Molecular biology of alpha-adrenergic receptors: implications for receptor classification and for structure-function relationships

Infarct size-reducing effect of ischemic preconditioning is related to alpha1b-adrenoceptors but not to alpha1a-adrenoceptors in rabbits

In rabbits and rats, both stimulation of alpha-adrenoceptors and ischemic preconditioning (PC) reduce infarct size. Activation of alpha1b-adrenoceptors play an important role in the PC effect on ventricular function in rats. However, the alpha1-adrenoceptors have not been reported to be related to the PC effect in rabbits, because the infarct size-reducing effect of PC is not blocked by the nonselective alpha-adrenoceptor antagonist, phenoxybenzamine (POB) or by the alpha1-adrenoceptor antagonist, BE2254. However, we speculated that alpha1b-adrenoceptors but not alpha1a-adrenoceptors may be related to the infarct size-reducing effect of PC in rabbit hearts. Thus we examined in rabbits whether the alpha1b-adrenoceptor blocker chloroethylclonidine (CEC), the alpha1a-adrenoceptor blocker 5-methylurapidil (5-MU), the selective alpha1-adrenoceptor antagonist bunazosin (BN), and the nonselective apha-adrenoceptor antagonist phenoxybenzamine (POB) can block the PC effect on infarct size. Eighty-eight anesthetized open-chest Japanese white male rabbits were subjected to 30-min coronary occlusion and 48-h reperfusion. In five PC groups, the rabbits were subjected to a single 5-min occlusion and 5-min reperfusion before 30-min sustained ischemia. In the PC groups, those with CEC (3 mg/kg, n = 10), 5-MU (3 mg/kg, n = 10), BN (0.3 mg/kg, n = 10), POB (4 mg/kg, n = 10), or placebo saline (n = 10) were pretreated before PC. In the non-PC groups, those with CEC (3 mg/kg, n = 7), 5-MU (3 mg/kg, n = 7), BN (0.3 mg/kg, n = 7), POB (4 mg/kg, n = 7), or placebo saline (n = 10) were pretreated before 30-min sustained ischemia. After a 48-h reperfusion, the infarct size was measured histologically and expressed as a percentage of the area at risk. PC caused a marked reduction of infarct size as compared with the non-PC control (10 +/- 3% vs. 42 +/- 2%; p < 0.05). The PC effect was completely blocked by CEC (36 +/- 2%) and by BN (42 +/- 4%) but not by 5-MU (14 +/- 1%) or POB (13 +/- 2%). None of the drugs by itself affected the infarct size. Stimulation of alpha1b-adrenoceptors but not of alpha1a-adrenoceptors during PC plays an important role in the PC effect on infarct size. This may explain the previous confusion concerning the PC blocking effect of various alpha1-blockers.

Molecular approaches to receptors as targets for drug discovery

The cloning of a great number of receptors and channels has revealed that many of these targets for drug discovery can be grouped into superfamilies based on sequence and structural similarities. This review presents an overview of how molecular biological approaches have revealed a plethora of receptor subtypes, led to new definitions of subtypes and isoforms, and played a role in the development of high selective drugs. Moreover, the diversity of subtypes has molded current views of the structure and function of receptor families. Practical difficulties and limitations inherent in the characterization of the ligand binding and signaling properties of expressed recombinant receptors are discussed. The importance of evaluating drug-receptor interactions that differ with temporally transient and distinct receptor conformational states is emphasized. Structural motifs and signal transduction features are presented for the following major receptor superfamilies: ligand-gated ion channel, voltage-dependent ion channel, G-protein coupled, receptor tyrosine-kinase, receptor protein tyrosine-phosphatase, cytokine and nuclear hormone. In addition, a prototypic receptor is analyzed to illustrate functional properties of a given family. The review concludes with a discussion of future directions in receptor research that will impact drug discovery, with a specific focus on orphan receptors as targets for drug discovery. Methods for classifying orphan receptors based upon homologies with members of existing superfamilies are presented together with molecular approaches to the greater challenge of defining their physiological roles. Besides revealing new orphan receptors, the human genome sequencing project will result in the identification of an abundance of novel receptors that will be molecular targets for the development of highly selective drugs. These findings will spur the discovery and development of an exciting new generation of receptor-subtype specific drugs with enhanced therapeutic specificity.

Familial studies of heritability of alpha1-adrenergic receptor responsiveness in superficial veins

BACKGROUND

Studies of monozygotic and dizygotic twins indicate an important genetic influence on the variability of responsiveness to norepinephrine in superficial human vein.

OBJECTIVES

Genetic aspects of variability of alpha1-adrenergic receptor responsiveness to norepinephrine in superficial veins were further investigated by studying the response to norepinephrine in the dorsal hand veins of parents and their children.

METHODS

Subjects were healthy nonsmoking adults (n = 24; age range, 40 to 52 years) and their biological children (n = 20; age range, 15 to 26 years) who were free from medications likely to modify vascular tone. Superficial vein responsiveness to norepinephrine was assessed by the linear variable differential transformer technique. The dose of norepinephrine required to constrict superficial vein diameter by 50% from baseline (ED50) was calculated for each subject. Heritability was estimated by standard techniques of regression of mid-parent/child (natural logarithm) ED50 values.

RESULTS

ED50 ranged from 5.6 to 254.6 ng/min in the parents and from 7.8 to 242.3 ng/min in the children. Heritability was calculated at 0.88.

CONCLUSIONS

These data confirm wide variability in superficial vein responsiveness to norepinephrine. The results confirm a major genetic influence in biological responsiveness of superficial vein to norepinephrine in healthy humans. Heritability of vascular alpha-adrenergic receptor responsiveness may influence vascular regulation during sympathetic stimulation and blockade.

Effects of niflumic acid on alpha1-adrenoceptor-induced vasoconstriction in mesenteric artery in vitro and in vivo in two-kidney one-clip hypertensive rats

The influence of niflumic acid (3 and 10 microM), a Cl- channel antagonist, on cirazoline-induced vasoconstriction in isolated perfused mesenteric artery (5 ml/min) from two-kidney one-clip (2K1C) hypertensive and sham normotensive rats was examined. In addition, the effect of a single i.v. bolus injection of niflumic acid (3 mg/kg) on cirazoline-mediated reduction in vascular conductance in superior mesenteric artery was determined in pentobarbital-anaesthetized hypertensive and normotensive rats. Bolus injections of cirazoline induced a dose-dependent transient increase in the perfusion pressure in vitro. In the presence of niflumic acid, cirazoline-mediated vasoconstriction was significantly inhibited. Cirazoline-induced vasoconstriction in isolated mesenteric beds was also significantly inhibited following perfusion with Cl(-)-free buffer. Pre-perfusion of mesenteric blood vessels with Cl(-)-free buffer resulted in a significantly greater inhibition of cirazoline-mediated vasoconstriction in sham normotensive rats than in hypertensive rats. We found that in Cl(-)-free buffer, cirazoline-mediated vasoconstriction could be further inhibited by niflumic acid. Intravenous infusion of cumulative doses of cirazoline in vivo caused a dose-dependent decrease in superior mesenteric vascular conductance. Pretreatment with niflumic acid significantly impaired cirazoline-mediated decreases in vascular conductance. Our results indicate that chloride ions play an important role in alpha1-adrenoceptor-mediated vasoconstriction in mesenteric blood vessels. In addition, the contribution of chloride ions in alpha1-adrenoceptor-mediated vasoconstriction in blood vessels from hypertensive rats appears to be reduced.

Subtype specific recognition of human alpha2C2 adrenergic receptor using monoclonal antibodies against the third intracellular loop

Monoclonal antibodies (Mabs) against human alpha2C2-adrenergic receptor (alpha2C2-AR) were raised in mice and characterized. Bacterially expressed fusion protein consisting a sequence from the putative third intracellular loop (amino acids 213-343) of human alpha2C2 and glutathione-S-transferase (GST) was used as antigen. Results from mass spectrometry of purified thrombin cleaved alpha2C2 polypeptide suggested that the epitope region would lie near the aminoterminal end of the 3rd intracellular loop of human alpha2C2-AR. Elevation of Mabs was detected with Western blotting from mouse blood samples. Three alpha2C2 specific cell clones were expanded to in vitro production in hollow fiber systems. The specificity of the Mabs was further determined by immunoprecipitation and immunocytochemistry. Scatchard analysis of thrombin digested, purified, Europium-labelled antigen (amino acids 213-343 of alpha2C2) revealed binding affinity constants of 0.4 x 10(9), 0.7 x 10(9) and 1.6 x 10(9) M(-1) and Kds of 2.6, 1.4 and 0.6 nM for the three Mabs 2B1, 3G3 and 7G1, respectively.

Cloning, functional expression and tissue distribution of rabbit alpha 1d-adrenoceptor

We have cloned a cDNA encoding rabbit alpha 1d-adrenoceptor from the rabbit liver cDNA library. The deduced amino-acid sequence of this clone encodes a protein of 576 amino acids that shows strong sequence homology to previously cloned human, rat and mouse alpha 1d-adrenoceptors. The pharmacological radioligand binding properties of this clone expressed in COS-7 cells were similar to those of rat alpha 1d-adrenoceptors. Competitive RT/PCR assays revealed wide tissue distribution of the alpha 1d-adrenoceptor mRNA in rabbit, especially abundant in vas deferens, aorta, prostate and cerebral cortex.

Phosphorylation of the third intracellular loop of the mouse alpha1b-adrenergic receptor by cAMP-dependent protein kinase

The third intracellular loop of adrenergic receptors has been implicated in their interaction with guanine nucleotide-binding proteins (G proteins). One of the mechanisms involved in the modulation of receptor function is the phosphorylation of specific residues by intracellular kinases. alpha1b-Adrenergic receptor is phosphorylated in vitro by cAMP-dependent protein kinase (PKA), although its physiological effect remains to be determined. We have produced fusion proteins formed by glutathione S-transferase and sequences of the third intracellular loop of mouse alpha1a-, alpha1b-, and alpha1d-adrenergic receptor subtypes, and used them as substrates for PKA. Only the fusion protein containing the alpha1b sequence was phosphorylated in vitro by this kinase. Site-directed mutagenesis of a serine (homologue to serine 278 of the rat sequence, RSS) to an alanine residue precluded phosphorylation by PKA.

Cloning, functional expression and tissue distribution of rabbit alpha1a-adrenoceptor

A cDNA clone, which has an open reading frame of 1398 nucleotides encoding a 466-amino-acid peptide, has been isolated from rabbit liver cDNA library. Compared with the peptide sequence, it shows high homology to alpha1a adrenoceptors of human, bovine and rat. We expressed this clone in COS-7 and investigated the pharmacological properties, revealing similarity to those of human alpha1a adrenoceptors. Competitive RT/PCR has detected the mRNA in variety of rabbit tissues, especially abundantly in liver, vas deferens, brain, and aorta, but not in heart.

Interaction of 5-methyl-urapidil with alpha 1-adrenoceptors in canine blood vessels: impact of pacing-induced heart failure

This study examines whether the alpha 1-adrenoceptors in canine endothelium-denuded dorsal pedal artery and endothelium-denuded saphenous vein can be differentiated by 5-methyl-urapidil before (non-paced) and at end-stage heart failure induced by rapid ventricular pacing. Noradrenaline and phenylephrine produced concentration-dependent contractions of the dorsal pedal artery and the saphenous vein which were enhanced at end-stage heart failure. In non-paced animals, 5-methyl-urapidil was shown to be insurmountable against noradrenaline with the artery being more sensitive compared to the vein. At end-stage heart failure, 5-methyl-urapidil was a competitive antagonist against noradrenaline in both the artery and the vein with pA2 values of 8.1 (7.9-8.4) and 8.6 (8.2-9.1), respectively. A different antagonist profile was seen against phenylephrine. Similar to noradrenaline, insurmountable antagonism was observed in the artery and the vein before the development of heart failure. In contrast to noradrenaline, at end-stage heart failure, no antagonism was seen with the concentrations of 5-methyl-urapidil tested against phenylephrine. These results suggest that the mechanisms mediating contractions in the dorsal pedal artery and saphenous vein to noradrenaline and phenylephrine are heterogeneous and dependent on the heart failure state.

Distribution of alpha 2-adrenoceptors in the human locus coeruleus

Recent reports of specific topographic patterns of cell loss in the locus coeruleus (LC) in psychiatric and neurologic disorders underscores the need for detailed neurochemical analyses of this cell group. In this study, the anatomical distribution of alpha 2-adrenoceptors and its relationship to the distribution of noradrenergic neurons in the human LC was studied. Quantitative autoradiography was used to assess the binding of [125I]p-iodoclonidine ([125I]PIC) to alpha 2-adrenoceptors coordinately with counts of neuromelanin-containing cells in tissue sections cryocut at 10-13 levels along the rostrocaudal axis of the LC. Pontine brain tissue was obtained postmortem from 7 subjects dying of natural or accidental causes, ranging in age from 26 to 78 years. Both the binding of [125I]PIC and number of neuromelanin-containing cells were differentially distributed along the LC axis (P < 0.01) with almost identical topographical patterns. The highest concentration of binding and the greatest number of neuromelanin-containing cells per section occurred near the middle portion of the nucleus. There was a significant correlation between the number of neuromelanin-containing cells per section and the specific binding of [125I]PIC at any particular level of the LC (r2 = 0.56; P < 0.0001). The highest and lowest amounts of [125I]PIC binding in the LC were observed in the youngest and oldest subjects, respectively, and this trend was parallelled by a significant negative correlation between the number of neuromelanin-containing cells at a given level and age (r2 = 0.85; P < 0.003). The uneven distribution of alpha 2-adrenoceptors in the LC demonstrates the importance of anatomical specificity when performing quantitative studies of LC protein chemistry in psychiatric and neurologic disorders.

(+/-)-tamsulosin, an alpha 1A-adrenoceptor antagonist, inhibits the positive inotropic effect but not the accumulation of inositol phosphates in rabbit heart

The influence of (+/-)-tamsulosin, a selective alpha 1A-adrenoceptor antagonist, on the positive inotropic effect and the accumulation of inositol phosphates that are induced via alpha 1-adrenoceptors was studied in comparison with that of another alpha 1A-adrenoceptor ligand oxymetazoline in the rabbit ventricular myocardium. Phenylephrine elicited a concentration-dependent positive inotropic effect via alpha 1-adrenoceptors in the presence of either (+/-)-bupranolol or S(-)-timolol. The mode of antagonism induced by (+/-)-tamsulosin on the effect of phenylephrine was dependent or the concentration applied: (+/-)-tamsulosin at 1 and 3 nM acted in a competitive manner, the slope of the regression line of the Schild plot being unity and the pA2 value being 9.12; at 10 nM, it shifted further the concentration-response curve to the right without affecting the maximal response but the slope became less than unity. At 100 nM and higher, it suppressed the maximal response to phenylephrine. (+/-)-Tamsulosin effectively antagonized the positive inotropic effect of phenylephrine even after inactivation of alpha 1B-adrenoceptors by treatment with chlorethylclonidine, which is an indication that the (+/-)-tamsulosin-sensitive subtype belongs to a class resistant to chlorethylclonidine. (+/-)-Tamsulosin, over the range of concentrations at which it antagonized the positive inotropic effect mediated by alpha 1-adrenoceptors, did not affect the accumulation of [3H]inositol phosphates that was induced by 10 microM phenylephrine. Oxymetazoline antagonized the positive inotropic effect of phenylephrine in a competitive manner without affecting the accumulation of inositol monophosphate induced by phenylephrine. These results indicate that the positive inotropic effect, mediated via (+/-)-tamsulosin- and oxymetazoline-sensitive subtype of alpha 1-adrenoceptors, is exerted by a subcellular mechanism that is independent of the accumulation of inositol phosphates.

Adrenoceptor- and cholinoceptor-mediated mechanisms in the regulation of 5-hydroxytryptamine release from isolated tracheae of newborn rabbits

1. Isolated tracheae of newborn rabbits were incubated in vitro and the outflow of 5-hydroxytryptamine (5-HT) was determined by h.p.l.c. with electrochemical detection. Evidence has previously been provided that this 5-HT outflow derives from neuroendocrine epithelial (NEE) cells of the airway mucosa. 2. Phenylephrine (1, 10 and 30 microM) enhanced the outflow of 5-HT by 80, 290 and 205%, respectively. 5-HT outflow evoked by 10 microM phenylephrine was not affected by the presence of the neurotoxin tetrodotoxin (1 microM). 3. Rauwolscine, ARC 239 (an alpha(2B)-adrenoceptor preferring antagonist), yohimbine and prazosin antagonized the effect of 10 microM phenylephrine in a concentration-dependent manner with IC50 values of 150, 295, 300 and 1,700 nM, respectively. Comparison of the ratios (between all antagonists) of the present IC50 values with the corresponding ratios of Ki values obtained in binding studies for the alpha(2A)-, alpha(2B)-, alpha(2C)- and alpha(2D)-adrenoceptor subtypes strongly suggests the involvement of an alpha(2B)-receptor. 4. 5-HT outflow evoked by 10 microM phenylephrine was inhibited by 65% in the presence of 1 microM forskolin and abolished in the presence of 10 microM forskolin. 5. 5-HT outflow evoked by 10 microM phenylephrine was inhibited by about 45 and 70% in the presence of 0.1 and 1 microM isoprenaline, respectively. The inhibitory effect of 1 microM isoprenaline was only marginally antagonized by 1 microM, but blocked by 10 microM propranolol. 6. 5-HT outflow was not affected by the muscarine receptor agonist oxotremorine (10 microM), but was enhanced by 175% by 100 microM nicotine. The effect of nicotine was blocked by 100 microM hexamethonium and prevented by 1 microM tetrodotoxin or 1 microM yohimbine. 7. In conclusion, 5-HT release from NEE cells of the rabbit trachea is stimulated via alpha-adrenoceptors most likely of the alpha(2B)-subtype localized directly at the NEE cells. Activation of beta-adrenoceptors as well as direct activation of adenylyl cyclase by forskolin exert inhibitory effects on 5-HT release. Activation of nicotinic, but not of muscarinic receptors, also evokes the release of 5-HT. However, the effect of nicotine appears to be mediated indirectly via the release of noradrenaline.

Distribution of alpha 2C-adrenergic receptor-like immunoreactivity in the rat central nervous system

The distribution of alpha 2C-adrenergic receptors (ARs) in rat brain and spinal cord was examined immunohistochemically by using an affinity purified polyclonal antibody. The antibody was directed against a recombinant fusion protein consisting of a 70-amino-acid polypeptide portion of the third intracellular loop of the alpha 2C-AR fused to glutathione-S-transferase. Selectivity and subtype specificity of the antibody were demonstrated by immunoprecipitation of [125I]-photoaffinity-labeled alpha 2-AR and by immunohistochemical labeling of COS cells expressing the individual rat alpha 2-AR subtypes. In both cases the antibody recognized only the alpha 2C-AR subtype, and immunoreactivity was eliminated by preadsorption of the antibody with excess antigen. In rat brain, alpha 2C-AR-like immunoreactivity (alpha 2C-AR-LI) was found primarily in neuronal perikarya, with some labeling of proximal dendrites; analysis by confocal microscopy revealed the intracellular localization of some of the immunoreactivity. Areas of dense immunoreactivity include anterior olfactory nucleus, piriform cortex, septum, diagonal band, pallidum, preoptic areas, supraoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, amygdala, hippocampus (CA1 and dentate gyrus), substantia nigra, ventral tegmental area, raphe (pontine and medullary), motor trigeminal nucleus, facial nucleus, vestibular nucleus, dorsal motor nucleus of the vagus, and hypoglossal nucleus. Labeling was found in specific laminae throughout the cortex, and a sparse distribution of very darkly labeled cells was observed in the striatum. At all levels of the spinal cord there were small numbers of large, darkly labeled cells in layer IX and much smaller cells in layer X. In general, the pattern of alpha 2C-LI throughout the neuraxis is consistent with previously published reports of the distribution of receptor mRNA detected by hybridization histochemistry.

Hemodynamic effects of L- and D-S-nitrosocysteine in the rat. Stereoselective S-nitrosothiol recognition sites

The vasorelaxant effects of the endothelium-derived relaxing factor S-nitrosocysteine (SNC) may not be simply due to its decomposition to NO. The biological actions of SNC may also involve the transnitrosation of amino acids in the blood and in plasma membranes. The possibility that the SNC moiety possesses biological activity prompted us to examine whether the hemodynamic effects of this S-nitrosothiol involves the activation of stereoselective S-nitrosothiol receptors within the cardiovascular system. We examined (1) the hemodynamic effects produced by intravenous injections of the L and D isomers of SNC (L- and D-SNC, respectively; 100 to 800 nmol/kg), the L and D isomers of the parent thiols (L- and D-cysteine, respectively; 100 to 800 nmol/kg), the oxidized thiol L-cystine (100 to 800 nmol/kg), and the NO donor sodium nitroprusside (SNP, 1 to 36 micrograms/kg) in conscious freely moving rats, (2) the baroreceptor reflex-mediated changes in heart rate elicited in response to the falls in arterial pressure produced by L- and D-SNC and SNP in conscious rats, and (3) the relative decomposition of L- and D-SNC to NO upon addition to heparinized rat blood or upon direct application to cultured porcine aortic smooth muscle (PASM) cells. We now report that (1) L-SNC is a more potent hypotensive and vasodilator agent within the mesenteric bed and sympathetically intact and sympathetically denervated hindlimb beds of conscious rats than is D-SNC, (2) L- and D-SNC markedly inhibit baroreceptor reflex-mediated tachycardia in conscious rats and D-SNC is considerably more effective than L-SNC, (3) the intravenous injections of L- and D-cysteine or L-cystine do not affect arterial blood pressure or vascular resistances, and (4) L- and D-SNC decompose equally to NO upon application to rat blood or cultured PASM cells. These results suggest that the hemodynamic effects of endogenous SNC may involve its interaction with stereoselective S-nitrosothiol recognition sites within the vasculature and the baroreflex arc. These findings provide tentative evidence that membrane-bound S-nitrosothiol receptors may exist within the cardiovascular system.

Chimeric mutagenesis of putative G-protein coupling domains of the alpha2A-adrenergic receptor. Localization of two redundant and fully competent gi coupling domains

We have investigated potential Gi and Gs coupling domains within the intracellular regions of the alpha2AAR subtype using a series of nine chimeric mutations. The second intracellular loop (ICL2, amino acids 133-149) and the amino- and carboxyl-terminal regions of the third intracellular loop (ICL3, amino acids 218-235 and 355-371, respectively) of the cloned human alpha2AAR were substituted with the analogous sequence from either the Gs-coupled beta2AR or the Gi-coupled serotonin type 1A receptor (5-HT1AR). Mutant and wild type alpha2AAR were stably expressed in Chinese hamster ovary cells and functional coupling of each receptor to Gi and Gs was assessed in membrane adenylyl cyclase assays. Substitution of 5-HT1AR sequence into ICL2 ablated coupling to Gs but not to Gi, whereas substitution of beta2AR sequence significantly depressed coupling to Gi but not to Gs. Thus, the ICL2 of the alpha2AAR contains elements essential for both signaling pathways. Substitution of either the amino- or carboxyl-terminal segments of ICL3 with 5-HT1AR sequence ablated agonist stimulation of adenylyl cyclase activity (without affecting inhibition), suggesting that both domains are necessary for alpha2AAR coupling to Gs. In contrast, individual substitution of beta2AR sequence into ICL3 amino or carboxyl termini had no appreciable effect on Gi coupling. Concomitant substitution of beta2AR sequence into both regions substantially impaired Gi coupling, implying that each is capable of independently supporting functional coupling. Substitution of 5-HT1AR at either locus had no effect on Gi coupling. Thus, for Gs coupling, these two domains within ICL3 are both required for functional coupling. However, for Gi coupling, the alpha2AAR appears to have two distinct regions within ICL3 that are capable of supporting Gi coupling independently. There has been no previous elucidation of a receptor having redundant, fully competent domains for coupling to a single class of G-protein. Such duplicity of functional domains within alpha2AR may suggest strong evolutionary pressure to maintain Gi coupling.

Noradrenergic mechanisms in stress and anxiety: I. Preclinical studies

There is considerable preclinical evidence for a relationship between noradrenergic brain systems and behaviors associated with stress and anxiety. The majority of noradrenergic neurons are located in the locus coeruleus (pons), with projections throughout the cerebral cortex and multiple subcortical areas, including hippocampus, amygdala, thalamus, and hypothalamus. This neuroanatomical formation of the noradrenergic system makes it well suited to rapidly and globally modulate brain function in response to changes in the environment, as occurs during the presentation of stressors. Stress exposure is associated with an increase in firing of the locus coeruleus and with associated increased release and turnover of norepinephrine in brain regions which receive noradrenergic innervation. Increased firing of the locus coeruleus is also associated with behavioral manifestations of fear, such as arched back and piloerection in the cat. Exposure to chronic stress results in long-term alterations in locus coeruleus firing and norepinephrine release in target brain regions of the locus coeruleus. Norepinephrine is also involved in neural mechanisms such as sensitization and fear conditioning, which are associated with stress. These findings are relevant to an understanding of psychiatric disorders, such as panic disorder and post-traumatic stress disorder (PTSD), the symptoms of which have been hypothesized to be related to alterations in noradrenergic function.

Subtype selectivity of a new alpha 1-adrenoceptor antagonist, JTH-601: comparison with prazosin

The existence of alpha 1-adrenoceptors with low affinity for prazosin (alpha 1L group: alpha 1L and alpha 1N subtypes) has been proposed in addition to alpha 1-adrenoceptor subtypes with high affinity for prazosin (alpha 1H group: alpha 1A, alpha 1B and alpha 1D subtypes). A newly synthesized alpha 1-adrenoceptor antagonist, JTH-601 (N-(3-hydroxy-6-methoxy-2,4,5-trimethylbenzyl)-N-methyl-2-(4-hydro xy-2-isopropyl-5-methyl-phenoxy) ethylamine hemifumarate) showed approximately a 10 times higher affinity for the alpha 1L group, a similar affinity for the alpha 1A subtype, but a more than 10 times lower affinity for the alpha 1B and alpha 1D subtypes when compared with prazosin. These results provide a further pharmacological evidence that alpha 1-adrenoceptors with low affinity for prazosin exist in addition to those with high affinity for prazosin, suggesting that JTH-601 may be useful for characterising the alpha 1-adrenoceptor subtypes.

Effect of left ventricular pressure and volume overload on alpha-adrenoceptor activity in patients with rheumatic heart valvular disease

We have investigated the possibility that the various left ventricular load conditions may exert different effects on the sympathetic function by comparing the influence of volume (VOL) and pressure (POL) overload on platelet alpha-adrenoceptor activity, plasma catecholamines and cAMP in 44 patients with rheumatic heart valvular disease. Receptor activity was determined by radioligand binding methods, catecholamines by HPLC using an electrochemical detector, and cAMP by radioimmunoassay. The mean alpha-adrenoceptor density (Bmax) of the control group (n = 29) was 4.71 +/- 0.41 fmol per 10(7) platelets and the corresponding dissociation constant (Kd) was 2.47 +/- 0.15 nM. In VOL patients, the density was elevated by 70% (P < 0.0001), but it remained unchanged in the POL patients. In contrast to the Bmax, the Kd of the VOL group was not changed, and it increased by 34% (P < 0.01) in the POL group. Norepinephrine was elevated by 91% (P < 0.05) in POL, and epinephrine increased by 65% (P < 0.05) in POL and 71% (P < 0.05) in VOL. These results suggest that the sympathetic nervous system responds to left ventricular volume overload by increasing alpha-adrenoceptor density with no apparent change in receptor affinity toward [3H]-yohimbine binding, and to left ventricular pressure overload by decreasing their binding affinity without a parallel decrease in receptor density. The increase in receptor density in VOL is accompanied by an increase in plasma epinephrine, and the decrease in binding affinity in POL is associated with increased plasma norepinephrine and epinephrine levels.

Differential responses of lateral and ventrolateral rat periaqueductal grey neurones to noradrenaline in vitro

1. The action of noradrenaline on the membrane properties of rat periaqueductal grey (PAG) neurones was examined using intracellular recordings in brain slices maintained in vitro. Morphological properties and the anatomical location of neurones were characterized by use of intracellular staining within biocytin. 2. Noradrenaline (0.3-100 microM) depolarized 66% (81/123) and hyperpolarized 30% (37/123) of neurones. The alpha 1- and alpha 2-adrenoceptor agonists phenylephrine and UK 14304 produced depolarizations and hyperpolarizations in all PAG neurones tested, respectively. Neurones depolarized by noradrenaline were more responsive to phenylephrine, whereas neurones hyperpolarized by noradrenaline were more responsive to UK 14304. 3. The UK 14304-induced hyperpolarizations reversed polarity at -108 +/- 2 mV (n = 11). The reversal potential increased when the extracellular potassium concentration was raised (slope = 57.8 mV/log[K+]o mM) in a manner similar to that predicted for potassium conductance. 4. The phenylephrine-induced depolarizations did not reverse polarity at negative potentials (n = 25), or did so at potentials (-119 +/- 2 mV, n = 13) more negative than the UK 14304-induced hyperpolarizations. Superfusion with low calcium (0.1 mM), high magnesium (10 mM) and either cobalt (2-4 mM), or cadmium (100 microM) usually reduced the response to phenylephrine and produced reversals near that predicted for potassium conductance. 5. The majority of the ventrolateral PAG neurones were depolarized by noradrenaline (85%, 62/73). In contrast, almost equal proportions of the lateral PAG neurones were hyperpolarized (54%, 20/37) and depolarized (46%, n = 17/37) by noradrenaline. PAG neurones depolarized or hyperpolarized by noradrenaline could not be differentiated on morphological grounds. 6. These results suggest that the net effect of noradrenaline on lateral and ventrolateral PAG neurones is to bias activity in favour of a ventrolateral PAG-mediated response pattern, which includes quiescence, hyporeactivity, hypotension and bradycardia.

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

Synthesis of a selective alpha-2A adrenoceptor antagonist, BRL 48962, and its characterization at cloned human alpha-adrenoceptors

The chiral synthesis of the potent and selective alpha-2A antagonist, BRL 48962, is described. Evaluation of BRL 48962 at cloned human alpha-adrenoceptors indicates that this antagonist has a selectivity in the order of 30-fold for the alpha-2A subtype.

Human adipocytes express alpha 2-adrenergic receptor of the alpha 2A-subtype only: pharmacological and genetic evidence

In the present study we have reinvestigated the subtype of alpha 2-adrenoceptors expressed in human adipocytes (from subcutaneous and internal fat deposits) by means of radioligand binding using subtype-selective antagonists, and RNase mapping using a set of specific probes prepared from human alpha 2-adrenoceptors genes (alpha 2C2, alpha 2C4 and alpha 2C10). Comparison of the pharmacological properties of the human adipocyte alpha 2-adrenoceptors with those of the different human adrenoceptors expressed in COS-7 cells demonstrated that: i) human adipocyte alpha 2-adrenoceptors displays a KD for [3H]RX821002 and [3H]MK912 identical to that found in COS-7 cells transfected with the alpha 2C10 gene; ii) yohimbine and oxymetazoline is 1,000-fold more potent than prazosin to inhibit [3H]antagonist binding. RNase protection assays on cellular RNA prepared from the three fat deposits showed the presence of substantial amounts of alpha 2C10 transcripts: in contrast, mRNAs from alpha 2C2 and alpha 2C4 genes were undetectable. Altogether these results definitively establish that human adipocytes express only one alpha 2-adrenoceptor which is of the alpha 2A-subtype and encoded by the alpha 2C10 gene.

Two distinct alpha 1-adrenoceptor subtypes in rabbit liver: a binding study

1. The characteristics of alpha 1-adrenoceptor subtypes present on rabbit liver membranes were determined by radioligand binding and compared with the characteristics of binding in rat liver. 2. In saturation experiments using rabbit liver, [3H]-prazosin bound to two distinct affinity sites (pKD = 10.3 +/- 0.19 and 8.13 +/- 0.17, Bmax = 11.6 +/- 3.3 and 657.8 +/- 198.0 fmol mg-1 protein, respectively). In studies using rat liver, [3H]-prazosin bound to a single affinity site (pKD = 9.98 +/- 0.27, Bmax = 190.5 +/- 38.5 fmol mg-1 protein). 3. In competition experiments, unlabelled prazosin displaced biphasically the binding of 200 pM [3H]-prazosin to the rabbit liver; the resulting two pK1 values (9.85 +/- 0.08 and 8.01 +/- 0.09) were consistent with the affinity constants obtained in the saturation experiments. Two sites were also recognized by doxazosin (pKI 9.73 +/- 0.78 and 8.12 +/- 0.34), 2-(2,6-dimethoxy phenoxyethyl)-aminomethyl-1,4-benzo-dioxane (WB4101) pKI (9.74 +/- 0.32 and 7.57 +/- 0.34) and 5-methylurapidil (pKI 8.69 +/- 0.27 and 6.75 +/- 0.35), and the population of low affinity sites for the three antagonists was approximately 70%. Two distinct affinity constants (pKI 8.55 +/- 0.09 and 7.90 +/- 0.09) were also calculated for alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy) ethyl)-amino)-propyl)-benzeneacetonitrile fumarate (HV723). 4. By contrast, [3H]-prazosin binding sites of rat liver membranes were detected as a single population with a high affinity for prazosin (pKI 10.01 +/- 0.08), and doxazosin (pKI 9.67 +/- 0.20) but with a low affinity for WB4101 (pKI 8.25 +/- 0.09), 5-methylurapidil (pKI 7.22 +/- 0.01) and HV723 (pKI 8.88 +/- 0.05). 5. These results indicate the presence of two distinct alpha 1-adrenoceptor subtypes in the rabbit liver, but only a single site in rat liver. The pharmacological characteristics of prazosin-high and -low sites in rabbit liver suggest identity with alpha 1A and putative alpha 1L subtypes, respectively. The site in rat liver is of the alpha 1B subtype.

Alpha 2C-adrenoceptor-modulated release of noradrenaline in human right atrium

1. The aim of the present study was to characterize the presynaptic alpha 2-autoreceptors in human right atrium in terms of the alpha 2A-D system. Segments of atrial appendages were preincubated with [3H]-noradrenaline and then superfused in the presence of cocaine and stimulated electrically. pEC30% values of eight alpha-adrenoceptor antagonists with discriminatory power were determined. pEC30% is the negative logarithm of the antagonist concentration that increased the stimulation-induced overflow of tritium by 30%. For four antagonists, the dissociation constant KD was determined, in addition to pEC30%, against the overflow-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) under autoinhibition-free conditions. 2. pEC30% and KD values yielded identical rank orders of antagonist affinity (rauwolscine > WB 4101 > phentolamine > prazosin) suggesting that both released noradrenaline and the exogenous agonist UK 14,304 activated the same receptor to inhibit release. 3. The eight antagonist pEC30% values obtained in right atrium correlated significantly with their pEC30% values, reported in the literature, at the presynaptic alpha 2C-autoreceptors in human kidney (r = 0.817; slope of the regression line 1.03). No significant correlation was obtained between pEC30% values at atrial autoreceptors and pKD values at previously characterized alpha 2A-autoreceptors in rabbit and alpha 2D-autoreceptors in rat, mouse and guinea-pig tissues. 4. Comparison of antagonist pEC30% values with their pKD values at native alpha 2 binding sites in cells or tissues that express a single subtype only, and with pKD values at alpha 2 binding sites in membranes of COS cells transfected with human alpha 2 subtype genes confirms the alpha 2C character of the atrial autoreceptors: significant correlations were obtained exclusively with the alpha 2C binding sites. 5. Ratios of KD values were computed for alpha 2-autoreceptors in human right atrium and for binding sites in COS cells transfected with human alpha 2 subtype genes. The autoreceptor ratios corresponded well with the respective ratios for the alpha 2C binding sites (maximal three fold deviation) but were, in part, markedly different from ratios calculated for alpha 2A and alpha 2B binding sites (up to 166 fold deviation). This outcome supports the alpha 2C designation of the autoreceptors. 6. In conclusion, the presynaptic alpha 2-autoreceptors in human right atrium are alpha 2C. In this they agree with the previously characterized alpha 2-autoreceptors in human kidney. The alpha 2C classification possibly separates, in general, human alpha 2-autoreceptors from those in lagomorph (rabbit) and rodent (rat, mouse, guinea pig) species that have been proposed to be predominantly alpha 2A or alpha 2D.

The alpha 1C-adrenoceptor in human prostate: cloning, functional expression, and localization to specific prostatic cell types

1. Benign prostatic hyperplasia (BPH) causes urinary obstruction in aging men that frequently requires surgery to relieve the symptoms of urinary retention, nocturia, and micturition. Smooth muscle tone which contributes to the urethral constriction in the enlarged gland appears to be mediated by the alpha 1-adrenoceptors. In this paper, molecular and pharmacological approaches are used to establish the role played by the alpha 1C-adrenoceptor subtype in the prostate. 2. The alpha 1-adrenoceptor subtype(s) expressed in human prostate were investigated by use of polymerase chain reaction (PCR), Northern blot, and in situ hybridization. The alpha 1C subtype was found in both prostate stromal and glandular cells while alpha 1B and alpha 1D subtypes were expressed in glandular cells. High expression levels for alpha 1C were observed in prostate cancer tissues in both stroma and glandular cells. 3. Full length alpha 1C-adrenoceptor cDNA was cloned from human prostate. Stable mammalian cell lines expressing human alpha 1B-, alpha 1C-, and alpha 1D-adrenoceptors were made. Membranes prepared from these cell lines and human prostate were used to evaluate the pharmacological profiles of human alpha 1B-, alpha 1C- and alpha 1D-adrenoceptors in comparison to human prostate. Leverage plot analysis of compound affinities determined by competition for [125I]-I-HEAT binding demonstrated that the alpha 1C subtype is the predominant alpha 1-adrenoceptor in human prostate. 4. The alpha 1-adrenoceptors cause smooth muscle constriction by coupling to IP3 turnover and intracellular Ca2+ release. Using stable cell lines to measure IP3 production in response to noradrenaline, alpha 1C stimulated IP3 production most efficiently, with alpha 1B at an intermediate level, while little IP3 above background could be detected with alpha 1D. These results supported a functional role of the alpha 1C-adrenoceptor on prostate smooth muscle constriction by noradrenaline stimulation.

Two human alpha 2-adrenoceptor subtypes alpha 2A-C10 and alpha 2B-C2 expressed in Sf9 cells couple to transduction pathway resulting in opposite effects on cAMP production

The baculovirus expression vector system utilizing the strong polyhedrin gene promoter of the Autographa californica nuclear polyhedrosis virus (AcNPV) was used for high level expression of the two alpha 2-adrenoceptor subtypes alpha 2A-C10 and alpha 2B-C2 in Spodoptera frugiperda (Sf-9) insect cells. For rapid screening of recombinant viruses the luciferase gene was expressed under the early ETL-promoter (early transcript large) in the same plasmid. Both receptor subtypes showed the same rank order of binding affinity for four agonists tested: dexmedetomidine > l-medetomidine = clonidine > noradrenaline. For the alpha 2A-C10 subtype, these agonists inhibited forskolin stimulated cAMP production through pertussis toxin sensitive G-proteins. In contrast, for the alpha 2B-C2 subtype the agonists stimulated both basal and forskolin stimulated cAMP production.

Cross talk between receptors mediating contraction and relaxation in the arterioles but not the dilator muscle of the rat iris

1. Sympathetic nerve stimulation causes contraction of the dilator muscle and the large arterioles of the iris via the activation of alpha 1B-adrenoceptors. We have investigated whether increases in adenosine 3': 5'-cyclic monophosphate (cyclic AMP) and the activation of receptors in these tissues can modulate these nerve-mediated contractions. 2. Increasing intracellular cyclic AMP with dibutyryl cyclic AMP (1 mM), forskolin (50 microM) or isobutylmethylxanthine (100 microM) produced relaxation of both the dilator and the arterioles, abolished the nerve-mediated constriction of the arterioles, but potentiated the nerve-mediated contraction of the iris dilator. 3. Pretreatment of the preparations with cholera toxin, to activate Gs permanently, caused a dilatation of the arterioles and abolished the nerve-mediated constriction but had no effect on the dilator muscle. 4. The beta-adrenoceptor agonist, isoprenaline (1 microM), the adenosine-A1,-A2 agonist, N-ethylcarboxamidoadenosine NECA (100 nM), in the presence of the A1-selective antagonist, 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX, 10 nM), and calcitonin gene-related peptide (CGRP, 10 nM) all separately caused a dilatation of the arterioles and abolished the nerve-mediated constriction, while only isoprenaline (1 microM) produced an effect on the dilator, i.e. a relaxation but a potentiation of the nerve-mediated contraction. These results suggest the presence of at least 3 types of receptor linked to Gs and an increase in cyclic AMP in the arterioles, i.e. beta-adrenoceptor, adenosine-A2 and CGRP, but only 1 Gs-linked receptor, i.e. beta-adrenoceptors, on the dilator muscle cells.2+ '

Modulation of Ca2+ channels by alpha 1A- and alpha 2A-adrenoceptors in vascular myocytes: involvement of different transduction pathways

Using subtype-selective agonists and antagonists, and antibodies directed against phosphatidylinositol and G-proteins, it has been shown in single myocytes of rat portal vein that both alpha 1A- and alpha 2A-adrenoceptors modulate Ca2+ channels through two distinct transduction pathways. alpha 1A-adrenoceptors couple with a Gq/G11 protein to activate a phospholipase C (PLC) which hydrolyses phosphatidylinositol to generate inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG). InsP3 releases intracellular stored Ca2+ as evidenced by microspectrofluorimetry with Fura-2. The large and transient increase in [Ca2+]i activates chloride channels leading to a membrane depolarization that opens voltage-gated Ca2+ channels. In addition, DAG activates transiently protein kinase C (PKC) which increases the opening probability of Ca2+ channels through a phosphorylation-dependent process. alpha 2A-adrenoceptors do not induce Ca2+ release from intracellular stores but promote sustained Ca2+ influx through voltage-gated Ca2+ channels. The coupling involves a Gi-protein and activation of PKC by DAG. These two transduction pathways may be involved in the physiological action of noradrenaline in vascular smooth muscles.

Factors determining specificity of signal transduction by G-protein-coupled receptors. Regulation of signal transfer from receptor to G-protein

Among subfamilies of G-protein-coupled receptors, agonists initiate several cell signaling events depending on the receptor subtype (R) and the type of G-protein (G) or effector molecule (E) expressed in a particular cell. Determinants of signaling specificity/efficiency may operate at the R-G interface, where events are influenced by cell architecture or accessory proteins found in the receptor's microenvironment. This issue was addressed by characterizing signal transfer from R to G following stable expression of the alpha 2A/D adrenergic receptor in two different membrane environments (NIH-3T3 fibroblasts and the pheochromocytoma cell line, PC-12). Receptor coupling to endogenous G-proteins in both cell types was eliminated by pertussis toxin pretreatment and R-G signal transfer restored by reconstitution of cell membranes with purified brain G-protein. Thus, the receptor has access to the same population of G-proteins in the two different environments. In this signal restoration assay, agonist-induced activation of G was 3-9-fold greater in PC-12 as compared with NIH-3T3 alpha 2-adrenergic receptor transfectants. The cell-specific differences in signal transfer were observed over a range of receptor densities or G-protein concentration. The augmented signal transfer in PC-12 versus NIH-3T3 transfectants occurred despite a 2-3-fold lower level of receptors existing in the R-G-coupled state (high affinity, guanyl-5'-yl imidodiphosphate-sensitive agonist binding), suggesting the existence of other membrane factors that influence the nucleotide binding behavior of G-protein in the two cell types. Detergent extraction of PC-12 but not NIH-3T3 membranes yielded a heat-sensitive, macromolecular entity that increased 35S-labeled guanosine 5'-O-(thiotriphosphate) binding to brain G-protein in a concentration-dependent manner. These data indicate that the transfer of signal from R to G is regulated by a cell type-specific, membrane-associated protein that enhances the agonist-induced activation of G.

Characteristics of the alpha 2/beta 2-adrenergic receptor-coupled adenylyl cyclase system in rat myometrium during pregnancy

alpha 2A- and alpha 2B-adrenoreceptors (AR), identified by Northern blotting in rat myometrium, showed a differential expression during the course of pregnancy. Indeed, the alpha 2A-AR transcript was present at mid-pregnancy, whereas high levels of alpha 2B-AR mRNA could be detected at term. The role of these subtypes in modulating beta 2-AR-stimulated adenylyl cyclase activity was investigated on myometrial membranes from mid-pregnancy and term. At nanomolar concentrations of clonidine (full alpha 2-AR agonist) or oxymetazoline (partial alpha 2A-AR agonist), adenylyl cyclase activity was inhibited by up to 50 +/- 7% at mid-pregnancy or 75 +/- 7% at term, whereas at micromolar concentrations, alpha 2-AR agonists potentiate adenylyl cyclase activity by 140-170% at mid-pregnancy. Both inhibitory and stimulatory components of this biphasic response were blocked by yohimbine, a selective alpha 2-AR antagonist. Preincubation of myometrial membranes with Gi2 and/or Gi3 antisera eliminated alpha 2-AR mediated attenuation or potentiation of isoproterenol-stimulated adenylyl cyclase, thus indicating that both the inhibitory and stimulatory components are mediated via Gi2 and Gi3. In addition, type II and IV adenylyl cyclases were identified by Northern blotting in the pregnant rat myometrium. Altogether these data strongly suggest that the alpha 2A-AR at mid-pregnancy potentiates adenylyl cyclase types II and IV through beta gamma released from Gi2 and Gi3 proteins, whereas the alpha 2B-AR expression at term may be related to persistent inhibition.

Expression of alpha 2-receptor-mediated responses by insulin in primary culture of rat hepatocytes

The effects of the alpha 2-adrenergic agonist, clonidine, on the glucagon-stimulated glucose output from serum-free cultures of adult rat hepatocytes were examined in vitro. When hepatocytes were cultured with 10 nM dexamethasone under the serum-free condition, 1 or 10 microM clonidine did not inhibit the glucagon-induced glucose production. In contrast, clonidine dose-dependently inhibited the activity concomitantly with suppression of hepatocyte cAMP production by glucagon when they were cultured with 10 nM dexamethasone and 10 nM insulin. The inhibitory effects of clonidine were completely blocked by prior treatment of hepatocytes with pertussis toxin (100 ng/ml). In addition, forskolin-stimulated cAMP production was also inhibited by alpha 2-adrenergic agonists (clonidine and oxymetazoline) in a dose-dependent manner when hepatocytes were cultured with 10 nM dexamethasone and 10 nM insulin. The inhibitory effects of alpha 2-adrenergic agonists on forskolin-stimulated cAMP production were specifically blocked when they were combined with the alpha 2-adrenergic antagonist yohimbine. Hepatocytes cultured with dexamethasone alone showed no response to the alpha 2-adrenergic agonists. The alpha 2-response was abolished when cycloheximide (0.5 microM) was added to the cultures. These results suggest that insulin develops alpha 2-adrenergic responsiveness through new protein synthesis during the primary culture of adult rat hepatocytes.

Vascular alpha-adrenoceptors: from the gene to the human

Adrenoceptors can be subdivided into three major types, the alpha 1-, alpha 2-, and beta-adrenoceptors. Each of these types can be further subdivided into three subtypes, based on pharmacological characteristics. Molecular cloning techniques have supported this subclassification. Recent data now suggest that alpha-adrenoceptor subtypes identified by pharmacological and molecular techniques correspond well, although species orthologs of several adrenoceptor subtypes have been identified. The secondary structure of the adrenoceptors has been elucidated and correlated with their interaction with second messenger molecules. alpha 1-Adrenoceptors, beta-adrenoceptors, and alpha 2-adrenoceptors mediate their actions through stimulation of inositol phosphate release, stimulation of adenylate cyclase, and inhibition of adenylate cyclase, respectively. Site-directed mutagenesis and the preparation of chimeric receptors have located the site of receptor--second messenger interaction to the third intracellular loop for each of these adrenoceptors. While subtypes of each of these classes all interact with the same second messenger, studies with recombinant alpha 2-adrenoceptors show subtype-related differences in receptor--second messenger interaction. Multiple alpha-adrenoceptor subtypes are expressed in vascular smooth muscle and are involved in various aspects of blood vessel function, including contraction, cellular growth, and proliferation. Various physiological factors can selectively influence responses to a particular subtype, and the relative roles of each subtype can vary between vascular beds and along an individual blood vessel as its caliber changes. Functional studies in blood vessels suggest the presence of additional alpha-adrenoceptor subtypes not yet identified via molecular techniques. Optimization of the therapeutic profile of an alpha-adrenoceptor antagonist may be possible via enhancement of selectivity for a particular subtype or by design of a specific profile of affinity for the individual subtypes.

Alpha 1-adrenoceptor subtype selectivity of tamsulosin: studies using livers from different species

The subtype selectivity of the alpha 1-adrenoceptor antagonist, tamsulosin, was tested using hepatocytes and liver membranes from guinea pigs and rabbits (expressing alpha 1-adrenoceptors with alpha 1A pharmacology) and rats (alpha 1B-adrenoceptors). Tamsulosin blocked the alpha 1-adrenergic activation of phosphorylase with higher affinity in hepatocytes from guinea pigs and rabbits than in those from rats. [3H]Tamsulosin binding to liver membranes was rapid, reversible and saturable. The Kd values obtained also indicated higher affinity for alpha 1A-adrenoceptors (70 and 140 pM, for liver membranes obtained from guinea pigs and rabbits, respectively) than for those of the alpha 1B-subtype (510 pM). Chloroethylclonidine potently and completely inactivated [3H]tamsulosin binding sites in membranes from rabbit and rat livers, but not those in guinea pig liver membranes. Binding competition and inactivation experiments were performed to further characterize the receptor subtypes present in the livers of these animals. In summary, tamsulosin is a very potent alpha 1-adrenoceptor antagonist that has higher affinity for alpha 1A-adrenoceptors than for those of the alpha 1B-subtype.

Renal aging change of alpha 1-adrenoceptor in Wistar rats

1. The aging changes of density of the alpha 1-adrenoceptors in the kidney were evaluated with Wistar rats of several ages (8, 52 and 104 weeks old). 2. [3H]prazosin and [3H]YM617 (newly synthesized alpha 1-blocker) were used for the ligand. The Bmax of [3H]prazosin was 74.0 +/- 9.5 fmol/mg/protein in 8 week, 52.1 +/- 7.3 fmol/mg protein in 52 week, and 31.3 +/- 4.2 fmol/mg protein in 104 week rats, and that of [3H]YM617 was 45.0 +/- 6.6 fmol/mg/protein in 8 week, 32.4 +/- 5.7 fmol/mg/protein in 52 week, and 19.3 +/- 5.5 fmol/mg/protein in 104 week rats. 3. The Bmax of both ligands for 104 week rats was significantly decreased compared to 8 week rats, however, 52 week rats showed no decrease of Bmax for both ligands. 4. The Kd values showed no difference in these three age groups for both ligands. 5. Autoradiographic study supported the result above mentioned. Furthermore, the binding sites of alpha 1-adrenoceptors were mainly in the cortex (vascular wall and peritubular area) and that alpha 1-adrenoceptors were chiefly chlorethylclonidine dihydrochloride (CEC) insensitive.

Pharmacological characterization of alpha 1-adrenoceptor subtypes in rat heart: a binding study

1. The alpha 1-adrenoceptor subtypes of rat heart were characterized in binding experiments performed with [3H]-prazosin as the radiolabel. The specific binding to the alpha 1-adrenoceptors was determined with 0.3 microM prazosin, because phentolamine (10 microM) was insufficient to inhibit completely the specific binding of high concentrations of [3H]-prazosin. 2. In saturation experiments, [3H]-prazosin bound to two distinct affinity sites (pKD = 10.39 and 8.19). The proportion of the low affinity sites was approximately 84% of total specific binding. Membranes pretreated with chloroethylclonidine (CEC, 10 microM) also showed two distinct affinity sites for [3H]-prazosin, although the maximum numbers of high and low affinity sites were reduced by 86 and 64%, respectively. 3. In competition experiments, [3H]-prazosin (100 pM) binding was inhibited by WB4101 (2-(2,6-dimethoxy-phenoxyethyl)aminomethyl-1,4-benzodioxane) and 5-methylurapidil. The inhibition curves displayed shallow slopes which could be subdivided into high and low affinity components (pKi = 10.43 and 8.36 for WB4101, 8.62 and 6.61 for 5-methylurapidil). However, unlabelled prazosin or HV723 (alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)-ethyl)amin o) propyl)benzeneacetonitrile fumarate) competed for [3H]-prazosin binding monophasically (pKi = 10.34 and 8.28, respectively). In CEC-pretreated membranes, prazosin, WB4101, 5-methylurapidil and HV723 antagonized the [3H]-prazosin (100 pM) binding monophasically (pKi = 9.70, 9.56, 8.60 and 8.82, for each antagonist). 4. On the other hand, 1000 pM [3H]-prazosin binding was inhibited by unlabelled prazosin biphasically (pKi = 10.49 and 8.49). HV723 did not discriminate both prazosin-high and low affinity sites (pKi = 8.18). 5. These results suggest the presence of at least three distinct alpha1-adrenoceptor subtypes in rat hearts(two prazosin-high affinity sites and one prazosin-low affinity site). According to the recent alpha l-adrenoceptor subclassifications, one of the former two sites corresponds to the alpha 1B subtype with low affinities for WB4101 and 5-methylurapidil and sensitive to CEC, while another site with relatively high affinities for WB4101 and 5-methylurapidil may be classical alpha 1A, cloned alpha 1c, alpha 1D subtypes or their mixture. The prazosin-low affinity site corresponds to putative alpha 1L subtype with low affinity for HV723,which may be predominantly involved in the positive inotropic response to phenylephrine.

Effect of receptor density on the receptor-effector coupling: use of cloned and stably expressed alpha 1B-adrenoceptors in CHO cells

Using Chinese hamster ovary cells stably expressing alpha 1B-adrenoceptor as a model, we investigated whether the changes in receptor density may influence the receptor-effector coupling relationship. Among the transfected cells, two clones which showed similar pharmacological properties but markedly differed in receptor density (Bmax were 1600 and 110,000 sites/cell, respectively), were examined. The phenoxybenzamine inactivation method showed that the alpha 1B-adrenoceptor occupancy and transients of cytosolic Ca2+ concentration ([Ca2+]i) response relationship was markedly nonlinear but similar in the two cell lines. The dose-response relationship for norepinephrine-induced [Ca2+]i response showed an increase in maximum effect with no change in agonist potency, and the increase in maximum effect was disproportionate to the difference in receptor density. The results indicate that the classical model of drug-receptor action cannot appropriately describe the coupling of alpha 1B-adrenoceptor to [Ca2+]i response in the single receptor expressing system.

Pharmacological characterization of noradrenaline-induced contractions of the porcine isolated palmar lateral vein and palmar common digital artery

1. The aim of this study was to examine the pharmacological characteristics of alpha-adrenoceptor-mediated contractions in two porcine isolated blood vessels, the palmar lateral vein (PLV) and the palmar common digital artery (PCDA). This was carried out with noradrenaline used as the agonist throughout, and either phentolamine (non-selective alpha-adrenoceptor antagonist), prazosin and YM-12617 (selective alpha 1-adrenoceptor antagonists) or rauwolscine and CH-38083 (selective alpha 2-adrenoceptor antagonists). 2. Noradrenaline (0.003-10 microM) produced concentration-dependent contractions in both vessels, with the PCDA (pD2 = 6.33 +/- 0.07, n = 10) being approximately 10 fold less sensitive to noradrenaline compared to the PLV (pD2 = 7.39 +/- 0.09, n = 8). Also, the maximal response to noradrenaline was greater in the PCDA compared to the PLV. Phentolamine (0.03-30 microM) produced parallel rightward shifts in the CRC to noradrenaline in both tissue preparations. The pA2 values were similar and slopes of the Schild plots were not significantly different from unity, indicating an interaction between phentolamine and a single receptor in each preparation. 3. In the PCDA the alpha 1-adrenoceptor antagonists, prazosin (0.01-1 microM) and YM-12617 (0.01-1 microM) produced non-parallel rightwards shifts in the CRC to noradrenaline, with the lower 10-15% of the CRC exhibiting greater resistance to the effects of these antagonists compared to the upper part. In contrast, rauwolscine (1-10 microM) and CH-38083 (10 microM) produced parallel displacement of the CRC to noradrenaline. In the PLV, low concentrations of either alpha l- (0.01 microM) or alpha2-adrenoceptor antagonists(0.1-1 microM) produced a large shift in the CRC, but subsequent higher concentrations had only small additional effects. Based upon pKB values estimated from the effects of the lower concentrations of antagonists, the results are consistent with a large population of alpha1-adrenoceptors in the PCDA and a mixture of alpha l- and alpha2-adrenoceptors in the PLV.4. In both tissues, when an ac,- and an a2-adrenoceptor antagonist were used in combination the effect produced was greater than that with either agent alone. In contrast, the combination of the alpha1-adrenoceptor antagonists (prazosin and YM-12617 together) or the alpha2-adrenoceptor antagonists (CH-38083 and rauwolscine together) were no more effective than that produced by the individual antagonists. These findings suggest the presence of functional alpha l- and alpha2-adrenoceptors in the PLV andPCDA.5. Phenoxybenzamine (0.3-3 microM, 60min exposure) produced a concentration-dependent reduction in the maximal response to noradrenaline which was more pronounced in the PCDA than the PLV. After a 60 min exposure to a combination of phenoxybenzamine (1 microM) and rauwolscine (1 microM), the remaining NA-induced contraction after washout was resistant to prazosin (0.1 microM) and sensitive to rauwolscine(1 microM) in both tissue preparations, indicating the existence of functional alpha2-adrenoceptors in both vessels.6. Evidence suggests that post-junctional alpha l- and alpha2-adrenoceptors contribute to noradrenaline-induced contractions in the PCDA and PLV, with the latter possessing a larger population of functional alpha2-adrenoceptors.

Alpha 2A-adrenoceptors mediate activation of non-selective cation channels via Gi-proteins in human erythroleukaemia (HEL) cells. No evidence for a functional role of imidazoline receptors in modulating calcium

Human erythroleukaemia (HEL) cells were investigated to characterize their alpha 2-adrenoceptor and imidazoline receptor sites. Membranes from HEL cells bound [3H]2-(2-methoxy-1, 4-benzodioxan-2yl)-2-imidazoline ([3H]RX821002) in a saturable and specific manner with a KD of 0.64 +/- 0.07 nM and a Bmax of 126 +/- 4 fmol/mg protein. [3H]RX821002 was displaced from HEL membranes by adrenergic drugs with the order of potency being yohimbine approximately oxymetazoline >> prazosin = 2-[2-[4-(o-methoxyphenyl)piperazin-1-yl]ethyl]-4,4-dimethyl- 1,3(2H,4H)-isochinolindione HCl (ARC 239), consistent with this site being an alpha 2A-adrenoceptor. HEL membranes also bound [3H]idazoxan in the presence of adrenaline to block alpha 2-adrenoceptors. This binding was saturable and specific with a KD of 3.5 +/- 1.0 nM and a Bmax of 31 +/- 6 fmol/mg protein. Adrenergic drugs from both the phenylethylamine and imidazoline classes increased high-affinity GTPase activity, an index of activation of regulatory heterotrimeric guanine-nucleotide binding proteins (G-proteins), and produced increases in cytosolic free calcium concentration ([Ca2+]i). The effects of these agonists in both systems were abolished by pertussis toxin pretreatment, and oxymetazoline and clonidine were antagonists. The potency of adrenergic drugs to inhibit 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304)-induced increases in [Ca2+]i was yohimbine approximately oxymetazoline >> ARC 239, consistent with the binding data and an action at alpha 2A-adrenoceptors. No evidence was found for a role of imidazoline receptors in stimulating G-proteins or modulating [Ca2+]i. The adrenergic agonist-induced increases in [Ca2+]i were due to both release of Ca2+ from intracellular stores and entry of extracellular Ca2+. Ca2+ entry was blocked by 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenylethyl)-1H- imidazole hydrochloride (SKF 96365), but not by nitrendipine. Adrenaline also stimulated Mn2+ entry in HEL cells. Taken together, these results suggest that HEL cells have alpha 2A-adrenoceptors that activate non-selective cation channels via pertussis toxin-sensitive G-proteins, i.e. Gi-proteins.

Use of recombinant alpha 1-adrenoceptors to characterize subtype selectivity of drugs for the treatment of prostatic hypertrophy

Several alpha 1-adrenoceptor antagonists have recently been developed for the treatment of benign prostatic hypertrophy because of their less frequent systemic side-effects compared to conventional alpha 1-adrenoceptor blockers. One potential explanation for their good tolerability would be the selectivity for a certain subtype of alpha 1-adrenoceptor. Utilizing COS-7 cells expressing the rat alpha 1A, the hamster alpha 1B and the human alpha 1C-adrenoceptors, we investigated affinities of alfuzosin, doxazosin, terazosin, indoramin and (+)- and (-)-5-[2-[[2-(o-ethoxyphenoxy)ethyl] amino]propyl]-2-methoxybenzesulfonamide HCl (YM 617) compared to prazosin. Radioligand binding studies showed that the affinities of alpha 1-adrenoceptor subtypes for alfuzosin (Ki value; alpha 1A: 2.4 nM, alpha 1B:1.4 nM, alpha 1C:4.2 nM), doxazosin (Ki value; alpha 1A:2.7 nM, alpha 1B:3.2 nM, alpha 1C:7.5 nM), terazosin (Ki value; alpha 1A:2.5 nM, alpha 1B:2.7 nM, alpha 1C:7.1 nM), indoramin (Ki value; alpha 1A:69 nM, alpha 1B:21 nM, alpha 1C:13 nM) and prazosin (Ki value; alpha 1A:0.16 nM, alpha 1B:0.19 nM, alpha 1C:0.2 nM) were equipotent to the three receptor subtypes. Unlike these antagonists, both (+)- and (-)-YM617 had relatively lower affinity for alpha 1B receptors compared to the other subtypes (Ki value; for (+)-YM617, alpha 1A:22 nM, alpha 1B:96 nM, alpha 1C:4.3 nM; for (-)-YM617, alpha 1A:0.11 nM, alpha 1B:0.7 nM, alpha 1C:0.035 nM). The data suggest that alpha 1-adrenoceptor antagonists currently used for the treatment of the benign prostatic hyperplasia do not show substantial subtype selectivity.

Pharmacological profiles of a novel alpha 1-adrenoceptor agonist, PNO-49B, at alpha 1-adrenoceptor subtypes

The effects of a newly synthesized compound, PNO-49B, (R)-(-)-3'-(2-amino-1-hydroxyethyl)-4'-fluoromethanesulfonanilide hydrochloride, on alpha 1-adrenoceptor subtypes were examined in various tissues in which the following distribution of alpha 1-adrenoceptor subtypes has been suggested: dog carotid artery (alpha 1B), dog mesenteric artery (alpha 1N), rabbit thoracic aorta (alpha 1B + alpha 1L), rat liver (alpha 1B), rat vas deferens (alpha 1A + alpha 1L), rat cerebral cortex (alpha 1A + alpha 1B) and rat thoracic aorta (controversial subtype). PNO-49B (0.1-100 microM) produced concentration-dependent contractions in dog mesenteric artery, rabbit thoracic aorta, rat thoracic aorta and rat vas deferens; and the maximal amplitudes of contraction were almost the same as or slightly less than those of noradrenaline. By contrast, the maximal response to PNO-49B in dog carotid artery was markedly smaller than the response to noradrenaline. In rabbit thoracic aorta, the contractile response to PNO-49B was not affected by inactivation of the alpha 1B subtype with chloroethylclonidine (CEC), although the response to noradrenaline was attenuated by that treatment. The dissociation constants (KA) of PNO-49B were not different among the rat thoracic aorta, dog carotid and mesenteric arteries and rabbit thoracic aorta (CEC-pretreated). The contractile responses to PNO-49B were inhibited competitively by prazosin, HV723 (alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)-ethyl)- amino(propyl)benzeneacetonitrile fumarate) and by WB4101 (2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4- benzodioxane).(ABSTRACT TRUNCATED AT 250 WORDS)

alpha2-adrenergic agonist-induced inhibition of cyclic AMP formation in transfected cell lines using a microtiter-based scintillation proximity assay

Human embryonic kidney cells (HEK-7) were transfected with the gene for the human alpha2C receptor (alpha2C4). Cells were grown in 96-well microtiter plates and cyclic AMP levels were measured by scintillation proximity assay, a modified radioimmunoassay technique. Radioactivity was quantified using a TopCount Scintillation detector. Cyclic AMP was increased in a dose-dependent manner by the addition of exogenous forskolin. The forskolin-induced enhancement of cyclic AMP was inhibited dose-dependently by the addition of alpha2-adrenergic agonists, and this inhibition was blocked by the addition of adrenergic antagonists. The extent of the inhibitory response caused by alpha2-adrenoceptor agonists was related to the receptor density in clonal cell lines derived from the transfected parental HEK-7 cells. By using cells grown in microtiter format, and employing the technological advantages of scintillation proximity assay and TopCount detection, it was possible to simultaneously evaluate the effects of multiple experimental permutations on cellular production of cyclic AMP with minimal disturbance of the cells and minimal and/or automated manipulation of the cyclic AMP formed. This combination of techniques should allow rapid testing of the actions of adrenergic agonists and antagonists on cells transfected with receptors linked to cyclic AMP formation.

Alpha 1B-receptors and intracellular calcium mediate sympathetic nerve induced constriction of rat irideal blood vessels

The present study has investigated the receptors involved in the non-cholinergic nerve mediated constriction of the larger blood vessels (30-50 microns) within the rat iris. This response was blocked by the alpha-adrenoceptor antagonist, benextramine (10(5) M). Furthermore, the response was more sensitive to blockade by the alpha 1 antagonist, prazosin (IC50 9 x 10(-10) M), than to blockade by the alpha 2 antagonist, yohimbine (IC50 2 x 10(-7) M), or the adrenergic antagonist, WB4101 (IC50 2 x 10(-8) M), and was abolished by chloroethylclonidine (10(-5) M). These results suggest the involvement of alpha 1B-adrenoceptors. The nerve mediated constriction was not blocked by the voltage-dependent calcium channel blocking drugs, nifedipine (10(-6) M), verapamil (10(-6) M) or diltiazem (10(-6) M), but was completely abolished by the intracellular calcium mobilizer, caffeine (10(-3) M), supporting the hypothesis that alpha 1B-adrenoceptors are activated following nerve stimulation. Dantrolene (10(-4) M), which interferes with calcium release from the sarcoplasmic reticulum, reduced the nerve mediated constriction by 40% as did thapsigargin (2 x 10(-6) M), which inhibits the calcium ATPase responsible for uptake of calcium into intracellular stores. When influx of calcium was blocked by verapamil (10(-6) M), thapsigargin, but not dantrolene, completely abolished the response. Noradrenaline (10(-5) M) produced a vasoconstriction in the presence or absence of external calcium although the latter response was significantly smaller than the former. Vasoconstriction produced by a submaximal concentration of noradrenaline (10(-6) M), was completely prevented by pretreatment with chloroethylclonidine. The data indicate that noradrenaline released from sympathetic nerves causes a constriction of arterioles in the iris by activating alpha 1B-adrenoceptors and releasing calcium from dantrolene sensitive and insensitive intracellular stores, followed by inflow of calcium through verapamil sensitive calcium channels. Applied noradrenaline also activates chloroethylclonidine sensitive receptors on the arteriolar surface.

Pharmacological characterization of alpha 1-adrenoceptor subtypes in the human prostate: functional and binding studies

OBJECTIVE

To characterize the alpha 1-adrenoceptor subtypes of the human benignly enlarged prostate using functional and binding studies.

MATERIALS AND METHODS

Strips of prostatic tissue taken from nine patients with benign prostatic hypertrophy who were undergoing open prostatectomy were used in the study.

RESULTS

The strips isolated from five prostates produced a large contraction in response to noradrenaline and phenylephrine but not to clonidine. The contractile response induced by noradrenaline was competitively antagonized by representative alpha 1-adrenoceptor antagonists (prazosin, WB4101, 5-methylurapidil and HV723), the dissociation constants (pKB) being < 8.5. Pre-treatment with chloroethylclonidine was without effect on the contractile response to noradrenaline. In saturation experiments with five prostates, [3H]-prazosin bound to the prostate membranes with two distinct affinities (pKD = 9.95 +/- 0.07 and 8.71 +/- 0.04, Bmax = 151 +/- 8 and 138 +/- 3 fmol/mg protein, respectively). Unlabelled prazosin and WB4101 biphasically displaced the binding of 200 pM [3H]-prazosin; the resulting high and low pKI values for each of the antagonists were consistent with the two pKD values obtained for [3H]-prazosin in the saturation experiments. 5-Methylurapidil and HV723 displaced the [3H]-prazosin binding monophasically with an affinity (pKI) close to 8.5.

CONCLUSIONS

These results suggest the presence of at least two distinct alpha 1-adrenoceptor subtypes (presumably an alpha 1C subtype with a high affinity for prazosin and WB4101, and a putative alpha 1L subtype with a low affinity for the antagonists) in the human prostate, in which the latter subtype may be predominantly involved in the contractile response to noradrenaline.

Identification of the alpha 1c-adrenoceptor in rabbit arteries and the human saphenous vein using the polymerase chain reaction

The expression of the alpha 1C-adrenoceptor subtype in human and rabbit blood vessels has been analyzed using the reverse transcriptase/polymerase chain reaction technique (RT/PCR). The 20 bp primers employed were designed from the bovine alpha 1C-adrenoceptor and flank a least conserved region--the putative third cytoplasmic loop. RT/PCR products generated from rabbit and human brain mRNA both had 93% homology to the bovine alpha 1C-adrenoceptor and were used as species and subtype specific probes in Southern blot analysis of vascular RT/PCR products. Poly A+ RNA was purified from the human saphenous vein and rabbit aorta, renal, pulmonary and central ear arteries and amplified by RT/PCR. Size analysis by agarose gel electrophoresis, together with Southern hybridization of the resulting cDNA products confirm the expression of the alpha 1C-adrenoceptor in these vessels.