Stimulation of bicarbonate secretion by alpha- and beta-adrenoceptor agonists in rat caecum in vitro

This study examines the effects of adrenergic drugs on bicarbonate secretion by the rat caecum in vitro. Noradrenaline, phenylephrine but not clonidine, stimulated secretion in a concentration-related manner. Noradrenaline responses were antagonised by alprenolol (20 microM) but not phentolamine (10 microM) whilst phenylephrine was antagonised by phentolamine (10 microM), prazosin (5 microM) but not yohimbine (5 microM), alprenolol or tetrodotoxin (1 microM). Replacement of mucosal Cl- abolished the phenylephrine response. Combined stimulation with maximum concentrations of phenylephrine and isoprenaline gave a response which was not greater than that to either agonist alone but it did involve both alpha- and beta-adrenoceptors as judged from the effects of alprenolol and phentolamine either alone or combined. Submaximum concentrations of the two agonists did show additive responses. The results show that alpha 1- but not alpha 2-adrenoceptor agonists stimulate bicarbonate secretion and may act on the same transport mechanism as beta-adrenoceptor agonists. Noradrenaline stimulates via beta-adrenoceptors.