The role of α‐ and β‐adrenergic receptors in some actions of catecholamines on intestinal smooth muscle

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Abstract
Experiments were carried out to determine the type of adrenergic receptor concerned in the action of noradrenaline [norepinephrine] in increasing the rate of exchange of K ions in the tenia of the guinea-pig cecum. The preparations were bathed in K-rich solutions in order to depolarize the muscle fibers and so eliminate changes in K flux secondary to alterations in membrane potential. Noradrenaline was more effective than isoprenaline in increasing both the influx and efflux of K42, suggesting that the [alpha]-receptors were involved. This action on Kpermeability was abolished by the [alpha]-blocking agent phentol-amine (10-7 g/ml.), the [beta]-blocking agent pronethalol (10-7 g/ml.) being without effect. In contrast, isoprenaline was about 30 times more active than noradrenaline in inhibiting calcium contractures of the depolarized tenia, 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. Carbachol contractures in quiescent preparations bathed in Krebs solution at 17[degree]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. 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.