Nitric oxide‐induced Cl− secretion in isolated rat colon is mediated by the release of thromboxane A2

Abstract

We have shown previously that thromboxane A₂ (TXA₂), which may be released by the anti‐tumour drug irinotecan and by platelet‐activating factor (PAF), causes Cl⁻ secretion in the isolated rat colon. In the present study, the involvement of TXA₂ in nitric oxide‐induced Cl⁻ secretion in isolated rat colon was investigated. In colonic mucosa set between Ussing chambers, the NO donor sodium nitroprusside (SNP; 100 μM) caused Cl⁻ secretion, an effect that was almost completely inhibited by the NO scavenger carboxy‐PTIO at 200 μM. The SNP‐induced Cl⁻ secretion was inhibited in a concentration‐dependent manner by the TXA₂ receptor antagonist ONO‐3708 (IC₅₀= 2 μM) and the TX synthase inhibitor Y‐20811 (IC₅₀= 0.4 μM). SNP significantly increased the release of TXA₂ (measured as TXB₂ release) from the mucosa. The SNP‐induced increases in Cl⁻ secretion and TXA₂ release were blocked by a NO‐sensitive guanylate cyclase inhibitor (ODQ). Dibutyryl cGMP (500 μM) also induced Cl⁻ secretion, which was sensitive to ONO‐3708 (10 μM) and Y‐20811 (1 μM), and increased the release of TXA₂ from the mucosa. PAF‐induced (10 μM) Cl⁻ secretion was inhibited by carboxy‐PTIO (200 μM) and ODQ (10 μM), whereas irinotecan‐induced (500 μM) Cl⁻ secretion was not significantly inhibited by these drugs. A stable TXA₂ analogue (STA₂) but not SNP (100 μM) changed the membrane potential of epithelial cells in isolated colonic crypts under the whole‐cell current‐clamp condition. These results indicate that PAF elicits the NO‐cGMP pathway and then stimulates the release of TXA₂, which is a stimulant of colonic Cl⁻ secretion. In contrast, the NO‐cGMP pathway is not involved in the TXA₂‐mediated Cl⁻ secretion induced by irinotecan.