Suppression of aggregation and calcium mobilisation in bovine platelets by external magnesium

We have examined the effect of increasing external Mg2+ and removing external Ca2+ on bovine platelet activation (measured by time taken for OD to recover to basal following shape change) and thrombin-induced Ca2+ transients. Increasing Mg2+ prolonged the duration of the recovery, both with and without external Ca2+, whereas Mg2+ removal shortened this parameter. In resting platelets, increasing external Mg2+ to 2.5 mM or above reduced basal [Ca2+](i), but had no significant effect when external Ca2+ was removed. Increasing external Mg2+ altered the time-course of the thrombin-induced rise in [Ca2+](i) with or without [Ca2+](o). There was no significant change in OD in a platelet suspension exposed to the highest dose (20 mM) of [Mg2+](o), indicating that the observed effects were not accompanied by significant changes in cell volume. These results suggest that (a) the effect of Mg2+ on bovine platelet function and Ca2+ mobilisation occurs through Ca2+-dependent and Ca2+-independent mechanisms, and (b) significant aggregation can occur in bovine platelets even in the absence of external Ca2+.