The effect of Na+ on Ca2+ homeostasis in unstimulated platelets.

Platelets maintain a low cytosolic free Ca2+ concentration by limiting Ca2+ influx from plasma and promoting Ca2+ efflux. The present studies examine the role of the plasma membrane Na+ gradient in these processes. The Na+ gradient in intact unstimulated platelets was altered by incubating the platelets with ouabain or by replacing extracellular Na+ with N-methyl-D-glucamine or choline. Ca2+ flux across the plasma membrane and the amount of exchangeable Ca2+ in the platelet cytosol were measured by observing 45Ca2+ influx and efflux under steady-state conditions. The cytosolic free Ca2+ concentration was measured with the fluorescent probe quin2. At extracellular Na+ concentrations below 50 mM, the size of the cytosolic exchangeable Ca2+ pool increased by 48%. The size of the exchangeable Ca2+ pool sequestered in the dense tubular system increased by 356%. Ca2+ flux across the plasma membrane increased by 38%. There was, however, no change in total platelet Ca2+ and little, if any, change in the cytosolic free Ca2+ concentration. Similar effects were produced by incubating platelets with ouabain. These observations demonstrate a marked influence of the plasma membrane Na+ gradient on Ca2+ homeostasis in platelets. The nature of the changes, however, suggests that Na+/Ca2+ exchange cannot be sole basis for Ca2+ efflux from platelets.