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

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Abstract
Thrombin-induced serotonin secretion from platelets 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 more 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.(ABSTRACT TRUNCATED AT 250 WORDS)