Thrombin-induced platelet aggregation is affected by external Na⁺ independently of the Na⁺/H⁺ exchange

Thrombin affects blood platelets by activation of Na⁺/H⁺ exchange and induction of aggregation, but the relationship between these effects is under debate. The present study attempts to clarify whether the activation of the exchanger activity is required for platelet aggregation. In apparent support of such a requirement, thrombin-induced aggregation is higher in Na⁺ medium than in N-methylglucamine⁺ medium and is inhibited by sphingosine, an inhibitor of protein kinase C known to regulate the Na⁺/H⁺ exchanger. However, the inhibition of aggregation by sphingosine occurs in both Na⁺-containing and Na⁺-free media, the aggregation is identical in Na⁺ and K⁺-containing media, and is not inhibited by 5-N-(3-aminophenyl)amiloride, at a concentration 10-fold higher than its K_i for platelet Na⁺/H⁺ exchange. Furthermore, at low concentration (0.005 U/ml) thrombin induces aggregation but does not activate the exchange. It is concluded that the activation of Na⁺/H⁺ exchange is not required for thrombin-induced platelet aggregation and that the apparent augmentation of aggregation by Na⁺ is due to an inhibitory effect of N-methylglucamine⁺.