Ouabain-sensitive and insensitive acetylcholine receptors on the membrane of the same neuron in Helix pomatia

Using internally dialyzed neurons of Helix pomatia as a model, the effect of structural analogs of acetylcholine (ACh) were investigated for their cholinomimetic properties on A- and B-types of ACh-responses. Specifically we analyzed choline esters of N- para- and ortho-alkoxybenzoyl-β-alanines, (CH 3O-, C 2H 5O-, C 3H 7O-, iso-C 3H 7O-, C 4H 9O-, iso-C 4H 9O-, C 5H 11O-, iso-C 5H 11O-), (in all, 16 combinations). The compounds evoked differing sensitivities of response to factors de-activating the Na–K-pump (ouabain and K-free solution). Most compounds resembled ACh: ionic currents caused by these compounds were inhibited by Na–K pump blockers in the case of A-type responses — B-type responses were insensitive to these factors. Ionic currents induced by choline esters of p-, o-propoxy- and iso-propoxybenzoyl-β-alanines were insensitive to ouabain and K-free solution in the case of A- and B-type responses. Ionic currents induced by the choline ester of p-butoxybenzoyl-β-alanine were inhibited by ouabain and K-free solution on both types of neuron. The results allow us to classify choline esters of p-, o-propoxy- and iso-propoxybenzoyl-β-alanines as N-cholinomimetics, while the choline ester of p-butoxybenzoyl-β-alanine can be considered as M-cholinomimetic. We conclude that both M- and N-type ACh receptors exist on the membrane of the same neurons.