?2-Adrenoceptors inhibit the cholera-toxin-induced intestinal fluid accumulation

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Abstract
The effects of adrenoceptor agonists and antagonists on the cholera-toxin-induced intestinal fluid accumulation and the mucosal levels of cAMP were investigated in vivo. Cholera toxin produced a marked fluid accumulation. Adrenaline inhibited the effect of the toxin in a dose-dependent manner. An α1-adrenoceptor blocking agent yohimbine antagonized the effect of adrenaline. The α1-adrenoceptor blocking agents prazosin and phenoxybenzamine failed to antagonize the effect of adrenaline. A high dose of a β-adrenoceptor blocking agent pindolol did not antagonize the effect of adrenaline. Yohimbine or pindolol alone did not produce any effects on the toxin-induced fluid accumulation. However, prazosin and phenoxybenzamine per se inhibited the toxin-induced fluid accumulation. An α2-selective agonist clonidine was slightly more potent than adrenaline, and was about 100-fold more potent than the α1-selective agonist methoxamine in inhibiting the cholera-toxin-induced intestinal secretion. Clonidine, adrenaline and methoxamine failed to reduce the mucosal levels of cAMP, while these α-adrenoceptor agonists inhibited the toxin-induced fluid accumulation in the same preparations. These results suggest that the stimulation of α2-adrenoceptors inhibit the cholera-toxin-induced intestinal secretion without reducing the whole mucosal levels of cAMP.