Evidence that an atypical β‐adrenoceptor mediates the inhibition of spontaneous rhythmical contractions of rabbit isolated jejunum induced by ritodrine and salbutamol

1 The nature of the adrenoceptors mediating the inhibitory action of noradrenaline, ritodrine and salbutamol on the spontaneous activity of longitudinal muscle of the rabbit jejunum in vitro was investigated by use of a range of adrenoceptor antagonists. 2 The actions of ritodrine and salbutamol were antagonized competitively by propranolol. The pA2 values of 6.4 and 6.6 respectively were smaller than those found elsewhere for β1‐ and β2‐adrenoceptors. 3 In contrast, the responses to ritodrine and salbutamol were antagonized only by high concentrations (>2.7 μm) of phentolamine and were unaffected by yohimbine (2.6 μm), mepyramine (2.5 μm) or cimetidine (4.0 μm). 4 Ritodrine which is less potent than salbutamol in tissues with typical β2‐adrenoceptors was found to be 8 times more potent than salbutamol in the rabbit jejunum. 5 It is suggested that in the rabbit jejunum ritodrine and salbutamol may act at an atypical β‐adrenoceptor, at which propranolol is a competitive but not very potent antagonist.