Influence of antimuscarinics on alpha-adrenoceptors in the female rabbit urethra.
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.
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
D000317:Adrenergic alpha-Antagonists; D010276:Parasympatholytics; D011942:Receptors, Adrenergic, alpha; D011976:Receptors, Muscarinic