Movement of sodium into human platelets induced by ADP.

1. In normal human platelets the concentrations of Na+ and K+ were 42.1 +/- 4.3 and 98.8 +/- 3.7 mequiv/l of platelet water respectively (mean +/- S.E. of 22 samples). 2. When platelet-rich plasma was incubated with 22Na+ at 37 degrees C for 2-3 h an increase in platelet Na+ concentration was found which was significant after 210 min. Platelet K+ concentration did not change significantly. The platelet 22Na+ radioactivity increased faster than did the total Na+ suggesting a Na+o-Na+ exchange process in unactivated platelets. 3. Addition of ADP to platelet-rich plasma resulted in platelet aggregation and a rapid rise (within seconds) in 22Na+-radioactivity within the platelets and after 300 s this increase diminished toward control levels. 4. Under the same experimental conditions, ADP did not bring about an increase of 36Cl- in the platelets. 5. Ouabain (10-(6) M) added to platelet-rich plasma induced an increase in Na+ concentration and 22Na+ radioactivity in the platelets, as well as a decrease in K+ concentration. ADP produced a further increase in 22Na+, which did not return toward control values, in the presence of ouabain. 6. The association of an increase in 22Na+ but not of 36Cl- accompanying aggregation by ADP suggests a selective mechanism for the movement of Na+ into platelets rather than a movement of NaCl together with water under an osmotic gradient.