title: Prejunctional alpha 2-adrenoceptor mediated inhibitory action of clonidine and B-HT 920, but not urapidil in guinea pig ileum.
abstract: Isolated guinea pig ileal longitudinal muscle was stimulated transmurally with a frequency of 0.1 Hz, duration of 0.5 msec, and supramaximal voltage (80-100 V). Transmural stimulation induces ileal contractions via activation of cholinergic neurons. alpha 2-Adrenergic agonists block the response to transmural stimulation via activation of prejunctional alpha 2 receptors which inhibit release of acetylcholine from cholinergic nerve terminals. Urapidil has been reported to have alpha 2-agonistic actions, and therefore was compared to the prototypic alpha 2 agonists, clonidine and B-HT 920. Clonidine and B-HT 920 depressed responses to transmural stimulation in the guinea pig ileum. Clonidine was the most potent inhibitor of the contractions, followed closely by B-HT 920. Very high concentrations of urapidil were necessary to suppress nerve-induced contractions of the ileum. The effects of clonidine and B-HT 920, but not urapidil, were antagonized by the selective alpha 2 antagonist, yohimbine. In unstimulated preparations, in which exogenous acetylcholine was used to elicit contractions of the ileum, urapidil depressed the response while clonidine and B-HT 920 had no effect. When PGF1 alpha was used to contract the ileum, no inhibitory effects were noted for urapidil, clonidine, or B-HT 920. Therefore urapidil, only in high concentrations, inhibits the contraction to transmural stimulation by depressing the response at a postjunctional cholinergic site. No evidence was found that urapidil can act as an agonist at a prejunctional alpha 2-receptor site.
authors.forename: R J
authors.initials: RJ
authors.lastname: Shebuski
authors.forename: B G
authors.initials: BG
authors.lastname: Zimmerman
mesh_terms: D000818:Animals; D001381:Azepines / Q000494:pharmacology*; D003000:Clonidine / Q000494:pharmacology*; D004558:Electric Stimulation; D005260:Female; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D010879:Piperazines / Q000494:pharmacology*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*
chemical_list: D001381:Azepines; D010879:Piperazines; D011942:Receptors, Adrenergic, alpha; C024763:talipexole; C015568:urapidil; D003000:Clonidine