Alpha-2 adrenergic inhibition of intestinal secretion induced by prostaglandin E1, vasoactive intestinal peptide and dibutyryl cyclic AMP in rat jejunum.

Effects of alpha adrenergic agents on intestinal secretion induced by prostaglandin E1 (PGE1), vasoactive intestinal peptide (VIP) and dibutyryl cyclic AMP (Bt2cAMP) were investigated in rat jejunum in vivo. Oxymetazoline and clonidine were more potent than epinephrine in inhibiting the PGE1-induced secretion. Methoxamine failed to inhibit the PGE1-induced secretion even with a 100-fold higher dose than that of clonidine. A high dose (1 mumol/kg) of oxymetazoline not only inhibited the PGE1- induced secretion but also enhanced net fluid absorption. Yohimbine reversed the inhibitory effect of clonidine, whereas phenoxybenzamine did not. These antagonists per se did not produce any effects on PGE1-induced secretion. Clonidine inhibited the intestinal secretion induced by VIP or Bt2cAMP, whereas methoxamine did not. The inhibitory effect of clonidine was reversed by yohimbine. Phenoxybenzamine per se inhibited intestinal secretion induced by either VIP or Bt2cAMP. Clonidine did not produce any significant effects on PGE1- augmented cAMP levels in jejunal mucosa in vivo. These results suggest that stimulation of alpha-2 adrenoceptors in rat jejunal mucosa inhibits some mechanisms distal to cAMP generation and in turn results in the inhibition of net water intestinal secretion. These findings also raise the question of general validity of a hypothesis that alpha-2 adrenoceptors regulate cellular functions through the inhibition of adenylate cyclase activity.