Effect of Extracellular Magnesium on Platelet Activation and Intracellular Calcium Mobilization

Abstract

A dose-dependent effect of magnesium on the inhibition of platelet aggregation and release of ATP from dense granules was observed in human platelets (in whole blood, platelet-rich plasma, or washed platelets) against various aggregation agents (ADP, U46619, collagen, or thrombin). The synthesis and release of the proaggregatory cyclooxygenase (CO) and lipoxygenase (LO) products, thromboxane A₂ (TXA₂) and 12-hydroxyeicosatetraenoic acid (12-HETE), respectively, in platelets were also inhibited by Mg in a dose-dependent manner (IC₅₀ 4 to 6 mmol/L). These Mg-mediated activities were further enhanced when platelets were preincubated with insulin (100 μU/mL). The effect of extracellular Mg on the change of intracellular calcium concentration ([Ca²⁺]_t ) was assessed using Fura-2/AM loaded cells in the presence or absence of extracellular Ca. Thrombin-stimulated influx of Ca ions decreased from 194 ± 30 nmol/L to 156 ± 21 nmol/L in the presence of 5 mmol/L Mg and to 111 ± 16 nmol/L in 10 mmol/L Mg. However, the intracellular Ca release (as determined in the presence of 5 mmol/L EGTA) was not affected by Mg. The intracellular Ca-dependent protein kinase C and myosin light chain kinase activities on the phosphorylation of endogenous p47 and p20 proteins studied after 2 min of thrombin addition decreased only 10 to 25% in the presence of 5 to 10 mmol/L Mg. Similar results were obtained when EGTA was added prior to the initiation of protein phosphorylation. We conclude that Mg can dose dependently inhibit a wide variety of agonists on platelet aggregation. Furthermore, insulin can potentiate the inhibitory effects of Mg on platelet activation. The inhibitory effect of Mg on platelet activation is most likely mediated by the inhibition of Ca influx, and by decreased synthesis and release of key eicosanoids involved in platelet aggregation. Am J Hypertens 1992;5:700-706