Phentolamine inhibition of rat seminal vesicle response to dopamine-mimetic drugs: alpha-adrenoceptor implication or lack of specificity?

Existence of dopamine D1 receptor on the sympathetic nerve endings in the guinea-pig vas deferens

The effects of selective dopamine receptor agonists and antagonists on sympathetic neuromuscular transmission were investigated in the guinea-pig vas deferens in order to test for the presence of presynaptic dopamine receptors. A single-pulse field stimulus induced a rapid monophasic contraction which was strongly inhibited by alpha,beta-methylene ATP, a P2X purinoceptor desensitizing agent. The contraction was also inhibited by 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14,304), a selective alpha2-adrenoceptor agonist. This inhibition was antagonized by idazoxan, an alpha2-adrenoceptor antagonist, but not by R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzaz epine hydrochloride (SCH-23390), a dopamine D1 receptor antagonist. Furthermore, the contractions were inhibited in a dose-dependent manner by R(+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride (SKF-38393) and (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-be nzazepine hydrobromide (SKF-82958), dopamine D1 receptor agonists, and the inhibition was antagonized by both SCH-23390 and idazoxan, but not by spiperone, a dopamine D2 receptor antagonist. The results suggest that dopamine D1 receptors are located on the sympathetic nerve endings of guinea-pig vas deferens.

Characterization of adrenergic receptors in membranes from the rat seminal vesicle

Characterization of adrenergic receptors in membranes from the rat seminal vesicle was studied by radioligand binding assay. Seminal vesicle membranes contained saturable and high affinity binding sites for the beta-adrenergic receptor antagonist 3H-dihydroalprenolol (3H-DHA) and for the alpha-adrenergic antagonist 3H-prazosin. The observed order of potency for adrenergic agonists in competing for the 3H-DHA binding sites: isoproterenol > epinephrine congruent to salbutamol > norepinephrine indicates that these membrane receptors have the properties of beta 2-adrenergic receptors. alpha 1-Adrenergic receptors were defined mainly as alpha 1A subtypes by demonstrating their insensitivity to pretreatment with chlorethylclonidine and the different rank orders of antagonist affinities. No significant binding sites for the alpha 2-adrenergic receptor agonist 3H-clonidine were observed. The GTP-induced reduction in the affinity of alpha 1-adrenergic receptors for epinephrine was significantly reversed by the muscarinic cholinergic agonist carbachol. Atropine effectively antagonized this effect of carbachol on the competitive inhibition of 3H-prazosin binding by epinephrine in the presence of GTP, which suggests that muscarinic cholinergic receptors regulate the affinity of alpha 1-adrenergic receptors by modulating the effect of guanine nucleotides. The effect of GTP on decreasing the affinity of beta 2-adrenergic receptors was not influenced by the addition of carbachol.

Evidence for alpha 1-adrenoceptor subtype predominance in the rat seminal vesicle

Noradrenaline (0.6-19 microM) and phenylephrine (2-130 microM) induced contractions in the rat seminal vesicle that were competitively antagonized by the alpha 1-adrenoceptor-selective antagonist corynanthine (120-920 nM). Yohimbine (60-450 nM), an alpha 2-adrenoceptor-selective antagonist, produced a non-competitive antagonism of noradrenaline responses, suggesting that the responses were not alpha 2-adrenoceptor mediated. It is concluded that the rat seminal vesicle has a predominance of alpha 1-adrenoceptors.

Characterization of the contractile activity of dopamine on the rat isolated seminal vesicle

The mechanism of the contractile effect of dopamine (DA) on the rat isolated seminal vesicle was studied. Cocaine (10 microM/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 alpha-adrenoceptors antagonized the contractile effect of DA, the rank order of potency being: prazosin greater than phentolamine greater than yohimbine greater than clonidine greater than sulpiride greater than apomorphine greater than 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 alpha-adrenoceptors as well; or, alternatively, binding presynaptic DA-receptors which have some links with alpha 2-adrenoceptors; the consequence being in either case the release of NA from sympathetic nerve endings.

Antagonist properties of phentolamine at both presynaptic alpha 2-adrenoceptors and presynaptic dopamine receptors using field stimulated right cat atria

Phentolamine, which is considered a specific alpha-adrenoceptor antagonist, was tested for antagonist properties at presynaptic dopamine receptors and presynaptic alpha 2-adrenoceptors present on sympathetic nerve terminals in isolated right cat atria. Field stimulation induced a transient tachycardia which was inhibited by stimulation of presynaptic dopamine receptors using apomorphine or by stimulation of presynaptic alpha 2-adrenoceptors using clonidine. The presynaptic inhibitory effects of apomorphine were competitively antagonized by sulpiride, which is considered a specific dopamine receptor antagonist, and by phentolamine and yohimbine which are considered alpha-adrenoceptor antagonists. The presynaptic inhibitory effects of clonidine were competitively antagonized by phentolamine and yohimbine but not by sulpiride. pA2 values for phentolamine against apomorphine and phentolamine against clonidine suggest that phentolamine may be an antagonist at both presynaptic dopamine receptors and presynaptic alpha 2-adrenoceptors.