Thromboxane A₂ (TXA₂) is well known as a potent inducer of platelet aggregation and vasoconstriction. In isolated rat colon, we found recently that Cl^– secretion induced by the anti-tumour drug irinotecan and platelet-activating factor (PAF) is mediated by the production of TXA₂ (Sakai et al. 1997; Suzuki et al. 2000). However, it has not been known how irinotecan and PAF stimulate the release of TXA₂ in the colon.

Here, we investigated effects of nitric oxide (NO) and cyclic GMP on the Cl^– secretion in isolated rat distal colon. Rats were humanely killed by stunning and cervical dislocation. Tissue preparation and Ussing chamber experiments were performed as previously described (Sakai et al. 1997; Suzuki et al. 2000). Data are shown as means ± S.E.M. Differences between groups were analysed by one-way ANOVA.

Isolated rat distal colon was fixed in Ussing chambers. Sodium nitroprusside (SNP), a NO donor (100 µM), caused Cl^– secretion, which was almost completely inhibited by carboxy-PTIO, a NO scavenger (300 µM, P < 0.01, n = 5). The SNP-induced Cl^– secretion was concentration-dependently inhibited by a TXA₂ receptor antagonist (ONO-3708; IC₅₀ = 2 µM, n = 6), and a thromboxane synthase inhibitor (Y-20811; IC₅₀ = 0.4 µM, n = 5). SNP significantly increased the release of TXA₂ (measured as TXB₂) from the colon into the serosal solution (P < 0.01, n = 4). The SNP-induced Cl^– secretion was blocked by a NO-sensitive guanylate cyclase inhibitor (ODQ; 10 µM, P < 0.01, n = 5). Dibutyryl cyclic GMP (500 µM) also caused Cl^– secretion, which was inhibited by ONO-3708 (10 µM, P < 0.01, n = 4) and Y-20811 (1 µM, P < 0.01, n = 4). Dibutyryl cyclic GMP increased release of TXB₂ (P < 0.01, n = 4). PAF (10 µM)-induced Cl^– secretion was inhibited by carboxy-PTIO (200 µM, P < 0.01, n = 5) and ODQ (10 µM, P < 0.01, n = 5), whereas irinotecan (500 µM)-induced Cl^– secretion was not significantly inhibited by these drugs (P > 0.05, n = 4).

These results indicate that PAF-induced Cl^– secretion is mediated by NO/cyclic GMP-elicited production of TXA₂. In contrast, irinotecan-stimulated production of TXA₂ is not mediated by the NO/cyclic GMP pathway.