Platelet free calcium concentrations measured with fura-2 are influenced by the transmembrane sodium gradient

The influence of the transmembrane Na+ gradient on the intracellular free calcium concentration, [Ca2+]i, was studied in Sepharose gel-filtered platelets from healthy human subjects, using the Ca-sensitive fluorescent dye, fura-2. Raising the internal Na+ concentration, [Na+]i, by Na+ pump inhibition with 0.05 mM ouabain, without changing external Na+ did not cause a significant increase in [Ca2+]i. Substitution of extracellular Na+ by iso-osmolar sucrose induced a rapid (half-time about 2 min) and significant rise in [Ca2+]i; this effect was amplified in Na-loaded platelets. Partial restitution of external Na+ in these cells with increased [Ca2+]i promoted a significant and rapid Na+ concentration-dependent fall in [Ca2+]i; little decline in [Ca2+]i was observed if K+ was used instead of Na+. These observations represent in vitro evidence for the existence of a Na/Ca exchange mechanism in human platelets that may, in vivo, participate in the control of [Ca2+]i.