The action of acetylcholine and other drugs on the efflux of potassium and rubidium from smooth muscle of the guinea-pig intestine

1. A method is described for measuring continuously the efflux of potassium or rubidium from smooth muscle of the guinea-pig.

2. Muscarinic drugs cause at maximum a 100-fold increase in the efflux rate, due to a direct increase in permeability and only to a minor extent secondary to depolarization. With acetylcholine the dose response curve for producing efflux is displaced to 1,000 times higher concentrations than that for contraction.

3. The shift varies with different agonists. The efflux and contractile responses to agonists are antagonized to an equivalent extent by atropine and several other reversible antagonists but benzhexol has a relatively greater effect on efflux. An estimate of spare receptors was obtained with benzilylcholine mustard and was similar for both responses. Dibenamine and local anaesthetics led to a parallel shift of the contraction dose response curve but a depression without shift in the efflux response.

4. The most satisfactory explanation of these results is that there are two types of the muscarinic receptor in the smooth muscle of the guinea-pig intestine.