Rapid Ca2+ extrusion via the Na+/Ca2+ exchanger of the human platelet

This communication reports the kinetics of the Na+/Ca2+ exchanger and of the plasma membrane (PM) Ca2+ pump of the intact human platelet. The kinetic properties of these two systems were deduced by studying the rate of Ca2+ extrusion and its Na+ dependence for concentrations of cytoplasmic free Ca2+ ([Ca2+]cyt) in the 1-10-microM range. The PM Ca(2+)-ATPase was previously characterized (Johansson, J.S. Haynes, D.H. 1988. J. Membrane Biol. 104:147-163) for [Ca2+]cyt < or = 1.5 microM with the fluorescent Ca2+ indicator quin2 (Kd = 115 nM). That study determined that the PM Ca2+ pump in the basal state has a Vmax = 0.098 mM/min, a Km = 80 nM and a Hill coefficient = 1.7. The present study extends the measurable range of [Ca2+]cyt with the intracellular Ca2+ probe, rhod2 (Kd = 500 nM), which has almost a fivefold lower affinity for Ca2+. An Appendix also describes the Mg2+ and pH dependence of the Kd and fluorescence characteristics of the commercially available dye, which is a mixture of two molecules. Rates of active Ca2+ extrusion were determined by two independent methods which gave good agreement: (i) by measuring Ca2+ extrusion into a Ca(2+)-free medium (above citation) or (ii) by the newly developed "ionomycin short-circuit" method, which determines the ionomycin concentration necessary to short circuit the PM Ca2+ extrusion systems. Absolute rates of extrusion were determined by knowledge of how many Ca2+ ions are moved by ionomycin per minute. The major findings are as follows: (i) The exchanger is saturable with respect to Ca2+ with a Km = 0.97 +/- 0.31 microM and Vmax = 1.0 +/- 0.6 mM/min. (ii) At high [Ca2+]cyt, the exchanger works at a rate 10 times as large as the basal Vmax of the PM Ca2+ extrusion pump. (iii) The exchanger can work in reverse after Na+ loading of the cytoplasm by monensin. (iv) The PM Ca2+ extrusion pump is activated by exposure to [Ca2+]cyt > or = 1.5 microM for 20-50 sec. Activation raises the pump Vmax to 1.6 +/- 0.6 mM/min and the Km to 0.55 +/- 0.24 microM. (v) The Ca2+ buffering capacity of the cytoplasm is 3.6 mM in the 0.1 to 3 microM range of [Ca2+]cyt. In summary, the results show that the human platelet can extrude Ca2+ very rapidly at high [Ca2+]cyt. Both the Na+/Ca2+ exchanger and Ca2+ pump activation may prevent inappropriate platelet activation by marginal stimuli.