Separation of putative alpha 1A- and alpha 1B-adrenoceptor mediated components in the tension response of the rat vas deferens to electrical field stimulation

Pupo A. Updates in the function and regulation of α1 -adrenoceptors

α1 -Adrenoceptors are seven transmembrane domain GPCRs involved in numerous physiological functions controlled by the endogenous catecholamines, noradrenaline and adrenaline, and targeted by drugs useful in therapeutics. Three separate genes, whose products are named α1A -, α1B -, and α1D - adrenoceptors, encode these receptors. Although the existence of multiple α1 -adrenoceptors has been acknowledged for almost 25 years, the specific functions regulated by each subtype are still largely unknown. Despite the limited comprehension, the identification of a single class of subtype-selective ligands for the α1A - adrenoceptors, the so-called α-blockers for prostate dysfunction, has led to major improvement in therapeutics, demonstrating the need for continued efforts in the field. This review article surveys the tissue distribution of the three α1 -adrenoceptor subtypes in the cardiovascular system, genitourinary system, and CNS, highlighting the functions already identified as mediated by the predominant activation of specific subtypes. In addition, this review covers the recent advances in the understanding of the molecular mechanisms involved in the regulation of each of the α1 -adrenoceptor subtypes by phosphorylation and interaction with proteins involved in their desensitization and internalization. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Contraction of Rat Cauda Epididymis Smooth Muscle to α1-Adrenoceptor Activation Is Mediated by α1A-Adrenoceptors

The cauda epididymis (CE), the site of sperm storage until the ejaculation, is densely innervated by the sympathetic nervous system. Contraction of CE smooth muscle via α₁-adrenoceptors (α₁-ARs) plays a key role during the seminal emission phase of ejaculation and α₁-AR antagonism has been suggested as a nonhormonal and reversible male contraceptive target. Since the α₁-AR subtype mediating contraction of rat CE is not known, this study investigates the expression and role of α₁-AR subtypes on the proximal and distal rat CE duct contraction to norepinephrine in vitro. Alpha_1a, α_1b, and α_1d transcripts were detected by real-time quantitative polymerase chain reaction in proximal and distal CE segments and α_1a and α_1d were shown to predominate over α_1b The inhibition of [³H]prazosin specific binding to intact CE segments from proximal and distal CE by RS 100329 and 5-methylurapidil (α_1A-selective) and BMY 7378 (α_1D-selective) showed that α_1A- and α_1D-ARs are expressed at similar densities. Norepinephrine-induced contractions of CE were competitively antagonized with high affinity by RS 100329 (pK_B ≈ 9.50) and 5-methylurapidil (pK_B ≈ 9.0) and with low affinity by BMY 7378 (pK_B ≈ 7.0) and the α_1B-selective L-765,314 (pA₂ < 7.0), suggesting contractions are mediated by α_1A-ARs. The clinically used α_1A/D-ARs antagonist tamsulosin potently (pA₂ ≈ 10.0) inhibited the norepinephrine-induced CE contractions. Altogether, our results show that α_1A- and α_1D-ARs are expressed in the CE duct and α_1A-AR is the main subtype mediating contraction to norepinephrine. Our results highlight the importance of α_1A-AR in the peripheral control of ejaculation and strengthen the α_1A-AR as a target for a nonhormonal approach to male contraception.

Expression and function of G-protein-coupled receptorsin the male reproductive tract

This review focuses on the expression and function of muscarinic acetylcholine receptors (mAChRs), α1-adrenoceptors and relaxin receptors in the male reproductive tract. The localization and differential expression of mAChR and α1-adrenoceptor subtypes in specific compartments of the efferent ductules, epididymis, vas deferens, seminal vesicle and prostate of various species indicate a role for these receptors in the modulation of luminal fluid composition and smooth muscle contraction, including effects on male fertility. Furthermore, the activation of mAChRs induces transactivation of the epidermal growth factor receptor (EGFR) and the Sertoli cell proliferation. The relaxin receptors are present in the testis, RXFP1 in elongated spermatids and Sertoli cells from rat, and RXFP2 in Leydig and germ cells from rat and human, suggesting a role for these receptors in the spermatogenic process. The localization of both receptors in the apical portion of epithelial cells and smooth muscle layers of the vas deferens suggests an involvement of these receptors in the contraction and regulation of secretion.

Alpha1-adrenoceptors are required for normal male sexual function

BACKGROUND AND PURPOSE

Alpha(1)-adrenoceptor antagonists are extensively used in the treatment of hypertension and lower urinary tract symptoms associated with benign prostatic hyperplasia. Among the side effects, ejaculatory dysfunction occurs more frequently with drugs that are relatively selective for alpha(1A)-adrenoceptors compared with other drugs of this class. This suggests that alpha(1A)-adrenoceptors may contribute to ejaculation. However, this has not been studied at the molecular level.

EXPERIMENTAL APPROACH

The physiological contribution of each alpha(1)-adrenoceptor subtype was characterized using alpha(1)-adrenoceptor subtype-selective knockout (KO) mice (alpha(1A)-, alpha(1B)- and alpha(1D)-AR KO mice) since the subtype-specific drugs available are only moderately selective. We analysed the role of alpha(1)-adrenoceptors in the blood pressure and vascular response as well as ejaculation by determining these variables in alpha(1)-adrenoceptor subtype-selective KO mice and in mice with all their alpha(1)-adrenoceptor subtypes deleted (alpha(1)-AR triple-KO mice).

KEY RESULTS

The pregnancy rate was reduced by 50% in alpha(1A)-adrenoceptor KO mice, and this reduction was dramatically enhanced in alpha(1)-adrenoceptor triple-KO mice. Contractile tension of the vas deferens in response to noradrenaline was markedly decreased in alpha(1A)-adrenoceptor KO mice, and this contraction was completely abolished in alpha(1)-adrenoceptor triple-KO mice. This attenuation of contractility was also observed in the electrically stimulated vas deferens.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that alpha(1)-adrenoceptors, particularly alpha(1A)-adrenoceptors, are required for normal contractility of the vas deferens and consequent sperm ejaculation as well as having a function in fertility.

Alpha-1B adrenoceptors mediate neurogenic constriction in mesenteric arteries of normotensive and DOCA-salt hypertensive mice

The contribution of alpha-1 adrenergic receptor (alpha1-AR) subtypes to neurogenic constrictions of mesenteric resistance arteries from SHAM and deoxycorticosterone acetate-salt (DOCA-salt) hypertensive mice was assessed. Frequency-response curves (0.5-30 Hz) for transmural stimulation-evoked contractions were examined in SHAM and DOCA-salt arteries in vitro in the absence (control) and presence of prazosin (0.1 microM), PPADS (10 microM), yohimbine (1 microM), 5-methylurapidil (5-MU; 0.1 microM), L-765,314 (1 microM) and BMY-7378 (0.3 microM); selective antagonists at alpha1-, P2X, alpha2-, alpha1A-, alpha1B-, and alpha1D-AR, respectively. In SHAM arteries, prazosin but not PPADS inhibited neurogenic responses. L-765,314 substantially inhibited neurogenic responses while 5-MU had a small inhibitory effect. BMY-7378 did not alter contractile responses at all. In DOCA-salt arteries, prazosin reduced neurogenic responses with no further significant inhibition seen with PPADS. L-765,314 antagonized neurogenic constrictions to a level similar to that seen in SHAM arteries. Furthermore, 5-MU and BMY-7378 did not affect these responses. The density of noradrenergic nerves (assessed using glyoxylic acid-induced fluorescence) or norepinephrine (NE) content was not altered by DOCA-salt hypertension. These results indicate that NE is the primary mediator of neurogenic constriction of murine mesenteric arteries. Nerve-released NE acts primarily at alpha1B-and to a lesser extent at alpha1A-ARs in SHAM arteries whereas NE mediates neurogenic constrictions in DOCA-salt arteries by acting at alpha1B-ARs.

Differences in alpha 1-adrenoceptor subtype-mediated vasoconstriction by tyramine and nerve stimulation in canine splenic artery

This study was designed to clarify the alpha(1)-adrenoceptor subtypes mediating the vasoconstrictor response to tyramine in isolated and perfused canine splenic artery. It was shown that tyramine potentiated the nerve stimulation-induced second peaked vasoconstriction that was readily suppressed by prazosin treatment. A bolus injection of tyramine (0.01-0.3 micromol) caused a vasoconstriction in a dose-related manner. The tyramine-induced vasoconstriction was inhibited by WB 4101 (10 and 100 nM), an alpha(1A)-and alpha(1D)-adrenoceptor antagonist, in a concentration-related manner. Neither BMY 7378 (100 nM), a selective alpha(1D)-adrenoceptor antagonist, nor chloroethylclonidine (60 microM), an alpha(1B)- and alpha(1D)-adrenoceptor antagonist, affected the tyramine-induced response. The results indicate that the noradrenaline released by tyramine may diffuse to the extrajunctional cleft, and thus it activates the extrajunctional alpha(1A)-adrenoceptors, because nerve stimulation-evoked second peaked vasoconstrictions were markedly inhibited by chloroethylclonidine but not by WB 4101.

Critical role for the alpha-1B adrenergic receptor at the sympathetic neuroeffector junction

The alpha-1 adrenergic receptors (alpha(1)ARs) are critical in sympathetically mediated vasoconstriction. The specific role of each alpha(1)AR subtype in regulating vasoconstriction remains highly controversial. Limited pharmacological studies suggest that differential alpha(1)AR responses may be the result of differential activation of junctional versus extrajunctional receptors. We tested the hypothesis that the alpha(1B)AR subtype is critical in mediating sympathetic junctional neurotransmission. We measured in vivo integrated cardiovascular responses to a hypotensive stimulus (induced via transient bilateral carotid occlusion [TBCO]) in alpha(1B)AR knockout (KO) mice and their wild-type (WT) littermates. In WT mice, after dissection of the carotid arteries and denervation of aortic baroreceptor buffering nerves, TBCO produced significant pressor and positive inotropic effects. Both responses were markedly attenuated in alpha(1B)AR KO mice (change systolic blood pressure 46+/-8 versus 11+/-2 mm Hg; percentage change in the end-systolic pressure-volume relationship [ESPVR] 36+/-7% versus 12+/-2%; WT versus KO; P<0.003). In vitro alpha(1)AR mesenteric microvascular contractile responses to endogenous norepinephrine (NE; elicited by electrical field stimulation 10 Hz) was markedly depressed in alpha(1B)AR KO mice compared with WT (12.4+/-1.7% versus 21.5+/-1.2%; P<0.001). In contrast, responses to exogenous NE were similar in alpha(1B)AR KO and WT mice (22.4+/-7.3% versus 33.4+/-4.3%; NS). Collectively, these results demonstrate a critical role for the alpha(1B)AR in baroreceptor-mediated adrenergic signaling at the vascular neuroeffector junction. Moreover, alpha(1B)ARs modulate inotropic responses to baroreceptor activation. The critical role for alpha(1B)AR in neuroeffector regulation of vascular tone and myocardial contractility has profound clinical implications for designing therapies for orthostatic intolerance.

Allosteric alpha 1-adrenoreceptor antagonism by the conopeptide rho-TIA

A peptide contained in the venom of the predatory marine snail Conus tulipa, rho-TIA, has previously been shown to possess alpha1-adrenoreceptor antagonist activity. Here, we further characterize its pharmacological activity as well as its structure-activity relationships. In the isolated rat vas deferens, rho-TIA inhibited alpha1-adrenoreceptor-mediated increases in cytosolic Ca2+ concentration that were triggered by norepinephrine, but did not affect presynaptic alpha2-adrenoreceptor-mediated responses. In radioligand binding assays using [125I]HEAT, rho-TIA displayed slightly greater potency at the alpha 1B than at the alpha 1A or alpha 1D subtypes. Moreover, although it did not affect the rate of association for [3H]prazosin binding to the alpha 1B-adrenoreceptor, the dissociation rate was increased, indicating non-competitive antagonism by rho-TIA. N-terminally truncated analogs of rho-TIA were less active than the full-length peptide, with a large decline in activity observed upon removal of the fourth residue of rho-TIA (Arg4). An alanine walk of rho-TIA confirmed the importance of Arg4 for activity and revealed a number of other residues clustered around Arg4 that contribute to the potency of rho-TIA. The unique allosteric antagonism of rho-TIA resulting from its interaction with receptor residues that constitute a binding site that is distinct from that of the classical competitive alpha1-adrenoreceptor antagonists may allow the development of inhibitors that are highly subtype selective.

In vivo and in vitro pharmacological studies of a new hypotensive compound (QF0301B) in rat: comparison with prazosin, a known alpha1-adrenoceptor antagonist

In this work, we studied the in vivo and in vitro pharmacological effects of the novel compound QF0301B (2-[2-(N-4-o-methoxyphenyl-N-1-piperazinyl)ethyl]-1-tetralone) and compared with those of prazosin. In anaesthetized normotensive rats, both QF0301B and prazosin (0.1-0.2 mg/kg iv) caused a pronounced and prolonged fall in mean arterial blood pressure accompanied by bradycardia. Neither QF0301B nor prazosin (0.2 mg/kg iv) significantly modified the cardiovascular effects of either 5-hydroxytryptamine (serotonin, 5-HT, 75 microg/kg iv) or the selective alpha(2)-adrenoceptor agonist B-HT 920 (0.2 mg/kg iv), but both markedly inhibited the hypertensive effect of noradrenaline (5 microg/kg iv), a nonselective alpha-adrenergic receptor agonist. In isolated rubbed rat aorta rings, QF0301B and prazosin showed marked alpha(1)-adrenoceptor blocking activity, with pA(2) values of 9.00+/-0.12 and 9.75+/-0.14, respectively. In addition, QF0301B reversed and competitively antagonized the inhibitory action produced by clonidine in electrically stimulated rat vas deferens and inhibited the force and rate of contraction in rat isolated atria (pA(2)=5.91+/-0.43), competitively antagonized the contractile effect of 5-HT in rat aorta (pA(2)=6.75+/-0.06) and in rat stomach fundus (pA(2)=7.13+/-0.48) and the contractions induced by histamine in isolated guinea pig longitudinal ileal muscle (pA(2)=7.40+/-0.40). QF0301B showed noncompetitive low action in 5-HT(3), muscarinic and nicotinic receptors, or as Ca(2+) antagonist. These results indicate that a alpha(1)-adrenoceptor blocking lead has been obtained with a new chemical structure and interesting pharmacological properties, which only alpha(1)-adrenoceptor blocking activity seems to be responsible for its cardiovascular effects.

Pharmacological analysis of functional neurovascular transmission in canine splenic arteries: role of neuropeptide Y

1 The effects of neuropeptide Y (NPY) upon the isolated vasculature are reviewed. 2 The vasconstrictor responses to periarterial nerve stimulation (PNS) and neurotransmission by noradrenaline (NA) and ATP are discussed and illustrated using canine isolated perfused splenic artery. 3 Modulation of the vascular responses to PNS by NPY via pre- and post-junctional NPY Y2 and Y1 receptors is discussed. 4 Evidence is presented for different alpha1-adrenoceptor subtypes mediating vasoconstriction to exogenous and endogenously released NA and their different locations in the neurovascular junction and extrajunctional regions. 5 Activation of NPY Y1-receptors potentiates sympathetic nerve activated alpha1-adrenoceptor vasoconstriction. The proposal that the postjunctional alpha1B adrenoceptor may be linked to the NPY Y1-receptor and is responsible for co-operation between sympathetic and NPYergic interactions in the vasculature is discussed.

Effects of L-765,314, a selective and potent alpha 1B-adrenoceptor antagonist, on periarterial nerve electrical stimulation-induced double-peaked constrictor responses in isolated dog splenic arteries

The periarterial nerve electrical stimulation (PNS) at a frequency of 1 or 4 Hz (30-s trains of pulses) readily caused a double peaked vasoconstriction in the canine splenic artery. The treatment with 1 microM L-765,314, a selective and potent alpha1B-adrenoceptor antagonist, markedly inhibited the second peaked constriction, whereas it did not modify the vasoconstrictor responses to exogenous noradrenaline (0.03-1 nmol) and A61603 (1-30 pmol), a selective alpha1A-agonist. A large dose of 10 microM L-765,314 significantly blocked exogenous noradrenaline- and A61603-induced responses. It is concluded that PNS-induced responses are mediated via the postjunctional alpha1B-adrenoceptor subtype.

Alpha1-adrenoceptor subtypes in rat epididymis and the effects of sexual maturation

We have characterized the expression of alpha1-adrenoceptor in epididymis from rats in different stages of sexual maturation: 40 (immature), 60 (young adult), and 120 (adult) days of age. Plasma testosterone levels were low in the immature animals but increased significantly in the 60- and 120-day-old animals. These changes were followed by a progressive increase in rat body weight and in caput and cauda epididymis relative weight. Reverse transcription polymerase chain reaction assay indicated that alpha1a-, alpha1b-, and alpha1d-adrenoceptor transcripts were present in both caput and cauda epididymis from adult rats. Ribonuclease protection assays further indicated that the expression of these alpha1-adrenoceptor transcripts differed with age and epididymal region analyzed. Prazosin (nonselective alpha1 antagonist), 5-methyl urapidil (alpha1A-selective), and BMY 7378 (alpha1D-selective) displaced [3H]prazosin binding curves in caput and cauda epididymis from 40- and 120-day-old rats. The potency order for these antagonists, as calculated from the negative logarithm of the inhibition constant (pK(i)) values for the high-affinity sites, indicated a predominant population of alpha1A-adrenoceptor subtype in caput and cauda epididymis from adult animals. Differences in pK(i) values in caput and cauda epididymis from immature and adult animals also suggested that the relative amount of alpha1-adrenoceptors, at the protein level, is modulated by sexual maturation. Taken together, the changes in alpha1-adrenoceptor expression during sexual maturation may suggest specific roles for these receptors in epididymal function.

Existence of different alpha(1)-adrenoceptor subtypes in junctional and extrajunctional neurovascular regions in canine splenic arteries

The present study attempted to characterize the alpha(1)-adrenoceptor subtypes mediating vasoconstrictor responses to administered and nerve stimulation-evoked noradrenaline (NA) release in the isolated and perfused canine splenic artery. A previous study demonstrated that periarterial electrical nerve stimulation (30 s trains of pulses at a frequency of 1, 4 or 10 Hz) induced a double peaked vasoconstriction consisting of an initial transient, predominantly P2X-purinoceptor-mediated constriction followed by a prolonged, mainly alpha(1)-adrenoceptor-mediated response in the canine splenic artery. The effects of alpha(1)-adrenoceptor subtype antagonists on neuronally-mediated second peaked vasoconstrictions were analysed. BMY 7378 (10 - 100 nM), a selective alpha(1D)-adrenoceptor antagonist produced a dose-dependent inhibition of the second peak responses at all frequencies used. BMY 7378 (100 nM) reduced these responses by approximately 30%. Exposure of tissues to chloroethylclonidine (CEC, 60 microM), a selective alpha(1B)-adrenoceptor antagonist attenuated the second peak response by approximately 60%, even in the presence of BMY 7378 (100 nM). On the other hand, WB 4101 (100 nM), a selective alpha(1A)-adrenoceptor antagonist potentiated nerve-stimulation-evoked double peaked vasoconstrictions, especially at low frequencies (1 and 4 Hz). Vasoconstrictor responses to administered NA were dose-dependently antagonized by WB 4101 (10 - 100 nM), but were not significantly affected by either BMY 7378 (10 - 100 nM) or by CEC (60 microM). The present results indicate that NA released from sympathetic nerves may junctionally exert its vasoconstrictor effect via activation of postjunctional alpha(1B)- and in part alpha(1D)-adrenoceptors, whereas exogenous NA extrajunctionally activates alpha(1A)-adrenoceptors to produce its vascular action in canine splenic arteries.

Periarterial electrical nerve stimulation-induced adrenergic vasoconstriction inhibited by adrenergic alpha1B-receptor blockade but not by alpha1A-blockade

The periarterial electrical nerve stimulation at a frequency of 4 Hz (30-s trains of pulses) induced a double-peaked vasoconstriction in the canine splenic artery. The treatment with chloroethylclonidine (CEC, 60 microM) markedly inhibited the second-peaked constriction, whereas it produced an insignificant effect on the first-peaked response. The vasoconstriction to noradrenaline (NA, 1 nmol) was not significantly influenced by 60 microM CEC. On the other hand, WB 4101 (1 microM) consistently abolished the vascular response induced by NA (1 nmol), but rather potentiated the double-peaked constriction. The results indicate that neuronal NA may junctionally exert its vasoconstrictor effect via an activation of postjunctional alpha1B-receptors, whereas exogenous NA may extrajunctionally activate alpha1A-receptors for its vascular action in the canine splenic artery.

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.

Investigation of the subtypes of alpha1-adrenoceptor mediating contractions of rat vas deferens

1 The subtypes of alpha1-adrenoceptor mediating contractions of rat vas deferens to endogenous and exogenous noradrenaline and to the exogenous agonists methoxamine, phenylephrine and A61603 have been examined. 2 The effects of antagonists on the shape of concentration-response curves, both tonic and phasic, to the four agonists were analysed. Prazosin produced parallel shifts in all cases. Particularly for RS 17053 against noradrenaline, there was some evidence for a resistant component of the agonist response. High concentrations of RS 17053 (1-10 microM) virtually abolished tonic contractions but phasic contractions were resistant. 3 A series of nine antagonists (the above and WB4101, benoxathian, phentolamine, BMY 7378, HV 723, spiperone) were investigated against contractions to noradrenaline. The correlation with the potency of the series of alpha1-adrenoceptor antagonists against contractions to noradrenaline was significant only for the alpha1A-adrenoceptor ligand binding site (r=0.88, n=9, P<0.01). 4 In epididymal portions (nifedipine 10 microM), the isometric contraction to a single electrical pulse is alpha1-adrenoceptor mediated. The correlation with ligand binding sites for 11 antagonists (the above plus ARC 239 and (+)-niguldipine) was significant only for the alpha1D-adrenoceptor subtype (r=0.65, n=11, P<0.05). 5 In conclusion, tonic contractions of rat vas deferens produced by exogenous agonists are mediated predominantly by alpha1A-adrenoceptors, although a second subtype of receptor may additionally be involved in phasic contractions. Nerve-stimulation evoked alpha1-adrenoceptor mediated contractions seem to predominantly involve non-alpha1A-adrenoceptors, and the receptor involved resembles the alpha1D-receptor.

Expression and pharmacological characterization of alpha1-adrenoceptors in rat seminal vesicle

In the present study, we investigated the alpha1-adrenoceptor subtypes mediating adrenaline-induced contractions of the rat seminal vesicle by using functional studies. The reverse transcription combined with polymerase chain reaction (RT-PCR) was also used to identify of alpha1-adrenoceptor mRNA subtypes. The rank order of potency of alpha1-adrenoceptor antagonists in blocking the contractile effects of adrenaline was: prazosin = WB 4101 >>> BMY 7378 > chloroethylclonidine, indicating the presence of alpha1A-adrenoceptors in the rat seminal vesicle. In the presence of nifedipine, there was a 76% reduction in the adrenaline-induced contractions. The nifedipine-insensitive component (24%) of the contractile response to adrenaline was unaffected by chloroethylclonidine. A small pool of spare alpha1-adrenoceptors for adrenaline (0.10%) was also detected. All three alpha1-adrenoceptor subtypes were amplified when RT-PCR was performed on total RNA isolated from rat seminal vesicle. In conclusion, these data indicate the presence of three alpha1-adrenoceptor mRNA subtypes, but only alpha1A-adrenoceptors are involved in the rat seminal vesicle contraction.

Characterization of alpha1-adrenoceptor subtypes mediating noradrenaline-induced contraction of rat epididymal vas deferens in calcium-free medium

The alpha1-adrenoceptor subtype mediating noradrenaline (NA)-induced contractions of rat epididymal vas deferens in Ca2+-free/EGTA (1 mM) medium was studied using competitive antagonists. The effects of chloroethylclonidine (CEC) was investigated in Ca2+-free and normal Krebs' medium and RT-PCR was used to identify alpha1-adrenoceptor specific mRNA in epididymal vas deferens. In Ca2+-free medium, NA evoked sustained contractions but was less potent (pD2, 5.9) than in normal Krebs' medium (pD2, 7.3). The contractions in Ca2+-free medium were inhibited by prazosin (pA2, 9.3), 5-methylurapidil (pA2, 8.4), spiperone (pA2, 7.6) and BMY 7378 (pK(B), 6.8) consistent with activation of alpha1A-subtype. Repeated pretreatment with CEC (100 microM) reduced the potency of NA and maximum contractions in normal and Ca2+-free media. CEC-sensitivity in normal Krebs' medium was enhanced by prior treatment with phenoxybenzamine. mRNA for alpha1a- and alpha1d- but not alpha1b-adrenoceptors were detected in epididymal vas deferens. These results suggest that NA contracts the tissue in Ca2+-free medium by the stimulation of alpha1A-adrenoceptors. Two factors affecting CEC-sensitivity of NA-induced contractions in this tissue are discussed.

Functional characterization of alpha1-adrenoceptor subtypes in longitudinal and circular muscle of human vas deferens

The alpha1-adrenoceptor subtype(s) mediating contraction to noradrenaline in longitudinal and circular muscle of human epididymal vas deferens was studied using competitive antagonists. The effects of the alkylating agents, phenoxybenzamine and chloroethylclonidine were also investigated. Noradrenaline evoked concentration-dependent contractions of longitudinal and circular muscle with comparable potencies (pD2; 5.6 and 5.5 respectively). The contractions in longitudinal and circular muscle respectively were inhibited by prazosin (pA2, 8.6 and pKB, 9.2), 5-methylurapidil (pKB, 8.7 and 9.1) and less potently by spiperone (pA2, 7.1) or BMY 7378 (pKB, 6.3 and 6.6). Contractions of the circular but not longitudinal muscle was comparatively insensitive to pretreatment with phenoxybenzamine. In contrast pretreatment with chloroethylclonidine reduced the contractions in both muscle types and also enhanced phenoxybenzamine-sensitivity in longitudinal but not circular muscle. The results suggest that contractions evoked by noradrenaline in both muscle types of human vas deferens is mediated via activation of alpha1-adrenoceptors with pharmacological profile of the alpha1A-subtype. However the involvement of alpha1A-adrenoceptor variants, such as the hypothesised alpha1L-subtype may underlie the differential effects of phenoxybenzamine in longitudinal and circular muscle. Factors contributing to chloroethylclonidine-sensitivity are discussed.

Autoinhibition, sympathetic cotransmission and biphasic contractile responses to trains of nerve stimulation in the rodent vas deferens

1. The present review critically discusses the evidence for and against the various hypotheses that have been proposed to explain the biphasic contractile response of the rodent vas deferens to trains of electrical field stimulation (EFS). 2. It is widely accepted that the initial component of the biphasic response of the rodent isolated vas deferens to trains of EFS is mediated by ATP and the second slower tonic contractions is mediated by noradrenaline (NA). This theory is based on the ability of antagonists of the post-junctional receptors for these neurotransmitters to inhibit the respective components of the biphasic response and on the ability of exogenous application of either ATP or NA to mimic the responses of each phase. 3. Prejunctional autoinhibition has also been proposed as the cause of the biphasic response. This is based primarily on the ability of alpha 2-adrenoceptor antagonists to transform responses from biphasic to monophasic and on the ability of neuronal NA uptake inhibitors to accentuate the separation of the two phases. 4. Atypical or extrajunctional NA receptors have also been proposed to be the mediators of the component of the response to nerve stimulation that is resistant to the traditional alpha-adrenoceptor antagonists. 5. Different contractile mechanisms and/or sources of calcium have also been postulated to cause the biphasic response. Blockers of intracellular Ca2+ mobilization are able to block the initial component, while blockers of extracellular Ca2+ entry inhibit the second tonic phase. 6. It is concluded that because alpha 1-adrenoceptor antagonists and blockers of P2 purinoceptors have also been shown to block both phases of the response to trains of EFS, prejunctional auto-inhibitory mechanisms perhaps provide the most sound explanation for the phenomenon of the biphasic contractile response to trains of EFS.

A1 adenosine receptor modulation of electrically-evoked contractions in the bisected vas deferens and cauda epididymis of the guinea-pig

1. The effects of adenosine receptor agonists upon both electrically-evoked and phenylephrine-induced contractile responses were investigated in the bisected vas deferens and the cauda epididymis of the guinea-pig. Electrical field-stimulation (10 s trains of pulses at 9 Hz, 0.1 ms duration, supramaximal voltage) elicited biphasic and monophasic contractile responses from preparations of bisected vas deferens and cauda epididymis, respectively; these responses were abolished by tetrodotoxin (300 nM). 2. In the prostatic half of the vas deferens the A1 selective adenosine receptor agonists, N6-cyclopentyladenosine (CPA) and (2S)-N6-[2-endo-norbornyl]adenosine ((S)-ENBA) and the non-selective A1/A2 adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA) inhibited electrically-evoked contractions (pIC50+/-s.e.mean values 6.15+/-0.24, 5.99+/-0.26 and 5.51+/-0.24, respectively). The responses to CPA were blocked by the A1 adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine, DPCPX (100 nM). 3. In the epididymal half of the vas deferens NECA potentiated (at < or = 100 nM) and inhibited (at > or = 1 microM) electrically-evoked contractions. In the presence of the non-selective alpha-adrenoceptor antagonist phentolamine (3 microM), the alpha1-adrenoceptor antagonist, prazosin (100 nM), or at a reduced train length (3 s) NECA inhibited electrically-evoked contractions (pIC50 values 6.05+/-0.25, 5.97+/-0.29 and 5.71 +/-0.27, respectively). CPA (at 10 microM) also inhibited electrically-evoked contractions in this half of the vas deferens. In the presence of prazosin (100 nM), CPA also inhibited electrically-evoked contractions (pIC50 6.14+/-0.67); this effect was antagonized by DPCPX (30 nM, apparent pK(B) 8.26+/-0.88). In the presence of the P2 purinoceptor antagonist, suramin (300 microM), CPA (up to 1 microM) potentiated electrically-evoked contractions. 4. NECA, CPA and APNEA potentiated electrically-evoked contractions in preparations of cauda epididymis (pEC50 values 7.49+/-0.62, 7.65+/-0.74 and 5.84+/-0.86, respectively), the response to CPA was competitively antagonized by DPCPX (100 nM) with an apparent pK(B) value of 7.64+/-0.64. 5. The alpha1-adrenoceptor agonist phenylephrine elicited concentration-dependent contractile responses from preparations of bisected vas deferens and cauda epididymis. NECA (1 microM) potentiated responses to phenylephrine (< or = 1 microM) in the epididymal, but not in the prostatic half of the vas deferens. In preparations of epididymis NECA (1 microM) shifted phenylephrine concentration response curves to the left (4.6 fold). In the presence of a fixed concentration of phenylephrine (1 microM), NECA elicited concentration-dependent contractions of preparations of the epididymal half of the vas deferens and of the epididymis (pEC50 values 7.57+/-0.54 and 8.08+/-0.18, respectively). NECA did not potentiate responses to ATP in either the epididymal half of the vas deferens or the epididymis. 6. These studies are consistent with the action of stable adenosine analogues at prejunctional A1 and postjunctional A1-like adenosine receptors. The prejunctional A1 adenosine receptors only inhibit the electrically-evoked contractions of purinergic origin (an effect predominant in the prostatic half of the vas deferens). At the epididymis, where electrically-evoked contractions are entirely adrenergic, the predominant adenosine receptor agonist effect is a potentiation of alpha1-adrenoceptor-, but not of ATP-induced contractility.

The alpha-adrenoceptor-mediated actions of chloroethylclonidine

1. Chloroethylclonidine (CEC) has an affinity for all 6 subtypes of alpha-adrenoceptor, but binds irreversibly particularly to alpha 1B-, alpha 1D-, alpha 2C-, and alpha 2A/D-adrenoceptors. 2. Functionally, CEC behaves as an irreversible alpha 1-adrenoceptor antagonist, reducing the maximum response to noradrenaline (NA), and shows subtype selectivity in that alpha 1A-adrenoceptors are relatively insensitive to CEC. CEC also behaves as an irreversible alpha 2-adrenoceptor agonist, both prejunctionally in the rat vas deferens and postjunctionally in the dog saphenous vein. 3. In the rat aorta, CEC does not produce direct contractions, but following exposure to CEC concentrations of NA of 10 microM and above produce contractions resistant to alpha 1- and alpha 2-adrenoceptor blockade. We have investigated this phenomenon in detail. 4. Receptor protection experiments were carried out in the rat aorta, in which the protecting agent was present prior to and during exposure to CEC. The component of the contraction to NA resistant to alpha-blockade was still present following receptor protection with the alpha 1-adrenoceptor antagonist prazosin, but absent following receptor protection with NA and reduced following receptor protection with alpha 2-adrenoceptor antagonists. The resistant response may represent an irreversible agonist interaction between CEC, NA, and normally silent alpha 2-adrenoceptors, that cannot be affected by subsequent competitive antagonism, but that can be prevented by receptor protection with the agonist NA prior to CEC. 5. CEC has two major classes of action at alpha-adrenoceptors: irreversible antagonism at alpha 1-adrenoceptors, and irreversible agonism at alpha 2-adrenoceptors. Both actions can be demonstrated in the rat aorta.

Alpha-adrenoceptor mediated responses of the cauda epididymis of the guinea-pig

1. The subtypes of alpha-adrenoceptor mediating the contractile responses of the cauda epididymis of the guinea-pig were investigated. The alpha 1-adrenoceptor agonist phenylephrine, but not the alpha 2-adrenoceptor agonist, xylazine (up to 10 microM), elicited concentration-dependent contractions from preparations of cauda epididymis (EC50 3.4 microM). The L-type Ca2+ channel antagonist, nifedipine (10 microM), reduced the maximal response to phenylephrine (by 77%). Preincubation of tissues with the alpha 1B-adrenoceptor-alkylating agent, chloroethylclonidine (50 microM, 30 min), shifted phenylephrine concentration-response curves to the right (4 fold) only when the alpha 2-adrenoceptor antagonist idazoxan (100 nM) was included during the pre-incubation with chloroethylclonidine. 2. Xylazine (1 microM) significantly shifted phenylephrine concentration-response curves to the left (3 fold); this effect was attenuated by idazoxan (100 nM). Both the incubation of preparations with nifedipine (10 microM) and the pre-incubation of preparations with chloroethylclonidine (50 microM, 30 min) attenuated the potentiating effects of xylazine (1 microM). Protection of alpha 2-adrenoceptors with idazoxan (100 nM) during the chloroethylclonidine (50 microM, 30 min) incubation restored the xylazine-mediated enhancement of phenylephrine concentration-response curves. Pertussis toxin (200 ng ml-1, 24 h) attenuated the xylazine (1 microM)-mediated potentiation of phenylephrine concentration-response curves. 3. Following the pre-incubation of preparations with chloroethylclonidine (50 microM, 30 min) 5-methylurapidil (10 nM to 3 microM) shifted phenylephrine concentration-response curves, in parallel, to the right with mean pKB values in the range of 8.27 (at 10 nM 5-methylurapidil) to 7.76 (at 3 microM 5-methylurapidil), the addition of idazoxan (100 nM) to the incubation medium did not significantly affect the 5-methylurapidil (10 to 300 nM) pKB values (8.41 to 7.64, respectively). In the presence of both idazoxan (100 nM) and nifedipine (10 microM), and following the pre-incubation with chloroethylclonidine (50 microM, 30 min), 5-methylurapidil (30 to 300 nM) still shifted phenylephrine concentration-response curves to the right (pKB values 7.77 to 7.36, respectively). 4. Phenylephrine (1 microM to 1 mM) increased the accumulation of [3H]-inositol phosphates (10 fold) in preparations of cauda epididymis (EC50 12 microM). This effect was sensitive to chloroethylclonidine pretreatment (50 microM, 30 min), antagonized with low affinity by 5-methylurapidil (- log pKi 7.8), but not potentiated by xylazine (1 microM). Xylazine (10 nM - 100 microM) reversed the forskolin (10 or 30 microM) stimulated accumulation of [3H]-adenosine 3':5'-cylic monophosphate (cyclic AMP) in preparations of cauda epididymis (by approximately 45%). Incubation of tissues with both pertussis toxin (200 ng ml-1, 24 h) and pertussis toxin vehicle increased the basal activity of adenylate cyclase (3 fold) but did not increase the capacity of forskolin (30 microM) to stimulate the accumulation of [3H]-cyclic AMP in these tissues. Xylazine did not significantly inhibit the forskolin-stimulated accumulation of [3H]-cyclic AMP in either vehicle or pertussis toxin treated tissues. 5. These studies indicate that the epididymis of the guinea-pig contains alpha 1- and alpha 2-adrenoceptors. On the basis of the actions of chloroethylclonidine and 5-methylurapidil the alpha 1-adrenoceptors of this tissue may be of the alpha 1A- and alpha 1B-subtypes and are linked to both the influx of extracellular Ca2+ and to phospholipase C. The alpha 2-adrenoceptors of this tissue are negatively coupled to adenylate cyclase, sensitive to pertussis toxin, but do not amplify phenylephrine-stimulated [3H]-inositol phosphate accumulation. Stimulation of the alpha 2-adrenoceptors of this tissue may selectively potentiate the influx of extracellular Ca2+.

Characterization of alpha-adrenoceptor subtype(s) mediating vasoconstriction in the perfused rabbit ovarian vascular bed

1. alpha 1-Adrenoceptor agonists, noradrenaline, phenylephrine, methoxamine, oxymetazoline and SDZ NVI 085 but not alpha 2-adrenoceptor agonists, UK 14304, tizanidine or clonidine evoked dose-dependent vasoconstriction of the isolated perfused rabbit ovarian vascular bed. The rank order of agonist potency was noradenaline > oxymetazoline > phenylephrine > SDZ NVI 085 > methoxamine. 2. Prazosin (10(-8) M - 10(-5) M) displaced agonist dose-response curves to the right. The pA2/pKB values ranged between 7.27 and 7.66 against noradrenaline, phenylephrine, methoxamine and SDZ NVI 085 and were not significantly different from each other. Prazosin was however significantly less potent against oxymetazoline (pA2 6.38). Yohimbine (10(-6) M - 10(-5) M) was not very effective against any of the agonists. 3. WB 4101 (10(-8) M - 10(-5) M) displaced agonist dose-response curves to the right. The pA2/ pKB values ranged between 7.08 and 7.93 against noradrenaline, phenylephrine, methoxamine and SDZ NVI 085. WB 4101 was significantly less potent against oxymetazoline (pKB 6.85). 4. SZL-49 (5 x 10(-6) M) but not chloroethylclonidine (3 x 10(-5) M) significantly reduced vasoconstrictor responses to all the agonists. 5. Electrical field stimulation of the ovarian bed produced frequency-dependent vasoconstrictor effects which were abolished by 6-OHDA. The responses were also antagonized in a concentration-dependent by prazosin (10(-7) M - 10(-5) M) and WB 4101 (3 x 10(-8) M - 3 x 10(-7) M). Yohimbine reduced the response to electrical stimulation by 20% at 10(-5) M. The vasoconstrictor effect was also inhibited by SZL-49 but not by chloroethylclonidine. 6. These results would suggest that the vasoconstrictor responses of the ovarian vascular bed to adrenergic agonists and to electrical stimulation are mediated via the alpha 1A-adrenoceptor subtype.

Selective enhancement by an adenosine A1 receptor agonist of agents inducing contraction of the rat vas deferens

The adenosine analogue N6-cyclopentyladenosine (CPA), acting via postjunctional A1 receptors, has been shown to enhance contractions of the rat vas deferens induced by adenosine 5'-triphosphate (ATP), the sympathetic cotransmitter in this tissue. The aim of the present study was to examine the ability of CPA to enhance contractions induced by other contractile agents. CPA (0.01-0.3 microM) enhanced contractions induced by exogenous ATP (10 microM), 5-hydroxytryptamine (5-HT) (3 microM), tyramine (10 microM), 2-methyl-5-hydroxytryptamine (2-Me-5-HT) (10 microM) and KCl (35 mM) and this enhancement was blocked by an A1-selective concentration (3 nM) of 1, 3-dipropyl-8-cyclopentylxanthine (DPCPX). CPA failed to enhance contractions induced by exogenous noradrenaline (NA) (1 microM or 10 microM), bradykinin (0.1 microM), phenylephrine (3 microM) or carbachol (10 microM). The contractions induced by ATP (10 microM), 5-HT (3 microM), 2-Me-5-HT (10 microM) and KCl (35 mM) were unaffected by tetrodotoxin (1 microM) as well as by desensitisation of the P2x-purinoceptors with the ATP analogue adenosine 5'-(alpha, beta-methylene) triphosphonate. The contractions induced by tyramine (10 microM) and 2-Me-5-HT (10 microM) were blocked by prazosin (100 nM) or by imipramine (1 microM). Ketanserin (10 nM) antagonised the response to 5-HT giving a dose-ratio of 12.9 corresponding to an apparent pA2 of 9.1. In conclusion, the A1-mediated effect was clearly selective for certain contractile agents and not due to a non-specific increase in contractility of the tissue. CPA enhanced contractions induced by both ATP and indirect sympathomimetics which release endogenous NA, and this enhancement of the two sympathetic cotransmitters may have a functional significance, and demonstrates the complexity of the neuromodulatory effects of adenosine in the rat vas deferens.

Endothelin-induced facilitation of sympathetic neurotransmission to the rat vas deferens: effects of suramin

Experiments were conducted to elucidate the mechanisms of action of endothelins in facilitating neurotransmission to the rat isolated vas deferens. Endothelin-1 and endothelin-3 potentiated field stimulation-induced contractions and those evoked by ATP and alpha, beta-methylene ATP. Responses to noradrenaline were unaffected. The C-terminal hexapeptide, endothelin-(16-21) was without effect on neurotransmission. The facilitation by endothelin-1 of responses to trains of stimulation (10 Hz for 10 s) was absent in the presence of the P2-purinoceptor antagonist, suramin, in concentrations which antagonised the contractile effects of alpha, beta-methylene ATP, but not those of noradrenaline. Suramin did not affect 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)- phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644)-induced potentiation of contractions in response to field stimulation. These results support the hypothesis that endothelin-induced facilitation of sympathetic neurotransmission to the vas deferens is due to potentiation of the postjunctional effects of the co-transmitter, ATP, acting at P2X-purinoceptors, and indicate that this effect is mediated through actions at endothelin receptors that are not of the ETB-subtype.

Physiology of the vas deferens

Seminal emission occurs in response to rhythmic contractions of male secondary sex organs, including the vas deferens. Although contraction of the vas is directly due to adrenergic mechanisms, numerous substances modulate the release of norepinephrine from sympathetic pathways. These substances include local endogenous factors and neurotransmitters such as acetylcholine and NPY. Many substances are capable of altering the contractility of the vas deferens by modulating neural transmitter release or the basal tone of this smooth muscle. Because multiple pathways and substrates are capable of affecting its contractility, it is not surprising that drugs and metabolic disorders influence the function of the vas deferens and, ultimately, male fertility.

Palytoxin-induced transmitter release in the autonomic nervous system of the rat

A combination of pharmacological, histochemical, immunocytochemical, neurochemical and microscopic techniques has been used to test the hypothesis that smooth muscle contractions caused by palytoxin are primarily a response to toxin-induced release of transmitter from motor nerve terminals. In the anococcygeus, palytoxin caused a dose-dependent contraction; the dose-response curve was particularly steep in the dose range 0.3-100 nM. This part of the response was abolished by phentolamine and absent in the muscles of reserpinized animals. A single, large dose of palytoxin (100 nM) caused a biphasic contraction. Phentolamine blocked the first phase by 50% and the second by > 80%. Transmitter overflow studies showed that the toxin enhanced the release of 3H after loading with 3H-NA, and immunofluorescence showed the loss of adrenergic transmitters from the innervation. In the vas deferens, palytoxin caused a biphasic contraction of the muscle. Phentolamine blocked first phase by > 80% and the second by 47%. Immunofluorescence studies showed that stores of adrenergic transmitter were depleted but stores of NPY were not greatly affected. Indirect evidence suggested that palytoxin did not cause the release of purinergic transmitters. A direct component to the contraction was apparently present in most preparations, but though variable in extent it was usually slight. It is concluded that the contractions of smooth muscle caused by palytoxin are primarily the result of toxin-induced transmitter release.

Pharmacological properties of the cloned alpha 1A/D-adrenoceptor subtype are consistent with the alpha 1A-adrenoceptor characterized in rat cerebral cortex and vas deferens

1. The pharmacological characteristics of cloned mammalian alpha 1A/D-, alpha 1B- and alpha 1C-adrenoceptor subtypes expressed in rat 1 fibroblasts were determined in comparison to the binding and functional properties of these subtypes in rat tissues. 2. Analysis of [3H]-prazosin binding to membrane homogenates from rat 1 fibroblast cells expressing each of the alpha 1-subtypes indicated high affinity binding to a single population of binding sites. Binding affinities were similar for alpha 1A/D-, alpha 1B- and alpha 1C-subtypes (Kds: 0.13, 0.10 and 0.15 nM respectively) although a higher density of alpha 1B- and alpha 1C-receptors (Bmax: 4068 and 10,323 fmol mg-1 protein respectively) were expressed in comparison to alpha 1A/D (838 fmol mg-1). 3. Displacement of [3H]-prazosin from membranes expressing cloned alpha 1-adrenoceptor subtypes revealed that 5-methyl-urapidil, WB 4101, benoxathian and phentolamine displayed high affinity and selectivity for alpha 1A/D- over alpha 1B-subtypes. These compounds also had high affinity and selectivity for alpha 1C- over alpha 1B-subtypes. 5-Methyl-urapidil showed selectivity for alpha 1C (Ki 0.60 +/- 0.16 nM) over both alpha 1A/D (Ki, 9.8 +/- 2.8 nM) and alpha 1B (Ki 57.2 +/- 12 nM) subtypes. Prazosin and doxazosin were not subtype selective. 4. In comparison to [3H]-prazosin a similar pharmacological profile was obtained with [125I]-HEAT using cloned alpha 1A/D-, alpha 1B- and alpha 1C-adrenoceptors expressed in rat 1 fibroblasts. 5. The affinities of prazosin, WB 4101, 5-methyl-urapidil, phentolamine and benoxathian at cloned alpha 1A/D-receptors were consistent with alpha 1A affinities determined with chlorethylclonidine-treated rat cortical membranes. Affinities at cloned XIB-receptors were consistent with alpha 1B affinities determined with rat liver membranes.6. Using the epididymal rat vas deferens as a functional measure of alpha 1A affinity, prazosin (pA29.23 +/- 0.28), WB 4101 (pA2 9.58 +/- 0.12), phentolamine (pKB 7.90 +/- 0.16), benoxathian (pKB 9.21 +/- 0.21)and 5-methyl-urapadil (pKB 8.51 +/-0.16) were potent antagonists of noradrenaline-induced contractions.7. At present, evidence from cloning studies suggests the existence of at least three alpha 1-adrenoceptor subtypes. In contrast to the recent proposal for alpha l-adrenoceptor classification, the pharmacology of the cloned alpha 1A/D (or alpha lD)-adrenoceptor is more consistent with that of an alpha 1A-adrenoceptor characterized in rat cerebral cortex and vas deferens.

Alpha 2-adrenoceptor binding sites vary along the length of the male reproductive tract: a possible basis for the regional variation in response to field stimulation

Field stimulation (60V, 1 ms, 10 Hz, 10 s) produced monophasic contractions of cauda epididymides that were enhanced by the noradrenaline uptake inhibitor, nisoxetine, but unaffected by the alpha 2-adrenoceptor antagonist, idazoxan. Similar stimulation of vas deferens produced biphasic contractions that in the presence of idazoxan were enhanced and became monophasic. Nisoxetine accentuated the separation of the two phases. Radioligand binding studies yielded dissociation constants (KD) for [3H]prazosin binding that were similar (0.1-0.2 nM) in membrane preparations of both halves of the vas deferens and in the cauda epididymis; maximum binding density (Bmax) was slightly lower in cauda epididymis (20 fmol/mg protein) than in vas deferens (approximately 50 fmol/mg protein). KD values for [3H]rauwolscine were similar in the two halves of the vas deferens but Bmax values were higher in the prostatic half of the tissue (39 fmol/mg protein) than in the epididymal half (22 fmol/mg protein). We were unable to detect specific binding of [3H]rauwolscine in cauda epididymis. Absence or masking of alpha 2-adrenoceptors in this tissue would explain our findings that alpha 2-adrenoceptor agonists and antagonists do not modulate the responses of the cauda epididymis to trains of field stimulation.

Alpha 1-adrenoceptors and calcium sources in adrenergic neurogenic contractions of rat vas deferens

1. The involvement of alpha 1-adrenoceptor subtypes in adrenergic neurogenic contractions of different type was studied in epididymal and prostatic portions of the rat vas deferens. 2. The adrenergic component of neurogenic contractions was isolated by suramin (300 microM). Twitch-like and tonic contractions were elicited by appropriate pulse patterns of electrical field stimulation, and contractions relying on intracellular calcium mobilization and calcium entry were isolated by means of nifedipine (10 microM) and ryanodine (20 microM), respectively. Increasing concentrations of 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)ethyl)- amino)-propyl)benzeneacetonitrile (HV 723), prazosin and 5-methylurapidil progressively, monophasically and with potency decreasing in that order reduced and finally abolished all types of contraction, with one exception: concentration-effect curves of 5-methylurapidil in epididymal segments in the presence of ryanodine levelled off at about 75% inhibition. In the presence of both nifedipine (10 microM) and ryanodine (20 microM), contractions were abolished. 3. Contractions elicited by exogenous noradrenaline were also studied in the presence of either nifedipine 10 microM (prostatic segments) or ryanodine 20 microM (epididymal segments). Increasing concentrations of tamsulosin, WB 4101, benoxathian, HV 723, prazosin, 5-methylurapidil and urapidil progressively, monophasically and with potency decreasing in that order reduced and eventually abolished both kinds of contraction, with two exceptions: in epididymal segments in the presence of ryanodine, the concentration-effect curve of 5-methylurapidil was biphasic and the curve of urapidil levelled off at only partial inhibition. 4. In slices prepared from the prostatic end and preincubated with [3H]-noradrenaline, WB 4101, HV 723, prazosin and 5-methylurapidil, at the highest concentrations tested against neurogenic contractions, increased only slightly the overflow of tritium elicited by trains of 50 pulses at 5 Hz. 5. It is concluded that two alpha l-adrenoceptor subtypes mediate adrenergic neurogenic contractions of rat vas deferens. The main one, pharmacologically alpha 1A, activates both calcium mobilization and entry. In addition there is a second receptor, not previously detected in the vas deferens and not corresponding to any named alpha l subtype, characterized by high and similar affinity for tamsulosin, WB 4101, benoxathian,HV 723 and prazosin and very low affinity for 5-methylurapidil and urapidil, and linked exclusively to calcium entry. Both subtypes and their respective transduction pathways also contribute to contractions elicited by exogenous noradrenaline. An alpha 1B-adrenoceptor-mediated contraction was not found under any experimental conditions.

Study of the mechanism of the relaxant action of (+)-glaucine in rat vas deferens

1. Effects of the aporphinoid alkaloid, (+)-glaucine, on rat vas deferens were investigated. 2. (+)-Glaucine (2-18 microM) competitively inhibited contractions induced by noradrenaline and methoxamine with a pA2 value of about 6. 3. (+)-Glaucine (2 and 18 microM) did not change the accumulation of tritium during incubation of the vas deferens with [3H]-noradrenaline. 4. (+)-Glaucine (0.3 nM-0.1 mM) inhibited specific [3H]-prazosin binding to membranes from rat vas deferens with a pKi value of 6.63, which is close to the pA2 value obtained against noradrenaline and methoxamine in functional studies. 5. In electrically-stimulated rat vas deferens, (+)-glaucine (0.3-10 microM) enhanced twitch contractions and competitively antagonized the inhibitory effect of clonidine with a pA2 value of 5.91. 6. In tissues incubated in depolarizing calcium-free high-potassium medium, (+)-glaucine (30-80 microM) inhibited Ca(2+)-induced contractions with depression of the maximal response at higher doses and with a pD'2 value of 3.65. Furthermore, (+)-glaucine (50 microM) did not modify basal 45Ca uptake but strongly inhibited the influx of 45Ca induced by K+. 7. These results suggest that (+)-glaucine has non-selective alpha 1- and alpha 2-adrenoceptor blocking properties. At higher doses, (+)-glaucine shows calcium antagonist activity which may be responsible, at least in part, for the inhibition of the contractions induced by Ca2+ in calcium-free high-potassium medium.

Investigation of the subtypes of alpha 1-adrenoceptor mediating contractions of rat aorta, vas deferens and spleen

1. The subtypes of alpha 1-adrenoceptor mediating contractions to exogenous noradrenaline (NA) or phenylephrine in rat vas deferens, spleen and aorta, and mediating contractions to endogenous NA in rat vas deferens have been examined. 2. In rat vas deferens, the competitive antagonists prazosin, WB 4101, benoxathian and 5-methyl-urapidil inhibited contractions to NA with pA2 values of 9.26, 9.54, 9.02 and 8.43, respectively. The irreversible antagonist chloroethylclonidine (CEC) (100 microM) failed to affect contractions to NA. 3. In rat vas deferens in the presence of nifedipine (10 microM), contractions to NA were significantly attenuated and under these conditions, CEC (100 microM) significantly reduced the maximum response to NA. 4. In rat spleen, the competitive antagonists prazosin, WB 4101 and benoxathian inhibited contractions to phenylephrine with pA2 values of 9.56, 8.85 and 7.60, respectively, and 5-methyl-urapidil had a KB of 6.62. CEC (100 microM) significantly reduced the maximum contraction to phenylephrine. 5. In rat aorta, the competitive antagonists, prazosin, WB 4101, benoxathian and 5-methyl-urapidil inhibited contractions to NA with pA2 values of 9.45, 9.21, 8.55 and 8.12, respectively. CEC (100 microM) produced an approximately parallel shift in the potency of NA, without significantly reducing the maximum response. 6. In epididymal portions of rat vas deferens in the presence of nifedipine (10 microM), the isometric contraction to a single electrical pulse was significantly reduced by CEC (100 microM), and by the competitive antagonists prazosin, WB 4101, benoxathian and 5-methyl-urapidil at concentrations of 1 nM. 7. In prostatic portions of rat vas deferens, the alpha l-adrenoceptor agonist, amidephrine, produced concentration-dependent increases in the isometric contraction to a single electrical stimulus and the maximum increase in the evoked response produced by amidephrine was unaffected by CEC (100 microM).8. Contractions of rat vas deferens produced by NA (and amidephrine) are mediated predominantly by alpha lA-adrenoceptors as shown by the high potency of alpha lA-adrenoceptor selective antagonists and the lack of effect of CEC. A small CEC-sensitive response, particularly in epididymal portions, was revealed in the presence of nifedipine. Contractions of rat spleen are mediated by alpha lB-adrenoceptors since alpha 1A selective antagonists showed low potency and CEC significantly reduced the maximum contraction to phenylephrine. Contractions of rat aorta to NA are mediated by non-alpha lA, non-alpha lB-adrenoceptors, due to the high potency of the aMA-selective antagonists and sensitivity to CEC.9. The noradrenergic contraction of epididymal portions of rat vas deferens in the presence of nifedipine is CEC-sensitive, but the alpha 1 A-selective antagonists showed high potency, suggesting that this response is mediated by non-alpha lA, non-alpha 1B-adrenoceptors.10. In conclusion, at least three subtypes of functional alpha 1-adrenoceptors have been demonstrated in these studies.

Effects of nifedipine and ryanodine on adrenergic neurogenic contractions of rat vas deferens: evidence for a pulse-to-pulse change in Ca2+ sources

1. The effects of nifedipine and ryanodine on the adrenergic component of neurogenic contractions of the rat isolated vas deferens were studied in an attempt to identify the sources of Ca2+ mediating the contraction. The tissue was electrically stimulated by single pulses or pairs of widely spaced pulses. The purinergic component of contraction was suppressed by the presence of 300 microM suramin. 2. In Mg(2+)-free medium, nifedipine (0.01-10 microM) reduced the first and, to a greater extent, the second twitch elicited by two pulses 3 s apart. This pattern of inhibition was observed both in the absence of rauwolscine (when twitch 2 was smaller than twitch 1) and in the presence of 0.1 microM rauwolscine (when, due to interruption of prejunctional alpha 2-adrenoceptor-mediated autoinhibition, twitch 2 was of similar height to twitch 1). Nifedipine reduced only twitch 2 but not twitch 1 in medium containing 1.2 mM Mg2+. 3. Single pulses of increasing current strength elicited increasing contraction. Nifedipine reduced contractions by about the same absolute extent at all current strengths, so that the relative contribution of the nifedipine-sensitive component decreased with increasing current strength. 4. When the pulse interval in a pair was increased from 5 to 60 s, the inhibition by nifedipine of the second twitch was most marked at an interval of 5 s and declined as the interval increased. 5. In contrast to nifedipine, 20 microM ryanodine reduced the first twitch of a pair to a greater extent than a second twitch 5 s later, so that twitch 2 became greater than twitch 1. The inhibition by ryanodine of twitch 2 increased with increasing pulse interval.6. In vasa deferentia preincubated with [3H]-noradrenaline, I microM nifedipine and 20 microM ryanodine did not change the electrically evoked overflow of tritium, whereas 10 microM nifedipine increased it.7. It is concluded that, when the sympathetic axons of the vas deferens are stimulated by a single pulse(or the first pulse of a pair) in Mg2+-free medium, both Ca2+ mobilization inside the smooth muscle cells and Ca2+ entry contribute to the ensuing adrenergic contraction. The relative contribution of Ca2+ entry is small at maximal stimulus strength but increases with decreasing stimulus strength. When a second pulse follows the first after an appropriate interval, a switch of Ca2+ sources occurs: intracellular Ca2+mobilization is decreased during twitch 2, whereas Ca2+ entry is increased.

Chloroethylclonidine: an irreversible agonist at prejunctional alpha 2-adrenoceptors in rat vas deferens

1. The possibility that chloroethylclonidine (CEC) activates prejunctional alpha 2-adrenoceptors was studied in the isolated vas deferens of the rat. Tissues were stimulated electrically and both the stimulation-evoked overflow of tritium (after preincubation with [3H]-noradrenaline) and the purinergic contraction component (isolated by prazosin 0.3 microM) were measured. 2. CEC (0.1-3 microM) concentration-dependently reduced the overflow of tritium evoked by trains of 6 pulses/100 Hz. The inhibition by CEC was not altered by prazosin (0.3 microM) but was prevented by pre-exposure to rauwolscine (0.3 microM). The inhibition, once established, did not fade upon washout of CEC, even when the washout fluid contained rauwolscine (0.3 microM). 3. CEC (0.1-3 microM) concentration-dependently reduced the purinergic component of contractions elicited by single pulses. The inhibition, again, was prevented by pre-exposure to rauwolscine (0.3 microM) and once established, did not fade upon washout of CEC, even when the washout fluid contained rauwolscine (0.3 microM). 4. CEC (3 microM) reduced the overflow of tritium evoked by 20 pulses/10 Hz, did not alter the overflow evoked by 100 pulses/10 Hz and increased the overflow evoked by 500 pulses/10 Hz. 5. CEC (3 microM) reduced the early peak, but increased the late plateau phase, of purinergic contractions elicited by 100 pulses/10 Hz. 6. It is concluded that CEC reduces the release of noradrenaline and a purinergic co-transmitter by irreversible activation of prejunctional alpha 2-adrenoceptors. CEC seems to be a partial alpha 2-agonist with an efficacy lower than that of noradrenaline. The prejunctional inhibitory effect limits the suitability of CEC for the characterization of postjunctional alpha 1-adrenoceptors mediating responses to sympathetic nerve stimulation.

Evidence for a noradrenergic innervation to alpha 1A-adrenoceptors in rat kidney

1. Experiments were undertaken to characterize the alpha 1-adrenoceptor subtype mediating vasoconstrictor responses to periarterial noradrenergic nerve stimulation (PNS) in the isolated perfused kidney of the rat. 2. Vasoconstrictor responses to nerve stimulation were inhibited by prazosin (10 nM), 5-methyl-urapidil (30 nM), and nitrendipine (1 microM) but were resistant to inhibition by chloroethylclonidine (100 microM). 3. 5-Methyl-urapidil (30 nM), chloroethylclonidine (100 microM) and nitrendipine (1 microM) did not inhibit the neuronal release of tritium from nerves loaded with [3H]-noradrenaline. 4. The results suggest that renovascular alpha 1A-adrenoceptors receive a noradrenergic innervation and that the innervated receptors are coupled to dihydropyridine-sensitive calcium channels.

Suramin: a selective inhibitor of purinergic neurotransmission in the rat isolated vas deferens

The effects of the putative P2 purinoceptor antagonist suramin on contractile responses of the rat isolated vas deferens to electrical field stimulation and exogenously applied drugs (alpha,beta-methylene ATP and noradrenaline) were investigated. Suramin was devoid of agonist activity in the rat vas deferens. The response of the vas deferens to single pulse field stimulation was characteristically biphasic with a large first component peaking between 260-300 ms after the stimulus followed by a second smaller component peaking at about 650 ms post-stimulus. Suramin (100 nM-1 mM) selectively impaired the first, purinergic phase of the response to single pulse field stimulation but was without effect on the second, noradrenergic component. The response of the vas deferens to trains of electrical field stimuli (10 Hz for 10 s) was also biphasic. Suramin (1 microM-1 mM) reduced the first (less than 1 s) phase of the response by 30%, the second (greater than 5 s) plateau phase by 50% and inhibited the intermediate (2-4 s) phase by 80%. Dose-contact periods of 20 or 30 min respectively were sufficient to achieve equilibration of the inhibitory effects of suramin against the responses to single pulse field stimulation or trains of pulses. Following 30 min incubation with 1 mM suramin, the remaining first and second phase components of the response to trains of pulses were impaired and subsequently abolished by the alpha 1-adrenoceptor antagonist WB4101 establishing their noradrenergic origin. Suramin (300 microM) abolished responses of the vas deferens to alpha,beta-methylene ATP but was without effect on those to noradrenaline. Suramin (30 microM) induced a rightward shift in the concentration-response relationship to alpha,beta-methylene ATP that was associated with a significant 40% increase in the maximum response, but did not modify responses to noradrenaline. The inhibitory effects of suramin (3-300 microM) on responses of the vas deferens to approximate EC50 concentrations of alpha,beta-methylene ATP were reversible on repeated washout for 40-60 min. These results reveal suramin to be a useful pharmacological tool for the study of purinergic neurotransmission in rodent vasa deferentia.