5-Methyl-urapidil discriminates between subtypes of the alpha 1-adrenoceptor

A Concise and Useful Guide to Understand How Alpha1 Adrenoceptor Antagonists Work

Adrenoceptors are the receptors for the catecholamines, adrenaline and noradrenaline. They are divided in α (α1 and α2) and β (β1, β2 and β3). α1-Adrenoceptors are subdivided in α1A, α1B and α1D. Most tissues express mixtures of α1-adrenoceptors subtypes, which appear to coexist in different densities and ratios, and in most cases their responses are probably due to the activation of more than one type. The three subtypes of α1-adrenoceptors are G-protein-coupled receptors (GPCR), specifically coupled to Gq/11. Additionally, the activation of these receptors may activate other signaling pathways or different components of these pathways, which leads to a great variety of possible cellular effects. The first clinically used α1 antagonist was Prazosin, for Systemic Arterial Hypertension (SAH). It was followed by its congeners, Terazosin and Doxazosin. Nowadays, there are many classes of α-adrenergic antagonists with different selectivity profiles. In addition to SAH, the α1-adrenoceptors are used for the treatment of Benign Prostatic Hyperplasia (BPH) and urolithiasis. This antagonism may be part of the mechanism of action of tricyclic antidepressants. Moreover, the activation of these receptors may lead to adverse effects such as orthostatic hypotension, similar to what happens with the antidepressants and with some antipsychotic. Structure-activity relationships can explain, in part, how antagonists work and how selective they can be for each one of the subtypes. However, it is necessary to develop new molecules which antagonize the α1-adrenoceptors or make chemical modifications in these molecules to improve the selectivity, pharmacokinetic profile and/or reduce the adverse effects of known drugs.

Urapidil, but not dihydropyridine calcium channel inhibitors, preserves the hypoxic pulmonary vasoconstriction: an experimental study in pig arteries

Hypoxic pulmonary vasoconstriction (HPV) is a protective mechanism maintaining blood oxygenation by redirecting blood flow from poorly ventilated to well-ventilated areas in the lung. Such a beneficial effect is blunted by antihypertensive treatment with dihydropyridine calcium channel inhibitors. The aim of the present study was to evaluate the effect of urapidil, an antihypertensive agent acting as an α₁ adrenergic antagonist and a partial 5-HT_1A agonist, on HPV in porcine proximal and distal pulmonary artery rings, and to characterize underlying mechanisms. Rings from proximal and distal porcine pulmonary artery were suspended in organ chambers and aerated with a 95% O₂ + 5% CO₂ gas mixture. HPV was induced by changing the gas to a 95% N₂ + 5% CO₂ mixture following a low level of pre-contraction with U46619. Hypoxia induced a contractile response in both proximal and distal pulmonary artery rings. This effect is observed in the presence of a functional endothelium and is inhibited by a soluble guanylyl cyclase inhibitor (ODQ), a NO scavenger (carboxy-PTIO), and by catalase in proximal pulmonary artery rings. The endothelium-dependent HPV is prevented by nicardipine and clevidipine but remained unaffected by urapidil in both proximal and distal pulmonary artery rings. These findings indicate that urapidil, in contrast to nicardipine and clevidipine, preserves the hypoxia-triggered vasoconstriction in isolated pulmonary arteries. They further indicate the involvement of the NO-guanylyl cyclase pathway and H₂ O₂ in HPV. Further research is warranted to determine the potential clinical relevance of the preserved hypoxia-induced pulmonary vasoconstriction by urapidil.

Urapidil, compared to nitroglycerin, has better clinical safety in the treatment of hypertensive patients with acute heart failure: a meta-analysis

Objectives

The application of urapidil for treating hypertensive patients with acute heart failure in the emergency department remains controversial. Our objective was to organize the relevant articles and assess the clinical indexes between urapidil and nitroglycerin.

Materials and methods

PubMed, EMBASE, the Cochrane Library and China National Knowledge Infrastructure were searched for randomized studies that compared urapidil treatment with nitroglycerin treatment for hypertensive patients with acute heart failure. The risk ratio, with 95% CI, was calculated by using a corresponding effects model, according to the value of I².

Results

Seven randomized controlled trials were identified, in order to compare the clinical indexes. On comparing the clinical indexes, the urapidil group was found to be better than the nitroglycerin group in regard to left ventricular ejection fraction, systolic blood pressure, N-terminal prohormone of brain natriuretic peptide, left ventricular end-diastolic volume, cardiac index, ALT, AST and health complications (P<0.05), but the indexes of creatinine were worse in the urapidil group. Furthermore, the two methods of treatment were comparable in diastolic blood pressure, left ventricular end-systolic volume, left ventricular end-systolic dimension, heart rate, fasting plasma glucose and total cholesterol levels (P>0.05).

Conclusion

Based on the current evidence, urapidil treatment had better clinical safety features than the traditional pharmaceutical treatment with nitroglycerin. For those indicators with a small amount of data, a greater number of randomized, high-quality controlled trials should be conducted in order to further verify the findings, which could give researchers a more comprehensive evaluation of urapidil treatment.

Interaction between nitric oxide and renal α1-adrenoreceptors mediated vasoconstriction in rats with left ventricular hypertrophyin Wistar Kyoto rats

Left ventricular hypertrophy (LVH) is associated with decreased responsiveness of renal α₁-adrenoreceptors subtypes to adrenergic agonists. Nitric oxide donors are known to have antihypertrophic effects however their impact on responsiveness of renal α₁-adrenoreceptors subtypes is unknown. This study investigated the impact of nitric oxide (NO) and its potential interaction with the responsiveness of renal α₁-adrenoreceptors subtypes to adrenergic stimulation in rats with left ventricular hypertrophy (LVH). This study also explored the impact of NO donor on CSE expression in normal and LVH kidney. LVH was induced using isoprenaline and caffeine in drinking water for 2 weeks while NO donor (L-arginine, 1.25g/Lin drinking water) was given for 5 weeks. Intrarenal noradrenaline, phenylephrine and methoxamine responses were determined in the absence and presence of selective α₁-adrenoceptor antagonists, 5- methylurapidil (5-MeU), chloroethylclonidine (CeC) and BMY 7378. Renal cortical endothelial nitric oxide synthase mRNA was upregulated 7 fold while that of cystathione γ lyase was unaltered in the NO treated LVH rats (LVH-NO) group compared to LVH group. The responsiveness of renal α_1A, α_1B and α_1D-adrenoceptors in the low dose and high dose phases of 5-MeU, CEC and BMY7378 to adrenergic agonists was increased along with cGMP in the kidney of LVH-NO group. These findings suggest that exogenous NO precursor up-regulated the renal eNOS/NO/cGMP pathway in LVH rats and resulted in augmented α_1A, α_1B and α_1D adrenoreceptors responsiveness to the adrenergic agonists. There is a positive interaction between H₂S and NO production in normal animals but this interaction appears absent in LVH animals.

Antidiuretic and antinatriuretic response to high salt load in normotensive Wistar-Kyoto rats: Role of alpha-1A-adrenoreceptors

Altered renal adrenergic responses have been recognized as pathophysiological responses to high salt intake. This study aims to investigate the influence of 6 weeks of high salt diet on α_1A -adrenoceptor regulation of renal tubular antinatriuretic and antidiuretic response in normal Wistar Kyoto rats. To achieve the above objective, antinatriuretic and antidiuretic response to phenylephrine was measured in the absence and presence of 5-methylurapidil (5-MeU) using the inulin clearance method. Systemic mean arterial blood pressure and renal haemodynamics were also measured simultaneously. Six weeks of high salt intake in Wistar-Kyoto (WKY) rats did not bring any significant increase in mean arterial blood pressure. WKY rat on high salt diet (WKYHNa) showed an exaggerated increase in absolute and fractional sodium excretion. There was a significant involvement of α_1A -adrenoceptor in carrying out renal tubular antinatriuretic and antidiuretic response in Wistar Kyoto rats on normal sodium diet (WKYNNa). However, α_1A -adrenoceptor played a minimal role in handling the tubular reabsorptive response in WKY rats on high salt diet.

Cystathione gamma lyase/Hydrogen Sulphide Pathway Up Regulation Enhances the Responsiveness of α1A and α1B-Adrenoreceptors in the Kidney of Rats with Left Ventricular Hypertrophy

The purpose of the present study was to investigate the interaction between H2S and NO (nitric oxide) in the kidney and to evaluate its impact on the functional contribution of α1A and α1B-adrenoreceptors subtypes mediating the renal vasoconstriction in the kidney of rats with left ventricular hypertrophy (LVH). In rats the LVH induction was by isoprenaline administration and caffeine in the drinking water together with intraperitoneal administration of H2S. The responsiveness of α1A and α1B to exogenous noradrenaline, phenylephrine and methoxaminein the absence and presence of 5-methylurapidil (5-MeU) and chloroethylclonidine (CEC) was studied. Cystathione gamma lyase (CSE), cystathione β synthase (CBS), 3-mercaptopyruvate sulphar transferase (3-MST) and endothelial nitric oxide synthase (eNOS) were quantified. There was significant up regulation of CSE and eNOS in the LVH-H2S compared to the LVH group (P<0.05). Baseline renal cortical blood perfusion (RCBP) was increased (P<0.05) in the LVH-H2S compared to the LVH group. The responsiveness of α1A-adrenergic receptors to adrenergic agonists was increased (P<0.05) after administration of low dose 5-Methylurapidil in the LVH-H2S group while α1B-adrenergic receptors responsiveness to adrenergic agonists were increased (P<0.05) by both low and high dose chloroethylclonidine in the LVH-H2S group. Treatment of LVH with H2S resulted in up-regulation of CSE/H2S, CBS, and 3-MST and eNOS/NO/cGMP pathways in the kidney. These up regulation of CSE/H2S, CBS, and 3-MST and eNOS/NO/cGMP pathways enhanced the responsiveness of α1A and α1B-adrenoreceptors subtypes to adrenergic agonists in LVH-H2S. These findings indicate an important role for H2S in modulating deranged signalling in the renal vasculature resulting from LVH development.

Activation of α1B-adrenoceptors contributes to intermittent hypobaric hypoxia-improved postischemic myocardial performance via inhibiting MMP-2 activation

Inhibition of matrix metalloproteinases-2 (MMP-2) activation renders cardioprotection from ischemia/reperfusion (I/R) injury; however, the signaling pathways involved have not been fully understood. Intermittent hypobaric hypoxia (IHH) has been shown to enhance myocardial tolerance to I/R injury via triggering intrinsic adaptive responses. Here we investigated whether IHH protects the heart against I/R injury via the regulation of MMP-2 and how the MMP-2 is regulated. IHH (Po2 = 84 mmHg, 4-h/day, 4 wk) improved postischemic myocardial contractile performance, lactate dehydrogenase (LDH) release, and infarct size in isolated perfused rat hearts. Moreover, IHH reversed I/R-induced MMP-2 activation and release, disorders in the levels of MMP-2 regulators, peroxynitrite (ONOO(-)) and tissue inhibitor of metalloproteinase-4 (TIMP-4), and loss of the MMP-2 targets α-actinin and troponin I. This protection was mimicked, but not augmented, by a MMP inhibitor doxycycline and lost by the α1-adrenoceptor (AR) antagonist prazosin. Furthermore, IHH increased myocardial α1A-AR and α1B-AR density but not α1D-AR after I/R. Concomitantly, IHH further enhanced the translocation of PKC epsilon (PKCε) and decreased the release of mitochondrial cytochrome c due to I/R via the activation of α1B-AR but not α1A-AR or α1D-AR. IHH-conferred cardioprotection in the postischemic contractile function, LDH release, MMP-2 activation, and nitrotyrosine as well as TIMP-4 contents were mimicked but not additive by α1-AR stimulation with phenylephrine and were abolished by an α1B-AR antagonist chloroethylclonidine and a PKCε inhibitor PKCε V1-2. These findings demonstrate that IHH exerts cardioprotection through attenuating excess ONOO(-) biosynthesis and TIMP-4 loss and sequential MMP-2 activation via the activation of α1B-AR/PKCε pathway.

Molecular and pharmacological characteristics of the gerbil α(1a)-adrenergic receptor

The spiral modiolar artery supplies blood and essential nutrients to the cochlea. Our previous functional study indicates the α(1A)-adrenergic receptor subtype mediates vasoconstriction of the gerbil spiral modiolar artery. Although the gerbil cochlea is often used as a model in hearing research, the molecular and pharmacological characteristics of the cloned gerbil α(1a)-adrenergic receptor have not been determined. Thus we cloned, expressed and characterized the gerbil α(1a)-adrenergic receptor and then compared its molecular and pharmacological properties to those of other mammalian α(1a)-adrenergic receptors. The cDNA clone contained 1404 nucleotides, which encoded a 467 amino acid peptide with a deduced sequence having 96.8, 96.4 and 91.6% identity to rat, mouse and human α(1a)-receptors, respectively. We transiently transfected the α(1a)-adrenergic receptor into COS-1 cells and determined its pharmacological characteristics by [(3)H]prazosin binding. Unlabeled prazosin had a K(i) of 0.89±0.1nM. The α(1A)-adrenergic receptor-selective antagonists, 5-methylurapidil and WB-4101, bound with high affinity and had K(i) values of 4.9±1 and 1.0±0.1nM, respectively. BMY-7378, an α(1D)-adrenergic receptor-selective antagonist, bound with low affinity (260±60nM). The 91.6% amino acid sequence identity and K(i)s of the cloned gerbil α(1a)-adrenergic receptor are similar to those of the human α(1a)-adrenergic receptor clone. These results show that the gerbil α(1a)-adrenergic receptor is representative of the human α(1a)-adrenergic receptor, lending validity to the use of the gerbil spiral modiolar artery as a model in studies of vascular disorders of the cochlea.

α1-adrenergic receptors positively regulate Toll-like receptor cytokine production from human monocytes and macrophages

Catecholamines released from the sympathetic nervous system in response to stress or injury affect expression of inflammatory cytokines generated by immune cells. α(1)-Adrenergic receptors (ARs) are expressed on innate immune cell populations, but their subtype expression patterns and signaling characteristics are not well characterized. Primary human monocytes, a human monocytic cell line, and monocyte-derived macrophage cells were used to measure expression of the proinflammatory mediator interleukin (IL)-1β responding to lipopolysaccharide (LPS) in the presence or absence of α(1)-AR activation. Based on our previous findings, we hypothesized that α(1)-AR stimulation on innate immune cells positively regulates LPS-initiated IL-1β production. IL-1β production in response to LPS was synergistically higher for both monocytes and macrophages in the presence of the selective α(1)-AR agonist (R)-(-)-phenylephrine hydrochloride (PE). This synergistic IL-1β response could be blocked with a selective α(1)-AR antagonist as well as inhibitors of protein kinase C (PKC). Radioligand binding studies characterized a homogenous α(1B)-AR subtype population on monocytes, which changed to a heterogeneous receptor subtype expression pattern when differentiated to macrophages. Furthermore, increased p38 mitogen-activated protein kinase (MAPK) activation was observed only with concurrent PE and LPS stimulation, peaking after 120 and 30 min in monocytes and macrophages, respectively. Blocking the PKC/p38 MAPK signaling pathway in both innate immune cell types inhibited the synergistic IL-1β increase observed with concurrent PE and LPS treatments. This study characterizes α(1)-AR subtype expression on both human monocyte and macrophage cells and illustrates a mechanism by which increased IL-1β production can be modulated by α(1)-AR input.

Evidence for the role of α1A-adrenoceptor subtype in the control of renal haemodynamics in fructose-fed Sprague-Dawley rat

AIM

To explore the hypothesis that high fructose intake results in a higher functional contribution of α1A-adrenoceptors and blunts the adrenergically and angiotensin II (Ang II)-induced renal vasoconstriction.

METHODS

Twelve Sprague-Dawley rats received either 20% fructose solution [FFR] or tap water [C] to drink ad libitum for 8 weeks. The renal vasoconstrictor response to noradrenaline (NA), phenylephrine (PE), methoxamine (ME) and Ang II was determined in the presence and absence of 5-methylurapidil (5-MU) (α1A-adrenoceptor antagonist) in a three-phase experiment (pre-drug, low- and high-dose 5-MU). Data, mean ± SEM were analysed by ANOVA or Student's unpaired t-test with significance at P < 0.05.

RESULTS

FFR exhibited insulin resistance (HOMA index), hypertension and significant increases in plasma levels of glucose and insulin. All agonists caused dose-related reductions in cortical blood perfusion that were larger in C than in FFR while the magnitudes of the responses were progressively reduced with increasing doses of 5-MU in both C and FFR. The degree of 5-MU attenuation of the renal cortical vasoconstriction due to NA, ME and Ang II was significantly greater in the FFR compared to C.

CONCLUSIONS

Fructose intake for 8 weeks results in smaller vascular response to adrenergic agonists and Ang II. The α1A-adrenoceptor subtype is the functional subtype that mediates renal cortical vasoconstriction in control rats, and this contribution becomes higher due to fructose feeding.

Multiple alpha1-adrenergic receptor subtypes support synergistic stimulation of vasopressin and oxytocin release by ATP and phenylephrine

Simultaneous exposure of explants of the hypothalamo-neurohypophyseal system (HNS) to ATP and the α(1)-adrenergic receptor (α(1)-R) agonist, phenylephrine (ATP+PE) induces a synergistic stimulation of vasopressin and oxytocin (VP/OT) release that is sustained for hours. The current studies confirm that the synergism is dependent upon activation of α(1)-R by demonstrating that an α(1)-R antagonist prevents the response. The role of the α(1)A, B, and D-adrenergic receptor subtypes in the synergistic effect of ATP+PE on intracellular calcium ([Ca(2+)](i)) in supraoptic nucleus (SON) neurons and VP/OT release from neural lobe was evaluated. The increase in [Ca(2+)](i) induced by PE in SON predominantly reflects release from intracellular stores and is mediated by activation of the α(1)A adrenergic receptor subtype. The α(1)A subtype is also required for the sustained elevation in [Ca(2+)](i) induced by ATP+PE. In contrast, although synergistic stimulation of VP/OT release was eliminated by removal of PE and was blunted by benoxathian, an α(1)-R antagonist that is not subtype selective, no single α(1)-R subtype selective antagonist prevented sustained stimulation of VP/OT release by ATP+PE. Thus, sustained activation of α(1)-R is essential for the synergistic VP and OT response to ATP+PE, but multiple α(1)-R subtypes can support the response. Redundancy amongst the α(1)-R subunits in supporting this response is consistent with the predicted importance of the response for sustaining the elevated VP release required to prevent cardiovascular collapse during hemorrhage and sepsis.

alpha(1)-Adrenergic receptor subtype function in fetal and adult cerebral arteries

In the developing fetus, cerebral artery (CA) contractility demonstrates significant functional differences from that of the adult. This may be a consequence of differential activities of alpha(1)-adrenergic receptor (alpha(1)-AR) subtypes. Thus we tested the hypothesis that maturational differences in adrenergic-mediated CA contractility are, in part, a consequence of differential expression and/or activities of alpha(1)-AR subtypes. In CA from fetal ( approximately 140 days) and nonpregnant adult sheep, we used wire myography and imaging, with simultaneous measurement of tension and intracellular Ca(2+) concentration ([Ca(2+)](i)), radioimmunoassay, and Western immunoblots to examine phenylephrine (Phe)-induced contractile responses. The alpha(1A)-AR antagonists (5-MU and WB-4101) completely inhibited Phe-induced contraction in adult but not fetal CA; however, [Ca(2+)](i) increase was reduced significantly in both age groups. The alpha(1D)-AR antagonist (BMY-7378) blocked both Phe-induced contractions and Ca(2+) responses to a significantly greater extent in adult compared with fetal CA. In both age groups, inhibition of alpha(1A)-AR and alpha(1B)-AR, but not alpha(1D)-AR, significantly reduced inositol 1,4,5-trisphosphate responses to Phe. Western immunoblots demonstrated that the alpha(1)-AR subtype expression was only approximately 20% in fetal CA compared with the adult. Moreover, in fetal CA, the alpha(1D)-AR was expressed significantly greater than the other two subtypes. Also, in fetal but not adult CA, Phe induced a significant increase in activated ERK1/2; this increase in phosphorylated ERK was blocked by alpha(1B)-AR (CEC) and alpha(1D)-AR (BMY-7378) inhibitors, but not by alpha(1A)-AR inhibitors (5-MU or WB-4101). In conclusion, in the fetal CA, alpha(1B)-AR and alpha(1D)-AR subtypes play a key role in contractile response as well as in ERK activation. We speculate that in fetal CA alpha(1B)-AR and alpha(1D)-AR subtypes may be a critical factor associated with cerebrovascular growth and function.

The alpha 1B/D-adrenoceptor knockout mouse permits isolation of the vascular alpha 1A-adrenoceptor and elucidates its relationship to the other subtypes

BACKGROUND AND PURPOSE

Mesenteric and carotid arteries from the alpha(1B/D)-adrenoceptor knockout (alpha(1B/D)-KO) were employed to isolate alpha(1A)-adrenoceptor pharmacology and location and to reveal these features in the wild-type (WT) mouse.

EXPERIMENTAL APPROACH

Functional pharmacology by wire myography and receptor localization by confocal microscopy, using the fluorescent alpha(1)-adrenoceptor ligand BODIPY FL-Prazosin (QAPB), on mesenteric (an 'alpha(1A)-adrenoceptor' tissue) and carotid (an 'alpha(1D)-adrenoceptor' tissue) arteries.

KEY RESULTS

Alpha(1B/D)-KO mesenteric arteries showed straightforward alpha(1A)-adrenoceptor agonist/antagonist pharmacology. WT had complex pharmacology with alpha(1A)- and alpha(1D)-adrenoceptor components. alpha(1B/D)-KO had a larger alpha(1A)-adrenoceptor response suggesting compensatory up-regulation: no increase in fluorescent ligand binding suggests up-regulation of signalling. alpha(1B/D)-KO carotid arteries had low efficacy alpha(1A)-adrenoceptor responses. WT had complex pharmacology consistent with co-activation of all three subtypes. Fluorescent binding had straightforward alpha(1A)-adrenoceptor characteristics in both arteries of alpha(1B/D)-KO. Fluorescent binding varied between cells in relative intracellular and surface distribution. Total fluorescence was reduced in the alpha(1B/D)-KO due to fewer smooth muscle cells showing fluorescent binding. WT binding was greater and sensitive to alpha(1A)- and alpha(1D)-adrenoceptor antagonists.

CONCLUSIONS AND IMPLICATIONS

The straightforward pharmacology and fluorescent binding in the alpha(1B/D)-KO was used to interpret the properties of the alpha(1A)-adrenoceptor in the WT. Reduced total fluorescence in alpha(1B/D)-KO arteries, despite a clear difference in the functionally dominant subtype, indicates that measurement of receptor protein is unlikely to correlate with function. Fewer cells bound QAPB in the alpha(1B/D)-KO suggesting different cellular phenotypes of alpha(1A)-adrenoceptor exist. The alpha(1B/D)-KO provides robust assays for the alpha(1A)-adrenoceptor and takes us closer to understanding multi-receptor subtype interactions.

Smooth muscle alpha1D-adrenoceptors mediate phenylephrine-induced vasoconstriction and increases in endothelial cell Ca2+ in hamster cremaster arterioles

BACKGROUND AND PURPOSE

alpha(1)-Adrenoceptor agonists induce Ca(2+)-transients in endothelial cells (ECs) of arterioles. However, the presence of alpha(1)-adrenoceptors on arteriolar ECs has not been excluded, and the identity of alpha(1)-adrenoceptor subtypes in arterioles only has been inferred from pharmacology. Therefore, we determined which subtypes were expressed by vascular smooth muscle cells (VSMCs) and ECs, and which subtype mediated alpha(1)-adrenoceptor-induced constriction.

EXPERIMENTAL APPROACH

EC Ca(2+)-transients in isolated, cannulated hamster cremasteric arterioles or freshly isolated ECs were studied using Fura 2. Arteriolar diameter was measured by video microscopy. alpha(1)-Adrenoceptor expression was assessed by western blot of whole-arteriolar homogenates and real-time RT-PCR on enzymatically isolated VSMCs and ECs.

KEY RESULTS

Phenylephrine-induced constriction and EC Ca(2+)-transients were abolished by the alpha(1)-adrenoceptor antagonist prazosin (30 nM) in arterioles. Phenylephrine-induced constriction was inhibited by the alpha(1D)-adrenoceptor antagonist BMY 7378 (K(B)=2.96 nM) and the alpha(1A)-adrenoceptor antagonist 5-methylurapidil (K(B)=4.08 nM), suggesting a significant role for alpha(1D)-adrenoceptors. Western blots confirmed alpha(1D)-adrenoceptor expression, but did not detect alpha(1A)-adrenoceptors. VSMCs expressed alpha(1D)- and alpha(1A)-, but not alpha(1B)-, adrenoceptor transcripts. No alpha(1)-adrenoceptor transcripts were detected in ECs. Neither phenylephrine (10 microM) nor noradrenaline (0.1-1 microM) elicited Ca(2+)-transients in freshly isolated ECs, whereas the endothelium-dependent vasodilators methacholine (1 microM) and substance P (100 nM) consistently increased Ca(2+).

CONCLUSIONS AND IMPLICATIONS

We reject the hypothesis that hamster cremasteric arteriolar ECs express alpha(1)-adrenoceptors and conclude that alpha(1)-adrenoceptor agonists predominantly act on VSMC alpha(1D)-adrenoceptors to cause vasoconstriction and a subsequent rise in EC Ca(2+).

Characterization of the alpha1-adrenoceptor subtype activating extracellular signal-regulated kinase in submandibular gland acinar cells

Alpha(1)-Adrenoceptors and extracellular signal-regulated kinases 1 and 2 (ERK1/2) regulate salivary secretion. However, whether alpha(1)-adrenoceptors couple to ERK1/2 activation and the specific alpha(1)-adrenoceptor subtypes involved in salivary glands is unknown. Western blotting of ERK1/2 phosphorylation showed phenylephrine activated ERK1/2 by 2-3-fold in submandibular gland slices and 3-4-fold in submandibular acinar (SMG-C10) cells with an EC(50) of 2.7+/-2 microM. ERK1/2 activation was blocked by either prazosin or HEAT, indicating alpha(1)-adrenoceptors stimulate ERK1/2 in native glands and SMG-C10 cells. Inhibition of [(125)I]HEAT binding by 5-methylurapidil (selective for alpha(1A) over alpha(1B/)alpha(1D)), but not BMY 7378 (selective for alpha(1D) over alpha(1A/)alpha(1B)), was biphasic and best-fit by a two-site binding model with K(i)(H) and K(i)(L) values for 5-methylurapidil of 0.64+/-0.3 and 91+/-7 nM, respectively, in SMG-C10 membranes. From these binding data, we obtained subtype-selective concentrations of 5-methylurapidil to determine the alpha(1)-adrenoceptor subtype/s activating ERK1/2 in SMG-C10 cells. 5-methylurapidil (20 nM) did not affect phenylephrine- or A-61603- (alpha(1A)-selective agonist) induced ERK1/2 activation; whereas, 30 microM chloroethylclonidine (alpha(1B)-selective antagonist) inhibited ERK1/2 activation by phenylephrine, indicating alpha(1B)-adrenoceptors, but not alpha(1A)-adrenoceptors, activate ERK1/2 in submandibular cells. We also examined alpha(1)-adrenoceptor location and dependence on cholesterol-rich microdomains for activating ERK1/2. Sucrose density gradient centrifugation showed 71+/-3% of alpha(1)-adrenoceptor binding sites were in plasma membranes. Cholesterol-disrupting agents filipin and methyl-beta-cyclodextrin inhibited phenylephrine-stimulated ERK1/2. These results show only alpha(1B)-adrenoceptors activate ERK1/2 and suggest subtype-specific ERK1/2 signaling by alpha(1B)-adrenoceptors may be determined by localization to cholesterol-rich microdomains in submandibular cells.

Sympathetic neurotransmission in the rat testicular capsule: functional characterization and identification of mRNA encoding alpha1-adrenoceptor subtypes

The rat testicular capsule is a thin tissue surrounding the testis, whose precise function is still unknown. We have studied the contractile effects of electrical field stimulation, noradrenaline, and the blockade by antagonists of adrenergic receptors, in order to characterize sympathetic neurotransmission, and adrenoceptor subtypes. In addition, reverse transcription polymerase chain reaction (RT-PCR) assays were made to check for the expression of the three known subtypes of alpha(1)-adrenoceptors. The effects of electrical field stimulation (2 to 20 Hz, 1 ms, 60 V) were almost totally abolished by depletion of neuronal noradrenaline storage with reserpine (10 mg/Kg), but not by the purinergic receptor antagonist suramin (10(-5) M), indicating that noradrenaline, but not ATP, was involved in contractions. The selective alpha(1)-adrenoceptor antagonist prazosin (10(-7) M) was more effective than the selective alpha(2)-adrenoceptor antagonist idazoxan (10(-7) M) to inhibit contractions induced by electrical field stimulation, pointing out a major involvement of alpha(1)-adrenoceptor. When noradrenaline was used instead of electrical field stimulation, it showed a high potency (pD(2)=7.9). Noradrenaline-induced contractions were competitively blocked by the selective alpha(1A)-adrenoceptor antagonists WB 4101 (pA(2)=8.88), phentolamine (pA(2)=8.39) and by the alpha(1B)-adrenoceptor antagonist spiperone (pA(2)=8.57), indicating the presence of functional alpha(1A)- and alpha(1B)-adrenoceptors. In addition, contractions were not blocked by the selective alpha(1D)-adrenoceptor antagonist BMY 7378 (up to 10(-6) M), while selective alpha(2)-adrenoceptor antagonists showed low pA(2) values (yohimbine, 7.25 and idazoxan, 7.49), suggesting a minor role, if any, for alpha(1D)- and alpha(2)-adrenoceptors. To check the proportionate role of alpha(1A)- and alpha(1B)-adrenoceptors, we blocked alpha(1B)-adrenoceptors with chloroethylclonidine (CEC, 30 microM, 45 min), that reduced the maximal effect of noradrenaline by about 60%. The remnant CEC-insensitive noradrenaline contraction was assumed to be unrelated to alpha(1B)-adrenoceptor, and was inhibited by 5-methyl-urapidil (pA(2)=8.94) and by the Ca(2+) channel blocker nifedipine (3 microM), confirming the involvement of alpha(1A)-adrenoceptors. The presence of mRNA encoding alpha(1A)- and alpha(1B)-adrenoceptor was also shown on RT-PCR assays. Unexpectedly, alpha(1D)-transcripts were also detected in these assays. Taken together, our results show that ATP co-transmission could not be detected, and that neurotransmission involves the interaction of noradrenaline with both alpha(1A)- and alpha(1B)-, but not with alpha(1D)- or alpha(2)-adrenoceptor. The fact that the functional alpha(1D)-adrenoceptor could not be detected in spite of the presence of the corresponding mRNA, remains to be investigated.

Ontogenesis of mRNA levels and binding sites of hepatic alpha-adrenoceptors in young cattle

Catecholamines affect hepatic glucose production through (alpha- and beta2-) adrenoceptors (AR). We studied mRNA abundance and binding of hepatic alpha-AR in pre-term (P0) calves and in full-term calves at day 0 (F0), day 5 (F5) and day 159 (F159) to test the hypothesis that gene expression and numbers of hepatic alpha-AR in calves are influenced by age and associated with beta2-AR and selected traits of glucose metabolism. mRNA levels of alpha1- and alpha2-AR were measured by real time RT-PCR. alpha1- and alpha2-AR numbers (maximal binding, Bmax) were determined by saturation binding of (3H)-prazosin and (3H)-RX821002, respectively. alpha1- and alpha2-AR subtypes were evaluated by competitive binding. alpha1A-AR mRNA levels were lower in P0 than in F0, F5 and F159 and alpha(2AD)-AR mRNA levels were lower in F159 than in P0, F0 and F5, while alpha2C-AR mRNA levels increased from P0 and F0 to F5 and F159. Bmax of alpha1-AR increased from P0 to F5, then decreased in F159. Bmax of alpha2-AR decreased from F0 to F159. Bmax of alpha1-AR was positively associated with mRNA levels of alpha1A-AR (r = 0.7), Bmax of beta2-AR (r = 0.5) and negatively with hepatic glycogen content (r = -0.6). Bmax of alpha2-AR was negatively associated with Bmax of beta2-AR (r = -0.4). In conclusion, mRNA levels and binding sites of alpha1- and alpha2-AR in calves exhibited developmental changes and were negatively associated with hepatic glycogen content.

Theoretical proton affinities of α1 adrenoceptor ligands

A systematic study has been performed of the proton affinity of a large family of agonists and antagonists of the alpha1-adrenoceptor at the B3LYP/6-31G* level of theory. After a conformational search, all the N atoms were considered as protonation sites and protonation energy values were determined. The inclusion of solvation by means of the Onsager model yielded stabilization in the proton affinity values obtained. In addition, a good correlation was found between the proton affinity values corresponding to the first protonation in gas phase of some of the compounds and their corresponding experimental affinity constants K(i) for the alpha1A adrenergic receptor.

Evidence for involvement of alpha1D-adrenoceptors in contraction of femoral resistance arteries using knockout mice

The role of alpha(1D)-adrenoceptors in vasoconstrictor responses to noradrenaline in mouse femoral resistance arteries was investigated using wire myography in alpha(1D)-adrenoceptor knockout (alpha(1D)-KO) and wild-type (WT) mice of the same genetic background.alpha(1D)-KO mice were 2.5-fold less sensitive than WTs to exogenous noradrenaline and BMY 7378 was significantly less potent against noradrenaline in alpha(1D)-KO mice than in WTs, showing a minor contribution of alpha(1D)-adrenoceptors in response to noradrenaline. Prazosin and 5-methyl-urapidil were equally effective against noradrenaline in alpha(1D)-KO and WT mice. Chloroethylclonidine produced a significantly greater attenuation of the response to noradrenaline in alpha(1D)-KO mice than in WTs. Responses to electrical field stimulation (EFS), at 2-20 Hz for 10 s and 0.09 ms pulse width were significantly smaller overall in alpha(1D)-KOs than in WTs although no significant differences were seen at the different frequencies.BMY 7378 produced significantly greater inhibition of responses at 2 and 5 Hz than at higher frequencies in WTs. In alpha(1D)-KOs, this greater sensitivity to BMY 7378 at lower frequencies was not apparent, confirming that the effect of BMY 7378 was due to blockade of alpha(1D)-adrenoceptors. Prazosin and 5-methyl-urapidil had similar inhibitory effects on responses to EFS in alpha(1D)-KO and WT mice. Chloroethylclonidine inhibited responses to EFS to a significantly greater extent in alpha(1D)-KO mice. The present study with alpha(1D)-KO mice shows that alpha(1D)-adrenoceptors contribute to vasoconstrictor responses to exogenous and neurally released noradrenaline in femoral resistance arteries.

Computational Study of Antagonist/α1A Adrenoceptor ComplexesObservations of Conformational Variations on the Formation of Ligand/Receptor Complexes

As selective antagonist inhibition may relieve the symptoms of benign prostatic hyperplasia, we have examined the interactions of antagonists including quinazoline and imidazolidinium/guanidinium compounds complexed with a homology model of the alpha(1A) adrenoceptor. Our approach involves docking of ligands of various structural classes followed by molecular dynamics simulations of antagonist/receptor complexes, which demonstrates that different structural classes of antagonist induce different receptor conformations upon binding with particular variations noted in the conformation of TM-V. Subsequently, we examined the interactions and the conformational flexibility of alpha(1) and alpha(1A) adrenoceptor antagonists, with the ligand-induced receptor conformers. This study indicated that a receptor conformation induced by one structural class of antagonist is not suitable for direct screening of another class. Our analysis indicates that computational high-throughput screening is likely to give inaccurate data on binding and selectivity and such studies need to consider conformational changes in the receptor.

Recent progress in alpha1-adrenergic receptor research

Alpha1-Adrenergic receptors (AR) play an important role in the regulation of physiological responses mediated by norepinephrine and epinephrine, particularly in the cardiovascular system. The three cloned alpha1-AR subtypes (alpha1A, alpha1B, and alpha1D) are G protein-coupled receptors that signal through the Gq/11 signaling pathway, each showing distinct pharmacological properties and tissue distributions. However, due to the lack of highly subtype-selective drugs, the functional roles of individual subtypes are still not clear. Development of new subtype-specific drugs will greatly facilitate the identification of the functions of each subtype. Conopeptide rho-TIA has been found to be a new alpha1B-AR selective antagonist with different modes of inhibition at alpha1-AR subtypes. In addition, recent studies using genetically engineered mice have shed some light on alpha1-AR functions in vivo, especially in the cardiovascular system and brain. Several proteins have been shown to interact directly with particular alpha1-AR, and may be important in regulating receptor function. Receptor heterodimerization has been shown to be important for cell surface expression, signaling and internalization. These new observations are likely to help elucidate the functional roles of individual alpha1-AR subtypes.

Insights into the functional roles of alpha(1)-adrenoceptor subtypes in mouse carotid arteries using knockout mice

1. alpha(1)-Adrenoceptor (AR) subtypes in mouse carotid arteries were characterised using a combination of agonist/antagonist pharmacology and knockout (KO) mice. 2. Phenylephrine (PE) was most potent in the alpha(1B)-KO (pEC(50)=6.9+/-0.2) followed by control (pEC(50)=6.3+/-0.06) and alpha(1D)-KO (pEC(50)=5.5+/-0.07). Both N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl] methanesulphonamide hydrobromide (A-61603) and 5-hydroxytryptamine (5-HT) were more potent in the alpha(1D)-KO (pEC(50)=7.4+/-0.27 and 7.4+/-0.05, respectively) than the control (pEC(50)=6.9+/-0.09 and 6.9+/-0.08, respectively) and equipotent with the control in the alpha(1B)-KO (pEC(50)=6.7+/-0.07 and 6.8+/-0.04). Maximum responses to PE and A-61603 were reduced in the alpha(1D)-KO compared to control; there was no difference in maximum responses to 5-HT. 3. In control arteries, prazosin and 5-methylurapidil acted competitively with pA(2) of 9.6 and 7.5, respectively. BMY7378 produced antagonism only at the highest concentration used (100 nM; pK(B) 8.3). 4. Prazosin, 5-methylurapidil and BMY7378 acted competitively in alpha(1B)-KO carotid arteries with pA(2) of 10.3, 7.6 and 9.6, respectively. 5. In the alpha(1D)-KO, against PE, 5-methylurapidil produced a pA(2) of 8.1. pK(B) values were calculated for prazosin (10.6) and BMY7378 (7.0). Against A-61603, 5-methylurapidil had a pA(2) of 8.5, prazosin 8.6, while BMY7378 had no effect. 6. In conclusion, the alpha(1B)-KO mediates contraction solely through alpha(1D)-ARs and the alpha(1D)-KO through alpha(1A)-ARs. Extrapolating back to the control from the knockout data suggests that all three subtypes could be involved in the responses, but we propose that the alpha(1D)-AR causes the contractile response and that the role of the alpha(1B)-AR is mainly regulatory.

Evidence for the presence of alpha1 adrenoceptor subtypes in the human ureter

AIM

Several studies have proposed a role for alpha1 adrenoceptors (ARs) in ureteral physiology, indicating that they are present in the ureter; however, few studies have been done to identify alpha1 AR subtypes present in this area. Thus, this study was carried out to characterize the alpha1 AR subtype gene and protein expression in proximal, medial, and distal region of the human ureter.

METHODS

Molecular characterization of alpha1 AR subtypes were analyzed by semi-quantitative RT-PCR. alpha1 AR protein expression was studied by saturation binding curves and by competition binding curves with selective antagonists. Analysis of data was performed using the GraphPad PRISM 4 software.

RESULTS

Analysis of saturation binding curves revealed a heterogeneous distribution of alpha1 AR binding sites, the B(max) for the distal ureter was indeed 52.5 +/- 5.4 fmol/mg prot, while a lower similar density of alpha1 ARs was demonstrated in the medial (25.2 +/- 1.7 fmol/mg prot) and proximal (23.4 +/- 0.4 fmol/mg prot) ureters. Molecular and pharmacological characterization of alpha1 AR subtypes indicated that each receptor was present, although with differences in terms of the amount expressed.

CONCLUSIONS

Human ureter was endowed with each alpha1 AR subtype, although alpha1D and alpha1A ARs were prevalent over alpha1B ARs. Radioligand binding results revealed that there were no significant differences in the K(d) between ureteral regions, while a heterogeneous distribution of alpha1 AR binding sites was detected, with the highest density of alpha1 ARs in the distal ureter and a lower similar density in the medial and proximal ureters.

Pharmacological characterization of α1-adrenoceptors in mouse isolated femoral small arteries

Arteries were isolated from male DBA/2 mice and mounted on a small vessel wire myograph for isometric recording. Responses to exogenous noradrenaline were inhibited with high affinity by prazosin (pKB, 9.3) and 5-methyl-urapidil (pKB, 9.2) and with low affinity by 8-[2-[4-(2-methoxyphenyl)-1 piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378) (pA(2), 6.7). Chloroethylclonidine (10 microM) produced only a small reduction in the maximum response to noradrenaline. Responses to electrical field stimulation were also inhibited with high affinity by prazosin (pIC50, 9.3-9.5) and 5-methyl-urapidil (pIC50, 8.0-8.3). Responses were sensitive to BMY 7378 at low frequencies of stimulation (pIC50 at 2 Hz, 8.2) but not at high frequencies (pIC50 at 20 Hz, 6.5). In conclusion, contractions to exogenous and endogenous noradrenaline in mouse femoral small arteries are mediated mainly by alpha1A-adrenoceptors. alpha1D-adrenoceptors are not involved in responses to exogenous noradrenaline but appear to be activated by neurally released noradrenaline at a low frequency of stimulation.

Subtype-selective Noncompetitive or Competitive Inhibition of Human α1-Adrenergic Receptors by ρ-TIA

The 19-amino acid conopeptide (rho-TIA) was shown previously to antagonize noncompetitively alpha(1B)-adrenergic receptors (ARs). Because this is the first peptide ligand for these receptors, we compared its interactions with the three recombinant human alpha(1)-AR subtypes (alpha(1A), alpha(1B), and alpha(1D)). Radioligand binding assays showed that rho-TIA was 10-fold selective for human alpha(1B)-over alpha(1A)- and alpha(1D)-ARs. As observed with hamster alpha(1B)-ARs, rho-TIA decreased the number of binding sites (B(max)) for human alpha(1B)-ARs without changing affinity (K(D)), and this inhibition was unaffected by the length of incubation but was reversed by washing. However, rho-TIA had opposite effects at human alpha(1A)-ARs and alpha(1D)-ARs, decreasing K(D) without changing B(max), suggesting it acts competitively at these subtypes. rho-TIA reduced maximal NE-stimulated [(3)H]inositol phosphate formation in HEK293 cells expressing human alpha(1B)-ARs but competitively inhibited responses in cells expressing alpha(1A)- or alpha(1D)-ARs. Truncation mutants showed that the amino-terminal domains of alpha(1B)- or alpha(1D)-ARs are not involved in interaction with rho-TIA. Alanine-scanning mutagenesis of rho-TIA showed F18A had an increased selectivity for alpha(1B)-ARs, and F18N also increased subtype selectivity. I8A had a slightly reduced potency at alpha(1B)-ARs and was found to be a competitive, rather than noncompetitive, inhibitor in both radioligand and functional assays. Thus rho-TIA noncompetitively inhibits alpha(1B)-ARs but competitively inhibits the other two subtypes, and this selectivity can be increased by mutation. These differential interactions do not involve the receptor amino termini and are not because of the charged nature of the peptide, and isoleucine 8 is critical for its noncompetitive inhibition at alpha(1B)-ARs.

Submandibular gland acinar cells express multiple alpha1-adrenoceptor subtypes

We evaluated an acinar cell line (SMG-C10) cloned from rat submandibular glands as a possible model for alpha(1)-adrenoceptor regulation of submandibular function. alpha(1)-Adrenoceptors are subdivided into three subtypes called alpha(1A), alpha(1B), and alpha(1D), which can be distinguished from one another by their differential affinity values for subtype-selective alpha(1)-adrenoceptor antagonists. Thus, alpha(1)-adrenoceptor subtypes in SMG-C10 cells were characterized with reverse transcription-polymerase chain reaction (RT-PCR) and [(3)H]prazosin binding in side-by-side experiments with native submandibular glands. RT-PCR identified mRNAs for alpha(1A)-, alpha(1B)-, and alpha(1D)-adrenoceptors in SMG-C10 cells and submandibular glands. The inhibition of [(3)H]prazosin binding by 5-methylurapidil (alpha(1A)-selective) was biphasic and fit best to a two-site binding model with 40 +/- 8% high (K(iH))- and 60 +/- 10% low (K(iL))-affinity binding sites in SMG-C10 cells, and 76% high- and 24% low-affinity binding sites in submandibular glands. Respective K(iH) and K(iL) values for 5-methylurapidil were 1.9 +/- 0.4 and 100 +/- 30 nM in SMG-C10 cells and 3.2 +/- 0.8 and 170 +/- 20 nM in submandibular glands. BMY-7378 [8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione dihydrochloride (alpha(1D)-selective)] bound with low affinity in SMG-C10 cells and submandibular glands with K(i) values of 81 +/- 20 and 110 +/- 20 nM, respectively. Chloroethylclondine, an irreversible alkylating agent selective for alpha(1B) adrenoceptors, reduced the density of [(3)H]prazosin binding sites by 42 and 26% in SMG-C10 and submandibular membranes, respectively. Thus, SMG-C10 cells and submandibular glands are similar in expressing receptor protein for alpha(1A)- and alpha(1B)-adrenoceptor subtypes, establishing SMG-C10 cells as a potential model for alpha(1)-adrenoceptor-mediated secretion.

The contribution of adrenoceptor subtype(s) in the renal vasculature of diabetic spontaneously hypertensive rats

1. Diabetes and hypertension are both associated with an increased risk of renal disease and are associated with neuropathies, which can cause defective autonomic control of major organs including the kidney. This study aimed to examine the alpha(1)-adrenoceptor subtype(s) involved in mediating adrenergically induced renal vasoconstriction in a rat model of diabetes and hypertension. 2. Male spontaneously hypertensive rats (SHR), 220-280 g, were anaesthetized with sodium pentobarbitone 7-day poststreptozotocin (55 mg x kg(-1) i.p.) treatment. The reductions in renal blood flow (RBF) induced by increasing frequencies of electrical renal nerve stimulation (RNS), close intrarenal bolus doses of noradrenaline (NA), phenylephrine (PE) or methoxamine were determined before and after administration of nitrendipine (Nit), 5-methylurapidil (5-MeU), chloroethylclonidine (CEC) and BMY 7378. 3. In the nondiabetic SHR group, mean arterial pressure (MAP) was 146+/-6 mmHg, RBF was 28.0+/-1.4 ml x min(-1) x kg(-1) and blood glucose was 112.3+/-4.7 mg x dl(-1), and in the diabetic SHR Group, MAP was 144+/-3 mmHg, RBF 26.9+/-1.3 ml(-1) min x kg(-1) and blood glucose 316.2+/-10.5 mg x dl(-1). Nit, 5-MeU and BMY 7378 blunted all the adrenergically induced renal vasoconstrictor responses in SHR and diabetic SHR by 25-35% (all P<0.05), but in diabetic rats the responses induced by RNS and NA treated with 5-MeU were not changed. By contrast, during the administration of CEC, vasoconstrictor responses to all agonists were enhanced by 20-25% (all P<0.05) in both the SHR and diabetic SHR. 4. These findings suggest that alpha(1A) and alpha(1D)-adrenoceptor subtypes contribute in mediating the adrenergically induced constriction of the renal vasculature in both the SHR and diabetic SHR. There was also an indication of a greater contribution of presynaptic adrenoceptors, that is, alpha(1B)-, and/or alpha(2)-subtypes.

Bunazosin hydrochloride reduces glutamate-induced neurotoxicity in rat primary retinal cultures

To study neuroprotective effects of bunazosin hydrochloride which is an alpha(1)-adrenoceptor antagonist used as an ocular hypotensive drug compared to other alpha(1)-adrenoceptor antagonists, and its mechanism of action. We evaluated the neuroprotective effects of bunazosin hydrochloride or seven other alpha(1)-adrenoceptor antagonists against glutamate-induced cell death in rat primary retinal cultures. We also evaluated the binding inhibition of bunazosin hydrochloride for 24 different receptors/channels and its effects on the Na(+) influx into cells induced by veratridine or glutamate. Bunazosin hydrochloride significantly inhibited glutamate-induced cell death at concentrations of 1 and 10 microM. Cells were also protected when treated with some alpha(1)-adrenoceptor antagonists, but not by the others. Bunazosin hydrochloride showed a high inhibition for Na(+) channels and inhibited the Na(+) influx induced by veratridine or glutamate. These findings indicate that in retinal cultures bunazosin hydrochloride has a neuroprotective effect against glutamate-induced cell death and that the inhibition of Na(+) channels by bunazosin hydrochloride may be partly responsible for this effect.

The role of alpha1D-adrenoceptors in prostatic contraction examined using protection studies

1 The aim of the study was to investigate the role of the alpha1D-adrenoceptor in alpha1-adrenoceptor-induced contraction of human prostate by means of protection experiments. 2 Responses of human prostate strips to noradrenaline were recorded, along with responses of rat aorta and vas deferens, tissues possessing predominantly alpha1D- and alpha1A-adrenoceptors respectively, for comparison. alpha1-adrenoceptors were then inactivated by incubation with the irreversible antagonist phenoxybenzamine. In some tissues alpha1A- or alpha1D-adrenoceptors were 'protected' from inactivation by incubation in the presence of the selective alpha1A- or 1D-adrenoceptor antagonists 5-methylurapidil and BMY 7378 before recording further responses to noradrenaline. 3 Phenoxybenzamine reduced the maximum noradrenaline-induced response and the potency of noradrenaline in all tissues. In rat vas deferens and human prostate, 5-methylurapidil protected alpha1A-adrenoceptors in a concentration-dependent manner. In rat aorta, 10 nM BMY 7378 almost fully protected alpha1D-adrenoceptors. However, concentrations of BMY 7378 up to 30-fold higher failed to protect receptors in the human prostate. 4 These results suggest that in human prostate functional alpha1D-adrenoceptors do not contribute to noradrenaline-induced contractile responses.

Selective and sustained occupancy of prostatic alpha1-adrenoceptors by oral administration of KMD-3213 and its plasma concentration in rats

This study examined the ex-vivo occupancy by KMD-3213 of alpha1-adrenoceptors in the prostate and other tissues of rats in terms of tissue selectivity and duration of occupancy in relation to plasma concentration. Oral administration of KMD-3213 (0.2-20.2 micromol kg(-1), 0.5 h) dose-dependently decreased [3H]prazosin binding sites (Bmax) in the prostate (42-74%) and submaxillary gland (54-88%) compared with the control value. In contrast, there was only a slight change in the Bmax values in the spleen and cerebral cortex of KMD-3213-treated rats. The alpha1-adrenoceptor occupancy in the prostate and submaxillary gland was increased, with plasma free concentration of KMD-3213 at 0.5 h after oral administration of KMD-3213 (0.6-20.2 micromol kg(-1)). The receptor occupancy in these tissues was much greater than that in the spleen, heart or cerebral cortex. After oral administration of KMD-3213 (6.1 micromol kg(-1)), the alpha1-adrenoceptor occupancy in the prostate and submaxillary gland occurred rapidly, in parallel with the rise in the plasma concentration of the drug, and it lasted for at least 24 h, despite a remarkable decrease in the plasma concentration. It is concluded that KMD-3213 may produce fairly selective and sustained occupancy of alpha1-adrenoceptors in the prostate, a target organ for treatment of bladder outlet obstruction in patients with benign prostatic hyperplasia.

Adrenergic pharmacology: focus on the central nervous system

Norepinephrine and epinephrine are involved in the control of several important functions of the central nervous system (CNS), including sleep, arousal, mood, appetite, and autonomic outflow. Catecholamines control these functions through activation of a family of adrenergic receptors (ARs). The ARs are divided into three subfamilies (alpha1, alpha2, and beta) based on their pharmacologic properties, signaling mechanisms, and structure. ARs in the CNS are targets for several therapeutic agents used in the treatment of depression, obesity, hypertension, and other diseases. Not much is known, however, about the role of specific AR subtypes in the actions of these drugs. In this paper, we provide an overview of adrenergic pharmacology in the CNS, focusing on the pharmacologic properties of subtype-selective AR agonists and antagonists, the accessibility of these drugs to the CNS, and the distribution of ARs in different areas of the brain.

Functional characterization of alpha(1)-adrenoceptor subtypes in human skeletal muscle resistance arteries

alpha(1)-adrenoceptor subtypes in human skeletal muscle resistance arteries were characterized using agonists noradrenaline (non-selective) and A61603 (alpha(1A)-selective), the antagonists prazosin (non-selective), 5-methyl-urapidil (alpha(1A)-selective) and BMY7378 (alpha(1D)-selective) and the alkylating agent chloroethylclonidine (preferential for alpha(1B)). Small arteries were obtained from the non-ischaemic skeletal muscle of limbs amputated for critical limb ischaemia and isometric tension recorded using wire myography. Prazosin antagonized responses to noradrenaline with a pA(2) value of 9.18, consistent with the presence of alpha(1)-adrenoceptors, although the Schild slope (1.32) was significantly different from unity. 5-Methyl-urapidil competitively antagonized responses to noradrenaline with a pK(B) value of 8.48 and a Schild slope of 0.99, consistent with the presence of alpha(1A)-adrenoceptors. In the presence of 300 nM 5-methyl-urapidil, noradrenaline exhibited biphasic concentration response curves, indicating the presence of a minor population of a 5-methyl-urapidil-resistant subtype. Contractile responses to noradrenaline were not affected by 1 microM chloroethylclonidine suggesting the absence of alpha(1B)-adrenoceptors. Maximum responses to noradrenaline and A61603 were reduced to a similar extent by 10 microM chloroethylclonidine, suggesting an effect of chloroethylclonidine at alpha(1A)-adrenoceptors at the higher concentration. BMY7378 (10 and 100 nM) had no effect on responses to noradrenaline. BMY7378 (1 microM) poorly shifted the potency of noradrenaline giving a pA(2) of 6.52. These results rule out the presence of the alpha(1D)-subtype. These results show that contractile responses to noradrenaline in human skeletal muscle resistance arteries are predominantly mediated by the alpha(1A)-adrenoceptor subtype with a minor population of an unknown alpha(1)-adrenoceptor subtype.

The alpha(1A)-adrenoceptor subtype mediates contraction in rat femoral resistance arteries

In this study, alpha(1)-adrenoceptor subtypes were characterised in rat femoral resistance arteries mounted on a small vessel myograph. A-61603 was found to be more potent than noradrenaline and phenylephrine in these arteries. Brimonidine (UK 14304) could not evoke any contractile responses and the sensitivity to noradrenaline and phenylephrine was not affected by (8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13a-decahydro-3-methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]-naphthyridine (RS 79948), ruling out the presence of alpha(2)-adrenoceptors. Prazosin, 5-methyl-urapidil and 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB 4101) produced rightward shifts in the sensitivity to noradrenaline, giving pA(2) values of 9.6, 9.4 and 10.4, respectively, in agreement with the presence of alpha(1A)-adrenoceptors. (8-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378; 1 microM) produced a small shift in the sensitivity of noradrenaline giving a pK(B) of 7.2. In the presence of 300 nM 5-methyl-urapidil, sensitivity to noradrenaline was not further shifted by 1 microM BMY 7378. Responses to noradrenaline were unaffected by the alpha(1B)-adrenoceptor alkylating agent chloroethylclonidine (1 microM). These results suggest alpha(1A)-adrenoceptors mediate contractile responses to noradrenaline in rat femoral resistance arteries.

Affinity of serotonin receptor antagonists and agonists to recombinant and native alpha1-adrenoceptor subtypes

Binding affinities of serotonin (5-HT)-receptor antagonists and agonists at human recombinant alpha1-adrenoceptor subtypes (alpha1a-, alpha1b- and alpha1d-subtypes) were examined and compared with the functional affinities obtained in rat caudal artery (alpha1A-subtype), dog carotid artery (alpha1B-subtype) and rat thoracic aorta (alpha1D-subtype). Most of the 5-HT-receptor antagonists and agonists tested showed relatively high affinity to three alpha1-adrenoceptor subtypes. The highest affinity close to 1 nM was seen for NAN-190 (5-HT1A antagonist) in binding and functional studies. 5-Methylurapidil (5-HT1A agonist) and BMY7378 (5-HT1A agonist) showed, respectively, alpha1a(alpha1A)- or alpha1d(alpha1D)-subtype selectivity in both binding and functional affinities, but spiperone (5-HT2A antagonist) showed alpha1b-selectivity only in binding affinity. Functional affinity of ritanserin (5-HT2A antagonist) to the alpha1B-subtype was approximately 500-fold lower than that of affinity to the alpha1b-subtype. The present results show that many 5-HT-receptor antagonists and agonists have high affinity to alpha1-adrenoceptors, but suggest that there is deviation between their functional affinities and binding affinities for some drugs.

Use of antisense oligonucleotides to verify the role of the alpha(1A)-adrenergic receptor in the contractility of the rat uterus post partum

The adrenergic system plays a major role in the regulation of the contractility of the uterus during pregnancy. This study investigated the role of the alpha(1A)-adrenergic receptor (AR) in this regulation. The use of partial phosphorothioate antisense oligodeoxynucleotides (AONs) permitted the sequence-selective inhibition of AR gene expression. AONs were injected together with a cationic liposomal carrier agent into the post partum rat uterus. Incubation for 12 or 24 h with the most effective AON (480-AON) caused a 58.7 or 53.0% inhibition, respectively, of the expression of the alpha(1A)-AR density, whereas incubation for 36 or 48 h resulted in only a 38.8 or 26.7% inhibition, respectively. The decrease of the alpha(1A)-AR density by 480-AON was demonstrated by Western blot analysis and a radioreceptor binding assay on rat uterus preparations 24 h after delivery. The changes in the contractility of the uterus after AON treatment were measured on isolated rat uterine tissue by electric field stimulation. The significant decrease in the ability of the uterus to contract was indicated by the area under the curve method. The electric field studies revealed that the specific alpha(1A)-blockers 5-methylurapidil and WB 4101 inhibited the rhythmic contraction by about 74 and 70% in the control uteri and by 25 and 20% in 480-AON-treated uteri, respectively. The curves for the beta-mimetic (terbutaline) and alpha(1D)-antagonist (BMY7370) inhibitors were unchanged after 480-AON treatment of the uteri. These results suggest the importance of the alpha(1A)-AR in the tocolytic effect exerted by the alpha(1)-antagonist, although high concentrations of antagonists can not exclude the role of alpha(1D)-ARs, too. Additionally, these prove that the knockdown transformation by AONs offers a useful animal model for the investigation of receptors controlling the function of uterine tissue.

Failure of AH11110A to functionally discriminate between alpha(1)-adrenoceptor subtypes A, B and D or between alpha(1)- and alpha(2)-adrenoceptors

The potency of the putatively alpha(1B)-adrenoceptor selective drug, 1-[biphenyl-2-yloxy]-4-imino-4-piperidin-1-yl-butan-2-ol (AH11110A), to antagonize contraction upon stimulation of alpha(1A)-adrenoceptors in rat vas deferens and rat perfused kidney, alpha(1B)-adrenoceptors in guinea-pig spleen, mouse spleen and rabbit aorta, and alpha(1D)-adrenoceptors in rat aorta and pulmonary artery was evaluated and compared to that of a number of subtype-discriminating antagonists. N-[3-[4-(2-Methoxyphenyl)-1-piperazinyl]propyl]-3-methyl-4-oxo-2-phenyl-4H-1-benzopyran-8-carboxamide (Rec 15/2739) and (+/-)-1,3,5-trimethyl-6-[[3-[4-((2,3-dihydro-2-hydroxymethyl)-1,4-benzodioxin-5-yl)-1-piperazinyl]propyl]amino]-2,4(1H,3H)-pyrimidinedione (B8805-033) were confirmed as selective for alpha(1A)-adrenoceptors, 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378), 8-[2-(1,4-benzodioxan-2-ylmethylamino)ethyl]-8-azaspiro[4.5]decane-7,9-dione (MDL 73005EF), and cystazosin were found to be selective for alpha(1D)-adrenoceptors, whereas spiperone was weakly selective for alpha(1B)-over alpha(1A)-adrenoceptors. However, from the functional affinity profile obtained for AH11110A at alpha(1A)-adrenoceptors (pA(2)=6.41 in rat vas deferens), alpha(1B)-adrenoceptors (pA(2)=5.40-6.54) and alpha(1D)-adrenoceptors (pA(2)=5.47-5.48), the affinity and presumed selectivity previously obtained for AH11110A in radioligand binding studies at native alpha(1B)- and cloned alpha(1b)-adrenoceptors (pK(i)=7.10-7.73) could not be confirmed. Additionally, AH11110A enhanced the general contractility of rat vas deferens, produced a bell-shaped dose-response curve of vasodilation in perfused rat kidney, and its antagonism in most other tissues was not simply competitive. The affinity of AH11110A for prejunctional alpha(2)-adrenoceptors in rabbit vas deferens (pA(2)=5.44) was not much lower than that displayed for alpha(1)-adrenoceptor subtypes, revealing that AH11110A, besides alpha(1)-adrenoceptors, also interacts with alpha(2)-adrenoceptors, and thus may be unsuitable for alpha-adrenoceptor subtype characterization, at least in smooth muscle containing functional studies.

Facilitation of transmitter release in the urinary bladders of neonatal and adult rats via alpha1-adrenoceptors

Age-dependent changes in the effects of the alpha1-adrenoceptor agonist, phenylephrine were investigated on neurally evoked contractile responses and basal tone in smooth muscle strips from rat urinary bladder. Phenylephrine facilitated the neurogenic contractions in both neonatal and 7-month-old adult rats. However, phenylephrine increased the basal tone in adult but not neonatal rats. In adult rats, phenylephrine-induced facilitation of neurally evoked contractions occurred before and after the block of cholinergic contractions with 1 microM atropine. In adult rats, the phenylephrine facilitation was reduced at stimulation parameters (20 Hz, 80 shocks and maximal voltage) which activated muscarinic receptor mediated facilitation of acetylcholine release. The results indicate that pre-synaptic alpha1-adrenoceptors facilitate the release of both acetylcholine and the non-cholinergic non-adrenergic transmitter. In summary, alpha1-adrenoceptor-mediated facilitation is less expressed when muscarinic M1 receptor mediated facilitation is functioning; pre-junctional alpha1-adrenoceptors are present in the bladder of both neonatal and adult rats, whereas post-junctional alpha1-adrenoceptors are expressed only in older adult rats.

Role of protein kinase C in mediating alpha-1-adrenoceptor-induced negative inotropic response in rat ventricles

The aim of this study was to determine the effect of protein kinase C (PKC) activation on intracellular Ca(2+) transient and its relation to alpha(1)-adrenoceptor (alpha(1)-AR)-stimulated negative inotropic response in rat ventricles. The electromechanical responses to phenylephrine (PE) in rat ventricular muscles were concomitantly examined using the conventional microelectrode method. The responses of intracellular Ca(2+) transient and cell contractions to PE in the absence of certain pharmacological interventions were ascertained in fura-2-loaded myocytes. The influence of PE on L-type Ca(2+) current (I(Ca,L)) was also examined using a voltage clamp in a whole-cell configuration. PE did not alter the action potential parameters during the negative inotropic phase. The negative inotropic effect (NIE) was inhibited by prazosin, chloroethylclonidine (CEC) and staurosporine, but was insensitive to pertussis toxin. Desensitization of PKC after prolonged pretreatment of rat ventricles with PDBu also abolished the NIE of PE. Caffeine modulated the NIE, but thapsigargin did not. The evoked intracellular Ca(2+) transient and cell contraction were initially decreased by PE, while I(Ca,L) was not altered. Prazosin and staurosporine significantly inhibited the responses. Our data indicated that alpha(1)AR-mediated NIE in rat ventricular muscles was due to the decrease of intracellular Ca(2+) transients by the modulation of PKC on Ca(2+)-releasing channels signaling through a CEC-sensitive alpha(1)AR subtype.

Smooth muscle and parasympathetic nerve terminals in the rat urinary bladder have different subtypes of alpha(1) adrenoceptors

Neurally evoked contractions and release of (3)H- acetylcholine (ACh) during electrical field stimulation were measured in rat urinary bladder strips. The alpha(1) agonist phenylephrine (PE, 2-8 microM) increased the amplitude of neurally evoked contractions, facilitated the release of ACh and increased the baseline tone of the bladder strips. The PE-induced facilitation of the contractions did not significantly change during a prolonged exposure to PE (120 min), whereas the PE-induced rise in baseline tone gradually decreased to 65% of the initial value. Low concentrations of specific alpha(1A) antagonists, 5-methyl urapidil (5-MU), REC15/2739 and WB-4101 competitively inhibited the facilitation of the neurally-evoked contractions (pA(2:) 8.77; 9.59 and 9.62, respectively), whereas higher concentrations of 5-MU (IC(50): 48 nM) were required to suppress the PE-rise in baseline. WB-4101 (100 microM) inhibited the PE-induced facilitation of ACh release. The irreversible alpha(1B) antagonist chloroethyl-clonidine (CEC, 10-50 microM) inhibited the PE-evoked rise in base line tone, but did not affect the PE-induced facilitation of the neurally evoked contractions nor the facilitation of ACh release. However, CEC increased the area and amplitude of the neurally-evoked contractions by 261+/-33 and 47.2+/-8.4%, respectively. Atropine significantly inhibited the CEC evoked increase in area and amplitude of the electrically evoked contractions (76.5+/-4.8 and 40.8+/-3%, respectively) indicating that CEC facilitated the cholinergic responses of the electrically stimulated bladder strips. It is concluded that alpha(1A) and CEC sensitive alpha(1B) and/or alpha(1D) adrenoceptors are expressed in the rat bladder in different locations. On the cholinergic nerve terminals alpha(1A) adrenoceptors mediate prejunctional facilitation, whereas postjunctional alpha(1B)/alpha(1D) adrenoceptors mediate smooth muscle contraction.

Both alpha(1A)- and alpha(1B)-adrenergic receptor subtypes couple to the transient outward current (I(To)) in rat ventricular myocytes

1. Regulation of transient outward current (I(To)) by alpha(1)-adrenergic (alpha(1)AR) plays a key role in cardiac repolarization. alpha(1)ARs comprise a heterogeneous family; two natively expressed subtypes (alpha(1A) and alpha(1B)) and three cloned subtypes (alpha(1a), alpha(1b) and alpha(1d)) can be distinguished. We have examined the electrophysiological role of each alpha(1)AR subtype in regulating I(To) in isolated rat ventricular myocytes. 2. Reverse transcription-PCR study revealed the presence of three subtype mRNAs (alpha(1a), alpha(1b) and alpha(1d)) in rat myocytes. 3. Radioligand binding assay using [(125)I]-HEAT showed that the inhibition curves for alpha(1A)AR-selective antagonists (WB4101, 5-methylurapidil, (+)-niguldipine and KMD-3213) in rat ventricles best fit a two-site model, with 30% high and 70% low affinity binding sites. The high affinity sites were resistant to 100 microM chloroethylclonidine (CEC), while the low affinity sites were highly inactivated by CEC. 4. Whole cell voltage clamp study revealed that methoxamine reduced a 4-aminopyridine(4-AP)-sensitive component of I(To) in the isolated rat ventricle myocytes. Lower concentrations of KMD-3213 (1 nM) or 5-MU (10 nM) did not affect the methoxamine-induced reduction of I(To). On the other hand, CEC treatment (100 microM) of isolated myocytes reduced the methoxamine-induced reduction of I(To) by 46%, and the remaining response was abolished by lower concentrations of KMD-3213 or 5-MU. 5. The results indicate that rat ventricular myocytes express transcripts of the three alpha(1)AR subtypes (alpha(1a), alpha(1b) and alpha(1d)); however, two pharmacologically distinct alpha(1)AR subtypes (alpha(1A) and alpha(1B)) are predominating in receptor populations, with approximately 30% alpha(1A)AR and 70% alpha(1B)AR. Although both alpha(1A) and alpha(1B)AR subtypes are coupled to the cardiac I(To), alpha(1B)ARs predominantly mediate alpha(1)AR-induced effect.

Vascular alpha 1D-adrenoceptors: are they related to hypertension?

Heterogeneity of vascular alpha 1-adrenoceptor subtypes has been revealed by pharmacological and molecular biology studies (i.e., alpha 1A-, alpha 1B-, and alpha 1D-adrenoceptors). The alpha 1D-adrenoceptor subtype is predominantly involved in the contraction of a variety of vessels and its role in the control of blood pressure has been suggested, a phenomenon probably related to aging. Recent advances in the use of young pre-hypertensive rats and adult spontaneously hypertensive rats with one kidney and Grollman-type renal hypertension suggest vascular alpha 1D-adrenoceptor involvement in the increased blood pressure. The possible role of alpha 1D-adrenoceptors in the genesis/maintenance of hypertension is discussed in this review.

Buspirone functionally discriminates tissues endowed with alpha1-adrenoceptor subtypes A, B, D and L

The affinity for functional alpha1-adrenoceptor subtypes of buspirone in comparison with its close structural analogs and selective alpha1D-adrenoceptor antagonists, BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]dec ane-7,9-dione) and MDL 73005EF (8-[2-(1,4-benzodioxan-2-ylmethylamino)ethyl]-8-azaspiro+ ++[4.5]decane-7,9-dione), was determined, namely at subtype A in rat vas deferens and perfused kidney, at subtype B in guinea-pig and mouse spleen, at subtype L in rabbit spleen, and at subtype D in rat aorta and pulmonary artery against noradrenaline-evoked contractions. BMY 7378 and MDL 73005EF were confirmed as 30- and 20-fold selective antagonists, respectively, for alpha1D- over both alpha1A- and alpha1B-adrenoceptors. Buspirone was a weak antagonist without intrinsic activity at alpha1A-adrenoceptors in rat vas deferens (pA2 = 6.12), at alpha1B-adrenoceptors in guinea-pig and mouse spleen (pA2 = 5.54 and 5.59) and at alpha1L-adrenoceptors in rabbit spleen (pA2 = 4.99), but caused partial vasoconstriction in rat kidney that was attenuable by the subtype D-selective adrenoceptor antagonist BMY 7378, but hardly by the subtype A-selective adrenoceptor antagonist B8805-033 ((+/-)-1,3,5-trimethyl-6-[[3-[4-((2,3-dihydro-2-hydroxymethyl)-1,4-be nzodioxin-5-yl)-1-piperazinyl]propyl]amino]-2,4(1H,3H)-pyrimidinedion e), confirming the additional presence of alpha1D-adrenoceptors mediating rat renal vasoconstriction. Buspirone behaved as a partial agonist at alpha1D-adrenoceptors in rat aorta (pD2 = 6.77, intrinsic activity (i.a.)= 0.40) and pulmonary artery (pD2 = 7.16, i.a. = 0.59). With buspirone as agonist in these tissues, the pA2 values of subtype-discriminating antagonists were consistent with their alpha1D-adrenoceptor affinity determined in rat aorta against noradrenaline and with published binding data on cloned alpha1d-adrenoceptors. The results provide pharmacological evidence that (1) in functional preparations for the A subtype, like rat vas deferens and perfused kidney, for the B subtype, like guinea-pig and mouse spleen, and for the L subtype, like rabbit spleen, buspirone is a weak antagonist without intrinsic activity, but (2) behaves as a partial agonist in rat aorta and pulmonary artery as models for the D subtype and (3) detects an additional vasoconstrictor alpha1D-adrenoceptor in rat kidney. Buspirone, like its close analogs BMY 7378 and MDL 73005EF, thus might also be a useful tool for functionally discriminating alpha1D- from alpha1A-, alpha1B- and alpha1L-adrenoceptors in various tissues.

Alpha1-adrenoceptor subtypes

Alpha1-adrenoceptors are one of three subfamilies of receptors (alpha1, alpha2, beta) mediating responses to adrenaline and noradrenaline. Three alpha1-adrenoceptor subtypes are known (alpha1A, alpha1B, alpha1D) which are all members of the G protein coupled receptor family, and splice variants have been reported in the C-terminus of the alpha1A. They are expressed in many tissues, particularly smooth muscle where they mediate contraction. Certain subtype-selective agonists and antagonists are now available, and alpha1A-adrenoceptor selective antagonists are used to treat benign prostatic hypertrophy. All subtypes activate phospholipase C through the G(q/11) family of G proteins, release stored Ca2+, and activate protein kinase C, although with significant differences in coupling efficiency (alpha1A > alpha1B > alpha1D). Other second messenger pathways are also activated by these receptors, including Ca2+ influx, arachidonic acid release, and phospholipase D. Alpha1-adrenoceptors also activate mitogen-activated protein kinase pathways in many cells, and some of these responses are independent of Ca2+ and protein kinase C but involve small G proteins and tyrosine kinases. Direct interactions of alpha1-adrenoceptors with proteins other than G proteins have not yet been reported, however there is a consensus binding motif for the immediate early gene Homer in the C-terminal tail of the alpha1D subtype. Current research is focused on discovering new subtype-selective drugs, identifying non-traditional signaling pathways activated by these receptors, clarifying how multiple signals are integrated, and identifying proteins interacting directly with the receptors to influence their functions.

Investigation of the effects of some alkaloidal alpha1-adrenoceptor antagonists on human hyperplastic prostate

The effects of N-allylsecoboldine, (-)-discretamine, ( )-govadine and [(+/-)-2,3,10,11-tetrahydroxytetrahydroproto-berberine HBr] ((+/-)-THP) on contractile responses were investigated in human hyperplastic prostate. They all inhibited, concentration dependently, the tension responses to phenylephrine and electrical field stimulation, and the pA2 and pIC50 values were calculated. The relative potencies of these four agents with reference to prazosin were obtained. The results showed that N-allylsecoboldine exhibited greater potency (4.1-fold), whereas (-)-discretamine, (+/-)-govadine and (+/-)-THP had similar potencies, against contractions elicited by electrical field stimulation and against contractions elicited by phenylephrine in human hyperplastic prostate. In addition, the potency ratios of N-allylsecoboldine, (-)-discretamine, (+/-)-govadine and (+/-)-THP against phenylephrine-induced contractions in rat vas deferens/spleen were 7.78, 0.89, 0.57, and 0.96, respectively. In the presence of prazosin (0.3 +/-M) to block alpha1-adrenoceptor-mediated responses, nifedipine (10 microM), but not the above four agents, significantly blocked KCl (60 mM)-induced tension responses in human hyperplastic prostate. It is suggested that N-allylsecoboldine exhibits greater potency against nerve-mediated contraction than against phenylephrine-induced contraction in human hyperplastic prostate and that this antagonistic effect is due mainly to its high affinity for the alpha1A-adrenoceptor subtype.

Halogenated derivatives of boldine with high selectivity for alpha1A-adrenoceptors in rat cerebral cortex

The selectivity of 3-nitrosoboldine and different halogenated derivatives of boldine (3-bromoboldine, 3,8-dibromoboldine and 3-chloroboldine) for alpha1-adrenoceptor subtypes was studied by examining [3H]-prazosin competition binding in rat cerebral cortex. In the competition experiments [3H]-prazosin binding was inhibited completely by all the compounds tested. The inhibition curves displayed shallow slopes which could be subdivided into high and low affinity components. The relative order of affinity and selectivity for alpha1A-adrenoceptors was 3-bromoboldine = 3,8-dibromoboldine = 3-chloroboldine > boldine > 3-nitrosoboldine. The competition curves for 3-bromoboldine remained shallow and biphasic following chloroethylclonidine treatment. Whereas the relative contribution of the high affinity sites increased, the 3-bromoboldine affinities at its high and low affinity sites remained similar to those obtained in untreated membranes. 3-Bromoboldine, 3,8-dibromoboldine, 3-chloroboldine and 3-nitrosoboldine did not significantly displace [3H]-(+)-cis-diltiazem binding to rat cerebral cortex membranes. This activity was lower than that shown by boldine. Compared to boldine, halogen (bromine or chlorine) substitution at position 3 increases the alpha1A-adrenoceptor subtype selectivity and decreases the affinity for the benzothiazepine binding site at the calcium channel. Further halogen substitution at position 8 did not significantly improve this activity with respect to 3-bromoboldine. In contrast, the NO substitution at position 3 of boldine (3-nitrosoboldine) gives a loss of affinity and selectivity for alpha1-adrenoceptor subtypes.