EFFECT OF THE TRANSMEMBRANE GRADIENT OF MAGNESIUM AND SODIUM ON THE REGULATION OF CYTOSOLIC-FREE MAGNESIUM CONCENTRATION IN HUMAN PLATELETS

1. To clarify the mechanism by which cytosolic free Mg2+ concentration s ([Mg2+](i)) are regulated in human platelets, we investigated platel et membrane permeability to Mg2+ by altering extracellular Mg2+ concen trations, and tested the existence of Na+-Mg2+ exchanger by manipulati ng the transmembrane Na+ gradient. 2. Platelet [Mg2+](i) was 421+/-52 mu mol/l in healthy men. [Mg2+](i) remained constant during a 120min e xposure to nominally zero (low) or 5mmol/l (high) external Mg2+ concen trations. 3. Preincubation of platelets with 10(-4) mol/l ouabain effe ctively decreased the transmembrane Na+ gradient, The ouabain-induced increase in [Mg2+](i) was statistically significant after 30min of exp osure (14.6+/-2.0%) and reached 24.6+/-4.5% after 60min. Similarly, th e replacement of extracellular Na+ with N-methyl-D-glucamine significa ntly increased by 48.5+/-3.9% and 78.8+/-12.5%, respectively. 4. These results suggest that [Mg2+](i) is well controlled in the presence of large transmembrane Mg2+ concentration gradients, and a Na+-Mg2+ excha nger may be involved in the regulation of [Mg2+](i) in human platelets .