Abstract
The mechanism of the contractile effect of dopamine (DA) on the rat isolated seminal vesicle was studied. Cocaine (10 uM/1 in the organ bath, 30 min before DA) and 6-OHDA (50 mg/Kg i.v. 24 hr before removal of the seminal vesicle) almost completely prevented the contractile effect of DA. Drugs known to have an affinity for DA receptors or for α-adrenoceptors antagonized the contractile effect of DA, the rank order of potency being: prazosin > phentolamine > yohimbine > clonidine > sulpiride > apomorphine > haloperidol. The antagonism was in each case greater against DA than against noradrenaline (NA), used for comparison; selectivity for DA being highest in the case of prazosin and sulpiride. Taken together, these findings indicate that DA makes the rat seminal vesicle contract mostly by means of an indirect mechanism, binding presynaptic DA-receptors and, in part, presynaptic α-adrenoceptors as well; or, alternatively, binding presynaptic DA-receptors which have some links with α 2-adrenoceptors; the consequence being in either case the release of NA from sympathetic nerve endings.
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