increased sensitivity to external Ca2+ and a decreased sensitivity to external Mg2+ and that an anomaly in the cellular response to physiological concentrations of Mg2+ may be important in the pathophysiology of essential hypertension.