Thrombin and NaF, but not epinephrine, raise cytosolic free Na + in human platelets

Abstract

We have investigated changes in [Na +] i in SBFI-loaded platelets stimulated at 37°C with thrombin, epinephrine, and NaF. Basal [Na +] i was 4.9 ± 1.3 mM ( n = 70). Stimulation of platelets with thrombin (0.1 U / ml) in the presence of 1 mM extracellular Ca 2+ rapidly raised [Na +] i by 27.3 ± 6 mM ( n = 16). Part of this increase (approx. 20–30%) is caused by Na + H + exchange, the rest is predominantly due to Na + influx. Epinephrine (20 μM) failed to change [Na +] i both in the absence and presence of fibrinogen. This is in agreement with earlier reports showing that epinephrine also fails to activate Na + H + exchange in human platelets. NaF which activates platelets via a direct effect on GTP-binding proteins induced a slow rise in [Na +] i to 9.5 ± 2.5 mM ( n = 4) and 33.0 ± 3.6 mM ( n = 12) at 10 and 20 mM NaF, respectively. This effect was completely blocked by SK&F 96365, a blocker of receptor-mediated Ca 2+ entry. Hence, the NaF-induced increase in [Na +] i is exclusively due to the opening of non-selective cation channels. This latter finding agrees with earlier observations which showed that NaF does not induce activation of Na + H + exchange in platelets.