Thromboxane A 2 -Induced Inhibition of Voltage-Gated K + Channels and Pulmonary Vasoconstriction

Abstract

Voltage-gated K + channels (K V ) and thromboxane A 2 (TXA 2 ) play critical roles in controlling pulmonary arterial tone under physiological and pathological conditions. We hypothesized that TXA 2 might inhibit K V channels, thereby establishing a link between these two major pathogenic pathways in pulmonary hypertension. The TXA 2 analogue U46619 inhibited I K(V) (E max =56.1±3.9%, EC 50 =0.054±0.019 μmol/L) and depolarized pulmonary artery smooth muscle cells via activation of TP receptors. In isolated pulmonary arteries, U46619 simultaneously increased intracellular Ca 2+ concentration and contractile force, and these effects were inhibited by nifedipine or KCl (60 mmol/L). U46619-induced contractions were not altered by the inhibitors of tyrosine kinase genistein or Rho kinase Y-27632 but were prevented by the nonselective protein kinase C (PKC) inhibitors staurosporine and calphostin C. Furthermore, these responses were sensitive to Gö-6983 but insensitive to bisindolylmaleimide I and Gö-6976. Based on the specificity of these drugs, we suggested a role for an atypical PKC in U46619-induced effects. Thus, treatment with a PKCζ pseudosubstrate inhibitor markedly prevented the vasoconstriction, the inhibition of I K(V) , and the depolarization induced by U46619. Western blots showed a transient translocation of PKCζ from the cytosolic to the particulate fraction on stimulation with U46619. These results indicate that TXA 2 inhibits I K(V) , leading to depolarization, activation of L-type Ca 2+ channels, and vasoconstriction of rat pulmonary arteries. We propose PKCζ as a link between TP receptor activation and K V channel inhibition.