STIMULATION OF BICARBONATE SECRETION BY ALPHA-ADRENOCEPTOR AND BETA-ADRENOCEPTOR AGONISTS IN RAT CECUM IN-VITRO

This study examines the effects of adrenergic drugs on bicarbonate sec retion by the rat caecum in vitro. Noradrenaline, phenylephrine but no t clonidine, stimulated secretion in a concentration-related manner. N oradrenaline responses were antagonised by alprenolol (20 mu M) but no t phentolamine (10 mu M) whilst phenylephrine was antagonised by phent olamine (10 mu M), prazosin (5 mu M) but not yohimbine (5 mu M), alpre nolol or tetrodotoxin (1 mu M). Replacement of mucosal Cl- abolished t he phenylephrine response. Combined stimulation with maximum concentra tions of phenylephrine and isoprenaline gave a response which was not greater than that to either agonist alone but it did involve both alph a- and beta-adrenoceptors as judged from the effects of alprenolol and phentolamine either alone or combined. Submaximum concentrations of t he two agonists did show additive responses. The results show that alp ha(1)- but not alpha(2)-adrenoceptor agonists stimulate bicarbonate se cretion and may act on the same transport mechanism as beta-adrenocept or agonists. Noradrenaline stimulates via beta-adrenoceptors.