Cyclic AMP-dependent Cl^- secretion induced by thromboxane A₂ in isolated human colon

Increased release of thromboxane A₂ (TXA₂) has been shown to be involved in inflammatory bowel diseases. In the present study, we have investigated the effect of a stable TXA₂ analogue (STA₂) on the electrical parameters in isolated human colonic mucosa. In the human mucosa set between Ussing chambers, STA₂ stimulated Cl^- secretion in a concentration-dependent manner with an EC₅₀ of 0.06 μM. The STA₂-induced Cl^- secretion was significantly inhibited by ONO-3708 (10 μM), a specific TXA₂ receptor antagonist. The effect of STA₂ (0.3 μM) was independent of the colonic segment from which the tissue was obtained, from caecum to rectum. Chromanol 293B, an inhibitor of the cAMP-dependent KvLQT1 channel, attenuated the STA₂-induced Cl^- secretion in the human colonic mucosa (IC₅₀ value 1.18 μM). We found that KvLQT1 mRNA and protein were expressed in all the tested segments of the human colon. The STA₂-induced Cl^- secretion was significantly inhibited by 8-bromo-2′-monobutyryladenosine-3′,5′-cyclic monophosphorothioate (50 μM), a membrane-permeant cAMP antagonist. STA₂ (0.3 μM) significantly increased the intracellular cAMP levels and the short-circuit current via TXA₂ receptor in a human colonic cell line. These results suggest that the TXA₂-induced Cl^- secretion in the colon is mediated via the cAMP pathway in addition to the Ca²⁺-calmodulin pathway which was previously reported.