Alpha-adrenergic receptors in liver membranes: delineation with subtype selective radioligands

α2-Adrenergic Receptor in Liver Fibrosis: Implications for the Adrenoblocker Mesedin

The noradrenergic system is proposed to play a prominent role in the pathogenesis of liver fibrosis. While α1- and β-adrenergic receptors (ARs) are suggested to be involved in a multitude of profibrogenic actions, little is known about α2-AR-mediated effects and their expression pattern during liver fibrosis and cirrhosis. We explored the expression of α2-AR in two models of experimental liver fibrosis. We further evaluated the capacity of the α2-AR blocker mesedin to deactivate hepatic stellate cells (HSCs) and to increase the permeability of human liver sinusoidal endothelial cells (hLSECs). The mRNA of α2a-, α2b-, and α2c-AR subtypes was uniformly upregulated in carbon tetrachloride-treated mice vs the controls, while in bile duct-ligated mice, only α2b-AR increased in response to liver injury. In murine HSCs, mesedin led to a decrease in α-smooth muscle actin, transforming growth factor-β and α2a-AR expression, which was indicated by RT-qPCR, immunocytochemistry, and Western blot analyses. In a hLSEC line, an increased expression of endothelial nitric oxide synthase was detected along with downregulated transforming growth factor-β. In conclusion, we suggest that the α2-AR blockade alleviates the activation of HSCs and may increase the permeability of liver sinusoids during liver injury.

Effect of 5-HT7 receptor blockade on liver regeneration after 60-70% partial hepatectomy

Background

Serotonin exhibits a vast repertoire of actions including cell proliferation and differentiation. The effect of serotonin, as an incomplete mitogen, on liver regeneration has recently been unveiled and is mediated through 5-HT₂ receptor. The aim of the present study was to investigate the effect of 5-HT₇ receptor blockade on liver regeneration after partial hepatectomy.

Methods

Male Wistar rats were subjected to 60-70% partial hepatectomy. 5-HT₇ receptor blockade was applied by intraperitoneal administration of SB-269970 hydrochloride two hours prior to and sixteen hours after partial hepatectomy and by intraperitoneal administration of SB-258719 sixteen hours after partial hepatectomy. Animals were sacrificed at different time points until 72 h after partial hepatectomy. Liver regeneration was evaluated by [³H]-thymidine incorporation into hepatic DNA, the mitotic index in hematoxylin-eosin (HE) sections and by immunochemical detection of Ki67 nuclear antigen. Reversion of 5-HT₇ blockade was performed by intraperitoneal administration of AS-19. Serum and liver tissue lipids were also quantified.

Results

Liver regeneration peaked at 24 h ([³H]-thymidine incorporation into hepatic DNA and mitotic index by immunochemical detection of Ki67) and at 32 h (mitotic index in HE sections) in the control group of rats. 5-HT₇ receptor blockade had no effect on liver regeneration when applied 2 h prior to partial hepatectomy. Liver regeneration was greatly attenuated when blockade of 5-HT₇ receptor was applied (by SB-258719 and SB-269970) at 16 h after partial hepatectomy and peaked at 32 h ([³H]-thymidine incorporation into hepatic DNA and mitotic index by immunochemical detection of Ki67) and 40 h (mitotic index in HE sections) after partial hepatectomy. AS-19 administration totally reversed the observed attenuation of liver regeneration.

Conclusions

In conclusion, 5-HT₇ receptor is a novel type of serotonin receptor implicated in hepatocyte proliferation.

Adrenergic receptors: classification, ligand binding and molecular properties

The interaction of catecholamines and drugs with adrenergic receptors leads to a set of biochemical reactions which ultimately results in a physiological response. A brief review is given of the classification of adrenergic receptors into subtypes and the use of ligand binding techniques for the identification and characterization of these receptors. Recent advances in the biochemistry of adrenergic receptors are reviewed with special reference to the interaction of the beta and alpha 2-receptors with guanine nucleotide regulatory proteins and adenylate cyclase. The role of calcium and phosphoinositides in the function of the alpha 1-receptor is also discussed.

Effects of pertussis toxin on extracellular signal-regulated kinase activation in hepatocytes by hormones and receptor-independent agents: evidence suggesting a stimulatory role of G(i) proteins at a level distal to receptor coupling

It was previously found that pertussis toxin (PTX) pretreatment inhibits the activation of extracellular signal-regulated kinases ERK1 (p44(mapk)) and ERK2 (p42(mapk)) in hepatocytes in response to either agonists that bind to heptahelical receptors or epidermal growth factor (EGF), suggesting a role of G(i) proteins in stimulatory mechanisms for ERK1/2. The present work shows that ERK1/2 is activated in a PTX-sensitive way not only by vasopressin, angiotensin II, prostaglandin (PG) F(2alpha), alpha(1)-adrenergic stimulation, and EGF but also by agents whose actions bypass receptors and stimulate protein kinase C (PKC) and/or elevate intracellular Ca(2+), such as 12-O-tetradecanoyl phorbol-13-acetate (TPA), exogenous phosphatidylcholine-specific phospholipase C (PC-PLC, from Bacillus cereus), thapsigargin, and the Ca(2+) ionophore A23187. Under the same conditions, PTX did not affect agonist stimulation of phosphoinositide-specific phospholipase C (PI-PLC) (IP(3) generation), and did not reduce the activation by these agents of phospholipase D (PLD). The results suggest that in hepatocytes a PTX-sensitive mechanism, presumably involving G(i) proteins, exerts a stimulatory effect on ERK at a level distal to receptor coupling, acting either as an integral part of the signaling pathway(s) or by a permissive, synergistic regulation.

Differential effects of adrenaline and noradrenaline on the hepatic expression of immediate early genes in mice

1. The effects of intraperitoneally (i.p.) administered noradrenaline and adrenaline on the hepatic expression of immediate early genes (IEGs) were studied in mice. 2. Intraperitoneal injections of various doses (0.2-2 mg kg(-1)) of noradrenaline and adrenaline dose-dependently induced hepatic c-fos and c-jun mRNA levels. The time-course study showed that there was an increase in c-fos and c-jun mRNA levels within 15 min, which reached a peak at 30 min, and returned to the basal levels 1-2 h after noradrenaline or adrenaline injection (2 mg kg(-1), i.p.). A Western blot assay revealed that c-Jun protein levels were maximally increased at 30 min and 1-2 h in noradrenaline- and adrenaline-treated mice, respectively. There was a slight increase in c-Fos protein, while 46-kDa Fra protein was prominently increased. Noradrenaline (2 mg kg(-1), i.p.) induced 46-kDa Fra within 15 min, which reached a maximum at 30 min and returned to the basal levels by 1 h. Adrenaline (2 mg kg(-1), i.p.) induced 46-kDa Fra at 30 min, which returned to the basal levels at 4 h. 3. Noradrenaline (2 mg kg(-1), i.p.)-induced increases in c-fos and c-jun mRNA expressions were inhibited by the pre-treatment with prazosin (alpha1-adrenergic antagonist; 0.5 mg kg(-1), i.p.), but not with yohimbine (alpha2-adrenoceptor antagonist; 1 mg kg(-1), i.p.) nor with propranolol (beta-adrenoceptor antagonist; 10 mg kg(-1), i.p.). Adrenaline (2 mg kg(-1), i.p.)-induced increases in c-fos and c-jun mRNA expressions were inhibited by the pre-treatment with prazosin or with propranolol, but not with yohimbine. Administration of ICI-118,551 (beta2-adrenoceptor antagonist; 2 mg kg(-1), i.p.), but not betaxolol (beta1-adrenoceptor antagonist; 2 mg kg(-1), i.p.), blocked adrenaline (2 mg kg(-1), i.p.)-induced increases in c-fos and c-jun mRNA expressions. 4. The results suggest that noradrenaline elicits the hepatic c-fos and c-jun mRNA responses by stimulating alpha1-adrenergic receptors, whereas in the case of adrenaline, this is elicited by stimulating both alpha1- and beta2-adrenergic receptors in mice. These catecholamine-induced hepatic IEG responses may be responsible for mediating some of the catecholamine actions in the liver.

Effects of clonidine and IBMX on sulfobromophthalein disposition in rats

Clonidine, an alpha 2-adrenoceptor agonist, inhibited the biliary excretion, reduced the plasma clearance and increased the hepatic retention of sulfobromophthalein (BSP) in a dose related fashion in rats. The maximal effects of clonidine on BSP disposition occurred about 4 h after pretreatment. The effects of clonidine on biliary excretion and hepatic retention of BSP were retained following laparotomy (with or without bile duct cannulation); however, the effect of clonidine on plasma disappearance profile was not retained following abdominal surgery. Isobutylmethylxanthine (IBMX) affected BSP disposition in a similar fashion as clonidine, in rats without bile duct cannulation only; no effect of IBMX could be observed in bile duct cannulation rats. Yohimbine, an alpha 2-adrenoceptor antagonist, reversed the effects of clonidine, but not of IBMX, on BSP disposition. It thus seems that clonidine and IBMX exert their effects on BSP disposition by different mechanisms and probably at different sites.

The endogenous cyclic AMP antagonist, cyclic PIP: its ubiquity, hormone-stimulated synthesis and identification as prostaglandylinositol cyclic phosphate

This report shows that the cyclic AMP antagonist cyclic PIP is present in all organs and tissues of the rat so far examined: brain, heart, lung, intestine, kidney, liver, spleen, skeletal muscle and fat. The synthesis of cyclic PIP is stimulated by insulin or noradrenaline (alpha-adrenergic action) in a dose-dependent fashion. Increasing cyclic PIP synthesis with increasing insulin concentrations matches the insulin receptor binding curves. Cyclic PIP levels in blood serum remain low after hormonal stimulation and no cyclic PIP can be detected in urine. As an indication of its ubiquity, cyclic PIP was even detected in yeast. Prostaglandin E (as shown by incorporation of [3H]PGE into cyclic PIP and demonstration of a constant specific activity), myo-inositol (as shown by acid hydrolysis of the dephosphorylated cyclic PIP and mass spectrometric identification of the products) and one phosphate (as shown by the ionic nature of cyclic PIP and its inactivation by phosphodiesterase plus phosphatase) are components of cyclic PIP. Chemical derivatization experiments of cyclic PIP suggest the phosphate to be bound to myo-inositol and the myo-inositol phosphate to the prostaglandin E by its C15-hydroxyl group.

Auditory-evoked response of the cortex after yohimbine administration: phase advance effect of central noradrenergic activation

The effects of central noradrenergic activation on an auditory-evoked cortical response were studied using systemic administration of yohimbine (2 mg/kg intravenously, IV), a noradrenergic stimulant, in 13 anesthetized rats. To analyze changes of the response, surface and intracortical evoked potentials (EP) as well as extracellular single-unit recordings with tungsten microelectrodes were employed. It was noted that the initial-positive wave of the surface EP corresponded to unit firing responses in a restricted area of the auditory cortex, where the surface EP was largest and a polarity inversion of the intracortical EP was observed. The following effects were produced by yohimbine: 1) The initial-positive surface potential (n = 10) and corresponding intracortical potential with inverted polarity (n = 6) both showed an increase in amplitude and a decrease in peak latency; 2) the unit firing response (n = 10) tended to show an increase in peak frequency and a decrease in peak firing latency; and 3) yohimbine produced an earlier ending of the firing period, and in paired stimulation experiments (n = 7) it prolonged the period during which the second response was suppressed, indicating an augmentation of postexcitation inhibition. Later histological examination suggested that most of the units recorded were pyramidal cells. These findings indicate that chemical stimulation of the central noradrenergic system by yohimbine enhances both the initial excitatory and following inhibitory processes in the auditory-evoked response of the cortical units (probably pyramidal cells), resulting not only in amplification of the response but also in advancement of the response phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by noradrenaline and adrenaline of the increase in glucose and lactate output and decrease in flow after sympathetic nerve stimulation in perfused rat liver: possible involvement of protein kinase C

In perfused rat liver stimulation of the hepatic nerve plexuses increased via alpha 1-receptors glucose and lactate output decreased flow and caused an overflow of noradrenaline into the hepatic vein. Infusion of noradrenaline and adrenaline also elicited similar metabolic and hemodynamic alterations via alpha 1-receptors, whereas infusion of isoproterenol via beta 2-receptors enhanced glucose output and slightly reduced lactate release without affecting flow. The influence of circulating catecholamines on the nerve stimulation-dependent changes was investigated. Noradrenaline (100 nmol/L) or adrenaline (40 nmol/L) but not isoproterenol (1 mumol/L), which themselves caused about half-maximal alterations, strongly inhibited the nerve stimulation-induced increase in glucose and lactate output and decrease in flow but had no effect on noradrenaline overflow. The protein kinase C activator (4 beta)phorbol 12-myristate, 13-acetate (100 nmol/L) but not its analog (4 alpha)phorbol 12,13-didecanoate (100 nmol/L) strongly inhibited the metabolic and hemodynamic changes caused by nerve stimulation or noradrenaline infusion. The protein kinase C inhibitor H7 (20 mumol/L) partially prevented the inhibition of the nerve actions by noradrenaline. The results lead us to conclude that noradrenaline and adrenaline inhibited the metabolic and hemodynamic nerve actions by means of a mechanism involving protein kinase C rather than presynaptic alpha-receptors or beta-receptors. The catecholamines apparently increased via alpha 1-receptors inositol 1,4,5-trisphosphate, which in turn enhanced cytosolic Ca2+ and thus altered metabolism and in part hemodynamics, and diacylglycerol, which in turn activated protein kinase C and thus feedback inhibited the signal chain from alpha 1-receptors via G proteins to phospholipase C.

Effects of the stereochemical orientation of phenethylamines and imidazolines on alpha-adrenergic receptor-mediated DNA synthesis in primary cultured rat hepatocytes

The hepatic alpha 1-adrenergic receptor mediates a variety of hepatic functions including respiration, glycogenolysis, gluconeogenesis, and growth. We have utilized a rat primary hepatocyte culture system to show that the alpha 1-adrenergic receptor can be activated in a stereoselective manner by a series of phenethylamines and catecholimidazolines resulting in the stimulation of DNA synthesis as determined by [3H]thymidine incorporation. The phenethylamines adhered to the Easson-Stedman hypothesis with a rank order of potency of (-)-(R)-norepinephrine (NE) greater than (+)-(S)-NE greater than the desoxy analog dopamine (DA) for the stimulation of DNA synthesis. However, the 2-substituted catecholimidazolines did not follow this trend and demonstrated an order of potency of the desoxy analog 3,4-dihydroxybenzyl imidazoline (DHT) greater than or equal to (-)-(R)-2-(3,4,alpha-trihydroxybenzyl)imidazoline (TBI) greater than (+)-(S)-TBI. 4-Substituted catecholimidazolines were less potent as inducers of DNA synthesis than the corresponding 2-substituted analogs with an order of potency of (+)-(R)-4-(3,4-dihydroxybenzyl)imidazoline (DBI) greater than (+,-)-(R,S)-DBI greater than (-)-(S)-DBI. When the beta-hydroxyl moiety of NE is replaced with an amino group as in 3,4-dihydroxyphenylethylenediamine, the isomers are less active than the beta-hydroxylated analogs and also demonstrate no stereoselectivity for the stimulation of DNA synthesis. These results demonstrate that the hepatic alpha 1-adrenergic receptor can recognize various isomeric forms of these compounds and that hepatocellular growth can be modulated in a stereoselective manner by phenethylamines and imidazolines.

Heparin and other polyanions uncouple alpha 1-adrenoceptors from G-proteins

Several polyanionic compounds antagonize the interaction between receptors and the G-proteins that regulate adenylate cyclase or K+ channels, possibly by binding to a basic stretch of the receptor that is proposed to mediate its interaction with the G-proteins. We have studied the effects of polyanions on the interaction between the liver alpha 1-adrenoceptor and the G-protein through which it stimulates polyphosphoinositide turnover. Heparin [concn. causing 50% of maximal effect (EC50) = 0.5 microM], Trypan Blue (EC50 7.1 microM) or suramin (EC50 2.1 microM) prevented formation of the high-affinity adrenaline-receptor-G-protein complex without affecting antagonist binding. After alkaline treatment of the membranes, previously reported to cause G-protein removal, binding of agonists was insensitive to both guanine nucleotides and heparin. We conclude that these polyanions uncouple the alpha 1-adrenoceptor from its G-protein, suggesting that similar coupling mechanisms may underlie receptor activation of the G-proteins that activate polyphosphoinositide hydrolysis and those that regulate adenylate cyclase. This action of heparin severely limits its utility as a selective antagonist of the Ins(1,4,5)P3 receptor in intact cells.

The formamidine pesticides chlordimeform and amitraz decrease hepatic glutathione in mice through an interaction with alpha 2-adrenoceptors

Recent studies have provided evidence that formamidine pesticides, such as chlordimeform (CDM; N'-4-chloro-o-tolyl-N,N-dimethylformamidine) or amitraz (AMZ; N'-2-4-(dimethylphenyl)-N-[((2,4-dimethylphenyl)imino)methyl]-N- methanimidamide) exert some of their toxic effects by an interaction with alpha 2-adrenoceptors. Since epinephrine and clonidine have been shown to decrease hepatic glutathione (GSH) by activating alpha 2-adrenoceptors, and alpha 2-antagonists partially antagonize GSH depletion and hepatotoxicity caused by bromobenzene and cocaine, we have investigated whether the formamidines would affect hepatic GSH levels in mice. Both CDM and AMZ decreased hepatic nonprotein sulfydryls (NPSH) to a maximum of about 40%, in a dose-dependent manner. The effect of AMZ was longer lasting than that of CDM. For both compounds, decrease of hepatic NPSH was antagonized by the alpha 2-antagonist yohimbine but not by the alpha 1-antagonist prazosin or the beta-antagonist propanolol. The alpha 2-agonist clonidine also caused a dose-dependent decrease of hepatic NPSH (to a maximum of 40%), which was prevented only by yohimbine. The effects of AMZ, CDM, and clonidine were not additive, suggesting that all compounds act on a common site and/or with a common mechanism. Adrenalectomy or destruction of peripheral sympathetic nerves with 6-hydroxydopamine did not alter the ability of CDM and AMZ to decrease hepatic NPSH. These results indicate that formamidine pesticides can affect the levels of hepatic GSH, possibly through a direct interaction with hepatic alpha 2-adrenoceptors.

Effect of detergent solubilization on the affinity of some quinazoline derivatives for the alpha 1-adrenoceptor

[3H]Prazosin bound to a single class of high-affinity sites in both bovine aortic and rat hepatic membranes. The absolute affinity values of displacing ligands (prazosin greater than doxazosin much greater than trimazosin greater than yohimbine) were the same for both tissues. After solubilization of the alpha 1-adrenoceptors with digitonin and 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate, an identical rank order potency was observed. However, solubilization significantly reduced ligand affinity. In both tissues the affinity of prazosin was reduced 10- to 13-fold, whereas the affinities of doxazosin, trimazosin and yohimbine were reduced two- to six-fold. There appeared to be no relationship between the lipophilicities of the ligands and the degree to which affinity is affected by solubilization. The results suggest that the reductions in affinity are the consequence of a conformational change in the alpha 1-adrenoceptor and appear to support the hypothesis that the alpha 1-adrenoceptor is so constructed that the spatial configuration of the binding site can change in response to an alteration in its microenvironment.

Correlations between hepatic monooxygenase system and glycogen storage

In experiments performed on female Wistar rats we have stated that simultaneously with glycogen mobilisation occurring immediately after epinephrine injection the amount of cytochrome P-450 (cP-450) was reduced. A characteristic cP-450 dependent function hexobarbital biotransformation was inhibited. No changes in hepatic protein and water content and serum bilirubin were measured. Glycogen and cP-450 loss and functional impairment depend on the administered epinephrine dose and time of treatment. We have proven that reduced activity of mixed function monooxygenase is related only to diminished glycogen content of the liver. Restitution of glycogen stores as well as amount and function of cP-450 were parallel.

Changes in number of alpha-adrenergic receptor subtypes in hepatocytes from rats fed 3'-methyl-4-dimethylaminoazobenzene

Changes in numbers of alpha 1- and alpha 2-adrenergic receptors in the plasma membranes of hepatocytes from female Donryu rats given feed containing 0.06% of the carcinogen 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB), were examined. alpha 1-Adrenergic receptors, measured in terms of [3H]prazosin binding, decreased to half of the control 2 weeks after the start of this diet, then gradually decreased for the next 22 weeks. alpha 2-Adrenergic receptors, measured in terms of [3H]clonidine binding, transiently increased 3-fold over the control at 2 weeks. These changes in the early period of the 3'-MeDAB diet intake may be related to hepatocarcinogenesis.

Alpha adrenoceptor sites in vascular smooth muscle. Differentiation by selective antagonist binding

The properties of alpha adrenoceptors in rat-tail artery membranes were studied using tritiated ligands that are selective for the alpha 1 and alpha 2 subtypes. High-affinity saturable binding was obtained for the alpha 1 antagonist prazosin yielding a Bmax of 144 +/- 31.6 fmol/mg protein (mean +/- SEM, N = 3) and a Kd of 0.17 +/- 0.04 nM, and also for the alpha 2 antagonist rauwolscine which yielded a Bmax of 141.3 +/- 19.3 fmol/mg protein and a Kd of 1.57 +/- 0.32 nM. The [3H]prazosin-labelled sites displayed a pharmacological profile characteristic of an alpha 1 adrenoceptor, whereas the [3H]rauwolscine-labelled sites exhibited the expected alpha 2 adrenoceptor profile. Agonist affinity for [3H]rauwolscine sites was reduced by Gpp(NH)p and Na+, and the effects appeared synergistic for adrenaline, but non-interactive for UK-14304. Agonist interaction with [3H]prazosin sites in the rat-tail artery was also regulated by Gpp(NH)p and Na+, although clearly in a qualitatively and quantitatively different manner from the [3H]rauwolscine sites. These results suggest that distinct binding sites for [3H]prazosin and [3H]rauwolscine could be differentiated with antagonist ligands. These distinct antagonist recognition sites demonstrate the pharmacological profile expected for alpha 1 and alpha 2 adrenoceptors, and the quantitatively differing abilities of Na+ and Gpp(NH)p to regulate agonist interactions with these sites are suggestive, but do not necessarily prove, that different G proteins may be involved in this regulation.

[3H]rauwolscine binding to myometrial alpha 2-adrenoceptors in pregnant guinea pig

Uterine sympathetic nerves can exert an excitatory influence in late pregnancy and during parturition. Neuronal norepinephrine release is increased at these times and a diminished alpha 2-adrenoceptor-mediated prejunctional inhibition could account for this. To assess whether an altered receptor population may contribute, [3H]rauwolscine was used to measure alpha 2-adrenoceptors in myometrial membranes at time intervals throughout pregnancy. High affinity [3H]rauwolscine binding [Kd = 11.4 +/- 1.5 nM (n = 42)] yielded linear Scatchard plots that in nonpregnant myometrium indicated a maximum binding density (Bmax) of 217 +/- 42.4 fmol/mg protein. alpha 2-Adrenoceptor density was increased twofold at midpregnancy (31 days) and thereafter fell sharply by up to 90% toward term (67 +/- 2 days). When uterine growth is accounted for and data are expressed in terms of total myometrial population (uncorrected for plasma membrane recovery), alpha 2-adrenoceptor number was eightfold (midpregnancy) and fourfold (term) greater than the nonpregnant value of 804 +/- 322.4 fmol/uterus. alpha 2-Adrenoceptors were also found to bind dopamine with high affinity [Ki = 3.66 +/- 0.45 microM (n = 3)]. These observations could indicate a pregnancy-related change in uterine sympathetic autoinhibitory capacity and, since alpha 2-adrenoceptors appear also to be located postjunctionally, explain in part reports of altered myometrial responsiveness to norepinephrine infusion and also the uterotonic actions of dopamine.

A comparison of adrenergic receptors of rat ascites hepatoma AH130 cells with those of normal rat hepatocytes

The pharmacological specificity of adrenergic receptors in the plasma membrane of rat ascites hepatoma AH130 cells was compared with that in normal rat hepatocytes. The number of [125I]iodocyanopindolol-binding sites was much greater in AH130 cells than in the hepatocytes. We characterized the alpha-adrenergic receptor subtypes using the alpha 1-selective ligand [3H]prazosin and the alpha 2-selective ligand [3H]clonidine. AH130 cells had fewer prazosin-binding sites than the hepatocytes and about 8 times as many clonidine-binding sites of high affinity. The results showed that the adrenergic receptors in AH130 cells have pharmacological properties that are very different from those of the receptors in normal rat hepatocytes.

Coupling of the alpha 1-adrenergic receptor to a guanine nucleotide-binding regulatory protein by a discrete domain distinct from its ligand recognition site

At rat hepatic membrane alpha 1-adrenergic receptors, the nonhydrolyzable GTP analogue p[NH]ppG causes a rightward shift of agonist competition curves and a loss of high-affinity binding. This p[NH]ppG effect is consistent with the involvement of a guanine nucleotide-binding regulatory protein (G-protein) in alpha 1-adrenergic receptor signalling. Although readily apparent in membranes prepared to avoid retention of endogenous nucleotides and activation of Ca2+-sensitive proteinases (+pi), this p[NH]ppG effect is not observed in membranes prepared without proteinase inhibitors (-pi), or in -pi membranes treated with Ca2+ (-pi, +Ca2+). In these various membrane preparations, different Mr forms of the receptor are also identified by photoaffinity labeling with [125I]CP65526, an aryl azide analog of the alpha 1-selective antagonist, prazosin, followed by SDS-polyacrylamide gel electrophoresis and autoradiography. Whereas a predominant Mr = 80,000 subunit is identified in +pi membranes, in -pi membranes a proteolytic Mr = 59,000 fragment is also observed. In -pi, +Ca2+ membranes, only this latter peptide is detected. To evaluate the ability of each of these forms of the receptor to couple with a G-protein, the effect of p[NH]ppG on the agonist-inhibition of [125I]CP65526 labelling was determined by laser densitometry scanning and computer analysis. At the Mr = 80,000 subunit, p[NH]ppG causes a rightward shift of agonist competition curves and a loss of high-affinity binding, even in -pi membranes. By contrast, agonist-binding at the Mr = 59,000 subunit is of low-affinity and was not affected by p[NH]ppG. These data indicate that the cleaved Mr = 59,000 fragment, while retaining hormone binding activity is unable to undergo G-protein coupling. Thus, the alpha 1-adrenergic receptor appears to contain a discrete domain necessary for G-protein coupling that is distinct from its ligand recognition site.

A water-soluble derivative of prazosin prazosinamine hydrochloride [1-(4'-amino-6',7'-dimethoxyquinazolin-2'-yl)-4-(6''-aminohexanoyl) piperazine hydrochloride], reversibly inhibits the calcium-mobilizing action of alpha 1-adrenergic agonists in the perfused rat liver

A newly-synthesized derivative of prazosin, prazosinamine hydrochloride, was examined for its ability to antagonize the interaction of the alpha 1-adrenergic agonist phenylephrine with liver cells. Using a Ca2--selective electrode to measure changes in perfusate Ca2+ concentration, prazosinamine was found to be as effective as prazosin in inhibiting the phenylephrine-induced efflux of Ca2+ from the perfused liver. Maximal and half-maximal inhibition occurred at 150 nM and 25 nM prazosinamine, respectively. Prazosinamine appears to share the alpha 1-specificity of prazosin, but has other unique and desirable properties. Its solubility in aqueous media is about three orders of magnitude higher than that of prazosin. Also, its antagonistic effects are rapid in onset, and are reversed within seconds of terminating its infusion into the liver. These attributes seem to make this agent more useful than prazosin for adrenergic receptor studies in perfused tissues. The molecule can also be readily coupled to other ligands.

Vasopressin, angiotensin and adrenergic receptors of rat liver Golgi fractions--molecular weight of the angiotensin-receptor irreversible complex after in vitro and in vivo labelling

The binding of vasopressin, angiotensin II and prazosin (alpha 1-adrenergic antagonist) to purified heavy (GH) and (intermediate + light) (GI + L) rat liver Golgi fractions was studied. The three types of ligands showed a saturable and specific binding in Golgi fractions; the maximal specific binding of [3H]vasopressin, [3H]prazosin and [125I]Sar-N3-Phe-angiotensin II was respectively 5-10%, 20-30% and 30-40% of that detected in purified plasma membranes. The apparent binding affinities of the three ligands were the same whether determined in Golgi fractions or plasma membranes. The presence of vasopressin, alpha 1-adrenergic and angiotensin receptors in very different proportions, as compared to the amount of receptor detected in plasma membranes, in GH and GI + L Golgi fractions was not compatible with the idea that a plasma membrane impurity accounted for the detection of receptor in the purified intracellular particulate fractions. In vivo injection of [125I]Sar-N3-Phe-angiotensin II resulted in a receptor-mediated endocytosis of the iodo-angiotensin analog into the GH and GI + L Golgi fractions. The apparent molecular weight of the irreversible complex, [125I]angiotensin-receptor, was estimated in subcellular fractions using SDS-PAGE electrophoresis. This value was identical after either in vivo or in vitro labelling (MW = 63,000) and was indistinguishable from the molecular weight of the irreversible hormone receptor complex present in the plasma membranes.

Prenatal exposure to methylmercury alters development of adrenergic receptor binding sites in peripheral sympathetic target tissues

In order to assess the impact of prenatal exposure to methylmercury on sympathetic neurotransmission, effects on development of adrenergic receptor binding sites in peripheral tissues were evaluated. In the liver, methylmercury produced a dose-dependent increase in alpha 1-, alpha 2- and beta-receptor binding of radioligands throughout the first 5 weeks of postnatal life. Similarly, renal alpha-receptor subtypes showed increased binding capabilities, but binding to beta-receptor sites was reduced. At least some of the changes in receptors appear to be of functional significance, as physiological reactivity to adrenergic stimulation is altered in the same directions in these two tissues. The actions of methylmercury displayed tissue specificity in that the same receptor populations were largely unaffected in other tissues (lung, heart). These results suggest that methylmercury exposure in utero alters adrenergic responses through targeted effects on postsynaptic receptor populations in specific tissues.

Similarity of central and peripheral alpha-1 adrenoceptors in rat and rabbit

In order to examine species and tissue differences in alpha 1 adrenoceptors, binding experiments were performed using 3H-prazosin and membrane homogenates of central nervous and peripheral tissues of rabbit (cortex and spleen), and rat (cortex, spleen, and liver). Saturation studies indicated one binding site for 3H-prazosin, with apparent log molar dissociation constants (pKD) ranging from 9.43 to 10.20. The rank orders of affinities of three competing antagonists (prazosin much greater than idazoxan greater than rauwolscine) and five agonists (cirazoline greater than clonidine approximately equal to (-)-norepinephrine greater than (-)-phenylephrine greater than (+)-norepinephrine) were typical of alpha 1 receptors in all tissues. There were small but significant differences in the mean affinities of rauwolscine, idazoxan and cirazoline among the five tissues. No significant differences in pseudo-Hill coefficients were observed among tissues, although agonist binding curves were shallow (.7 to .85) and prazosin competition curves were significantly steeper (greater than .85). Guanine nucleotide did not affect the position or slope of the (-)-norepinephrine competition profile in rat cortex. These results demonstrate a qualitative similarity among central and peripheral alpha 1 receptors of the rat and rabbit, with small differences observed between central and peripheral sites in both species.

Effects of neonatal methylmercury exposure on adrenergic receptor binding sites in peripheral tissues of the developing rat

Neonatal exposure to methylmercury produces changes in patterns of tissue growth and function, in part, due to alterations in adrenergic neuronal input. To explore the mechanisms by which these changes come about, newborn rats were exposed to methylmercury (1 or 2.5 mg/kg per day) throughout the preweaning stage and the ontogeny of adrenergic receptor binding sites evaluated in liver, kidney, heart and lung, using [3H]prazosin (alpha 1-receptors), [3H]rauwolscine (alpha 2-receptors) and [125I]pindolol (beta-receptors). In the kidney, methylmercury caused decreases in beta- and alpha 1-receptor binding and increases in alpha 2-binding; previous work has shown that beta-receptor-mediated responses are generally enhanced in methylmercury-exposed pups, and the down-regulation of beta-receptor binding thus probably represents a compensatory action secondary to alterations in post-receptor coupling mechanisms. The effects of methylmercury on hepatic adrenergic receptors were different from those seen in the kidney, with substantial elevations in beta- and alpha 1-receptor binding apparent in the preweaning stage; this agrees also with the differences in effects of the mercurial on trophic reactivity and growth in the 2 tissues. Despite the fact that methylmercury causes activation of neonatal cardiac sympathetic nerves, beta-receptor binding sites in the heart were unaffected by methylmercury exposure; the failure to down-regulate cardiac postsynaptic receptors in the face of increased nerve activity again represents an anomaly of synaptic regulatory function. These results indicate that methylmercury alters adrenergic responsiveness, in part, through actions on the development of receptor binding sites, and further, that the organ-specificity and receptor subtype-selectivity are consistent with subsequent effects of the organomercurial on adrenergic participation in target organ growth; however, changes in receptor binding alone do not account for all of the effects of methylmercury on synaptic activity or trophic responses.

Agonist-induced isomerization of the alpha 1-adrenergic receptor: kinetic analysis using broken-cell and solubilized preparations

The affinity of agonists but not antagonists at hepatic membrane alpha 1-adrenergic receptors is temperature dependent; a 100-fold higher affinity is observed at 4 degrees C than at 37 degrees C. The relationship between these two agonist affinity states was investigated by using a strategy that allows the kinetics of this transition to be examined under equilibrium conditions. When competition assays are performed at 37 degrees C for varying intervals and the reaction mixture is then rapidly cooled by freezing, allowed to thaw, and further equilibrated at 4 degrees C, a rapid and progressive decrease (t1/2 of 1-2 min) in agonist affinity occurs, the extent of which is directly related to the incubation time at 37 degrees C. This decrease in agonist affinity is sustained as long as agonist is present but can be reversed by its subsequent removal. In contrast, no change in affinity is seen in identical experiments when antagonists are employed as the competing ligand. High-affinity binding of agonists is also demonstrated in short-term nonequilibrium experiments, indicating that the low-temperature incubations do not induce, but rather stabilize, a receptor conformation of high affinity for agonists. These findings suggest that the predominantly low-affinity binding of agonists to alpha 1-adrenergic receptors demonstrated in equilibrium studies at physiological temperatures may be the result of a ligand-driven decrease in affinity. Since the transition in receptor affinity for agonists occurs not only in broken-cell preparations but also after detergent solubilization of the membrane receptor, it most likely is due to an agonist-induced change in the conformation of the receptor protein per se.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1- and alpha 2-adrenoceptors in the smooth muscle of male and female rabbit urethra

[3H]Prazosin and [3H]rauwolscine, specific alpha 1- and alpha 2-antagonists respectively, were used to label alpha-adrenoceptors in membranes from male and female rabbit urethra. In the male rabbit, [3H]prazosin bound with high affinity (Kd = 0.56 nM) to a single population of sites with a capacity of 73 fmol/mg protein. [3H]Rauwolscine bound with a lower affinity (Kd = 2.24 nM) to another single class of sites with a capacity of 41 fmol/mg protein. The order of potencies of various adrenergic compounds in inhibiting radioligand binding suggested that [3H]prazosin and [3H]rauwolscine interacted in the urethra with sites having the characteristics of alpha 1- and alpha 2-adrenoceptors, respectively. In addition, studies on the female rabbit urethra showed that alpha 2-adrenoceptor density and affinity were respectively 6 times higher and 2 times lower than in the male. No significant sex difference was observed for urethral alpha 1-adrenoceptors.

An endogenous Ca2+-sensitive proteinase converts the hepatic alpha 1-adrenergic receptor to guanine nucleotide-insensitive forms

An iodoazido[125I]prazosin analogue was employed to photoaffinity label alpha 1-adrenergic receptors in rat liver plasma membranes. Labeled proteins were separated by gradient polyacrylamide gel electrophoresis in sodium dodecyl sulfate, and (-)-epinephrine displacement of [3H]prazosin binding was concurrently measured in the presence or absence of guanosine 5'-O-(gamma-thiotriphosphate) (GTP[gamma S]). Inclusion of EGTA and/or proteinase inhibitors during membrane preparation and incubation increased the effect of GTP[gamma S] on alpha 1-adrenergic agonist binding and this could be correlated with increased concentrations of a 78 kDa photoaffinity labeled protein. In contrast, omission of EGTA or addition of exogenous Ca2+ diminished or abolished the effect of GTP[gamma S] on binding and caused loss of the 78 kDa form and the appearance of lower molecular weight labeled proteins. Age-dependent differences in GTP[gamma S] effects on alpha 1-adrenergic agonist binding were abolished when membranes were prepared and incubated in the presence of EGTA and proteinase inhibitors. However, the 78 kDa photoaffinity labeled protein observed in adult rats (over 225 g body weight) was not apparent in membranes from younger rats (50-75 g), even when the membranes were prepared and incubated in the presence of EGTA and proteinase inhibitors. Instead, a 68 kDa species was the major labeled protein. These data suggest that GTP effects on alpha 1-adrenergic agonist binding in rat liver membranes require the presence of either a 68 or 78 kDa alpha 1-adrenergic binding protein. Failure to inhibit proteolysis in the membranes leads to the generation of lower-molecular-weight binding proteins and the loss of GTP effects on alpha 1-adrenergic agonist binding, although [3H]prazosin binding characteristics are not changed. It is suggested that either the proteolyzed forms of the alpha 1-adrenergic receptor are unable to couple to a putative guanine nucleotide-binding regulatory protein, or that such a protein is concurrently proteolyzed and is thus unable to couple to the receptor.

Age-related change in adrenergic regulation of glycogen phosphorylase in rat hepatocytes

The effects of development on adrenergic regulation of glycogenolysis were studied in rat liver. In isolated hepatocytes, the activation of glycogen phosphorylase by alpha-adrenergic stimulation decreased moderately with advancing age. However, activation by beta-adrenergic receptors more markedly declined and almost disappeared in isolated hepatocytes from 6-month-old rats. Furthermore, the ability to glucagon to stimulate phosphorylase activity and cAMP generation was found to decrease with increasing age. The age-related changes in the pattern of stimulation of glycogen phosphorylase by catecholamines in isolated hepatocytes were accompanied by parallels changes in the numbers of alpha 1- and beta-adrenoceptors in membranes prepared from the isolated hepatocytes. A progressive decrease in the total number of alpha 1-receptors measured with [3H]-prazosin and beta-receptors measured with [125I]cyanopindolol (CYP) was found with increasing age. The loss of beta-adrenergic receptors was much more dramatic. Our results suggest that age-related alterations of hepatic glycogen phosphorylase activation by catecholamines may in part be explained by the changes in the expression adrenergic receptors.

Characterization of alpha-adrenoceptors in myometrium of preparturient rats

We describe three methods for the quantitative analysis of the alpha-adrenoceptor subtypes in preparturient rat myometrial membrane fractions. A non-subtype-selective antagonist radioligand. [3H]dihydroergocryptine ([3H]DHE), was used to label all of the alpha-receptors. [3H]DHE bound to both alpha 1- and alpha 2-receptors with indistinguishable affinity. Computer modelling of competition curves of unlabeled selective antagonists or agonists was then required in order to determine reliably alpha 1 and alpha 2 affinities and proportions: the alpha 1-receptors represent 45% and the alpha 2-receptors 55% of the entire alpha-receptor population in rat uterus. The second approach involved the administration of phenoxybenzamine (POB) that irreversibly blocks the alpha 1-adrenoceptors. Myometrial membranes obtained from rats 1 h after the administration of varying amounts of POB showed a dose-dependent reduction in specific [3H]DHE binding. This reduction was accompanied by a progressive increase of the value of the dissociation constant. Our data indicate that a dose of 1 mg of POB left the alpha 2-receptors intact while entirely blocking the alpha 1-receptors in rat myometrium. The third approach utilized the selective radioligand antagonists [3H]prazosin ([3H]PRAZ) and [3H]rauwolscine ([3H]RAUW). The results obtained with these radioligands confirmed our observations on the alpha-adrenoceptor subtypes in experiments with [3H]DHE. The results obtained with the 3 methods are in good agreement. Each approach appears valid and applicable to the characterization of alpha 1- and alpha 2-adrenoceptor subtypes in rat uterus, but the method using [3H]PRAZ and [3H]RAUW demonstrates more directly the presence of the two receptor subtypes.

Effects of guanine nucleotides and cations on agonist affinity of alpha 1-adrenoceptors in brown adipose tissue

In order to investigate the coupling mechanism of alpha 1-adrenoceptors in brown adipose tissue, the effects of guanine nucleotides and cations on agonist binding were studied with a membrane fraction obtained from hamsters. The affinity of the alpha 1-receptor for the adrenergic agent was followed in competition experiments with [3H]prazosin. It was found that the addition of GTP diminished the affinity of the alpha 1-receptor for norepinephrine but not for the alpha-antagonist phentolamine. This effect seemed to be dependent upon the presence of Mg2+ and could not be observed in isolated cells, indicating an intracellular site of action. A reduction of the Mg2+ concentration from the conventional 10 mM to the more physiological level of 1 mM markedly increased the affinity of the receptor for norepinephrine; this effect again was agonist-specific. The addition of Na+ (150 mM) decreased the agonist affinity of the receptor. It is suggested that the (not adenylate cyclase-coupled) alpha 1-adrenergic pathway in brown adipose tissue also is regulated by guanine nucleotides and modulated by the cations Mg2+ and Na+, indicating an involvement of a guanine nucleotide binding protein. Such a system may therefore be a general mechanism for the transduction of hormone stimulation over the cell membrane.

Molecular comparison of alpha 1- and alpha 2-adrenergic receptors suggests that these proteins are structurally related "isoreceptors"

The structures of human platelet alpha 2-adrenergic receptors and rat liver alpha 1-adrenergic receptors were compared by utilizing isoelectric focusing, NaDodSO4/PAGE, and monoclonal antibody crossreactivity. Digitonin-solubilized alpha 1- and alpha 2-adrenergic receptors have an identical isoelectric point of 4.6. Under reducing conditions in NaDodSO4/polyacrylamide gels, the alpha 1-adrenergic receptor has an apparent molecular mass of 85 kDa. Similarly, the alpha 2-adrenergic receptor, which had been affinity-labeled with [3H]phenoxybenzamine and partially purified by isoelectric focusing or photoaffinity-labeled with p-[3,5-3H]azidoclonidine, was also found to have an apparent molecular mass of 85 kDa. One hybridoma, developed from a fusion between SP2/O myeloma cells and splenic lymphocytes from BALB/c mice immunized with human platelet alpha 2-adrenergic receptors, secreted a monoclonal antibody (alpha 2-116p) against the ligand binding site of alpha 2-adrenergic but not alpha 1-adrenergic receptors. In contrast, three monoclonal antibodies raised against the alpha 1-receptor polypeptide backbone but not the ligand binding site were found to specifically immunoprecipitate human platelet alpha 2-adrenergic receptors. These data suggest that the alpha 1- and alpha 2-adrenergic receptors are "isoreceptors," sharing immunogenic and, by implication, structural determinants that most likely evolved as a result of gene duplication.

Differential effects of adrenergic agonists and phorbol esters on the alpha 1-adrenoceptors of hepatocytes and aorta

Epinephrine, norepinephrine and phenylephrine stimulate phosphatidylinositol labeling with [32P]Pi in both rat hepatocytes and rabbit aorta. Methoxamine was a full agonist for this effect in rabbit aorta whereas cirazoline and oxymetazoline were partial agonists. In contrast, these three agents (methoxamine, cirazoline and oxymetazoline) were unable to stimulate phosphatidylinositol labeling in rat hepatocytes. Furthermore, cirazoline and oxymetazoline were able to displace the dose-response curve to epinephrine in rat hepatocytes, i.e., they behaved as antagonists. Binding competition curves of these agents with labeled adrenergic ligands indicate that the affinity of alpha 1-adrenergic receptors in these two tissues (aorta and liver) for the different agents tested was very similar. In addition it was observed that phorbol myristate-acetate inhibited in a dose-dependent fashion the epinephrine-mediated stimulation of phosphatidylinositol labeling in hepatocytes but was without effect on the action of the amine in aorta. Our data suggest that stereochemical differences for alpha 1-adrenergic activation in liver and aorta may exist and indicate that the ability of phorbol esters to inhibit alpha 1-adrenergic effects is not universal.

Possible involvement of two mechanisms of signal transduction in alpha 1-adrenergic action. Selective effect of cycloheximide

We have previously suggested that the effects of alpha 1-adrenergic agents on hepatocyte metabolism involve two pathways: (a) a calcium-independent, insulin-sensitive process which is modulated by glucocorticoids; and (b) a calcium-dependent, insulin-insensitive process which is modulated by thyroid hormones. Cycloheximide stimulated ureogenesis through a prazosin-sensitive mechanism in liver cells (alpha 1-adrenergic). The effect of cycloheximide was insulin-insensitive and calcium-dependent. Furthermore, a clear effect of cycloheximide was observed in hepatocytes obtained from adrenalectomized animals, whereas no effect was observed in cell from hypothyroid rats. The effects of epinephrine and cycloheximide were blocked by phorbol esters in all the conditions tested. Binding competition experiments indicated that cycloheximide interacts with only a fraction of the alpha 1-adrenergic sites labeled with [3H]prazosin. It is suggested that cycloheximide activates preferentially one of the pathways involved in the alpha 1-adrenergic action in liver cells.

Adrenergic receptor subtypes in rabbit iris-ciliary body membranes: classification by radioligand studies

Receptor subclasses in iris-ciliary body cell membranes were determined by direct binding of radioactive dihydroalprenolol (DHA), yohimbine (YOH), WB-4101 WB) and prazosin (PRZ), classified respectively as beta 1 + beta 2, alpha 2, alpha 1, and alpha 1 subtype selective ligands (based on binding to brain adrenergic receptors). Specific binding was defined with appropriate unlabelled agonist and antagonist drugs in each case. Binding data were analysed by library programs of the PROPHET computer system. Subclass specificity was also determined indirectly by binding competition of the labelled ligand (at a three- to 10-fold Kd concentration) with increasing concentrations of 'cold' agonist or antagonist. Ligand binding parameters of unlabelled drugs were obtained from Dixon and Scatchard plots of the data. The adrenergic receptor density of iris-ciliary body is approx. 600 fmol mg-1 protein, of which 20-25% are primarily beta 2 receptors. Three distinct subpopulations of alpha-receptors, representing 10, 25 and 40-45% of the total, bind PRZ, WB and YOH, respectively, each with high specificity for its corresponding ligand but with 10- to 1000-fold lower specificity for the other two ligands. The majority of alpha-adrenergic receptors are of the alpha 2-subtype. A small population of receptors are similar to vascular postsynaptic alpha 1-receptors, while a larger subpopulation may have characteristics intermediate between alpha 1- and alpha 2-subtypes.

Involvement of guanine nucleotide and sodium in regulation of yohimbine and clonidine binding sites in the HT29 human colon adenocarcinoma cell-line

Sodium and GppNHp, a GTP analogue, decrease the specific binding of the alpha 2-adrenergic agonist [3H]clonidine on membranes from the HT29 cells; affinity of the ligand for the receptor is also decreased. The effects of the two agents are additive. By contrast, combination of sodium and GppNHp increases the number of binding sites for the alpha 2-adrenergic antagonist [3H]yohimbine. Sodium and GppNHp also decrease the potency of epinephrine and clonidine to displace [3H]yohimbine from the alpha 2-adrenoceptors of the HT29 cells

Reduction in hepatic but not in renal and vascular vasopressin receptor number in hyperinsulinemic mice and rats

Hepatic plasma membranes of female obese mice C57 BL-6 orl ob/ob (ob/ob mice) completely lack vasopressin (VP) receptors of the V1 type whereas kidney VP receptors are normally expressed and functionally coupled to adenylate cyclase. To discover if these alterations are linked to a genetic defect of the V1 receptor, we have studied the binding of VP on liver and kidney membranes of two other models, female diabetic mice C57 BL-6 orl db/db (db/db mice) and female Zucker rats Fatty/orl fa/fa (fa/fa rats), which exhibit different temporal pattern of obesity, hyperinsulinemia and insulin resistance. In addition, since VP is known to exert its vascular response through stimulation of V1 receptors, we have studied the reactivity of VP of isolated tail artery in the three different models, ob/ob and db/db mice and fa/fa rats, and in their respective controls. In all cases, VP kidney receptors and VP vascular reactivity are normal. db/db mice exhibit a marked decrease in hepatic VP receptors whereas a 50% decrease was observed in 32 week fa/fa rats. Angiotensin II and prazosin binding sites are still present as well as the adenylate cyclase response to glucagon. These results suggest that the specific alteration in liver VP receptors is not related to a defect in V1 receptor genetic expression but is specific for liver and appears to parallel the level of hyperinsulinemia and/or insulin resistance.

A comparison of the binding characteristics of the beta-adrenoceptor antagonists 3H-dihydroalprenolol and 125I-iodocyanopindolol in rat liver

The binding characteristics of 3H-dihydroalprenolol and 125I-iodocyanopindolol have been compared in a particulate fraction from regenerating rat liver. When total 3H-dihydroalprenolol binding and inhibition of total 3H-dihydroalprenolol binding by (-)isoprenaline, (-)alprenolol and (+/-)cyanopindolol was investigated, it was found that all agents were bound to two classes of saturable binding sites. In the inhibition studies, the presence of two binding components was not obvious until the data were transformed into Hofstee plots and these were decomposed, except in the case of (+/-)cyanopindolol. Only (+/-)cyanopindolol was found to distinguish clearly between the two saturable binding sites identified by 3H-dihydroalprenolol, as indicated by a broad plateau in the inhibition curve. When 125I-iodocyanopindolol was used as radioligand, only one saturable binding site was identified, even in the presence of less selective inhibiting ligands. The lower affinity component of 3H-dihydroalprenolol binding could be inhibited by 10 microM phentolamine. However, binding experiments with 3H-prazosin indicated that the lower affinity component was not identical with the alpha-adrenoceptor. Phentolamine did not influence 125I-iodocyanopindolol binding. Thus, due to its higher specific activity and a high degree of selectivity, 125I-iodocyanopindolol appears to be the ligand of choice.

Alpha 2-adrenoceptors in human lymphocytes: direct characterisation by [3H]yohimbine binding

[3H]yohimbine, a potent and selective alpha 2-adrenergic antagonist was used to label alpha-adrenoceptors in intact human lymphocytes. Binding of [3H]yohimbine was rapid (t1/2 1.5 -2.0 min) and readily reversed by 10 microM phentolamine (t1/2 = 5 - 6 min) and of high affinity (Kd = 3.7 +/- 0.86 nM). At saturation, the total number of binding sites was 19.9 +/- 5.3 fmol/10(7) lymphocytes. Adrenergic agonists competed for [3H]yohimbine binding sites with an order of potency: clonidine greater than (-) epinephrine greater than (-) norepinephrine greater than (+) epinephrine much greater than (-) isoproterenol; adrenergic antagonists with a potency order of yohimbine greater than phentolamine greater than prazosin. These results indicate the presence of alpha 2-adrenoceptors in human lymphocytes.

Interactions of endogenous and exogenous norepinephrine with alpha 2 adrenoceptor binding sites in rat cerebral cortex

The specific binding of (3H)yohimbine to alpha 2 adrenoceptors in rat cerebral cortex varied significantly with methods of membrane preparation. Membranes prepared in a sucrose containing buffer showed markedly lower Bmax values than those prepared in hypotonic buffer without any change in affinity (Kd) for (3H)yohimbine. Higher concentrations of residual endogenous norepinephrine were found in sucrose prepared membranes. In the presence of exogenous norepinephrine (10(-8), 10(-7) M), membranes prepared in hypotonic buffer showed apparent reduced receptor densities similar to those observed in sucrose prepared cerebral membranes. The presence of exogenous norepinephrine did not produce any apparent change in the overall Kd for (3H)yohimbine. The depression of (3H)yohimbine binding capacities could, in all cases, be reversed by performing incubations in the presence of 200 mM NaCl and 10 microM Gpp(NH)p which synergistically dramatically reduce the affinity of norepinephrine for cerebral alpha 2 adrenoceptors. It is concluded, therefore, that the lower (3H)yohimbine binding capacities in sucrose prepared membranes appears to be due to the occupancy of receptor sites by residual norepinephrine in a pseudo non competitive manner. The reduced affinity of retained endogenous norepinephrine for the alpha 2 adrenoceptor, in the presence of Na+ and guanine nucleotides, would seem to be the cause of the apparent increase in (3H)yohimbine binding sites produced by these modulators in sucrose prepared membranes.

RX781094 a potent and selective alpha 2-adrenergic antagonist. Effects in adipocytes and hepatocytes

The potency and selectivity of RX781094, 2-(2-(1,4-benzodioxanyl]-2-imidazoline HCl, as alpha 1- and alpha 2-adrenergic antagonist was studied using rat hepatocytes and hamster adipocytes. The alpha 1-adrenergic stimulation of ureogenesis produced by epinephrine in rat hepatocytes was slightly diminished by 10(-4) M RX781094. On the contrary the alpha 2-adrenergic effect of epinephrine in hamster adipocytes (inhibition of adenylate cyclase) was antagonized dose-dependently by RX781094. This agent was approximately 10-fold more potent than yohimbine. Radioligand binding studies also showed that RX781094 was more potent and selective than yohimbine at alpha 2-adrenergic sites. It is concluded that RX781094 is a potent and selective alpha 2-adrenergic antagonist.