The role of alpha- and beta- adrenergic receptors in some actions of catecholamines on intestinal smooth muscle

Abstract

1. Experiments were carried out to determine the type of adrenergic receptor concerned in the action of noradrenaline in increasing the rate of exchange of potassium ions in the taenia of the guinea-pig caecum. The preparations were bathed in potassium-rich solutions in order to depolarize the muscle fibres and so eliminate changes in potassium flux secondary to alterations in membrane potential.2. Noradrenaline was more effective than isoprenaline in increasing both the influx and efflux of (42)K, suggesting that the alpha-receptors were involved. This action on potassium permeability was abolished by the alpha-blocking agent phentolamine (10(-7) g/ml.), the beta-blocking agent pronethalol (10(-7) g/ml.) being without effect.3. In contrast, isoprenaline was about 30 times more active than noradrenaline in inhibiting calcium contractures of the depolarized taenia, suggesting that here the beta-receptors were concerned. This was confirmed by showing that this action of noradrenaline was effectively antagonized by pronethalol, again at 10(-7) g/ml., but not by the same concentration of phentolamine.4. Carbachol contractures in quiescent preparations bathed in Krebs solution at 17 degrees C could be inhibited by both noradrenaline and isoprenaline. Noradrenaline was found to be more active under these conditions, and this action was antagonized by phentolamine (and piperoxane), but not by pronethalol, at the same concentrations as before.5. These findings confirm that the alpha- and beta-receptors are quite distinct in their actions. Nevertheless, under physiological conditions, both may contribute to the inhibitory actions of catecholamines on intestinal smooth muscle.