The characteristics of adrenoceptors mediating relaxation to catecholamines in rat isolated jejunum were investigated. 2. Catecholamines and BRL 37344 produced relaxation of the KCl-contracted strips with an order of potency of isoprenaline (1.0) > BRL 37344 (0.63) > noradrenaline (0.1) > adrenaline (0.04). 3. In the presence of both prazosin (1 microM) and propranolol (1 microM) only small dextral shifts of the concentration-response curves to agonists were observed and an order of potency of BRL 37344 (2.5) > isoprenaline (1.0) > noradrenaline (0.2) > adrenaline (0.1) was obtained. 4. In the presence of prazosin (1 microM) and propranolol (1 microM), cyanopindolol (0.1-10 microM) produced a concentration-dependent rightward shift of the concentration-response curve to adrenaline with a Schild slope not significantly different from unity and a mean pA2 value of 7.01. 5. The resistance of relaxant responses to propranolol, the relatively high potency of BRL 37344 compared to catecholamines and the competitive antagonism of relaxant responses to adrenaline by cyanopindolol suggest that beta-adrenoceptors in rat small intestine are mainly atypical in nature.
