Thrombin activation of the Na⁺/H⁺ exchanger in vascular smooth muscle cells. Evidence for a kinase C-independent pathway which is Ca²⁺-dependent and pertussis toxin-sensitive

The mechanism by which human α-thrombin activates the Na⁺/H⁺ exchanger was studied in cultured neonatal rat aortic smooth muscle cells. Thrombin (0.4 unit/ml) caused a rapid cell acidification followed by a slow, amiloride-inhibitable alkalinization (0.10-0.14 ΔpH(i) above base line). In protein kinase C down-regulated cells (exposed to phorbol 12-myristate 13-acetate for 24 or 72 h), the ΔpH(i) induced by thrombin was only partially attenuated. This protein kinase C-independent activation of the Na⁺/H⁺ exchanger was blocked by pertussis toxin (islet activation protein (IAP)), reducing ΔpH(i) by 50%. IAP did not directly inhibit Na⁺/H⁺ exchange activity as assessed by the response to intracellular acid loading. Thrombin also stimulated arachidonic acid release by 2.5 fold and inositol trisphosphate release by 6.2 fold. IAP inhibited both of these activities by 50-60%. Intracellular Ca²⁺ chelation with 120 μM quin2 prevented the thrombin-induced Ca²⁺ spike, inhibited thrombin-induced arachidonic acid release by 75%, and inhibited thrombin-induced activation of the Na⁺/H⁺ exchanger in protein kinase c-deficient cells by 65%. Increased intracellular [Ca²⁺] alone was not sufficient to activate the Na⁺/H⁺ exchanger, since ionomycin (0.3-1.5 μM) failed to elevate cell pH significantly. 10 μM indomethacin inhibited thrombin-induced ΔpH(i) in both control and protein kinase C down-regulated cells by 30-50%. Thus, thrombin can activate the Na⁺/H⁺ exchanger in vascular smooth muscle cells by a Ca²⁺-dependent, pertussis toxin-sensitive pathway which does not involve protein kinase C.