| 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 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>homatropine>emeprone>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. |
