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 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.