Different pathways for control of Na+/H+ exchange via activation of the thrombin receptor.

The aim of the present study was to clarify the control of Na+/H+ exchange in platelets activated via the thrombin receptor. When human BCECF-loaded platelets were stimulated with the thrombin-receptor-activating peptide (TRAP; amino acid sequence SFLLRN), which activates the receptor independently of proteolysis, the cytosolic pH (pHi) rose from 7.13 +/- 0.04 (n = 6) to 7.27 +/- 0.04 (n = 5), followed by a rapid decrease to resting values. Trypsin, which cleaves the receptor, induced a rapid and irreversible rise in pHi to 7.31 +/- 0.06 (n = 5). gamma-Thrombin, which cleaves the receptor but is unable to bind to the hirudin-like domain, induced a slow and irreversible rise in pHi to 7.31 +/- 0.04 (n = 14). alpha-Thrombin, which cleaves the receptor and binds to its hirudin-like domain, induced a rapid and irreversible rise in pHi to 7.31 +/- 0.04 (n = 22). Changes in pHi induced by TRAP, trypsin, gamma- and alpha-thrombin were accompanied by similar changes in cytosolic Ca2+ concentration ([Ca2+]i) and 32P-pleckstrin, a substrate of protein kinase C (PKC). The separate chelation of Ca2+i (30 microM BAPTA-AM) or inhibition of PKC (1 microM staurosporine) induced about 50% inhibition of the pHi responses triggered by TRAP, trypsin, gamma- and alpha-thrombin, but the combination induced complete inhibition. Thus the different types of activation of the thrombin receptor control Na+/H+ exchange via the same mechanism. Binding of thrombin to the hirudin-like domain accelerates exchange activation, whereas proteolysis of the receptor is essential for a sustained increase in pHi.