Heterogeneity of alpha 1 receptors associated with vascular smooth muscle: evidence from functional and ligand binding studies

Alpha1L-adrenoceptors in canine pulmonary artery

The aim of this study was to characterize the alpha-adrenoceptors of the canine pulmonary artery. Arterial rings from lower lung lobes were suspended for isometric-tension recording in the presence of cocaine (5 x 10(-6) M), hydrocortisone (3 x 10(-5) M), propranolol (5 x 10(-6) M), and rauwolscine (10(-7) M) to inhibit neuronal uptake, extraneuronal uptake, and beta- and alpha2-adrenoceptors, respectively. Prazosin was more potent against contractions evoked by phenylephrine (pA2 of 9.7) compared with methoxamine (pA2 of 8.4). SZL49 (10(-8) and 3 x 10(-8) M), an irreversible alpha1-adrenergic antagonist, inhibited responses to phenylephrine but not methoxamine. With norepinephrine, low concentrations of prazosin (3 x 10(-10) M and 10(-9) M) caused inhibition of the concentration-response curve; a higher concentration (3 x 10(-9) M) failed to produced further inhibition, whereas increasing the concentration of the antagonist (to 10(-8) and 3 x 10(-8) M) caused further rightward shifts in the concentration-response curve. The Arunlakshana and Schild plot revealed two components corresponding to pA2 values of 9.8 and 8.4. After SZL49 (3 x 10(-8) M), the Arunlakshana and Schild plot for the interaction between norepinephrine and prazosin was linear and generated a pA2 of 8.3. Contractions evoked by phenylephrine were inhibited by the alpha1B/alpha1D-adrenoceptor antagonist, chloroethylclonidine (10(-5) M), or by the alpha1B-antagonist, risperidone (pA2 value of 8.5), but were relatively resistant to inhibition by the selective alpha1D-antagonist, BMY7378 (-log K(B) of 6.1). The results suggest that two alpha1-adrenoceptor subtypes mediate contraction of the canine pulmonary artery. One subtype has high affinity for prazosin (alpha1H, likely to be alpha1B), is activated by phenylephrine, and is inhibited by SZL49. The other subtype has lower affinity for prazosin (alpha1L), is stimulated by methoxamine, and is relatively resistant to SZL49. The physiologic agonist, norepinephrine, causes contraction by activating both subtypes.

Alpha-adrenoceptors and vascular regulation: molecular, pharmacologic and clinical correlates

This manuscript is intended to provide a comprehensive review of the alpha-adrenoceptors (ARs) and their role in vascular regulation. The historical development of the concept of receptors and the division of the alpha-ARs into alpha 1 and alpha 2 subtypes is traced. Emphasis will be placed on current understanding of the specific contribution of discrete alpha 1- and alpha 2-AR subtypes in the regulation of the vasculature, selective agonists and antagonists for these receptors, the second messengers utilized by these receptors, the myoplasmic calcium pathways activated to initiate smooth muscle contraction, as well as the clinical uses of agonists and antagonists that work at these receptors. New information is presented that deals with the molecular aspects of ligand interactions with specific subdomains of these receptors, as well as mRNA distribution and the regulation of alpha 1- and alpha 2-AR gene transcription and translation.

Recent advances in the molecular pharmacology of the alpha 1-adrenergic receptors

This review is intended to discuss recent developments in the molecular pharmacology of the alpha 1-adrenergic receptor (alpha 1-AR) subtypes. After a brief historical development, we will focus on the more contemporary issues having to do with this receptor family. Emphasis will be put on recent data regarding the cloning, nomenclature, signalling mechanisms, and genomic organization of the alpha 1-AR subtypes. We will also highlight recent mutational studies that identify key amino acid residues involved in ligand binding, as well as the role of the alpha 1-AR subtypes in regulating physiologic processes.

Vascular alpha-1 adrenergic receptor subtypes in the regulation of arterial pressure

Alpha 1 (alpha 1)-adrenoceptors can be found at numerous end organs in the autonomic nervous system, especially vascular smooth muscle. The tonic sympathetic activation of vascular alpha 1-adrenoceptors maintains vascular resistance and is vital to the regulation of arterial pressure. Recent evidence clearly demonstrates that alpha 1-adrenoceptors are a heterogenous class of receptors and that each subtype may subserve specific cardiovascular functions. Elucidation of the physiological role of each subtype in the regulation of vascular resistance and arterial pressure will enhance our understanding of the cardiovascular system and may facilitate the development of therapeutics with improved efficacy and tolerability.

Alpha 1b-adrenoceptors mediate renal tubular sodium and water reabsorption in the rat

1. It is known that activation of alpha 1-adrenoceptors causes renal vasoconstriction and increased tubular Na+ and water reabsorption, with the alpha 1a-subtype mediating the constrictor effect. 2. This study examines which subtype of alpha 1-adrenoceptors mediates tubular Na+ and water reabsorption in pentobarbitone-anaesthetized rats. In order to avoid systemic effects, phenylephrine (0.3 to 30 micrograms kg-1), methoxamine (0.1-10 micrograms kg-1) and vehicle were infused into the right renal artery (via the suprarenal artery) of three groups of rats. Two other groups of rats were continuously infused with the irreversible selective alpha 1b-adrenoceptor antagonist, chloroethylclonidine (3 mg kg-1 h-1) for 1 h, prior to the construction of dose-response curves to phenylephrine or methoxamine. Another group was continuously infused with the irreversible selective alpha 1a-adrenoceptor antagonist, SZL-49 (10 micrograms kg-1 h-1) for 1 h, prior to the construction of dose-response curves to phenylephrine. Mean arterial pressure (MAP), heart rate (HR), urine flow, Na+ and K+ excretion, and urine osmolality were monitored. 3. Phenylephrine and methoxamine did not affect MAP or HR but dose-dependently and significantly decreased urine flow, urine osmolality as well as Na+ excretion and, slightly increased K+ excretion, although this was significant only for phenylephrine. 4. The antidiuretic, antinatriuretic and kaliuretic effects of phenylephrine were abolished by pretreatment with chloroethylclonidine, but were not inhibited by SZL-49. The inhibitory effects of methoxamine on urine flow and Na+ excretion were also almost totally abolished by chloroethylclonidine. 5. Our results show that alpha 1b-adrenoceptors mediate renal tubular Na+ and water reabsorption.

Pharmacological characterization of two distinct alpha 1-adrenoceptor subtypes in rabbit thoracic aorta

1. alpha 1-Adrenoceptor subtypes in rabbit thoracic aorta have been examined in binding and functional experiments. 2. [3H]-prazosin bound to two distinct populations of alpha 1-adrenoceptors (pKD,high = 9.94, Rhigh = 79.2 fmol mg-1 protein; pKD,low = 8.59, Rlow = 215 fmol mg-1 protein). Pretreatment with chloroethylclonidine (CEC, 10 microM) almost inactivated the prazosin-high affinity sites and reduced the number of the low affinity sites without changing the pKD value. 3. In the displacement experiments with CEC-untreated membranes, unlabelled prazosin, WB4101 and HV723 displaced the binding of 200 pM [3H]-prazosin monophasically; the affinities for WB4101 (pK1 = 8.88) and HV723 (8.49) were about 10 times lower than that for prazosin (9.99). In the CEC-pretreated membranes also, the antagonists inhibited the binding of 1000 pM [3H]-prazosin monophasically; the pK1 values for prazosin, WB4101 and HV723 were 9.09, 8.97 and 8.17, respectively. These results suggest that the prazosin-high and low affinity sites can be independently appraised in the former and latter experimental conditions. Noradrenaline, but not methoxamine, showed slightly higher affinity for the prazosin-high affinity site than for the low affinity site. 4. In the functional experiments, noradrenaline (0.001-100 microM) and methoxamine (0.1-100 microM) produced concentration-dependent contractions. Pretreatment with CEC inhibited the contractions induced by low concentrations of noradrenaline but without effect on the responses to methoxamine. Prazosin inhibited the concentration-response curves for noradrenaline in the CEC-untreated aorta in a manner which was not consistent with competitive antagonism at a single site, and two distinct affinity constants(pKB = 9.71 and 8.74) were obtained. However, after CEC-pretreatment, Schild plots for prazosin were not significantly different from unity (pKB = 8.50). WB4101 and HV723 competitively inhibited the noradrenaline-induced contraction with low pKB values (approximately 8.30), irrespective of CEC pretreatment.Methoxamine-induced contractions were competitively inhibited by prazosin, WB4101 and HV723 with low pKB values similar to those obtained when noradrenaline was used as the agonist.These were not affected by CEC-pretreatment.5. The affinity constant for noradrenaline (pKA = 6.40) in CEC-untreated aorta was slightly greater than that obtained in CEC-pretreated aorta (5.78). On the other hand, methoxamine showed a similar affinity in CEC-untreated and pretreated aortae (pKA = approximately 4.5).6. Nifedipine (1 microM) slightly attenuated the contractile responses to noradrenaline and methoxamine in CEC-untreated and pretreated aortae, suggesting that nifedipine cannot discriminate between alpha 1-adrenoceptors involved in CEC-sensitive and -resistant contractions.7. From these results it is suggested that in the rabbit thoracic aorta there are two distinct alpha 1-adrenoceptor subtypes (presumably alpha 1B and alpha 1L subtypes according to recently proposed subclassification),both of which are involved in noradrenaline-induced contraction. The alpha 1L subtype predominantly mediates the contraction induced by methoxamine.

Errors introduced in radioligand binding studies due to displaceable, cation dependent, [3H]prazosin binding to glass-fibre filters and glass surfaces

[3H]prazosin not only specifically and homogeneously labels alpha 1-adrenoceptors, but also binds to glass surfaces and non-linearly to the glass-fibre filters, commonly used in radioligand binding experiments. Binding to filters can be modulated by unlabeled alpha-adrenergic compounds and cations. If no correction is applied for displaceable filter binding, analysis of [3H]prazosin binding experiments leads to erroneous results. Analysis of [3H]prazosin saturation experiments on guinea-pig cerebral cortex membranes with correction for filter binding before the non-linear fit procedure indicated that [3H]prazosin labels a homogeneous population of alpha 1-adrenoceptors (Rtot: 8.33 fmol.mg-1 wet tissue) with a dissociation constant of 1.28 x 10(-10) M. However, analysis of the same data after correction for non-specific binding, (determined in parallel experiments by adding 10 microM phentolamine to the incubation medium) resulted in a best fit to a model in which [3H]prazosin labels two alpha 1-adrenoceptor subpopulations (R1: 15.0 fmol.mg-1 and R2: 14.6 fmol.mg-1 wet tissue) with dissociation constants of respectively 1.78 x 10(-10) and 5.63 x 10(-9) M. The discrepancy between the two methods of analysis is due to displacement of the radioligand from the filters by phentolamine. Prazosin and oxymetazoline are also able to displace filter-bound [3H]prazosin. The extent to which displaceable filter binding distorts the proper results depends on the actual magnitude of the error and also on the method of analysis.

Vascular α-adrenoceptor affinity variation is not due to varying populations of subtypes distinguished by WB 4101 and chlorethylclonidine

Interaction with chlorethylclonidine has been used to subdivide populations of alpha 1-adrenoceptors in some tissues. WB 4101 can distinguish high and low affinity states of the receptor. The present study was carried out to determine if different populations or affinity states of alpha 1-adrenoceptors distinguished by either of these compounds, could explain the variation in alpha 1-adrenoceptor agonist affinity found amongst rabbit arteries. Five arteries were studied whose affinity for noradrenaline vary between 4.8 and 6.4. These were the thoracic aorta, renal, superior mesenteric, ear and ovarian arteries. WB 4101 was found to be equally effective in antagonizing noradrenaline on all arteries. Chlorethylclonidine caused a 20-fold rightward shift of the noradrenaline dose-contraction curve in the thoracic aorta; but had little or no effect on the other vessels. Thus, the combination of different proportions of subsets of alpha 1-adrenoceptors distinguished by WB 4101 or chlorethylclonidine does not explain the variation in alpha 1-adrenoceptor affinity found in these rabbit arteries.

Three distinct binding sites for [3H]-prazosin in the rat cerebral cortex

1. The putative alpha 1-adrenoceptor subtypes of rat cerebral cortex membranes were characterized in binding. 2. Specific binding of [3H]-prazosin was saturable between 20-5000 pm. Scatchard plots of the binding data were non-linear, indicating the presence of two distinct affinity sites for prazosin (pKD, high = 10.18, Rhigh = 308 fmol mg-1 protein; pKD, low = 8.96, Rlow = 221 fmol mg-1 protein). 3. In the membranes pretreated with chlorethylclonidine (CEC) two affinity sites for prazosin were also observed: the affinities were similar to those without CEC pretreatment, but the maximum numbers of binding sites were reduced by CEC pretreatment to 23 and 62% for prazosin-high (Rhigh) and low affinity sites (Rlow), respectively. 4. The prazosin-high affinity sites were further subdivided into two subclasses by WB4101(2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane) and phentolamine; the low affinity sites for WB4101 and phentolamine were more potently inactivated by CEC as compared with the high affinity sites. On the other hand, prazosin, HV723 (alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)ethyl)- amino )-propyl)benzeneacetonitrile fumarate) and yohimbine inhibited [3H]-prazosin binding to prazosin-high affinity sites monophasically. 5. In addition to the high affinity sites, the prazosin-low affinity sites were labelled at high concentrations of [3H]-prazosin. Thus, prazosin and WB4101 showed shallow displacement curves. On the other hand, HV723 and yohimbine did not discriminate between prazosin-high and low affinity sites. 6. Two distinct alpha 1-adrenoceptor subclassifications have been recently proposed (alpha 1A, alpha 1B subtypes and alpha 1H, alpha 1L, alpha 1N subtypes). 5. In both motoneurones and dorsal root fibres, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) effectively depressed the depolarization induced by kainoids, and neither 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1- phosphonic acid (CPP) nor picrotoxin blocked or affected the depolarization, but there were some differences in pharmacological potencies of glutamate antagonists between both preparations.6. MFPA, HFPA and acromelic acids should provide valuable pharmacological tools for analysis of physiological functions of excitatory amino acids, in particular, as specific agonists for some subtypes of kainate receptors.

Two distinct alpha 1-adrenoceptor subtypes involved in noradrenaline contraction of the rabbit thoracic aorta

1. Recently, alpha 1-adrenoceptors in blood vessels have been classified into three subtypes (alpha 1H, alpha 1L and alpha 1N). We examined which subtype (or subtypes) is involved in the noradrenaline-induced contraction of rabbit thoracic aorta. 2. Noradrenaline produced a concentration-dependent contraction in the rabbit isolated thoracic aorta. Prazosin antagonized the contractions to noradrenaline, resulting in a rightward displacement of the concentration-response curve. However, the shift was not proportional to the concentration of prazosin; Schild plots showed that the inhibition by prazosin was biphasic, implying that noradrenaline acted through two receptor populations. Two affinity constants (pKB values of 10.02 and 8.83) were determined for prazosin at these sites. 3. However, under continuous treatment with 1 nM prazosin, or in strips pretreated with chlorethylclonidine (CEC; an alpha 1H inactivating agent) to remove the contribution of one receptor population, prazosin showed a single pKB or pA2 value of approximately 8.3. 4. Yohimbine also produced biphasic antagonism of noradrenaline-induced contractions, resulting in two affinity constants (pKB = 6.52 and 6.17). However, a monophasic Schild plot was obtained for yohimbine either in the presence of 1 nM prazosin (pA2 = 6.08) or in strips pretreated with CEC (pA2 = 6.03). 5. The Schild plot for HV723 (a selective alpha 1N-antagonist) yielded a monophasic slope (pKB = 8.47) and the inhibition was not affected by 1 nM prazosin or CEC-pretreatment. 6. [3H]-prazosin bound to alpha 1-adrenoceptors of the aortic membrane preparations with two different affinities (pKD = 9.94 and 8.37). The high but not the low affinity site was completely masked by 1 nM prazosin and inactivated by pretreatment with CEC. 7. These results strongly suggest that noradrenaline-induced contraction of the rabbit thoracic aorta is mediated through two distinct alpha l-adrenoceptor subtypes, designated alpha 1H and (alpha lL*

Central alpha 1 adrenoceptor subtypes in narcolepsy-cataplexy: a disorder of REM sleep

The present study suggests the specific involvement within the central nervous system of an alpha 1 adrenoceptor subtype in a behavior, the control of cataplexy, a pathological analogue of rapid eye movement (REM) sleep atonia. Experiments have shown that prazosin, an alpha 1 antagonist, dramatically aggravates canine narcolepsy-cataplexy through a central mechanism, and that [3H]prazosin binding sites are increased in the amygdala of narcoleptic dogs. However, the corresponding Scatchard plots were curvilinear and best fit was obtained with a two-site model, suggesting the existence of two [3H]prazosin binding sites. These two sites (high and low affinity [3H]prazosin binding sites) met the criteria for authentic receptors and were respectively very similar to the alpha 1a and alpha 1b (high and low affinity for WB4101, respectively) subtypes recently described in the rat and rabbit. Our results of in vivo pharmacology and in vitro [3H]prazosin binding in canine narcolepsy now clearly implicate the low affinity [3H]prazosin binding site (alpha 1b) in canine narcolepsy: (1) Prazosin, an alpha 1 antagonist with similar affinity for both subtypes, was much more potent in increasing cataplexy than WB4101, a compound with more affinity for the alpha 1a receptor. (2) Chlorethylclonidine and phenoxybenzamine, two irreversible blockers of the alpha 1 receptors with more affinity for the alpha 1b receptors, aggravate cataplexy for up to two weeks. (3) The alpha 1 receptor upregulation previously reported by our group in the amygdala of narcoleptic dogs was due to a selective increase in the low affinity [3H]prazosin binding sites. A role for noradrenaline in REM sleep regulation has been suspected for many years, but has never been clearly elucidated.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for multiple [3H]prazosin binding sites in canine brain membranes

Two classes of alpha 1 adrenoceptors were identified in canine brain and liver using conventional radioligand binding methods. Scatchard plots of specific [3H]prazosin binding to brain and liver membranes prepared from 100-150-day-old Doberman pinscher dogs were consistently curvilinear and best fit a two-site binding model (frontal cortex, Kd1 = 57.7 +/- 10.0 pM, Bmax1 = 64.6 +/- 17.1 fmol/mg protein, Kd2 = 1.5 +/- 0.5 nM, Bmax2 = 159 +/- 37.6 fmol/mg protein; liver, Kd1 = 82.6 +/- 36 pM, Bmax1 = 7.0 +/- 5.1 fmol/mg protein, Kd2 = 0.8 +/- 0.2 nM, Bmax2 = 62.1 +/- 8.7 fmol/mg protein). Kinetically derived affinity constants from association and dissociation experiments agreed with those obtained by Scatchard analyses of equilibrium binding data. Binding sites were saturable, heat labile, bound ligand reversibly, and appeared to be appropriately distributed in relation to endogenous catecholamine. [3H]Prazosin also bound with high affinity to two classes of binding site in porcine and bovine brain membrane but [3H]prazosin binding in monkey and rat brain was best described by a single-site binding model. Affinities obtained were in between values obtained for high and low affinity Kds in the other species. Competitions for [3H]prazosin binding sites in canine frontal cortex were conducted with the following antagonists: WB-4101, corynanthine, phentolamine, benoxathian, phenoxybenzamine, chlorethylclonidine, thymoxamine, prazosin, yohimbine and agonists: methoxamine, (-)-norepinephrine, and clonidine. All ligands but prazosin, norepinephrine and clonidine competed for specific [3H]prazosin binding in a statistically significant biphasic manner. Benoxathian and WB-4101 displayed the highest affinities (benoxathian: Ki1 = 0.26 nM, WB-4101: Ki1 = 0.20 nM) and selectivity (high affinity/low affinity: benoxathian = 1640, WB-4101 = 13204) for the high affinity [3H]prazosin binding site; chlorethylclonidine had highest affinity (Ki2 = 91 nM) and selectivity (low affinity/high affinity = 405) for the lower affinity [3H]prazosin binding site. As defined, the two sites were similar to the alpha 1a and alpha 1b recently described in the rat and rabbit. A noticeable difference was that the subtypes described in dog brain had a 30-fold difference in affinity for prazosin.

Buffer effects on high affinity [3H]-prazosin binding in brain and spinal cord

[3H]-Prazosin binding was characterized in cortical and spinal membranes from Fischer 344N and Sprague-Dawley rats. Estimates of Bmax and Kd values were comparable with earlier studies of these regions in the central nervous system (CNS). However, the Kd obtained using Tris buffer system was greater than when HEPES or phosphate buffer was used. These data indicate that high affinity [3H]-prazosin binding in the homogenates of tissue from the CNS is affected critically by buffer selection.