title: Influence of antimuscarinics on alpha-adrenoceptors in the female rabbit urethra.
abstract: The effects of the tertiary amine atropine and its structural analogues homatropine and scopolamine, as well as the quarternary amine emeprone, were evaluated on noradrenaline (NA)-induced contractions of isolated female rabbit urethral ring preparations. In addition, the abilities of these antimuscarinics to inhibit 3H-dihydro-alpha-ergocryptine (3H-DHE) binding to alpha-adrenoceptors were studied on a crude membrane preparation from the female rabbit bladder base and urethra. Atropine and homatropine depressed the NA-induced contractions in a concentration-dependent way, whereas this was not seen with scopolamine. Emeprone 10(-5) and 10(-4) M augmented the contractions, an effect possibly attributable to a NA-uptake blocking effect. All antimuscarinics displaced specific 3H-DHE binding, the order of potency being atropine greater than homatropine greater than emeprone greater than scopolamine. In general a good correlation was seen between the binding and mechanical activity studies for atropine, homatropine and scopolamine, while this was not found for emeprone. It is concluded that alpha-adrenoceptor blockade by atropine can be observed at concentrations exceeding 10(-7) M. Scopolamine, showing alpha-adrenoceptor blocking properties only in high concentrations, may be used as an alternative for blockade of muscarinic cholinoceptors.
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D005260:Female; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects*; D010276:Parasympatholytics / Q000494:pharmacology*; D011817:Rabbits; D011869:Radioligand Assay; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D011976:Receptors, Muscarinic / Q000187:drug effects; D014521:Urethra / Q000187:drug effects
chemical_list: D000317:Adrenergic alpha-Antagonists; D010276:Parasympatholytics; D011942:Receptors, Adrenergic, alpha; D011976:Receptors, Muscarinic