Capacitative calcium influx and intracellular pH cross-talk in human platelets

This study focuses on the potential interrelationships between changes in pH and capacitative calcium entry in stimulated platelets and on the participation of SOCs in the control of intracellular pH (pH(i)). Extracellular acidification reduces the Mn(2+) entry, measured by the slope of the quenching of FURA 2 fluorescence at the isoemissive wavelength of 360 nm. In thrombin-stimulated platelets Mn(2+) entry is reduced by acidosis (pH(o) = 6.89) to 17 +/- 4% of control (pH(o) = 7.32). In platelets treated with thapsigargin (TG) to induce the opening of store-operated channels (SOCs) the rate of quenching was reduced by acidosis to 31 +/- 5 % of control. Calcium entry was measured as the peak of [Ca(2+)](i) response to extracellular calcium readmission after mobilization of calcium from intracellular stores. Changes in pH(o) of platelet suspension media markedly alters the calcium entry evoked by thrombin that reach a 16 +/- 6 % of control in acidosis (pH(o) = 6.89) and 150 +/- 15% of control in alkalosis (pH(o) = 7.62). The SERCA inhibitor TG was used to study the effect of pH(o) on Ca(2+) influx. Acidosis decreases and alkalosis increases the capacitative calcium entry to 22 +/- 4 % and 129 +/- 1% of control respectively. These changes in pH(o) also produced changes in pH(i). Treatment of platelets with titrated solutions of trimethylamine causes intracellular alkalinization without changes in pH(o) increasing the capacitative calcium entry to 120 +/- 5%. TG itself produces an intracellular alkalinization that is further increased by calcium entry. Blockage of the Na(+)/H(+) exchanger reverted TG effect on pH(i) without changes in capacitative calcium entry.