Abstract 16945: Role of Thromboxane A 2 -Activated Nonselective Cation Channels in Hypoxic Pulmonary Vasoconstriction of Rat

Abstract

Hypoxia-induced pulmonary vasoconstriction (HPV) is a physiological mechanism responsible for controlling the ventilation-perfusion ratio in the lungs. Maintenance of the ventilation-perfusion ratio at 0.8-1 is critical for the optimization and maximization of gas exchange in the lung. Inhibition of K + channels has been shown to be a mechanism for hypoxia-induced membrane depolarization in pulmonary arterial smooth muscle cells (PASMCs). Pharmacological inhibition of K + channels, however, does not induce significant contraction in isolated rat pulmonary arteries, but significantly enhances HPV. These results suggest that, in addition to K + channel inhibition, other mechanisms are involved in hypoxia-induced membrane depolarization. We recently found that a partial contraction by thromboxane A 2 (TXA 2 ) is required to induce HPV. Therefore we hypothesize that TXA 2 receptor (TP) stimulation is involved in eliciting membrane polarization by activating nonselective cation (NSC) channels. Consistently, we found that 5-10 nM U46619, a stable agonist of TP, was essential for the contraction of isolated rat pulmonary arteries by 4-aminopyridine, a blocker of voltage-gated K + channel (Kv). Whole-cell voltage clamp experiments with rat PASMCs revealed that U46619 induced a weakly outward-rectifying NSC current (I NSC,TXA2 ), which was blocked by ruthenium red (RR), an antagonist of the transient receptor potential vanilloid (TRPV) subfamily. In addition, 2-aminoethoxydiphenyl borate, an agonist for TRPV1-3, activated NSC channels. In contrast, agonists for TRPV1 (capsaicin), TRPV3 (camphor), or TRPV4 (4α-PDD) rarely increased the membrane conductance. RT-PCR analysis showed the expression of transcripts for TRPV2 and TRPV4 in rat PASMCs. Finally, it was confirmed that the pretreatment with RR and tranilast (a TRPV2 inhibitor) both significantly inhibited HPV in the presence of U46619. Taken together, these data suggest that the concerted effects of I NSC,TXA2 increase and Kv inhibition during under hypoxia are required to induce membrane depolarization sufficient for eliciting HPV. Our results also imply that TXA 2 -activated NSC might be formed by TRPV2 in rat PASMCs and contribute to the hypoxia-induced membrane depolarization in PASMCs.