Effects of St-587 on the alpha-adrenoceptors in the bisected rat vas deferens

Potential serotonergic and noradrenergic involvement in the discriminative stimulus effects of the selective imidazoline I2-site ligand 2-BFI

The functional significance of imidazoline I2 binding sites is unknown but microdialysis studies have indicated that the administration of I2-site ligands leads to an increase in extracellular levels of monoamines. The specific I2-site ligand 2-(-2-benzofuranyl)-2-imidazoline (2-BFI) generates a cue in drug discrimination, thereby indicating functional consequences of I2-site ligand binding. In the present work, we explored the ability of selective noradrenergic and serotonergic ligands to substitute for 2-BFI. Hooded Lister rats were trained in two-lever operant chambers with condensed milk reward to distinguish 2-BFI (7 mg/kg) from saline vehicle, by pressing the correct lever to a predetermined success criterion. Training sessions were then interspersed with sessions in which animals were administered test substances and the proportion of lever presses on the 2-BFI-associated lever (substitution) recorded. Several agents exhibited significant partial substitution for 2-BFI: The monoamine-releasing agents D-amphetamine and fenfluramine dose-dependently substituted for 2-BFI, while norepinephrine (desipramine, reboxetine) and serotonin (clomipramine, citalopram) reuptake inhibitors substituted at one or more doses. Further investigation using specific receptor agonists and antagonists indicated a possible role for activation of alpha1-adrenoceptors but failed to support involvement of alpha2-adenoceptor, beta-adrenoceptor or 5-HT1A receptor activation. These results support the concept that the 2-BFI cue may contain both noradrenergic and serotonergic components.

Noradrenergic response in vas deferens from rats submitted to acute and repeated stress

1. This work investigated the effects of androgens on the norepinephrine sensitivity of vasa deferentia from rats submitted to acute or repeated stress, as well as the participation of alpha1-adrenoceptors in the response of intact and bisected vasa deferentia from adult normal rats submitted to acute or repeated stress. 2. The acute stress produced subsensitivity to norepinephrine only in intact vasa deferentia from adult normal rats, which was prevented by lack of androgens, suggesting that the sensitivity may be dependent on the physiological level of androgen. 3. No change was observed in intact vas deferens sensitivity to norepinephrine in repeated stress, suggesting the occurrence of adaptation to elevated norepinephrine levels or a mild decrease in androgen levels or both. 4. The changes in sensitivity observed in acute and repeated stress may also be due to alterations in alpha1-adrenergic receptors that are located in the prostatic portion of the vas deferens.

Sources of calcium and alpha 1-adrenoceptor-mediated contraction in rat tail artery

1. The relative importance of intracellular and extracellular Ca2+ on alpha 1-adrenoceptor-mediated contraction by noradrenaline and St-587 has been studied and correlated with the binding characteristics in intact tail artery from Sprague-Dawley rats. 2. Noradrenaline and St-587 behaved as full agonists inducing a concentration-dependent vasoconstriction. 3. Nifedipine (1 microM and 10 microM) blocked by 50% (P < 0.001) and 75% (P < 0.001) respectively, the maximum contraction (Emax) induced by St-587. Nevertheless, to reach 40% inhibition of Emax on noradrenaline responses (P < 0.01), 10 microM nifedipine was necessary. 4. Both agonists induced a concentration-dependent accumulation of inositol phosphates. Noradrenaline behaved as a full agonist and St-587 as a partial agonist for this response. 5. [3H]-prazosin binding to intact tail artery rings was saturable and of high affinity (KD = 4.44 +/- 0.46 nM; Bmax = 36.35 +/- 4.22 fmol mg-1 tissue). 6. Competition curves for inhibition of specific [3H]-prazosin binding by WB-4101 suggest that the rat tail artery contains two alpha 1-adrenoceptor subtypes in an approximate ratio of 60:40. 7. After irreversible alkylation of alpha 1B-adrenoceptors with 100 microM chloroethylclonidine (CEC), nifedipine (1 microM) influenced to a greater extent the St-587- than the noradrenaline-induced contraction. 8. Our results indicate that the degree of participation of intracellular and extracellular Ca2+ sources, on the alpha 1-adrenoceptor-mediated contraction, depends on the agonist used. The two alpha 1-adrenoceptor subtypes observed in binding experiments seem to be unrelated to the Ca2+ sources used for contraction.

Modelling the changes induced by chronic desipramine treatment on the factors governing the agonism at prejunctional alpha 2-adrenoceptors

1. The adaptational changes induced after chronic desipramine treatment on functional responsiveness of alpha 2-adrenoceptor activation were investigated in prostatic portions of the rat vas deferens. 2. For this purpose, clonidine and xylazine were studied for their effects on twitch contractions elicited by electrical field stimulation of prostatic portions removed 48 h after the last injection to the animals of vehicle or desipramine (10 mg kg-1, i.p.; 14 days). Operational model-fitting and the nested hyperbolic method were used to analyse the effects of irreversible receptor alkylation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 300 nM) on the alpha 2-adrenoceptor-mediated effects of clonidine, either in vehicle- or in desipramine-treated animals. 3. Treatment with desipramine decreased the potency (increased the EC50) of clonidine and xylazine by about 12 and 9 fold respectively. However, the treatment did not modify the maximal effect (alpha) elicited by either agonist. The estimates of apparent affinity for clonidine did not depend on the method of calculation as the 'null' method and the 'operational' method gave similar answers. Estimates of tau values for both agonists revealed that chronic desipramine treatment resulted in significant decreases in the efficacy of agonists. However, desipramine treatment was not associated with significant changes in the affinity constant for clonidine while for xylazine, the operational model provided a higher estimate of KA (lower affinity) after desipramine treatment. 4. The results indicate a large receptor reserve at prejunctional alpha 2-adrenoceptors which is modulated by chronic desipramine treatment. 5. The comparison of results obtained after chronic desipramine exposure with those by using EEDQ suggests that chronic desipramine treatment is not a useful experimental intervention for the purpose of estimating agonist affinities and efficacies.

Effect of NG-nitro-L-arginine methylester (L-NAME) on functional and biochemical alpha 1-adrenoceptor-mediated responses in rat blood vessels

1. The modulation by NG-nitro-L-arginine methylester (L-NAME) of alpha 1-adrenoceptor-mediated contraction was investigated on isolated segments of rat tail artery and aorta. The influence of L-NAME on inositol phosphates accumulation by alpha 1-adrenoceptor agonists was also investigated to elucidate the intracellular mechanism responsible for this modulation. 2. In aorta but not in tail artery L-NAME (30 microM) enhanced the sensitivity (3.3 times) and the maximum contraction (Emax) induced by the full agonist, phenylephrine. 3. St-587, a partial alpha 1-adrenoceptor agonist, behaved as a weak agonist in the aorta (22.2% of phenylephrine Emax). However, when the same agonist was studied in tail artery rings a maximum contraction that was 78.4% of the phenylephrine induced Emax was reached. 4. L-NAME increased (3.3 times) the Emax for St-587 contraction in the aorta but not in the tail artery. Sensitivity to St-587 was slightly but significantly (P < 0.001) enhanced (1.9 times) by L-NAME in tail artery segments. 5. Contractile responses to phenylephrine after partial alkylation with phenoxybenzamine were analyzed by the nested hyperbolic null method. To elicit 50% of Emax for contraction only 1.1% of the receptors in the tail artery and 21% of the receptors in the aorta need to be occupied. These results indicate a higher receptor reserve for the tail artery than the aorta. 6. In the tail artery but not in the aorta, St-587 activates phosphoinositide turnover. The presence of L-NAME was without effect on inositol phosphates accumulation induced by this partial alpha 1-adrenoceptor agonist. 7. The maximum contraction induced by phenylephrine, after partial alpha-adrenoceptor alkylation, was enhanced by L-NAME in tail artery rings. However, the NO synthase inhibitor was unable to modify the phenylephrine-induced accumulation of inositol phosphates in the presence of phenoxybenzamine. 8. These results indicate that the differences in St-587-induced contraction and the modulation by L-NAME of alpha 1-adrenoceptor-mediated contraction observed between the tail artery and aorta are associated with differences in receptor reserve. In addition, our biochemical studies indicate that the potentiating effect of L-NAME is independent of intracellular calcium release via phosphatidylinositol turnover.

Analysis of agonism at functional prejunctional alpha 2-adrenoceptors of rat vas deferens using operational and null approaches

The alpha 2-adrenoceptors located prejunctionally on the postganglionic neurons that innervate the smooth muscle of the prostatic portion of the rat vas deferens were examined. For this purpose, three imidazolidine derivatives (structurally related to clonidine) were studied for their effects on twitch contractions elicited by electrical field stimulation of this tissue. In this study, operational model-fitting and the nested hyperbolic method were used to analyse the effects of irreversible receptor alkylation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on the alpha 2-adrenoceptor-mediated effects of clonidine (2-[2,6-dichlorophenylimino]imidazolidine) in stimulated vas deferens. The operational model provided an estimate of KA for clonidine which was not significantly different from the estimate obtained by using the nested hyperbolic method (null approach). The data indicate a large receptor reserve at prejunctional alpha 2-adrenoceptors for clonidine. The estimates of apparent affinity for St-587 (2-[2-chloro-5-trifluoromethylphenylimino]imidazolidine) and St-591 (2-[2-chloro-5-methylphenylimino]imidazolidine) did not depend on the method of calculation as the 'null' method and the 'operational' method gave similar answers. Further, estimates of the ratio of tau values for these partial agonists with respect to clonidine were numerically the same as those of their relative efficacies. Therefore, no limitations in the ability of the operational model to fit experimental data and provide reproducible estimates of affinity and efficacy have been revealed for agonists acting at prejunctional alpha 2-adrenoceptors.

Modulation of alpha 1-adrenoceptors and functional consequences in the bisected rat vas deferens following chronic inhibition of neuronal noradrenaline uptake

1. The adaptational changes induced after chronic inhibition of neuronal noradrenaline uptake on both functional responsiveness of alpha 1-adrenoceptor activation and [3H]-prazosin binding were investigated in prostatic and epididymal portions of the rat vas deferens. 2. Contractile concentration-response curves to phenylephrine and saturation isotherms of [3H]-prazosin binding to homogenates of each of the portions of the bisected rat vas deferens were determined 48 h after the last injection of desipramine, nomifensine or nisoxetine (10 mg kg-1; i.p. for 14 days). 3. Treatment with both nomifensine and nisoxetine decreased the potency (pD2) of phenylephrine by about 10 and 8 fold respectively in the epididymal portion. However, administration of desipramine only reduced the potency of the alpha 1-adrenoceptor agonist by about 1.8 fold. None of the treatments modified the maximal effect (Emax) elicited by phenylephrine in this portion of the vas deferens. In the prostatic portion only the treatment with nomifensine (1.4 fold) and nisoxetine (1.8 fold) decreased the potency of phenylephrine; the maximal contraction elicited by the agonist after the treatments was also reduced. 4. Chronic treatment with either nomifensine or nisoxetine did not change the KD for [3H]-prazosin binding in either epididymal or prostatic membranes. However, these two treatments resulted in a significant decrease in the [3H]-prazosin Bmax in membranes in both portions of rat vas deferens. The reduction in density of alpha 1-adrenoceptors was higher in the epididymal than the prostatic half. Desipramine reduced the Bmax only in the epididymal portion. 5. These results indicate that differential regulation of ax-adrenoceptors in either portion of the rat vas deferens could result from a greater degree of activation of these receptors in the epididymal half after chronic inhibition of neuronal noradrenaline uptake. The different functional consequences of the loss of alpha l-adrenoceptors in each portion seems to be explained on the basis of a different relationship between the occupancy of the receptor and the response elicited.

Identification of alpha 1-adrenoceptor subtypes in the rat vas deferens: binding and functional studies

1. The alpha 1-adrenoceptor subtypes of the prostatic and epididymal portion of rat vas deferens were characterized in binding and functional experiments. 2. In saturation experiments, [3H]-prazosin bound to two distinct affinity sites in the epididymal portion of rat vas deferens (pKD = 10.1 +/- 0.13 and 9.01 +/- 0.15, Bmax = 507 and 1231 fmol mg-1 protein, respectively). In the prostatic portion [3H]-prazosin bound to a single affinity site (pKD = 9.82 +/- 0.04, Bmax = 924 fmol mg-1 protein). 3. In the displacement experiments, unlabelled prazosin displaced biphasically the binding of 200 pM [3H]-prazosin to the epididymal portion; the resulting two pKI values were consistent with the affinity constants obtained in the saturation experiments. WB4101 (2-(2,6-dimethoxy-phenoxyethyl)-amino-methyl-1,4-benzodioxane) and benoxathian also discriminated the two affinity sites in the epididymal portion and the population of low affinity sites for the three antagonists was approximately 40%. On the other hand, the prostatic portion predominantly showed a single affinity site for prazosin, WB4101 and benoxathian, although the presence of a small proportion (less than 10%) of the low affinity site could be detected. HV723 (alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)ethyl)-a min o)- propyl) benzeneacetonitrile fumarate) displaced the [3H]-prazosin binding monophasically with a low affinity in both halves. 4. Pretreatment with chlorethylclonidine (CEC) at concentrations higher than 1 microM inhibited 700 pM [3H]-prazosin binding to the prostatic portion by approximately 50%. However, the inhibition in the epididymal portion was much less (approximately 21% at 50 microM CEC).5. In the functional study, the contractile response to noradrenaline was competitively inhibited by prazosin, WB4101, benoxathian and HV723 with similar and low affinities (pKB value ranging from 8.0to 9.0) in the epididymal portion of rat vas deferens. In the prostatic portion of rat vas deferens,noradrenaline also produced a contraction, but the maximal amplitude of contraction developed was approximately one-fourth of that in the epididymal portion. Prazosin and WB4101 also inhibited the contractile response of the prostatic portion with the pKB values similar to those obtained in the epididymal portion. The contractions to noradrenaline in both portions were potently attenuated by 1 LM nifedipine but were not affected by pretreatment with 1O LM CEC.6. Under conditions where P2x-purinoceptors and prejunctional M2-adrenoceptors were blocked, electrical transmural stimulation produced a rapidly developing phasic contraction and a subsequent tonic contraction in the epididymal portion of rat vas deferens. The phasic and tonic contractions were inhibited in a concentration-dependent manner by prazosin (ICs = 25.7 and 25.9 nm, respectively),WB4101 (ICo= 7.27 and 7.58 nM), benoxathian (ICs = 10.9 and 8.66 nM) and HV723 (ICs = 15.9 and 14.9 nM). Nifedipine selectively attenuated the tonic contraction induced by electrical stimulation, and the residual phasic response was inhibited by the antagonists mentioned above with similar affinities to those in the absence of nifedipine. CEC (10 gM) had little effect on the adrenergic neurogenic contractions.7. The present results indicate the presence of two distinct alpha&-adrenoceptor subtypes in the rat vas deferens, which show respectively high and low affinities for each of prazosin, WB4101 and benoxathian,and presumably correspond to putative MIA and alL subtypes according to the recent am-adrenoceptorsubclassifications. The contractions induced by exogenous and endogenous noradrenaline seem to be predominantly mediated through the alL subtype. The heterogeneous distribution of the low affinity sites(alL subtype) may well explain differences in functional responsiveness between the two portions of rat vas deferens.

Mechanisms underlying the differential sensitivity to alpha 1-adrenoceptor activation in the bisected rat vas deferens

1. The factors underlying the different responsiveness of the prostatic and epididymal portions of rat vas deferens to alpha 1-adrenoceptor stimulation were investigated. 2. The alpha 1-adrenoceptors in membranes of both halves of rat vas deferens were labelled with [3H]-prazosin and the affinities of agonists and antagonists for these receptors were determined. In saturation studies, the Bmax and KD values for [3H]-prazosin in membranes of both portions were the same. 3. In competition studies, the inhibition curves for phentolamine were biphasic and consistent with the presence of both alpha 1a- and alpha 1b-adrenoceptor subtypes. The proportions of binding sites with high and low affinity for phentolamine in both halves of rat vas deferens were similar and in good agreement with the percentages of binding sites for WB-4101 and phentolamine previously reported in the whole rat vas deferens. 4. The phenylethylamines displaced [3H]-prazosin with a shallow inhibition curve. The data are compatible with the assumption of two affinity states for the binding sites. For the imidazoline compounds no such distinct affinity states could be demonstrated. 5. The affinity for, and the relative intrinsic efficacy on postsynaptic alpha 1-adrenoceptors of both portions of rat vas deferens were studied for noradrenaline, phenylephrine and methoxamine by irreversible inactivation of the alpha 1-adrenoceptors by phenoxybenzamine. The parameters for partial agonists were determined by comparing the responses to the partial agonist to those of a full agonist in the same tissue. Homogeneous estimates of the equilibrium dissociation constants (Ka) were obtained, indicating that these agonists bind to the receptors of both tissues in an identical manner. Further, estimation of the intrinsic efficacy of agonists relative to noradrenaline, indicated no differences between the two halves of rat vas deferens. 6. Ka values for agonist activation of the functional alpha 1-adrenoceptors were compared with K, values for agonist inhibition of specific [3H]-prazosin binding. The K. values were well correlated with the low affinity K, values for phenylethylamines in both portions of rat vas deferens, suggesting that the initial event in signal transduction by alpha 1,-adrenoceptors is the binding to the low affinity state of the receptor. 7. There was a non-linear relationship between response and receptor occupancy in both halves of rat vas deferens but the occupancy-response coupling was more efficient in the epididymal than in the prostatic portion. This fact may account for the differences observed in the functional responses.