The action and interaction of β-phenethylamines and imidazolines on prejunctional α2-adrenoceptors of guinea-pig ileum in the presence of the non-competitive antagonist benextramine

Abstract

Abstract The effects of benextramine, a selective non-competitive, irreversible antagonist of α-adrenoceptors, against α2-adrenoceptor agonists has been investigated in isolated field-stimualted guinea-pig ileum. Benextramine was equipotent against the imidazoline, clonidine and the thiazoloazepine B-HT920, however, higher concentrations of benextramine were required for an equivalent non-competitive antagonism of the β-phenethylamine, α-methylnoradrenaline. Using benextramine to differentially block the agonist effects of clonidine but not α-methylnoradrenaline it was shown that clonidine can competitively antagonize the effects of α-methylnoradrenaline. From these and previous results on the differential effects of imidazoline-like and β-phenethylamine-like drugs on α2-adrenoceptors, it is proposed that two distinct populations of these receptors exist.