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Abstract

We have investigated changes in [Na⁺]_i in SBFI-loaded platelets stimulated at 37 degrees 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 Ca²⁺ 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 microM) 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 Ca²⁺ 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.