Responses Of Rat Vas Deferens Research Articles

Huwentoxin-XVI, an analgesic, highly reversible mammalian N-type calcium channel antagonist from Chinese tarantula Ornithoctonus huwena

N-type calcium channels play important roles in the control of neurotransmission release and transmission of pain signals to the central nervous system. Their selective inhibitors are believed to be potential drugs for treating chronic pain. In this study, a novel neurotoxin named Huwentoxin-XVI (HWTX-XVI) specific for N-type calcium channels was purified and characterized from the venom of Chinese tarantula Ornithoctonus huwena. HWTX-XVI is composed of 39 amino acid residues including six cysteines that constitute three disulfide bridges. HWTX-XVI could almost completely block the twitch response of rat vas deferens to low-frequency electrical stimulation. Electrophysiological assay indicated that HWTX-XVI specifically inhibited N-type calcium channels in rat dorsal root ganglion cells (IC50 ∼60 nM). The inhibitory effect of HWTX-XVI on N-type calcium channel currents was dose-dependent and similar to that of CTx-GVIA and CTx-MVIIA. However, the three peptides exhibited markedly different degrees of reversibility after block. The toxin had no effect on voltage-gated T-type calcium channels, potassium channels or sodium channels. Intraperitoneal injection of the toxin HWTX-XVI to rats elicited significant analgesic responses to formalin-induced inflammation pain. Toxin treatment also changed withdrawal latency in hot plate tests. Intriguingly, we found that intramuscular injection of the toxin reduced mechanical allodynia induced by incisional injury in Von Frey test. Thus, our findings suggest that the analgesic potency of HWTX-XVI and its greater reversibility could contribute to the design of a novel potential analgesic agent with high potency and low side effects.

Effect of noni (Morinda citrifolia Linn.) fruit and its bioactive principles scopoletin and rutin on rat vas deferens contractility: an ex vivo study.

This study examined the effect of methanolic extract of Morinda citrifolia Linn. (MMC) and its bioactive principles, scopoletin and rutin, on dopamine- and noradrenaline-evoked contractility in isolated rat vas deferens preparations. MMC (1–40 mg/mL), scopoletin (1–200 μg/mL), and rutin hydrate (0.6–312.6 μg/mL) dose-dependently inhibited the contractility evoked by submaximal concentrations of both dopamine and noradrenaline, respectively. Haloperidol and prazosin, reference dopamine D2, and α 1-adrenoceptors antagonists significantly reversed the dopamine- and noradrenaline-induced contractions, respectively, in a dose-dependent manner. Interestingly, MMC per se at higher doses (60–100 mg/mL) showed dose-dependent contractile response in rat vas deferens which was partially inhibited by high doses of haloperidol but not by prazosin. These results demonstrated the biphasic effects of MMC on dopaminergic system; that is, antidopaminergic effect at lower concentrations (<40 mg/mL) and dopaminergic agonistic effect at higher concentrations (>60 mg/mL). However, similar contractile response at high doses of scopoletin (0.5–5 mg/mL) and rutin hydrate (0.5–5 mg/mL) per se was not observed. Therefore, it can be concluded that the bioactive principles of MMC, scopoletin, and rutin might be responsible for the antidopaminergic and antiadrenergic activities of MMC.

Effect of meptazinol on evoked responses in rat vas deferens.

Responses of rat isolated vas deferens to electrical stimulation through field electrodes (400 mA, 1 ms duration, single shocks at 5 min intervals) were potentiated by meptazinol (10 to 300 microM) in whole tissues and also in the separated prostatic and epididymal portions. The effect was fast in onset, reproducible and easily reversed by washing. Prazosin (0.1 microM) practically abolished the response of the epididymal portion to electrical stimulation while the response of the prostatic portion was only slightly reduced (less than 20%). In the presence of prazosin, meptazinol still produced potentiation of the response of the prostatic portion. Nifedipine (2 microM) practically abolished the response of the prostatic portion to electrical stimulation while the response of the epididymal portion was only slightly reduced (less than 20%). In the presence of nifedipine, meptazinol no longer produced potentiation of the response of the epididymal portion. Exogenous ATP (5 microM to 1 mM) and phenylephrine (1 to 50 microM) produced a contractile response which was potentiated in the presence of meptazinol (100 microM) but in the presence of meptazinol (100 microM) and nifedipine (5 mM) together, potentiation of phenylephrine no longer occurred. It is suggested that potentiation by meptazinol of electrically induced responses in this tissue is due to a direct action on the smooth muscle.

Evaluation of the cholinergic and adrenergic effects of two tropane alkaloids from Erythroxylum pervillei

Pervilleine A is an aromatic ester tropane alkaloid from Erythroxylum pervillei that has shown promising activity as a multidrug resistance inhibitor. Due to its structural similarity with the well known (-)-hyoscyamine and (-)-cocaine, the cholinergic and adrenergic activities of pervilleine A were evaluated. At 30 microm (+/-)-pervilleine A hydrochloride exhibited non-competitive inhibition of the cholinergic response in the guinea-pig ileum and did not affect the carbachol-induced contraction of the rat anococcygeus smooth muscle. (+/-)-Pervilleine A hydrochloride blocked nonspecifically the vascular response of (-)-norepinephrine in the rat aorta ring, while the contractile response of rat vas deferens to (-)-norepinephrine was not affected significantly at a 100 microm concentration. An analogue of pervilleine A, (+/-)-pervilleine H, without a 6-O-trans-3,4,5-trimethoxycinnamoyl ester substituent required for anti-multidrug resistance activity, did not exhibit any effects in these experiments. The data suggest that (+/-)-pervilleine A hydrochloride has weak nonspecific anticholinergic and vascular antiadrenergic activities. The lack of significant cholinergic and adrenergic receptor-mediated activities may be considered advantageous for the further development of pervilleine A as a new adjuvant in cancer chemotherapy.

Specific H +/K +-ATPase inhibitors decreased contractile responses of isolated rat vas deferens

The effect of H +/K +-ATPase inhibitors on rat vas deferens contractility was investigated in vitro. Omeprazole (100–300 μM), lansoprazole (100–300 μM) and SCH 28080 (10–100 μM) (2-methyl-8-(phenylmethoxy)-imidazo[1,2-a]pyridine-3-acetonitrile) decreased contractile responses of vas deferens to electrical field stimulation, high K + (80 mM) and phenylephrine in a reversible, reproducible and concentration-dependent manner. The inhibitory potency of lansoprazole on vas deferens contractility was increased in relatively acidic solution (pH 6.9), suggesting that the site of action may be related to H +/K +-ATPase. However, lansoprazole-induced inhibition on contractility was unaltered in K + free solution, indicating that the mechanism of action is independent from H +/K +-ATPase. Reversible nature of omeprazole and lansoprazole-induced inhibition on contractility also suggests that the effects are not due to inhibition of H +/K +-ATPase, since both compounds are irreversible inhibitors of the enzyme. Presence of ouabain (5 μM) did not decrease lansoprazole-induced inhibition on contractility but potentiated the inhibitory effect of lansoprazole, suggesting that lansoprazole-induced inhibition is not mediated by the inhibition of Na +/K +-ATPase. Calcium-induced contractions in high K +–Ca 2+ free medium were completely antagonized by lansoprazole, implying that lansoprazole inhibits Ca 2+ entry through voltage-gated channels. In conclusion, three H +/K +-ATPase inhibitors decreased contractile responses of rat vas deferens to various stimulants in vitro. They may act on a common mechanism, which plays a crucial role in regulating rat vas deferens contractility and this mechanism is probably involved in the regulation of intracellular Ca 2+.

Function and Solution Structure of Huwentoxin-X, a Specific Blocker of N-type Calcium Channels, from the Chinese Bird Spider Ornithoctonus huwena

Huwentoxin-X (HWTX-X) is a novel peptide toxin, purified from the venom of the spider Ornithoctonus huwena. It comprises 28 amino acid residues including six cysteine residues as disulfide bridges linked in the pattern of I-IV, II-V, and III-VI. Its cDNA, determined by rapid amplification of 3' and 5' cDNA ends, encodes a 65-residue prepropeptide. HWTX-X shares low sequence homology with omega-conotoxins GVIA and MVIIA, two well known blockers of N-type Ca2+ channels. Nevertheless, whole cell studies indicate that it can block N-type Ca2+ channels in rat dorsal root ganglion cells (IC50 40 nm) and the blockage by HWTX-X is completely reversible. The rank order of specificity for N-type Ca2+ channels is GVIA approximately HWTX-X > MVIIA. In contrast to GVIA and MVIIA, HWTX-X had no detectable effect on the twitch response of rat vas deferens to low frequency electrical stimulation, indicating that HWTX-X has different selectivity for isoforms of N-type Ca2+ channels, compared with GVIA or MVIIA. A comparison of the structures of HWTX-X and GVIA reveals that they not only adopt a common structural motif (inhibitor cystine knot), but also have a similar functional motif, a binding surface formed by the critical residue Tyr, and several basic residues. However, the dissimilarities of their binding surfaces provide some insights into their different selectivities for isoforms of N-type Ca2+ channels.

COMPARISON OF PERIPHERAL INHIBITORY EFFECTS OF CLOMIPRAMINE WITH SELECTIVE SEROTONIN RE-UPTAKE INHIBITORS ON CONTRACTION OF VAS DEFERENS: IN VITRO AND IN VIVO STUDIES

Purpose: We compared the peripheral inhibitory effects of the tricyclic antidepressant clomipramine with those of various selective serotonin re-uptake inhibitors on the contractile response of the vas deferens.Materials and Methods: The contractile responses of 17 circular smooth muscle strips of human vas deferens to 10−4 M. norepinephrine were observed in the absence and presence of clomipramine, and the selective serotonin re-uptake inhibitors fluoxetine, sertraline and paroxetine. The intraluminal pressure response of rat vas deferens to electrical stimulation of the hypogastric nerve was measured in 5 rats in the central plus peripheral effect group before and after the intravenous injection of 4.2 mg./kg. clomipramine or 8.3 mg./kg. sertraline. The pressure response to each agent was also observed after the transection of all proximal sympathetic input to the hypogastric nerve in 5 animals in the peripheral effect group.Results: Clomipramine was about 100-fold more potent than sertraline, fluoxetin...

Influence of castration on the response of the rat vas deferens to fluoxetine

Antidepressant drugs such as desipramine and fluoxetine increase norepinephrine (NE) contractile response in rat vas deferens by inhibiting neuronal amine uptake. Fluoxetine, unlike other antidepressants, also inhibits calcium fluxes, which results in an inhibition of maximal NE effect. Since the contractile response of the reproductive tract is under the influence of testosterone, the effect of fluoxetine could be modified according to the endocrine status of the animal. In the present study we evaluated the influence of castration and testosterone replacement (1 mg per 100 g body wt.) on the peripheral action of fluoxetine. Castration was followed by a decrease in vas deferens weight and the appearance of spontaneous activity. Testosterone replacement reversed these effects. Concentration–response curves to NE and calcium were obtained in the absence and the presence of fluoxetine in vasa deferentia from normal, castrated and testosterone-treated castrated rats. After castration the effect of fluoxetine on vas deferens contractility was markedly altered. The spontaneous activity that appears after castration was prevented by fluoxetine and the stimulatory effect on NE-induced contractions was not observed. In contrast, the inhibitory action of fluoxetine on maximal NE effect was increased. Testosterone replacement restored vas deferens response to NE in the presence of fluoxetine. Fluoxetine did not modify the binding parameters of [3H]prazosin in vasa deferentia from normal or castrated animals. Cocaine shifted the NE concentration–response curve to the left in all groups, suggesting that the changes in fluoxetine effect following castration were not the result of an alteration of the neuronal uptake mechanism. The nitric oxide synthase inhibitor l-NMMA did not modify vas deferens response to NE in castrated animals either in the absence or presence of fluoxetine. An increased sensitivity to the inhibitory effect of fluoxetine was observed in the calcium concentration–response curves in vasa deferentia from castrated rats, an effect that was reversed by testosterone replacement. The results suggest that the alteration in the responsiveness of vasa deferentia from castrated rats to calcium could be responsible for increased sensitivity to the inhibitory effect of fluoxetine. It is concluded that vas deferens contractile response is testosterone dependent and that this behaviour modifies the effect of drugs such as fluoxetine that have dual effect on contractility.

Characterisation of calcitonin gene-related peptide receptors in rat atrium and vas deferens: evidence for a [Cys(Et) 2,7]hCGRP-preferring receptor

The present study was performed in order to characterise calcitonin gene-related peptide (CGRP) receptor subtypes in rat left atrium and vas deferens by using [ R-( R*, S*)]- N-[2-[[5-amino-1-[[4-(4-pyridinyl)-1-piperazinyl]carbonyl]pentyl]amino]-1-[(3,5-dibromo-4-hydroxyphenyl)methyl]-2-oxoethyl]-4-(1,4-dihydro-2-oxo-3(2 H)-quinazolinyl)-,1-Piperidinecarboxamide (BIBN4096BS), a novel CGRP receptor antagonist. When CGRP was used as an agonist, BIBN4096BS exhibited an almost 10-fold higher affinity for CGRP receptors in rat left atrium compared to those in the vas deferens, indicating that CGRP acts through different CGRP receptor subtypes in these two tissues. In addition, BIBN4096BS was almost 10-fold more potent in antagonizing [Cys(Et) 2,7]hCGRPα and human adrenomedullin-induced responses than CGRP-induced responses in rat vas deferens. This might indicate receptor heterogeneity in rat vas deferens. Accordingly, the present work provides first experimental evidence that the rat vas deferens contains two CGRP-like receptor subtypes. Namely, the CGRP 2 receptor and a “novel” receptor that possesses low efficacy for CGRP and that is selectively stimulated by [Cys(Et) 2,7]hCGRP or adrenomedullin and which can be blocked with high affinity by BIBN4096BS.

Differential effects of drugs interacting with autonomic transmitters on responses of rat vas deferens to field stimulation.

1. Frequency-response curves (0.1-30 Hz) were obtained in the epididymal portion of rat vas deferens. At low frequencies (0.1-1 Hz), the parameters evaluated were the first twitch and the fourth twitch at each frequency. The responses to trains of stimuli at intermediate (2-5 Hz) and high (10-30 Hz) frequencies were biphasic consisting of phase I (the first rapid phase of tetanus) and of phase II (the secondary slowly developing one). 2. Prazosin inhibited the first and the fourth twitch but not when the frequency was < 1 Hz. Suramin inhibited the first twitch while substantially depressing the fourth one. The combination of prazosin and suramin almost completely abolished all the twitches evoked by a train of stimuli at low frequencies. Nifedipine left almost unaltered the first twitch while markedly depressing the fourth one, especially at relatively high frequency (1 Hz). Verapamil was devoid of any inhibitory action. Papaverine depressed the first twitch while only at the highest concentration used (1 x 10(-4) M) markedly inhibited the fourth one. Chloroethylclonidine (CEC) depressed the first twitch and increased the fourth. 3. When intermediate (2-5 Hz) and high (10-30 Hz) frequencies are considered, prazosin and suramin partially inhibited both phase I and phase II, while in combination they almost completely abolished both phases. Nifedipine and verapamil selectively suppressed phase II, leaving phase I unaffected. Papaverine completely abolished both phase I and phase II. CEC was able to completely abolish phase I but increased phase II. 4. These results suggest that the response to the first twitch of a train at low frequency is prevailingly noradrenergic, prazosin-sensitive, while when the twitches are close enough (i.e. at 1 Hz) a summation of stimuli takes place and a predominant purinergic component, both suramin- and nifedipine-sensitive, becomes evident. 5. At high frequencies, both phases are due to the concomitant release of noradrenaline and adenosine triphosphate (ATP). The noradrenergic component of phase I is nifedipine-insensitive and CEC-sensitive, resembling the pharmacological profile of the endogenously released noradrenaline by single pulse, while that of phase II, nifedipine-sensitive and CEC-insensitive, is similar to that produced by exogenously applied noradrenaline.

The novel pyridoxal-5′-phosphate derivative PPNDS potently antagonizes activation of P2X 1 receptors

Pyridoxal-5′-phosphate-6-(2′-naphthylazo-6′-nitro-4′,8′-disulfonate) (PPNDS) potently antagonized P2X 1 receptor-mediated responses in rat vas deferens (p K B=7.43) and Xenopus laevis oocytes (pIC 50=7.84). It showed lower (up to 20,000-fold) inhibitory potency on ecto-nucleotidase in Xenopus oocytes and on P2Y 1 receptors in guinea-pig ileum (p A 2=6.13). PPNDS did not interact with α 1A-adrenoceptors, adenosine A 1 and A 2B, histamine H 1 and muscarinic M 3 receptors. Thus, PPNDS is a novel, specific P2 receptor antagonist and represents the pyridoxal-5′-phosphate derivative with the highest potency at P2X 1 receptors.

Effect of venlafaxine hydrochloride in different preparations of isolated guinea-pig and rat organ tissues.

A study was undertaken to know better the effects of venlafaxine hydrochloride on the responses of isolated rat vas deferens to noradrenaline and dopamine, those of isolated rat uterus to serotonin and histamine, and those of isolated guinea-pig ileum to acetylcholine and histamine. Venlafaxine hydrochloride increased the response of rat vas deferens to noradrenaline but not to dopamine. Venlafaxine did not alter the response of rat isolated uterus to serotonin. In rat uterus, venlafaxine did not modify the response to histamine but was able to increase it in guinea-pig ileum. An anticholinergic effect was observed with the lowest concentration tested. Although venlafaxine is a selective serotonine reuptake inhibitor in the central nervous system, serotonin uptake was not seen in the rat uterus. The anticholinergic effects observed in the present study might be consistent with some of the side-effects associated with venlafaxine.

Extraneuronal uptake inhibitor U-0521 decreases contractile responses in rat vas deferens

1. 1. The influence of catechol-O-methyltransferase inhibitor U-0521 on isotonic contraction of isolated rat vas deferens was examined to determine optimal concentration and nonspecific effects. 2. 2. Maximum responses to (-)-epinephrine were increased at 0.4 μM and 1 μM concentrations of U-0521. Epinephrine responses were progressively decreased in the presence of higher concentrations (10 μM, 30 μM and 100 μM) of U-0521. 3. 3. The response to the nonadrenergic agonist neurokinin A was similarly depressed in the presence of 100 μM U-0521. 4. 4. U-0521 not only inhibits COMT, at concentrations above 1 μM it nonspecifically depresses contraction of the rat vas deferens by both adrenergic and nonadrenergic agonists.

NS 1619 activates Ca 2+-activated K + currents in rat vas deferens

The effects of NS 1619, a newly developed activator of large-conductance Ca 2+-activated K + channels, were investigated on single smooth muscle fibers dissociated enzymatically from rat vas deferens and on contractions of the epididymal half of vas deferens. K + currents were recorded using whole-cell patch-clamp methods in near-physiological K + solutions (5.4 mM extracellular K +/145 mM intracellular K +). When cell membrane voltage was stepped to test potentials (−60 to +60 mV) from a holding potential of −10 mV, NS 1619 increased the outwardly rectifying K + current in a concentration-dependent manner. The increased portion of the K + current by NS 1619 was totally abolished by charybdotoxin (100 nM) but not by glibenclamide (3 μM). NS 1619 reduced electrically stimulated contractile responses of rat vas deferens in a concentration-dependent manner, and charybdotoxin but not glibenclamide partially inhibited the effect of NS 1619. NS 1619 (50 μM) inhibited the noradrenaline-induced contraction. Charybdotoxin (100 nM) partially reduced the NS 1619-induced inhibition while glibenclamide (3 μM) had no effect. NS 1619 (10–100 μM) reduced the high K +-induced contractions in a noncompetitive manner. The present results indicate that NS 1619 activates charybdotoxin-sensitive Ca 2+-activated K + channels and probably inhibits Ca 2+ influx. These two effects might account largely for the observed mechanical inhibition induced by NS 1619 in the epididymal half of isolated rat vas deferens. © 1997 Elsevier Science B.V.

Regional difference in sympathetic neurotransmitter-and Ca 2+ channel-mediated responses in rat vas deferens

1. 1. Pharmacological properties were compared in the epididymal and prostatic portions of the rat vas deferens. 2. 2. Contractile response to norepinephrine (NE) was larger in the epididymal portion, in spite of the smaller diameter in this region. In contrast, contraction evoked by ATP or α,β-methylene ATP (α,β-mATP) was larger in the prostatic portion. The sensitivities to NE but not to α,β-mATP are different in these portions. 3. 3. ATP or NE facilitated the contraction induced by the other agonist, suggesting that they cooperatively elicit smooth muscle contraction. This cooperation was observed in both portions. 4. 4. Neither the contraction elicited by the addition of Ca 2+ to smooth muscle depolarized with 63.7 mM extracellular K + nor the relaxation by nifedipine of the depolarized smooth muscle precontracted with 1.8 mM extracellular Ca 2+ was regionally different. However, Bay k 8644 elicited contraction only in the epididymal portion. A combination of 5 mM K + with Bay k 8644 also caused oscillatory contraction in the prostatic portion. 5. 5. A radioligand binding study was performed using the microsomal fraction prepared separately from these portions. Both the dissociation constant and the maximum binding for 3H-nimodipine were smaller in the epididymal fraction than in the prostatic fraction. 6. 6. These results suggest that 1. the NE-elicited contraction is more pronounced in the epididymal portion, 2. the purinoceptor-mediated contractions along the vas deferens are less heterogeneous, and 3. although the sensitivity to Bay k 8644 is higher in the epididymal portion, Ca 2+ channel-mediated responses are not regionally different when they are fully activated.

Synthesis and biological characterization of a series of analogues of omega-conotoxin GVIA.

The 27-residue polypeptide omega-conotoxin GVIA (omega-CgTx), from the venom of the cone shell Conus geographus, blocks N-type neuronal calcium channels. It contains three disulphide bridges. We report here the synthesis and biological characterization of a series of analogues in which one disulphide has been replaced by substitution of appropriate Cys residues with Ser, viz. [Ser1,16]-omega -CgTx, [Ser8,19]-omega-CgTx, [Ser15,26)-omega-CgTx, [Ser16]-omega-CgTx8-27 and [Ser15]-omega-CgTx1-19. All syntheses were conducted manually using either Boc or Fmoc methodology. Deprotected peptides were oxidized to their bridged forms using either aerial oxidation or aqueous dimethyl sulphoxide. Peptides were purified using RP-HPLC, and their purity and identity were checked by RP-HPLC, capillary electrophoresis and mass spectrometry. Inhibition of neuronal N-type calcium channels was assessed as the inhibition of the twitch responses of rat vas deferens stimulated with single electrical pulses at 20 second intervals. None of these analogues was biologically active, suggesting that the disulphides play an important role in maintaining biological activity.

Pharmacological evaluation of N-methyl-actinodaphnine, a new vascular α-adrenoceptor antagonist, isolated from Illigera luzonensis

The pharmacological activity of N-methyl-actinodaphnine, isolated from Illigera luzonensis, was determined by functional and binding experiments with peripheral tissues. In a functional study, N-methyl-actinodaphnine was a simple competitive antagonist of contractions elicited by phenylephrine (pA 2 = 7.11) in rat thoracic aorta; it also competitively antagonised the clonidine-induced inhibition of the twitch response of rat vas deferens (pA 2 = 5.01). In addition, [ 3H]inositol monophosphate formation caused by noradrenaline (3 μM) in rat isolated thoracic aorta was concentration dependently inhibited by N-methyl-actinodaphnine (1 and 10 μM); however, it had no effect on cyclic AMP and cyclic GMP contents. Additionally, N-methyl-actinodaphnine had extremely low affinity for thromboxane receptors, prostaglandin receptors, Ca 2+ channels, muscarinic receptors, histamine receptors, β-adrenoceptors, neurokinin and leukotriene receptors in vitro. However, N-methyl-actinodaphnine also possessed 5-hydroxytryptamine (5-HT) receptor blocking activity. Its potency for blocking 5-HT receptors was about 14 times less than that for blocking α 1-adrenoceptors. In binding experiments, N-methyl-actinodaphnine displaced biphasically the binding of 0.2 nM [ 3H]prazosin to cultured A10 cells. The selectivity for α 1-adrenoceptor subtypes was also investigated in rat vas deferens and spleens. The contractile response in rat vas deferens to noradrenaline was competitively inhibited by N-methyl-actinodaphnine with a pA 2 value of 6.58; N-methyl-actinodaphnine also competitively antagonized the phenylephrine-induced contraction in rat spleen with a pA 2 value of 7.38. These results indicate that N-methyl-actinodaphnine is a selective α 1-adrenoceptor antagonist. Furthermore, it is more selective for the α 1B- than for the α 1A-adrenoceptor subtype.

Characterization of alpha 1-adrenoceptor subtypes in tension response of human prostate to electrical field stimulation.

1. The effects of various alpha 1-adrenoceptor antagonists and nifedipine on tension responses of human prostate to electrical field stimulation were evaluated in this study. 2. Prazosin (3 x 10(-10) to 10(-8) M) and 5-methyl-urapidil (10(-9) to 3 x 10(-8) M) blocked concentration-dependently the tension responses to electrical field stimulation and completely abolished them in the maximal concentrations (10(-8) M and 3 x 10(-8) M, respectively); in contrast, chloroethylclonidine (CEC), in the maximal concentration of 100 microM, blocked these effects by only 50%. 3. The contractile responses of rat vas deferens and spleen to exogenously-applied alpha 1-adrenoceptor agonists were competitively inhibited by prazosin and 5-methyl-urapidil; in addition, the pA2 values were calculated and the relative potencies with reference to prazosin were obtained. The relative potency of 5-methyl-urapidil in human prostate (0.105) was close to that in rat vas deferens (0.257), which contains primarily putative alpha 1A-adrenoceptors. However, it was much more than that in rat spleen (0.011), which contains primarily putative alpha 1B-adrenoceptors. 4. Nifedipine (10(-8) to 10(-6) M) inhibited concentration-dependently the contractile responses to electrical field stimulation in human prostate; in addition, the inhibition percentages were similar to those to exogenously-applied noradrenaline in rat vas deferens. In contrast, CEC (10 microM), which almost flattened the concentration-response curve of the rat spleen to phenylephrine, only partially inhibited (by 33.1%) the nerve-mediated contraction of human prostate. 5. The involvement of prejunctional alpha 2-adrenoceptors situated on the sympathetic nerve terminals of human prostate was also examined. Clonidine (3 x 10-9 to 3 x 10- M) blocked concentration-dependently the contractile response to electrical field stimulation of human prostate and this inhibitory effect was reversed by yohimbine (10-7 M). Additionally, the inhibitory effect of CEC (3 x 10-6 to 3 x 10-4 M)to the nerve-mediated contraction was also partially reversed by yohimbine (10-7 M).6. It is suggested that the putative czA-adrenoceptors in human prostate may be functionally confined to the synaptic region whereas only minor populations of the putative alpha 1B- and/or alpha 1c-adrenoceptors exist in this region.

Comparison of inhibition by phentolamine, verapamil and cromakalim of noradrenaline and naphazoline responses in rat vas deferens

Comparison of inhibition by phentolamine, verapamil and cromakalim of noradrenaline and naphazoline responses in rat vas deferens

Neurotransmitter feedback is not important in modulating the noradrenergic component of responses of rat vas deferens to twin pulse electrical field stimulation.

1. Drug effects on the full time-course of tension responses of the rat vas deferens to challenges of twin pulse field stimulation (TPFS) were examined. A microprocessor-controlled system was used to regulate stimulus delivery, on-line data collection and subsequent data analysis. 2. The second, noradrenergic phase of the response to the second stimulus of TPFS was missing when the interpulse interval was set at 3 s but was progressively restored as the interpulse interval was extended to 120 s. 3. With a 3 s interpulse interval, the missing second phase of the response to the second stimulus was not restored by the selective alpha 2-adrenoceptor antagonists yohimbine, imiloxan or idazoxan, indicative that alpha 2-adrenoceptor-mediated feedback inhibition of noradrenaline release is not the predominant mechanism modulating this response component. 4. Incubation with the P1-purinoceptor antagonist 8-phenyl-theophylline also failed to restore the missing noradrenergic component in the response to the second stimulus of TPFS. 5. Nevertheless, both responses to TPFS were impaired by the selective alpha 2-adrenoceptor agonist clonidine and by the P1-purinoceptor agonist 2-chloroadenosine, indicating the presence of functional presynaptic receptors of both types. These agonist-induced inhibitory effects were readily reversed by those antagonists which had failed to restore the missing noradrenergic component in the second response to TPFS.