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

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 TYA₂ receptor antagonist ONO-3708 (IC₅₀ = 2 μM) and the TX synthase inhibitor Y-20811 (IC₅₀ =0.4 μM). SNP significantly increased the release of TYA₂ (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-damp 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.