Conformational requirements of alpha 2-adrenergic receptors.

The conformationally restrained trans-extended and cis-folded isomers of 2-(3,4-dihydroxyphenyl)cyclobutylamine were used to establish the conformational requirements of presynaptic alpha 2-adrenergic receptors in the field-stimulated guinea pig ileum. The trans-extended isomer produced a concentration-dependent inhibition of the twitch response with an EC50 of 34 microM. The cis-folded analogue failed to produce a significant inhibition of the twitch response at concentrations up to 1 mM. These results suggest that the presynaptic alpha 2-adrenergic receptor prefers phenethylamines in the trans-extended conformation over the cis-folded conformation. The conformational requirements of alpha 1- and alpha 2-adrenergic receptors appear to be similar.