Stimulation of phosphate uptake in human platelets by thrombin and collagen. Changes in specific 32P labeling of metabolic ATP and polyphosphoinositides.

The uptake of [32P]phosphate by human, gel-filtered blood platelets and its incorporation into cytoplasmic ATP and polyphosphoinositides was studied. In unstimulated platelets, uptake was Na+o-dependent and saturable at approximately 20 nmol/min/10(11) cells with a half-maximal rate at 0.5 mM extracellular phosphate. Upon stimulation with thrombin or collagen, net influx of [32P]Pi was accelerated 5- to 10-fold. With thrombin, [32P]Pi efflux was also increased. After the first 2 min, efflux exceeded influx, resulting in the net release of [32P]Pi from the platelets. Since the stimulus-induced burst in [32P]Pi uptake paralleled the secretory responses, it might be an integral part of stimulus-response coupling in platelets. The stimulus-induced burst in net [32P]Pi uptake led to an enhanced labeling of metabolic ATP, which was already detectable at 5 s after stimulation with thrombin. Concomitantly, the incorporation of [32P]Pi into phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was accelerated. The thrombin-induced increase in specific 32P radioactivity of cytoplasmic ATP fully accounted for the simultaneous increase in specific 32P radioactivity of these phosphoinositides. In studying the extent of 32P labeling of phosphorylated compounds in response to a cellular stimulus, it is therefore essential to measure the effect of the stimulus on the specific radioactivity of cytoplasmic ATP.