[A comparative study of the stereoselectivity of cholinoreceptors]

Muscarine-sensitive cholinoreceptors (M-ChR) of higher vertebrates exhibit high stereoselectivity which is also revealed with respect to enantiomers of a very potent muscarinomimetic methyldilvasen (F-2268), the stereospecific index (SSI) being about 100. M-ChRs in the neuronal membrane of the gastropod mollusc Planorbarius corneus and in the hearts of the bivalve molluscs Mercenaria stimpsoni and Anadara broughtoni are highly sensitive to methyldilvasen (10(-9)-10(-10) M), but their sensitivity to its enantiomers is identical. In heart atria of the tortoise Testudo horsfieldi, frog Rana temporaria, and fishes Siluris glanis, Cyprinus carpio, as well as in ventricles of tadpoles, high SSI was revealed. These data are consistent with a hypothesis that during evolution of vertebrates no significant changes took place in the active center of M-ChR. Possibly, the lack of stereoselectivity in the investigated molluscan M-ChRs, together with their other peculiarities (they are not blocked by atropine), indicate "immaturity" of these receptors.