The action and interaction of beta-phenethylamines and imidazolines on prejunctional alpha 2-adrenoceptors of guinea-pig ileum in the presence of the non-competitive antagonist benextramine.
The effects of benextramine, a selective non-competitive, irreversible antagonist of alpha-adrenoceptors, against alpha 2-adrenoceptor agonists has been investigated in isolated field-stimulated 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 beta-phenethylamine, alpha-methylnoradrenaline. Using benextramine to differentially block the agonist effects of clonidine but not alpha-methylnoradrenaline it was shown that clonidine can competitively antagonize the effects of alpha-methylnoradrenaline. From these and previous results on the differential effects of imidazoline-like and beta-phenethylamine-like drugs on alpha 2-adrenoceptors, it is proposed that two distinct populations of these receptors exist.
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000818:Animals; D001381:Azepines / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology; D003538:Cystamine / Q000031:analogs & derivatives* / Q000494:pharmacology; D004558:Electric Stimulation; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects; D007093:Imidazoles / Q000494:pharmacology*; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D008771:Nordefrin / Q000494:pharmacology; D010627:Phenethylamines / Q000494:pharmacology*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D001381:Azepines; D007093:Imidazoles; D010627:Phenethylamines; D011942:Receptors, Adrenergic, alpha; C029998:benextramine; C024763:talipexole; D003000:Clonidine; D003538:Cystamine; D008771:Nordefrin