[Relation between the changes in Na+/H+-exchange and cytoplasmic Ca levels in platelet activation]

Abstract

In experiments on human and rat platelets the changes in cytoplasmic pH (pHi) and Ca2+ concentration (Ca2+) have been studied by the use of fluorescent probes BCECF and quin-2, respectively. Inhibition of Na+/H+ exchange resulted in removal of external Na+ (equimolar substitution by cholin) induced a considerable reduction of Ca2+-signal caused by 10 mMPAF, and a slight decrease in Ca2+-signal elicited by 0.1 mu/ml thrombin. In the control Na+ and Ca2+ containing medium both PAF and thrombin induced first a decrease then an increase of pHi above its original level. The latter phase being much more pronounced in the case of thrombin action. Removal of Ca2+ from the external solution suppressed pHi increase and correspondingly it enhanced initial decrease. Addition of Ni2+ also suppressed stimulus-induced pHi increase. A treatment of platelets by Ca-ionophore A23187 caused a rise of pHi without its initial decrease; in medium without Ca2+ the changes of pHi were inhibited. The results obtained suggest that in platelets there exist a mutual interdependence between Ca2+ influx and change in pHi: Ca2+ influx enhanced the activation of Na+/H+ exchange by agonist; in turn Na+/H+ exchange activation enhances the stimulus-induced Ca2+ influx.