Changes in cytoplasmic pH and in membrane potential in thrombin-stimulated human platelets.

The response of human platelets to stimulation by a specific aggregant such as thrombin has been postulated to proceed sequentially via induction of response at the membrane, followed by execution of shape change, secretion, and aggregation of the platelets. We have shown earlier that the platelet response includes a depolarization of the membrane which starts within less than 5 s and is thrombin-dose-dependent up to 4.5 nM alpha-thrombin. This depolarization may be measured by the distribution of either a fluorescent or a tritium-labeled lipophilic cation. We present here an adaptation of techniques for intracellular pH measurements to the human platelet. These show that stimulation with thrombin also induces a rapid change in the platelet transmembrane pH gradient as measured using either a weak base or a fluorescein derivative as a probe. The pH gradient undergoes a time-dependent and thrombin-dose-dependent change which parallels that exhibited by the membrane potential and by serotonin secretion.