Selective Adrenoceptor Antagonist Research Articles

NITRIC OXIDE, ERECTILE DYSFUNCTION AND BETA‐BLOCKER TREATMENT (MR NOED STUDY): BENEFIT OF NEBIVOLOL VERSUS METOPROLOL IN HYPERTENSIVE MEN

1. Hypertensive men treated with beta-blockers frequently complain of erectile dysfunction. The present study investigated the effects of two beta(1)-adrenoceptor-selective antagonists, namely nebivolol and metoprolol, on erectile function in hypertensive men. 2. Male out-patients (age range 40-55 years) with newly diagnosed or existing stage 1 essential hypertension (mean seated systolic blood pressure 140-159 mmHg; diastolic blood pressure 90-99 mmHg) were enrolled in the study. All patients lived in a stable, heterosexual partnership and had no history of sexual dysfunction. After a 2-week placebo run-in period, patients were randomized double-blind to either Treatment group A (comprising nebivolol 5 mg once daily for 12 weeks, followed by placebo for 2 weeks and then metoprolol succinate 95 mg once daily for 12 weeks) or Treatment group B (comprising metoprolol succinate 95 mg for 12 weeks, placebo for 2 weeks and then nebivolol 5 mg for 12 weeks). An international index of erectile function (IIEF) questionnaire and a diary documented patients' sexual function and activity. 3. Nebivolol and metoprolol lowered blood pressure to a similar extent. Metoprolol, but not nebivolol, significantly decreased the IIEF erectile function subscore by 0.92 in the first 8 weeks after onset of beta-blocker treatment. In contrast with metoprolol, nebivolol improved secondary sexual activity scores and other IIEF subscores. 4. Despite similar antihypertensive efficacy of the cardioselective beta(1)-adrenoceptor antagonists nebivolol and metoprolol, nebivolol may offer additional benefits by avoiding erectile dysfunction in male hypertensive patients on long-term beta-adrenoceptor antagonist therapy.

Peripheral Sympathetic Component of the Temporomandibular Joint Inflammatory Pain in Rats

The aim of this study was to further validate our carrageenan-induced temporomandibular joint (TMJ) inflammatory hyperalgesia model in rats by showing that administration of indomethacin before the initiation of inflammation would diminish the TMJ hyperalgesia. Using this model, we investigated whether norepinephrine and local β-adrenoceptors contribute to the development of inflammatory TMJ hyperalgesia. Carrageenan-induced TMJ hyperalgesia was assessed by measuring the behavioral nociceptive responses, such as rubbing the orofacial region and flinching the head, induced by the injection of a low dose of 5-hydroxytryptamine into the TMJ sensitized 1 h before by a TMJ injection of carrageenan. Blockade of prostaglandin synthesis by indomethacin prior to initiation of inflammation by carrageenan significantly attenuated the TMJ hyperalgesia. The guanethidine depletion of norepinephrine or the blockade of β 2 but not the blockade of the β 1 - adrenoceptor by the selective adrenoceptor antagonists ICI 118.55 and atenolol, respectively, significantly reduced carrageenan-induced TMJ hyperalgesia. In the present study, we further validated our carrageenan-induced TMJ hyperalgesia model to study the mechanisms involved in inflammatory TMJ hyperalgesia and to test the analgesic effect of different types of peripheral analgesics. We also demonstrated that norepinephrine released at the site of injury contributes to the development of the inflammatory TMJ hyperalgesia by the activation of β 2 -adrenoceptors. Perspective The findings that local sympathomimetic amines contribute to the inflammatory TMJ hyperalgesia by activating β 2-adrenoceptors may be relevant to clinical TMJ inflammatory pain states less sensitive to nonsteroidal anti-inflammatory drugs.

Alpha1A/L‐adrenoceptors mediate contraction of the circular smooth muscle of the pig urethra

Sympathetically mediated urethral tone is essential for the maintenance of continence and involves the activation of postjunctional alpha(1)-adrenoceptors. This study characterizes the alpha(1)-adrenoceptor subtypes responsible for mediating contraction of the urethral circular smooth muscle of the pig. The potency order of a number of agonists and the affinities of several receptor selective antagonists were determined on pig-isolated circular smooth muscle strips in the presence of cocaine (1 microm) and corticosterone (10 microm) to inhibit amine uptake and propranolol (1 microm) to antagonize beta-adrenoceptors. The potency order for agonists was N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl]methanesulphonamide (A61603) > noradrenaline = phenylephrine = M6434 > methoxamine with pEC(50) values of 7.3, 5.8, 5.7, 5.6 and 5.0 respectively. 4 The alpha(1D)-adrenoceptor-selective antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]decane-7,9-dione (BMY7378) caused rightward shifts of the concentration-response curves to noradrenaline, yielding a low affinity estimate (6.6) for the urethral receptor. The alpha(1A)-adrenoceptor-selective antagonists, RS100329 and 5-methylurapidil, gave relatively high affinity estimates (9.6 and 8.8 respectively) for this receptor. All three antagonists produced Schild plots with slopes close to unity but did reduce maximum responses at higher concentrations. Prazosin antagonized responses of the urethra to noradrenaline, yielding a mean affinity estimate of 9.0. Although the Schild plot for prazosin again had a slope of unity, this drug also reduced maximum responses to noradrenaline at all concentrations examined (10-100 nm). N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethanamide (RS17053), which discriminates between responses mediated via alpha(1A) (high affinity) and alpha(1L)-adrenoceptors (low affinity) at concentrations up to 3 microm, failed to antagonize responses of the urethra. 5 These results suggest that contraction of urethral circular smooth muscle in the pig is mediated via a single population of adrenoceptors with the pharmacological characteristics of the alpha(1A/L)-adrenoceptor, most probably the alpha(1L)-adrenoceptor.

A Selective α1A-Adrenoceptor Antagonist Inhibits Detrusor Overactivity in a Rat Model of Benign Prostatic Hyperplasia

Alpha(1)-adrenoceptor antagonists relax the obstructed prostatic urethra and suppress the irritative symptoms frequently observed in patients with benign prostatic hyperplasia. We investigated the effects of 3 alpha(1)-adrenoceptor antagonists on urodynamics in rats with hormone induced benign prostatic hyperplasia to determine which alpha(1)-adrenoceptor subtype selective antagonists would suppress irritative symptoms. Rats were treated with testosterone and 17beta-estradiol by weekly intramuscular injections. After 4 weeks a pressure flow study was done and the effects of the alpha(1)-adrenoceptor antagonists KMD-3213 silodosin, tamsulosin and prazosin on urodynamics were compared. We especially investigated the involvement of the bladder and prostatic urethra to clarify the mechanism of detrusor overactivity expression. Hormone treatment induced benign prostatic hyperplasia and resulted in detrusor overactivity, as determined by cystometry. Baseline perfusion urethral pressure and the phenylephrine induced increase in it were significantly higher in rats with vs without benign prostatic hyperplasia. Cystometry in hormone treated female rats did not show detrusor overactivity. KMD-3213 decreased detrusor overactivity, similar to other alpha(1)-adrenoceptor antagonists. These results suggest that an excessive response to sympathetic nerve stimulation, which is mainly mediated via alpha(1A)-adrenoceptor, in the hypertrophied prostate gives rise to detrusor overactivity. Furthermore, the alpha(1A)-adrenoceptor selective antagonist KMD-3213 would be suitable for improving irritative symptoms in patients with benign prostatic hyperplasia.

Influence of Cardioselective Beta-Blockade to the Occurrence of Exercise Induced Arterial Hypoxemia

Exercise induced arterial hypoxemia (EIAH) at or near sea level is recognized to occur in a significant number of fit, healthy subjects of both sexes and of varying ages. PURPOSE: The aim of this study was to compare the effect of a single dose of the highly ?1-selective adrenoceptor antagonist bisoprolol with those of placebo on the occurrence of an EIAH. METHODS: Sixteen healthy male subjects (mean±SD age: 23.5±3.6 yrs; height: 183.2±6 cm; weight: 78±7.9 kg) randomly received oral placebo or bisoprolol (5 mg) 6–10 hours before a cardiopulmonary exercise test using a double-blind cross-over design. The subjects performed an incremental cycle ergometer test until exhaustion to determine cardiorespiratory parameters and partial pressure of oxygen (PO2), partial pressure of carbon dioxide (PCO2), oxygen saturation (SO2) and pH at rest, at every workload step and at maximum performance. RESULTS: Bisoprolol significantly reduced maximal heart rate; no differences between bisoprolol and placebo were found for maximal values for oxygen uptake, carbon dioxide production, power output, minute ventilation and blood lactate concentration. Seven of the 16 subjects sustained signs of an EIAH. The PO2 of more than 10 mmHg lower than at rest is by one definition referred as EIAH, occurred in one individual using placebo and in two individuals using bisoprolol. The SO2 of more than 3% lower than at rest serves as another definition of EIAH, occurred in two subjects under beta blockade. Two other individuals achieved these two criteria for EIAH when using each of the drugs. CONCLUSION: 43.75% of the tested population showed signs of an EIAH, which is in agreement with the findings of other investigators. A single dose of the ?1-selective adrenoceptor antagonist bisoprolol leads to a higher occurrence of EIAH at submaximal and maximal performance. To ascertain why EIAH occurs at these workloads, additional investigations are warranted.

Influence of Ephedrine and the Role of ?? Subtype Adrenoreceptors in the Vascular Bed of the Cat Lung

In a university research laboratory and in separate experiments, the effects of phentolamine, the alpha-adrenergic antagonist; prazosin, an alpha1-adrenoceptor antagonist; 5-methyl-urapidil, the selective alpha1A-subtype adrenoceptor antagonist; chloroethylclonidine, an alpha1B- and alpha1D-subtype adrenoceptor antagonist; and BMY 7378, a selective alpha1D-subtype adrenoceptor antagonist were analyzed in an attempt to identify any significant effect on pulmonary arterial responses to ephedrine and other agonist agents in the pulmonary vascular bed of the cat. Under constant flow conditions, lobar arterial perfusion pressure and systemic pressure were continuously monitored, electronically averaged, and permanently recorded. In the isolated left lower lobe of the pulmonary feline vascular bed, ephedrine induced a dose-dependent vasoconstrictor response that was not significantly altered following administration of 5-methyl-urapidil. The vasopressor activity as a result of ephedrine was significantly decreased after administration of phentolamine, prazosin, chloroethylclonidine, and BMY 7378. Further, when the alpha1B- and alpha1D-subtype adrenoceptor antagonist chloroethylclonidine was given, there was almost complete elimination of the ephedrine-induced vasoconstrictor response. The results of this study suggest that ephedrine causes a dose-dependent vasopressor response in the feline pulmonary vascular bed and that this activity may be mediated or modulated by both alpha1B- and alpha1D-subtype adrenoceptor sensitive pathways.

The α1D‐adrenoceptor antagonist BMY 7378 is also an α2C‐adrenoceptor antagonist

1 We have investigated the actions of the alpha(1D)-adrenoceptor selective antagonist BMY 7378 in comparison with yohimbine at alpha(1)- and alpha(2)-adrenoceptors. 2 In rat aorta (alpha(1D)-adrenoceptor), BMY 7378 (pA(2) of 8.67) was about 100 times more potent than yohimbine (pA(2) of 6.62) at antagonizing the contractile response to noradrenaline. 3 In human saphenous vein (alpha(2C)-adrenoceptor), BMY 7378 (pA(2) of 6.48) was approximately 10 times less potent than yohimbine (pA(2) of 7.56) at antagonizing the contractile response to noradrenaline. 4 In prostatic portions of rat vas deferens, BMY 7378 (10 mum) did not significantly affect the concentration-dependent inhibition of single pulse nerve stimulation-evoked contractions by xylazine (an action at prejunctional alpha(2D)-adrenoceptors). 5 In ligand-binding studies, BMY 7378 showed 10-fold selectivity for alpha(2C)-adrenoceptors (pK(i) of 6.54) over other alpha(2)-adrenoceptors. 6 It is concluded that BMY 7378, in addition to alpha(1D)-adrenoceptor selectivity in terms of alpha(1)-adrenoceptors, shows selectivity for alpha(2C)-adrenoceptors in terms of alpha(2)-adrenoceptors.

Possible Role of Dexamethasone in Sensitizing the Beta-2-Adrenergic Receptor System in vivo in Calves during Concomitant Treatment with Clenbuterol

β₂-Agonists blunt the function of the β-adrenoceptor G-protein adenylate cyclase-signalling system, whereas glucocorticoids reverse the agonist-mediated diminished β-adrenergic responses; however, these effects have not been reported in vivo in calf lymphocytes. In this study, we first investigated the presence of the β₂-adrenergic receptors on calf lymphocytes, and second we tested the effects of either clenbuterol alone or in combination with dexamethasone on receptor expression and function (isoproterenol-induced intracellular adenosine 3′,5′-cyclic monophosphate (cAMP) formation) in vivo. (–)-[¹²⁵I]-Iodocyanopindolol (ICYP) binding to intact calf lymphocytes was rapid, saturable (maximal number of binding sites 987 ± 89 ICYP-binding sites/cell, n = 4) and of high affinity (K_D value 17.23 ± 2.8 pmol/l, n = 4). These binding sites were of the β₂-subtypes of adrenoceptors as indicated by the fact that β-agonists inhibited ICYP binding with an order of potency: (–)-isoproterenol > (–)-adrenaline > (–)-noradrenaline. Furthermore, the selective β₂-adrenoceptor antagonist ICI 118.551 was about >1,500 times more potent in inhibiting ICYP binding than was the β₁-selective adrenoceptor antagonist CGP 20712A. Consequently, calves were treated with clenbuterol (1.0 µg/kg b.i.d., i.v.) for 9 days alone or simultaneously with dexamethasone (0.1 mg/kg, i.v., once a day for 4 days). Clenbuterol decreased the number of lymphocyte β₂-adrenergic receptors by about 40–50% after only 48 h of drug administration. This was accompanied by a decrement in isoproterenol-induced lymphocyte cAMP formation. Upon application of both drugs, dexamethasone restored the clenbuterol-mediated decrease in β₂-adrenoceptors and cAMP production. Dexamethasone elevated the number of β₂-adrenoceptors and cAMP almost 1.5- to 2-fold at 24 h of drug administration, an effect that persisted for up to 24 h following drug withdrawal. Neither clenbuterol nor the combination with dexamethasone had an influence on the affinity of the receptor for the ligand. The present results demonstrate that dexamethasone in vivo upregulates the number and function of calf lymphocyte β₂-adrenoceptors, and thus enhances the sensitivity of the β₂-adrenoceptor signal-transduction pathway for clenbuterol during concomitant treatment with both drugs.

Pharmacological and biochemical characterization of the beta-adrenergic signal transduction pathway in different segments of the respiratory tract

Although in the respiratory system there is great therapeutic interest in manipulating and understanding the β-adrenoceptor–G-protein–adenylate cyclase (AC) signal transduction pathway, little is known on segmental differences among lung, bronchus, and trachea with regard to the receptor concentration and interaction to G-proteins and coupling to AC. In this study, patterns of distribution and absolute quantities of β-adrenoceptor subtypes β 1 and β 2 were determined in membranes of equine lung parenchyma, bronchial and tracheal epithelium with the underlying smooth muscle by saturation and competition binding assays using the radioligand (−)-[ 125 I ]-iodocyanopindolol (ICYP). Additionally, the functional coupling of β-adrenoceptors to G-proteins (assessed by β-agonist competition binding in the presence and absence of GTP) as well as the coupling efficiency and biochemical activities of AC was investigated in each region. The specific ICYP binding was rapid, reversible, saturable with time and of high affinity. The radioligand binding identified more total β-adrenoceptors in the lung than in bronchus or trachea (428±19, 162.4±4.8, 75.6±1.2 fmol/mg protein, respectively) with about 40% of receptors in the high affinity state. The β 2-adrenoceptor subtype predominated in all segments (approximately 74–80%), as the highly selective β 2-adrenoceptor antagonist ICI 118,551 was about 10,000 times more potent in inhibiting ICYP binding than was the β 1-selective adrenoceptor antagonist CGP 20712A, and β-adrenoceptor agonists inhibited ICYP binding with an order of potency: (−)-isoprenaline>(−)-adrenaline>(−)-noradrenaline. The dissociation constant ( K d ) was higher in the trachea than in bronchus or lung (13.0±0.9 pM vs. 20.0±2.3 pM vs. 30.8±4.4 pM, P<0.05, respectively). The β 2-adrenoceptor-mediated AC response was tissue-dependent; stimulants acting on β-adrenoceptor (isoproterenol), G-protein (GTP, NaF) and AC (forskolin, Mn 2+) enhanced AC responses in all three regions, but the AC activity was higher in tracheal crude membranes than in bronchus or lung ( trachea⋙ bronchus> lung ), hence, the number of β 2-adrenoceptors correlated inversely with the amount of AC. We conclude that (1) the stoichiometry of components within the pulmonary β-adrenoceptor–G-protein complex is segment-dependent, and (2) the receptor number or AC activity is possibly the rate-limiting factor in the β-adrenoceptor–G-protein–AC-mediated physiological responses. Thus, it is speculated that this could have important therapeutic consequences in β-adrenoceptor agonist-induced receptor regulation in bronchial asthma.

Lack of inverse agonistic activity of nebivolol, its d- and l-enantiomers and of in vivo metabolized nebivolol in human myocardium

The pharmacological profile of nebivolol may be mediated by its enantiomers and/or its hydroxylated metabolites. Therefore, the cardiac effects of the nebivolol enantiomers as well as of serum specimens containing hydroxylated nebivolol metabolites were studied in human myocardium. For control, the β 1-adrenoceptor selective antagonist metoprolol was used. After pre-stimulation of force of contraction with forskolin (0.3 μM) or isoprenaline (0.01 μM), force developement was decreased only at high concentrations (≥300 nM) of nebivolol or its enantiomers in isolated trabeculae. Nebivolol and its enantiomers, in contrast to metoprolol (0.4 μM: −72% basal force), produced only minor negative intropic effects in isolated trabeculae under basal conditions. Basal force of contraction was not decreased by in vivo metabolized nebivolol in pharmacological concentrations. Neither d- nor l-nebivolol (30 μM) influenced myofibrillar Ca 2+ responsiveness. Nebivolol and the d-enantiomer, but not the l-enantiomer (all 0.5 μM), improved the frequency-dependent force generation. d-Nebivolol, in contrast to l-nebivolol, was a β 1-adrenoceptor selective compound in membrane preparations from non-failing donor hearts. In conclusion, nebivolol and its enantiomers as well as in vivo metabolized nebivolol produce only minor negative inotropic effects. This and the finding that nebivolol and its d-enantiomer improve the frequency-dependent force generation may be of particular advantage when treating patients with already compromised cardiac function.

Investigation of postjunctional alpha1- and alpha2-adrenoceptor subtypes in vas deferens from wild-type and alpha(2A/D)-adrenoceptor knockout mice.

1. The subtypes of alpha(1)- and alpha(2)-adrenoceptor mediating contractions of vas deferens have been examined in wild-type and alpha(2A/D)-adrenoceptor knockout mice. 2. Maximum contractions to noradrenaline but not phenylephrine were significantly greater in vas from wild-type. The alpha(1A)-adrenoceptor antagonist RS100329 (5-methyl-3-[3-[4-[2-(2,2,2,-trifluoroethoxy)phenyl]-1-piperazinyl]propyl]-2,4-(1H)-pyrimidinedione) (10 nM) significantly shifted the potency of noradrenaline. The alpha(2D)-adrenoceptor antagonist BRL 44408 (2-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-2,3-dihydro-1-methyl-1H-isoindole) significantly reduced the maximum contraction to noradrenaline in wild-type but not in knockout. 3. Following prazosin (0.1 micro M), a component of the contraction to noradrenaline in wild-type but not in knockout was sensitive to the alpha(2)-adrenoceptor antagonist yohimbine. 4. Nifedipine (10 micro M) or suramin (100 micro M) reduced the contraction to 10 Hz stimulation for 4 s to an early peak and small maintained response. The peak was abolished by the alpha(1)-adrenoceptor antagonist prazosin. 5. RS100329 or prazosin inhibited 10 Hz stimulation evoked contractions with a U-shaped concentration-response curve: inhibiting responses up to 0.1 micro M, with a reversal of inhibition above this concentration. In the presence of suramin or nifedipine, the reversal of inhibition by high concentrations of prazosin was reduced. 6. The alpha(1D)-adrenoceptor selective antagonist BMY7378 (8-[2-(4-(2- methoxyphenyl)piperazin-1-yl)ethyl]-8-azaspiro[4,5]decane-7,9-dione) produced inhibition of 10 Hz evoked contractions only in high concentrations. 7. In conclusion, contractions to nerve stimulation in mouse vas deferens involve largely alpha(1A)-adrenoceptors and purinoceptors. alpha(1)-Adrenoceptor antagonists in high concentrations increase the purinergic response presumably by blocking prejunctional alpha(2)-adrenoceptor-mediated inhibition. In the presence of nifedipine, responses are predominantly alpha(1)-adrenoceptor mediated. Contractions to exogenous noradrenaline involved both alpha(1A)- and alpha(2A/D)-adrenoceptors in wild-type mice.

The β-adrenoceptor antagonists metipranolol and timolol are retinal neuroprotectants: comparison with betaxolol

β-adrenoceptor antagonists are used clinically to reduce elevated intraocular pressure in glaucoma which is characterised by a loss of retinal ganglion cells. Previous studies have shown that the β 1-selective adrenoceptor antagonist, betaxolol, is additionally able to protect retinal neurones in vitro and ganglion cells in vivo from the detrimental effects of either ischemia-reperfusion or from excitotoxicity, after topical application. The neuroprotective effect of betaxolol is thought not to be elicited through an interaction with β-adrenoceptors, but by its ability to reduce influx of sodium and calcium through voltage-sensitive calcium and sodium channels. In the present study it is shown that the non-selective β-adrenoceptor antagonists, metipranolol and timolol behave like betaxolol. When topically applied they all attenuate the detrimental effect of ischemia-reperfusion. Protection of the retina was determined by evaluating changes in the electroretinogram and by assessing the loss of mRNA for Thy-1, which is expressed in retinal ganglion cells. In addition, studies conducted on neurones in mixed retinal cultures demonstrated that metipranolol, betaxolol and timolol were all able to partially counteract anoxia-induced cell loss and viability reduction. The influence of timolol was, however, not significant. Within the confines of these investigations, an order of neuroprotective efficacy was delineated for the three β-adrenoceptor antagonists: betaxolol>metipranolol>timolol. The ability of the β-adrenoceptor antagonists to attenuate ligand-induced stimulation of calcium and sodium entry into neuronal preparations showed a similar order of effectiveness. In conclusion, the ability to confer neuroprotection to retinal neurones is a common feature of three ophthalmic β-adrenoceptor antagonists (betaxolol, metipranolol and timolol). A comparison of the effectiveness of the individual compounds in protecting retinal cells in vivo was not possible in these studies. However, in vitro studies show that the capacity of the individual β-adrenoceptor antagonists to act as neuroprotectants appears to relate to their capacity to attenuate neuronal calcium and sodium influx.

High non-specific binding of the β1-selective radioligand 2-125I-ICI-H

As results of cardiac biopsies suggest, myocardial beta(1) -adrenoceptor density is reduced in patients with chronic heart failure. However, changes in cardiac beta(2)-adrenoceptors vary. With suitable radiopharmaceuticals single photon emission computed tomography (SPECT) and positron emission tomography (PET) offer the opportunity to assess beta-adrenoceptors non-invasively. Among the novel racemic analogues of the established beta(1)-selective adrenoceptor antagonist ICI 89.406 the iodinated 2-I-ICI-H showed high affinity and selectivity to beta(1)-adrenoceptors in murine ventricular membranes. The aim of this study was its evaluation as a putative sub-type selective beta(1)-adrenergic radioligand in cardiac imaging. Competition studies in vitro and in vivo were used to investigate the kinetics of 2-I-ICI-H binding to cardiac beta-adrenoceptors in mice and rats. In addition, the radiosynthesis of 2-(125)I-ICI-H from the silylated precursor 2-SiMe(3)-ICI-H was established. The specific activity was 80 GBq/ micro mol, the radiochemical yield ranged from 70 to 80%. The unlabelled compound 2-I-ICI-H showed high beta(1)-selectivity and -affinity in the in vitro competition studies. In vivo biodistribution studies apparently showed low affinity to cardiac beta-adrenoceptors. The radiolabelled counterpart 2-(125)I-ICI-H showed a high degree of non-specific binding in vitro and no specific binding to cardiac beta(1)-adrenoceptors in vivo. Because of its high non-specific binding 2-(125)I-ICI-H is no suitable radiotracer for imaging in vivo.

Mechanistic investigation of the adrenergic induction of ventral prostate hyperplasia in mice.

The norepinephrine (NE) analog phenylephrine has previously been shown to induce atypical prostate hyperplasia in rats. The objective of the present study was to provide further insight into the mechanism of phenylephrine-induced prostate growth. Adult male C57/BL6 mice were given daily subcutaneous injection of phenylephrine, isoproterenol, or phenylephrine in combination with BMY7378, cyclazosin, RS100329, or yohimbine, and the effects on ventral prostate histology, and proliferative and apoptotic indices determined. Phenylephrine was also administered in combination with testosterone in castrated mice. Atypical prostatic hyperplasia characterized by piling up and/or papillary infolding of epithelial cells with concomitant stromal smooth muscle hyperplasia was seen in adult mice given subcutaneous injection of phenylephrine daily for 26 days. Phenylephrine induced hyperplasia was more severe proximally and was associated with significantly reduced rates of apoptosis (but no change in cell proliferation) in both stromal and epithelial compartments. Only the alpha(1A)-adrenoceptor selective subtype antagonist RS100329 abrogated the phenylephrine-induced hyperplasia. Using selective antibodies, the alpha(1A-1)-adrenoceptor subtype was predominantly localized to the stromal compartments of the mouse and rat ventral prostates. The effects of phenylephrine were mediated independent of testicular androgens. Prostatic hyperplasia in mice occurs as a consequence of subchronic administration of the sympathomimetic phenylephrine. Response to phenylephrine is mediated by the alpha(1A)-adrenoceptor, which predominates in the stroma of the rodent ventral prostate. Conceivably, therefore, phenylephrine could directly modulate prostate stromal growth, and indirectly modulate epithelial growth in a paracrine fashion. We cannot, however, rule out the contribution of other indirect effects such as hypoxia/reperfusion or effects on intermediary metabolism.

Basolateral adrenoceptor activation mediates noradrenaline-induced Cl- secretion in M-1 mouse cortical collecting duct cells.

Noradrenaline (NA) released from efferent renal sympathetic nerves may directly affect renal tubular transport. Here we examined the effect of NA on transepithelial ion transport of cultured M-1 mouse cortical collecting duct cells using equivalent short-circuit current ( I(SC)) measurements. Steady-state I(SC) averaged 87.5+/-2.9 microA cm(-2) (n=185) and was reduced by 97.1+/-0.1% (n=80) by apical amiloride (100 microM) confirming that the predominant electrogenic transport across M-1 monolayers is sodium absorption via the epithelial sodium channel (ENaC). Basolateral addition of NA (10 microM) induced a biphasic change in I(SC) characterized by an initial transient peak increase of 18.1+/-0.9 microA cm(-2) with a subsequent decline to a plateau 1.4+/-0.3 microA cm(-2) (n=20) above baseline. Apical application of NA had no effect. The response to basolateral NA was concentration dependent and was preserved in the presence of apical amiloride. In contrast, the response was largely reduced in the presence of apical diphenylamine-2-carboxylic acid (1 mM) and in the absence of extracellular Cl(-). The peak response to NA was reduced in the presence of the alpha-adrenoceptor antagonist phentolamine (100 microM), whereas the beta-antagonist propranolol (100 microM) reduced the secondary plateau phase while failing to influence the peak response. The alpha(1)-adrenoceptor-selective antagonists prazosin (10 nM) and corynanthine (1 microM) reduced the NA-induced peak response by about 75% and 70%, respectively, while the alpha(2)-adrenoceptor antagonist yohimbine (100 nM) was ineffective. We conclude that in M-1 cells NA stimulates Cl(-) secretion probably involving both alpha(1)- and beta-adrenoceptors located basolaterally.

Efficient synthesis of the optically active dihydropyrimidinone of a potent α1A- selective adrenoceptor antagonist

The convergent synthesis of a potent α1A-selective adrenoceptor antagonist is described. Salient features of the synthesis include the enzymatic resolution of a racemic dihydropyrimidinone and the use of a palladium coupling reaction in the synthesis of 2,4'-dipyridyl.Key words: dihydropyrimidinone, enzymatic resolution, palladium coupling.

α1A-Adrenoceptors Mediate Sympathetically Evoked Pupillary Dilation in Rats

Evidence suggests that in some species (cats, rabbits, and possibly humans) alpha-adrenoceptors in the iris dilator muscle are "atypical" in that they cannot be readily classified by conventional criteria. This study was undertaken in an attempt to characterize the alpha-adrenoceptor subtype(s) mediating sympathetically elicited mydriasis in rats. Frequency-response pupillary dilator curves were generated by stimulation of the preganglionic cervical sympathetic nerve (1-32 Hz) in pentobarbital-anesthetized rats. Evoked responses were inhibited by systemic administration of nonselective alpha-adrenergic antagonists, phentolamine (0.3-10 mg/kg) and phenoxybenzamine (0.03-1 mg/kg). The selective alpha(1)-adrenergic antagonist, prazosin (0.01-1 mg/kg), also was effective, although alpha(2)-adrenergic antagonism with rauwolscine (0.1-1 mg/kg) was not. alpha(1A)-Adrenoceptor-selective antagonists, 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB-4101; 0.1-1 mg/kg) and 5-methylurapidil (0.1-1 mg/kg), as well as the alpha(1D)-adrenoceptor-selective antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY-7378; 1-3 mg/kg), were used to determine the subtype(s) involved. Evoked mydriasis was significantly antagonized by both WB-4101 and 5-methylurapidil but not by BMY-7378. These results suggest that, unlike some other species, adrenoceptors in the rat iris dilator mediating neurogenic mydriasis are "typical" and, in addition, can be characterized as being primarily of the alpha(1A)-adrenoceptor subtype.

Activation of alpha 1-adrenoceptors is not essential for the mediation of ischaemic preconditioning in rat heart.

1. The aim of the present study was to clarify the role of alpha1-adrenoceptors in the mechanism of ischaemic preconditioning (IP). 2. Rat isolated perfused hearts were either non- preconditioned, preconditioned with 5 min ischaemia or treated for 5 min with alpha1-adrenoceptor agonists (50 micromol/L phenylephrine; 0.1, 0.5 and 1 micromol/L methoxamine) before being subjected to 45 min of sustained ischaemia followed by 60 min reperfusion. 3. Within each of the above protocols, hearts were divided into groups to which alpha1-adrenoceptor antagonists (prazosin, 5'-methyl urapidil and chloroethylclonidine (CEC)) were administered. Functional recovery and infarct size were used as indices of the effects of ischaemia. Ischaemic contracture characteristics and maximal diastolic pressure during reflow were also assessed. 4. Blockade of alpha(1)-adrenoceptors with prazosin or the subtype-selective antagonists 5'-methyl urapidil and CEC did not abolish the protective effect of IP with respect to both functional recovery and infarct size reduction. 5. Protection afforded by phenylephrine was attenuated in hearts treated with prazosin or the alpha(1B)-adrenoceptor- selective antagonist CEC, but not in those treated with the alpha(1A)-adrenoceptor-selective antagonist 5'-methyl urapidil. 6. Treatment with low concentrations of methoxamine, considered to be alpha(1A)-adrenoceptor selective, did not confer any protection to the ischaemic myocardium. 7. A close relationship between accelerated ischaemic contracture and enhanced cardioprotection was observed. 8. The results suggest that alpha1-adrenoceptor stimulation mimics IP, but it is not an essential component in the mechanism behind the protective effect of IP in rat heart. In addition, the present study demonstrates that stimulation of the alpha(1B)- but not the alpha(1A)-adrenoceptor subtype is responsible for the catecholamine-induced protection of ischaemic myocardium in rat.

Nebivolol, bucindolol, metoprolol and carvedilol are devoid of intrinsic sympathomimetic activity in human myocardium.

1. The present study investigated whether or not there may be differences in the direct cardiac actions of the novel, highly beta(1)-selective adrenoceptor antagonist nebivolol (NEB) in comparison to metoprolol (MET), bisoprolol (BIS), carvedilol (CAR) and bucindolol (BUC) in human myocardium (n=9). 2. The rank order of beta(1)-selectivity as judged by competition experiments to (3)H-CGP 12.1777 in the presence of CGP 207.12 A (300 nmol l(-1), K(i)beta(2)) or ICI 118.551 (50 nmol l(-1), K(i)beta(1)) were NEB(K(i)beta(2)/K(i)beta(1): 40.7) > BIS(15.6) > MET(4.23) > CAR(0.73) > BUC(0.49). 3. The rank order of the negative inotropic potency of the beta-adrenoceptor antagonists measured in left ventricular trabeculae (dilated cardiomyopathy, DCM) as judged by the concentration needed to induce a 50% decrease in isoprenaline (1 micromol l(-1))-stimulated force (IC(50)) was: MET (0.6 micromol l(-1)) > CAR (4.1 micromol l(-1)) > NEB (7.0 micromol l(-1)). 4. NEB, BUC, MET and CAR did not not exert an intrinsic sympathomimetic activity (ISA) as determined by measurements of force development in forskolin (0.3 micromol l(-1)) pre-treated left ventricular trabeculae, nor by measuring adenylate cyclase activity in forskolin (0.3 micromol l(-1))-stimulated assays (crude membranes). This also holds true for radioligand binding assays with or without guanine nucleotide guanyl-5'-yl imidodiphosphate (Gpp(NH)p). 5. Although all studied beta-adrenoceptor antagonists lack intrinsic sympathomimetic activity (ISA), they differ in the beta(1)-selectivity as well as in their direct negative inotropic action. These differences as well as the mode of extracardiac action may have an impact on outcome of patients treated with beta-adrenoceptor antagonists.

Endogenous adrenergic control of reflexes evoked by mechanical stimulation of the heel in the decerebrated rabbit

In decerebrated rabbits, reflexes were evoked in medial gastrocnemius motoneurones by mechanical stimulation of the heel, using four pinch strengths from 183 (innocuous) to 4577 (noxious) mN. The α 2-adrenoceptor antagonist idazoxan (1–156 μg intrathecal (i.th.) significantly increased responses to pinch strengths of 607 mN and above. Subsequent administration of the α 1 adrenoceptor selective antagonist prazosin (200 μg i.th.) decreased reflexes to 4577 mN pinches but had no other significant effects. The opioid antagonist (−)-quadazocine (25 μg i.th.) caused no further changes in reflexes. Spinal section at L1 in the presence of this drug combination enhanced gastrocnemius responses to 183 and 607 mN stimuli, had no effect on reflexes to 1866 mN and significantly decreased responses to 4577 mN pinches. These data confirm that reflexes evoked by ‘natural’ stimulation of heel mechanoreceptors are subject to powerful tonic descending inhibition mediated by α 2-adrenoceptors. For the highest strength stimulus, the results of α 2 blockade involved enabling of descending facilitation as well as reduction of descending inhibition.