Endogenous noradrenaline masks beta-adrenergic receptors in rat heart membranes via tight agonist binding

Regulation of beta-adrenoceptors in a rat model of cardiac failure: effect of perindopril

This study investigated the effects of myocardial infarction (MI)-induced cardiac failure and treatment with an angiotensin-converting enzyme (ACE) inhibitor perindopril (2 mg/kg/day) on rat beta-adrenoceptor (beta-ar) subtypes in anatomically defined regions of infarcted left ventricular (LV) free wall and noninfarcted tissue from right ventricle (RV) by using autoradiography. After 5 weeks of MI, rats with large MI size (>42%) had developed cardiac failure and beta1-ars were significantly decreased (-59%; p < 0.01) in the border region of the infarcted LV and almost abolished in the infarcted area (-90%; p < 0.005) compared with normal LV from sham-operated controls. The beta-ar changes were not found in the noninfarcted area of the same LV or in RV. MI did not significantly alter the number of beta2-ar subtypes in any region of the ventricles. Perindopril treatment for 4 weeks reduced mean cardiac region weights but did not affect beta-ar density in any cardiac region in either sham-operated or MI rats. These results indicate that cardiac failure due to MI causes significant downregulation of beta1-ars only in border and infarcted regions of rat LV and no change in beta2-ar in any area. It also suggests that the improved response of the infarcted rat heart to isoprenaline stimulation after ACE inhibitors does not result from changes in the numbers of cardiac beta-ars.

Chronic (-)-isoprenaline infusion down-regulates beta 1- and beta 2-adrenoceptors but does not transregulate muscarinic cholinoceptors in rat heart

Regulation of beta-adrenoceptor (beta-ar) subtypes and transregulation of muscarinic cholinoceptors (mAchr) was examined in regions of rat heart after chronic infusion of (-)-isoprenaline (450 micrograms/kg per hour) for 14 days. Following (-)-isoprenaline infusion systolic blood pressure was reduced for 10 days but then gradually returned to control levels, whereas heart rate was increased for 7 days before declining to a level significantly above control. Heart weight to body weight ratio was increased in (-)-isoprenaline treated rats. beta-ar subtype densities were measured by quantitative autoradiography with [125I]-cyanopindolol (CYP) in sinoatrial node (SA), atrioventricular node (AV), bundle of His (BH), left (LB) and right (RB) bundle branches, interventricular (IVS) and interatrial (IAS) septa, right atria (RA), apex (AX) and mitral valve (MV). beta 1-ars were reduced by 59.1-74.2% in the AV conducting regions, 53.4% in the SA node and 43.3-53.4% in myocardial areas, beta 2-ars were markedly reduced in myocardial regions (93.2-98.5%) and in pacemaker and conducting regions (87.7-97.8%). No changes in mAchr densities measured using [3H]-N-methyl scopolamine (NMS) occurred in the AV node, BH, LB, RB, IVS and IAS following (-)-isoprenaline infusion. Densities of beta 1- and beta 2-ars and mAchrs were also measured in ventricular homogenates from control and (-)-isoprenaline treated animals. beta-ar levels were significantly reduced (P < 0.05) in treated animals and the ratio of beta 1- to beta 2-ars increased after treatment. mAchr density in ventricular homogenates measured using either [3H]-NMS or [3H]-quinuclidinyl [phenyl-4-3H]benzilate (QNB) was unchanged. Homogenates of left and right ventricle also showed no change using [3H]-NMS. Organ bath studies were used to investigate the effect of (-)-isoprenaline infusion on negative inotropic and chronotropic effects of the non-selective muscarinic receptor agonist bethanechol in left and right atria, respectively. Lower concentrations of bethanechol (3 x 10(-10) to 10(-6) M) produced a negative inotropic response in isolated electrically driven left atria from (-)-isoprenaline treated rats, but not from control rats, with the slope of the curves being significantly different between groups (ANCOVA, P = 0.037). At concentrations of bethanechol from 10(-6) to 3 x 10(-4) M the negative inotropic response was not changed between (-)-isoprenaline treated and control animals. Bethanechol also produced a negative chronotropic response at lower concentrations (10(-10) to 10(-6) M) in (-)-isoprenaline treated rats, but not in controls. A second, steeper phase of the negative chronotropic response occurred at concentrations of bethanechol greater than 10(-6) M and was also seen in control rats. Expression of M2 (cardiac) mAchrs (m2Achr) in left and right ventricular tissues measured using a quantitative non-competitive polymerase chain reaction (PCR) assay showed a significant (P = 0.001) 28.5% increase in expression in left ventricle and a significant (P = 0.003) 21.5% decrease in expression in right ventricle after (-)-isoprenaline treatment, compared to controls. There was no significant difference in total ventricular m2Achr expression between the two groups of rats. The results suggest that chronic beta-ar stimulation down-regulates both beta 1- and beta 2-ars, and appears to differentially transregulate m2Achr expression, but not mAchr protein. Following (-)-isoprenaline infusion, muscarinic receptor mediated responses were sensitised, with no change in receptor densities, suggesting changes occur in the cell signalling system beyond the level of the receptor.

beta-Adrenoceptor regulation in rat heart, lung and skin after chronic treatment with (--)-tertatolol or (--)-propranolol

1. The effect of long-term treatment with the beta-adrenoceptor antagonists (--)-tertatolol and (--)-propranolol was studied. Sprague-Dawley rats were treated with either (--)-tertatolol (50 micrograms kg-1 hr-1), (--)-propranolol (250 micrograms kg-1 hr-1) or vehicle (1 mM HCl) for 14 days with osmotic minipumps implanted subcutaneously. 2. The mean daily systolic blood pressure and heart rate of rats treated with either (--)-tertatolol (108 +/- 1 mmHg/330 +/- 3 bpm) or (--)-propranolol (103 +/- 1 mmHg/330 +/- 2 bpm) were lower than in the control (126 +/- 1 mmHg/405 +/- 3 bpm, P < 0.001, n = 8-10) indicating the effectiveness of drug delivery. 3. Autoradiographic studies in areas of heart, lung and skin showed that beta-adrenoceptor populations were not significantly affected by the drug treatment (all regions P > 0.05). Nevertheless, the receptor population in the homogenates of (--)-tertatolol treated lung were halved (194 +/- 28 fmol mg protein-1 compared with a control value of 388 +/- 54 fmol mg protein-1, P < 0.01, n = 6). 4. In the presence of CGP 20712A, the left atrial inotropic and right atrial chronotropic responsiveness to (--)-isoprenaline were hypersensitive in both (--)-tertatolol and (--)-propranolol-treated groups (P < 0.005, ANCOVA). 5. (--)-Propranolol treated left ventricular free wall had lower basal [3H]-forskolin binding to adenylate cyclase (14.45 +/- 1.20 fmol mg protein-1 compared with a control value of 18.91 +/- 0.78 fmol mg protein-1, P = 0.01, n = 6). (--)-Tertatolol treatment had no effect on the basal binding. In the presence of the G-protein activators NaF and Gpp(NH)p, the enhancement of [3H]-forskolin binding did not differ between control and the drug treated groups. 6. Chronic (--)-tertatolol or (--)-propranolol treatment therefore did not produce an increase in receptors in heart, lung or skin but the beta-adrenoceptor-mediated responses were enhanced. In addition, [3H]-forskolin binding did not increase suggesting that the hypersensitivity was not due to changes in the number of receptors or adenylate cyclase. Hypersensitivity following beta-adrenoreceptor antagonist administration may therefore involve enhanced coupling of receptors to G-proteins.

Beta-adrenoceptor regulation and functional responses in the guinea-pig following chronic administration of the long-acting beta 2-adrenoceptor agonist formoterol

Formoterol is a long acting beta 2-adrenoceptor agonist designed for the alleviation of the symptoms of asthma. This study examined the effects of 14 day administration of formoterol (200 micrograms/kg/day i.p.) on beta 1- and beta 2-adrenoceptors in guinea-pig cardiac and lung tissue. Quantitative autoradiography was used to measure changes in receptor density and organ bath studies determined alterations in functional response. Formoterol treatment produced marked reductions of between 43% and 77% in beta 2-adrenoceptor density in all regions of the heart (atrioventricular node, bundle of His, right and left bundle branches, interventricular and interatrial septa, right and left atria, ventricles and apex) and lung (bronchial and vascular smooth muscle and parenchyma) (P < 0.01, n = 6). beta 1-Adrenoceptor density remained unchanged in all cardiac and lung regions. In functional studies (-)-isoprenaline was 4 fold less potent at causing relaxation of carbachol (1 microM) precontracted tracheal smooth muscle (pD2: control 8.49 +/- 0.03, formoterol 7.91 +/- 0.10, P < 0.001, n = 4), but formoterol treatment did not change the ability of (-)-isoprenaline to elicit a maximum response. The pKB values for ICI 118,551, 7.33 +/- 0.08 in the control and 7.20 +/- 0.01 in formoterol treated animals, were between those expected for beta 1- and beta 2-adrenoceptors suggesting involvement of both subtypes in the response. In spontaneously beating right atria and electrically paced left atria, tissues in which responses are largely mediated by beta 1-adrenoceptors, there was no significant change in responses to (-)-isoprenaline (right atria pD2: control 8.45 +/- 0.02; formoterol 8.42 +/- 0.11; P = 0.77, n = 4) (left atria pD2: control 8.25 +/- 0.03; formoterol 8.47 +/- 0.08; P = 0.09, n = 4). In the presence of CGP 20712A (100 nM) the pKB values did not change with formoterol treatment (left atria: control 9.59 +/- 0.12, formoterol 9.66 +/- 0.12; P = 0.70, n = 4) (right atria: control 8.93 +/- 0.11, formoterol 9.11 +/- 0.07; P = 0.25, n = 4). The doses and route of administration of formoterol used in this study differs from those used clinically. However, this study demonstrates that chronic formoterol administration produces selective down-regulation of beta 2-adrenoceptors in the lung and heart. The changes in the lung are accompanied by a shift to the right in the concentration-response curve to beta-agonist stimulation with no change in the maximum response.

Spatiotemporal alterations of central alpha 1-adrenergic receptor binding sites following amygdaloid kindling seizures in the rat: autoradiographic studies using [3H]prazosin

Noradrenergic neurons are thought to be involved in the process of seizure development and long-term central nervous system plasticity associated with kindling and epilepsy. These processes involve actions of noradrenaline at alpha 1-, alpha 2- and beta 1-adrenergic receptors. In this study, quantitative in vitro autoradiography was used to investigate possible changes in the density of brain alpha 1-adrenergic receptors in a kindling model of epilepsy in the rat. Kindling was produced by daily unilateral stimulation of the amygdala. The alpha 1A+alpha 1B subtypes of adrenergic receptors were labelled with the alpha 1-selective antagonist, [3H]prazosin and alpha 1B receptors, detected in the presence of 10 nM WB4101 to selectively occupy alpha 1A receptors, accounted for 50% of total alpha 1 receptors in cerebral cortex. Autoradiographic studies identified significant and long-lasting, ipsilateral increases in specific [3H]prazosin binding throughout layers I-III of the cortex in sham-operated, unstimulated rats, presumably caused by the surgical implantation of the stimulating electrode within the basolateral amygdaloid nucleus. Binding to alpha 1A + alpha 1B receptors and alpha 1B receptors was increased by an average of 35 and 60%, respectively under these conditions. Stimulation-evoked seizures produced dramatic bilateral increases in specific [3H]prazosin binding to alpha 1A + alpha 1B receptors and particularly to alpha 1B receptors in layers I-III of all cortical areas examined. These changes were rapidly induced and the largest increases (range alpha 1A + alpha 1B 80-340%; alpha 1B 165-380%) occurred at 0.5-2 h after the last stage 5 kindled seizure. At 1 and 3 days after the last seizure, increases were measured for both alpha 1A + alpha 1B and alpha 1B receptors in layers I-III of particular cortical regions, but not overall (e.g. 60-210% increase in perirhinal cortex at both times, with increases also in retrosplenial, hindlimb, occipital, parietal and temporal cortices). Between 2-8 wk post-stimulation specific receptor binding levels were equivalent to those in sham-operated, unstimulated rats. In contrast to the large and widespread increases in outer cortical [3H]prazosin binding, smaller increases were detected in the inner cortex (layer V-VI) at individual times (65-75% increase at 30 min), while no significant changes occurred in several other brain regions examined, including thalamus, which contained a high density of alpha 1A and alpha 1B receptors, or hippocampus which has a low density of both alpha 1 receptor subtypes.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of chemical sympathectomy on (-)-isoprenaline-induced changes in cardiac beta-adrenoceptor subtypes in the guinea-pig and rat

1. Quantitative autoradiography was used to determine beta-adrenoceptor densities in cardiac regions of guinea-pigs and rats after chemical sympathectomy with 6-hydroxydopamine, and to examine how chemical sympathectomy affected beta-adrenoceptor changes following infusion of (-)-isoprenaline (400 micrograms kg-1 hr-1, 7 days). 2. Seven days after 6-hydroxydopamine (100 mg kg-1, i.v.), cardiac tissue levels of noradrenaline were reduced by 94.0 +/- 3.5% (guinea-pig) and 86.0 +/- 7.0% (rat). The blood pressure increase in rats to tyramine (0.5 mg, i.v.) was reduced from 118 mmHg in controls to 4.4 mmHg in 6-hydroxydopamine-treated animals. 3. There were no changes 7 and 14 days after 6-hydroxydopamine treatment in total, beta 1-and beta 2-adrenoceptor density in the atrioventricular conducting system and atrial and ventricular myocardium in both species. 4. In control animals, (-)-isoprenaline infusion produced selective reductions in beta 2-adrenoceptor density, whilst beta 1-adrenoceptor density remained unchanged. 5. In 6-hydroxydopamine treated guinea-pigs or rats, (-)-isoprenaline infusion caused no change in beta 1-adrenoceptors except in the right bundle branch whilst beta 2-adrenoceptors were reduced in the atrioventricular conducting system (atrioventricular node, bundle of His, right and left bundle branches) and myocardium (interventricular septum and atria). 6. The differential effect of (-)-isoprenaline on beta 1- and beta 2-adrenoceptors is not therefore due to the occupation of beta 1-adrenoceptors by noradrenaline or to prior down-regulation of beta 1-adrenoceptors by noradrenaline, since it persists after depletion of noradrenaline.

Regional stimulatory and inhibitory effects of guanine nucleotides on [125I]galanin binding in rat brain: relationship with the rate of occupancy of galanin receptors by endogenous galanin

Galanin has been shown to stimulate feeding or modulate neuroendocrine secretions when administered centrally. In the present work, using quantitative autoradiography, we documented the existence of [125I]galanin specific binding sites in several hypothalamic nuclei expected to mediate these effects. In standard binding conditions, [125I]galanin specific binding can be visualized in the hypothalamic ventromedial nucleus, stria terminalis, piriform cortex, central amygdaloid nucleus and medial amygdaloid nucleus, while it is almost undetectable in most neuroendocrine or autonomic hypothalamic areas. We hypothesized that high endogenous galanin levels in these regions might mask galanin receptors. We first showed that a high ionic strength/acid wash of brain slices is effective in removing more than 80% of specifically prebound [125I]galanin in all tested regions. After such treatments, specific binding sites could be revealed in the hypothalamus namely in the parvocellular paraventricular nucleus, periventricular nucleus, arcuate nucleus and median eminence. In contrast, regions already labeled in standard conditions exhibited a slight decrease in [125I]galanin binding. Thus, regions were ranked from low to high rate of occupancy of galanin receptors by endogenous galanin, the rate of occupancy of galanin receptors being maximal in median eminence (greater than 90%). We thus studied the regional effect of guanine nucleotides on [125I]galanin specific binding. A high concentration (100 microM) of guanyl 5'-yl imidodiphosphate, a nonhydrolyzable analog of GTP directly added to the incubation medium, inhibited [125I]galanin binding in all telencephalic regions. On the same sections and only in regions of high index of galanin receptor occupancy (arcuate nucleus, median eminence, dorsomedial nucleus, paraventricular nucleus, and periventricular hypothalamic nucleus), guanyl 5'-yl imidodiphosphate paradoxically enhanced [125I]galanin binding. The effects of acid preincubation and guanyl 5'-yl imidodiphosphate incubation on [125I]galanin binding were strongly correlated in these hypothalamic areas (r = 0.97). In all regions, guanyl 5'-yl imidodiphosphate increased the rate of dissociation of [125I]galanin. In competition studies, guanyl 5'-yl imidodiphosphate decreased the IC50 s of unlabeled galanin which were homogenized around 4 nM in most telencephalic and hypothalamic regions. Thus, the guanyl 5'-yl imidodiphosphate-induced stimulation of [125I]galanin specific binding measured in the neuroendocrine and autonomic hypothalamus is linked to an increase in receptor capacity and not to a rise in receptor affinity. Both inhibitory and stimulatory guanyl 5'-yl imidodiphosphate effects observed in [125I]galanin equilibrium binding studies were dose-dependent and guanine nucleotide-specific with guanyl 5'-yl imidodiphosphate more potent than GTP or GDP.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of (-)-[125I]cyanopindolol binding in guinea-pig heart: characteristics of non-beta-adrenoceptor related binding in cardiac pacemaker and conducting regions

Receptor autoradiography was used in guinea-pig heart to locate binding sites for the beta-adrenoceptor ligand (-)[125I]cyanopindolol (CYP) resistant to blockade by the beta-adrenoceptor antagonist (-)-propranolol (1 microM). Highly localized binding was observed to regions closely associated with the sinoatrial node, atrioventricular node and bundle of His but was not observed on myocardial, pacemaker, conducting cells or adipose tissue. Free [125I] also bound to identical sites. Binding was enhanced in the presence of ascorbic acid but was completely inhibited by (-)-isoprenaline (100 microM), serotonin (5-HT) (10 microM) and phentolamine (10 microM).

Functional analysis of rabbit anti-peptide antibodies which mimic autoantibodies against the beta 1-adrenergic receptor in patients with idiopathic dilated cardiomyopathy

A synthetic peptide corresponding to the second extracellular loop of the beta 1-adrenergic receptor was used as an antigen for antibody production in three rabbits. Antibodies of high titers were obtained in all rabbits. Only one rabbit yielded antibodies which decreased radioligand binding on the receptor in a similar way to that described for autoantibodies in patients with dilated cardiomyopathy. These antibodies recognized the receptor protein in immunoblots. Epitope mapping indicated that the N-terminal sequence of the loop used as antigen was the target of the major antigen fraction. Incubation of antibodies with C6 glioma cell membranes or inner membranes of E. coli, which express the human beta 1-adrenergic receptor, resulted in a decrease in number of radioligand binding sites. This decrease was dependent on the concentration of antibody and of Mg++ ions. It was not affected by the GTP analog GppNHp or the beta 1 subtype-specific antagonist metoprolol. The agonist, isoproterenol, also induced a decrease but the effects of antibody and agonist were not additive. These results suggest that the antibodies induce a Mg(++)-dependent, 'active', labile conformation of the receptor, independent from coupling to the GTP regulatory protein, but similar to that induced by the agonist isoproterenol. This interpretation was corroborated by the beta 1-adrenergic receptor agonist-like effect of the antibodies on cardiomyocytes in culture.

Characterization of beta 1- and beta 2-adrenoceptors in rat skeletal muscles

Binding studies with (-)-[125I]cyanopindolol (ICYP) were conducted to characterize beta-adrenoceptors in plantaris and soleus muscles of rats (male, 250-300 g). The distribution of beta 1- and beta 2-adrenoceptors in different muscle fiber types, identified in serial sections by succinic dehydrogenase (SDH) staining, was studied by autoradiography. The densities of binding sites (Bmax, fmol/mg protein) were 5.4 +/- 0.9 (mean +/- SEM) in plantaris and 11.5 +/- 2.0 in soleus muscle. In plantaris muscle, monophasic competition curves were observed when binding experiments were performed using CGP20712A (50 pM to 0.5 mM), a beta 1-adrenoceptor selective antagonist, or ICI 118,551 (50 pM to 20 microM), a beta 2-adrenoceptor selective antagonist, to compete for ICYP binding. Analysis with LIGAND revealed a single binding site with a KD value of 2.41 +/- 0.56 nM (mean +/- SEM) for ICI 118,551 and 8.93 +/- 3.00 microM for CGP 20712A, indicating the presence of a homogeneous population of beta 2-adrenoceptors. In soleus muscle, competition curves were biphasic with 16-21% beta 1-adrenoceptors. Autoradiographic studies supported the findings from binding studies with membrane homogenates. The ICYP binding pattern was associated closely with the muscle fiber types identified by SDH staining. Propranolol-resistant binding sites were observed, and these sites were associated with muscle fibers positive to SDH staining.

The beta 1- and beta 2-adrenoceptor affinity and beta 1-blocking potency of S- and R-metoprolol

1. The beta-adrenoceptor affinity and blocking potency of the two enantiomers and the racemate of metoprolol were investigated in vitro, by use of a receptor-binding technique, and in vivo in the anaesthetized cat. 2. The enantiomeric purity of the S- and R-form was: greater than 99.2% and greater than 99.9%, respectively. 3. The beta 1- and beta 2-adrenoceptor affinity (-log equilibrium dissociation constant) of the enantiomers was determined from competition binding experiments (radioligand: [125I]-(S)-pindolol) performed in membranes prepared from the guinea-pig left ventricular free wall (predominantly beta 1) and soleus muscle (beta 2). The beta 1-adrenoceptor affinity was (means +/- s.d.): 7.73 +/- 0.10 and 5.00 +/- 0.06 for the S- and R-form of metoprolol, respectively. The corresponding values for beta 2-adrenoceptors were 6.28 +/- 0.06 (S) and 4.52 +/- 0.09 (R). Thus, the difference in affinity for the two enantiomers was greater on beta 1- (about 500) than on beta 2-adrenoceptors (about 50). The beta 1-adrenoceptor selectivity of the S-form (about 30) was similar to that of the racemic metoprolol, while the R-form was almost non-selective (3 fold beta 1-selective). 4. In the anaesthetized cat, the (-log) intravenous doses (mumol kg-1) of S- and R-metoprolol causing a 50% reduction (ED50) in the heart rate response to sympathetic nerve stimulation were determined. The doses inducing a 25% depression (DD25) of the basal myocardial contractility were also estimated. For the two enantiomers, the beta 1-blocking potency (-log ED50) was 7.04 +/- 0.16 (S) and 4.65 +/- 0.16 (R). A significant cardiodepressive effect was observed at high doses (-log DD25): 4.18 + 0.20 (S) and 4.08 + 0.10 (R). 5. It is concluded that the binding of metoprolol to beta 1-adrenoceptors has a stricter steric requirement than that for the binding of this beta l-blocker to beta 2-adrenoceptors. Furthermore, the non-specific cardiodepressive effect of metoprolol was observed at equally high doses for the two enantiomers.

Comparison of the effects of xamoterol and isoprenaline on rat cardiac beta-adrenoceptors: studies of function and regulation

1. The effects of the beta 1-selective partial agonist xamoterol and the full agonist isoprenaline on rat cardiac beta-adrenoceptors were compared in functional studies of heart rate response in vivo and in vitro. In addition, the ability of both agents to cause receptor down-regulation in the rat heart following chronic (6 days) subcutaneous infusions was assessed by radioligand binding with [125I]-pindolol. 2. In the functional studies, xamoterol produced a maximal effect equivalent to approximately 65% of that of isoprenaline and was overall less potent than the full agonist. 3. Compared to saline control, the density of beta-adrenoceptors was reduced approximately 39% in ventricular membranes prepared from animals after 6 days of isoprenaline infusion but was unaffected by xamoterol. The relative proportions of the beta-adrenoceptor subtypes were unchanged by either active treatment. 4. Plasma xamoterol level at the end of the infusion period was equivalent to that associated with maximum tachycardia in vivo and to the concentration producing maximal stimulation of the rat isolated atrium in vitro. Thus suggesting 100% beta-adrenoceptor occupancy during the period of xamoterol infusion. 5. These results indicate that in this animal model xamoterol does not induce cardiac beta-adrenoceptor down-regulation during chronic treatment, with doses that produce a maximal functional response both in vitro and in vivo.

Regional differences in the effect of guanine nucleotides on agonist and antagonist binding to adenosine A1-receptors in rat brain, as revealed by autoradiography

The effect of increasing doses of GTP on agonist and antagonist binding to adenosine A1-receptors in different regions of rat brain was studied by autoradiography. A high concentration of GTP (100 microM) practically eliminated the binding of the agonist [3H]N6-cyclohexyladenosine in all regions. However, there were regional differences in the effects of low concentrations of GTP (0.1-10 microM). In some regions, for example the hippocampus, all concentrations of GTP decreased [3H]N6-cyclohexyladenosine binding, by decreasing the Bmax. In other structures, e.g. the superior colliculus, there was a biphasic response to GTP. Concentrations of 0.1-3 microM increased agonist binding, apparently due to a decrease in KD, whereas higher concentrations also decreased binding in these regions. The effects of GTP were mimicked by the stable GTP analogue guanosine-5'-O-(3-thiotriphosphate). GTP (0.5-100 microM) increased the binding of the antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine in all regions, but most markedly in those where GTP had a biphasic effect on agonist binding. Decreasing the levels of endogenous adenosine by increasing the concentration of adenosine deaminase and adding the 5'-nucleotidase inhibitor alpha-beta-methylene adenosine-5'-diphosphate gave an increase in [3H]8-cyclopentyl-1,3-dipropylxanthine binding and diminished the response to GTP. In sections treated with adenosine deaminase and alpha-beta-methylene adenosine-5'-diphosphate, GTP steadily decreased [3H]N6-cyclohexyladenosine binding in all regions. Thus, the GTP-induced increase in both agonist and antagonist binding may be due to a displacement of endogenous adenosine. In the presence of 1 mM EDTA, GTP had a monophasic effect on the binding of [3H]N6-cyclohexyladenosine in all regions. In the presence of 2 mM MgCl2 a biphasic response to GTP was seen in all regions. In EDTA washed sections, the effect of MgCl2 on [3H]N6-cyclohexyladenosine binding was more pronounced in the superior colliculus, where we had observed a biphasic response to GTP. The results suggest that there are regional differences in the effects of GTP on adenosine A1-receptor binding in rat brain, that reflect regional differences in the magnesium-dependent binding of endogenous adenosine, which is bound to the receptor by tight binding, is very difficult to remove, and easily interferes with radioligand binding in in vitro experiments. There may be regional differences in the sensitivity of A1-receptor-G-protein complexes to magnesium, that reflect a heterogeneity of the G-proteins to which the A1-receptors are coupled.

Decrease in rat cardiac beta 1- and beta 2-adrenoceptors by training and endurance exercise

The cardiac beta-adrenoceptor adaptation to physical activity was investigated in rats which were subjected to a six-week endurance swimming training (ET; n = 7) and a training of high intensity (MT; n = 7). In addition, the effect of a single bout of endurance exercise without preceding training (EE; n = 7) was evaluated. These groups were compared with a sedentary control group (C; n = 9). Beta-adrenergic receptors in rat myocardial membranes were labelled using the high affinity antagonist radioligand (-)125iodocyanopindolol (ICYP). Computer modelling techniques provided estimates of the maximal binding capacity (Bmax) and the dissociation constants (KD). Tissue was constantly kept at temperatures of less than or equal to 4 degrees C and incubated at 4 degrees C for 18 h in buffer containing 100 microM GTP so as to prevent masking of beta-adrenoceptors by endogenous norepinephrine. In comparison with the C group (Bmax = 43.2 +/- 1.6 fmol/mg protein, KD = 11.7 +/- 1.5 pM) computerized coanalyses of saturation binding data of ET, MT, and EE revealed a 13.0%, 25.5%, and 16.6% decrease in Bmax (P less than 0.01), respectively, without significantly differing KD values (10.6 pM, 9.0 pM, 10.5 pM, respectively). We provide the first evidence that acute exercise lowers the sarcolemmal beta-adrenoceptor number in the rat heart. In the competition radioligand binding, CGP20712A and ICI118.551 were employed as subtype-selective antagonists of beta 1- and beta 2-adrenoceptors, respectively, to determine the relative proportions of the receptor subtypes. The ratio of beta 1-/beta 2-adrenoceptors in C was 67.5:32.5 and no statistically significant variation occurred in animals subjected to physical activity. On the basis of published data we assume that acute exercise induces a sequestration of beta-adrenoceptors from the cell surface to some intracellular compartment, whereas the molecular basis of the chronic beta-adrenoceptor down-regulation may involve a training-induced reduction in receptor synthesis. Our findings on cardiac beta-adrenoceptor adaptation to physical activity may represent one of the mechanisms underlying the relative bradycardia in trained subjects.

Myocardial slice: a physiological approach to beta-adrenergic ([3H] CGP-12177) receptor binding in hamster and guinea pig heart

A new technique is described for the characterization and quantification of beta-adrenergic receptors in biologically viable slices of myocardium from the hamster right ventricle using the hydrophilic radioligand, [3H]CGP-12177 (CGP). Binding was stereospecific, saturable, of high affinity, reversible, displaceable by appropriate drugs, and highly positively correlated with increasing tissue concentrations. Bmax for CGP binding to myocardial slices from 50-day-old male Golden Syrian hamsters was 3.28 +/- 0.15 fmol/mg wet weight, while Kd was 0.21 +/- 0.02 nM. Freezing resulted in a close to 50% loss of receptor number with no apparent change in affinity. The slice preparation may be utilized to detect in vivo changes in myocardial cell surface receptors, as evidenced by the fact that the number of receptors in slices from ischemic guinea pigs was increased (Bmax = 15.5 +/- 1.25 fmol/mg wet wt) compared with sham-operated controls (Bmax = 10.4 +/- 0.38 fmol/mg wet wt). The minimal tissue disruption associated with this procedure, as well as its speed, simplicity, and relatively low cost, suggest that the myocardial slice preparation provides an important methodology for the study of beta-adrenergic receptor binding in the semiintact myocardium.

Genetic influences on agonist binding to cardiac beta-receptors

Regulation of beta-adrenoceptor-agonist function in the Maudsley Reactive (MR/Har) and the Maudsley Non-Reactive (MNRA/Har) rat strains was assessed by comparison of isoproterenol competition for [125I]iodocyanopindolol (ICYP) binding sites in crude left ventricular homogenate preparations. Non-linear, least-squares analysis of isoproterenol competition for ICYP binding in the absence of guanine nucleotide revealed different proportions of high- and low-affinity receptors in the two strains; MR/Har rats (59 +/- 3.3%) had a significantly greater proportion of receptors in the high-affinity state than the MNRA/Har rats (41 +/- 4.5%). Addition of the non-hydrolyzable guanine nucleotide analog guanylylimidodiphosphate (Gpp(NH)p) converted receptors to the low-affinity state. Analysis of Gpp(NH)p concentration-response curves in left ventricular homogenates of the two strains revealed that the MR/Har strain had a significantly (P less than 0.02) lower EC50 for guanyl nucleotide inhibition of isoproterenol competition for ICYP binding than the MNRA/Har. Confirming previous experimental results, a significantly (P less than 0.04) greater density of ventricular beta-receptors was found in MR/Har rats (13.16 +/- 0.92 fmol/mg protein) than in MNRA/Har rats (10.81 +/- 0.63 fmol/mg protein). Left ventricular catecholamine levels were found to be correlated inversely with beta-adrenoceptor density in the two strains; norepinephrine (NE) and epinephrine (EPI) concentrations (ng/mg protein) in left ventricle were 12.19 +/- 0.94 for NE and 0.165 +/- 0.038 for EPI in MNRA/Har, and 8.73 +/- 0.95 and 0.018 +/- 0.018, respectively, in MR/Har. All other parameters of agonist interactions with the cardiac beta-adrenoceptor for the MR/Har and MNRA/Har rat strains were similar [the IC50 for displacement of ICYP binding by isoproterenol, the accompanying Hill coefficients in the Gpp(NH)p present and absent condition, the Kd of the high- and low-affinity states in the absence of Gpp(NH)p, and the Kd of the uniform low-affinity state in the presence of Gpp(NH)p]. We hypothesize that the strain-dependent differences in high-affinity state formation reported here may account for some of the in vivo differences in cardiovascular function previously demonstrated in the Maudsley rats.

The beta 1- and beta 2-adrenoceptor affinity of atenolol and metoprolol. A receptor-binding study performed with different radioligands in tissues from the rat, the guinea pig and man

The radioligand binding technique was used to perform a systematic investigation of the beta 1- and beta 2-adrenoceptor affinity of atenolol and metoprolol in tissues from the rat, the guinea pig and man. Radioligands, [125I](+/-)hydroxybenzylpindolol, [125I](-)pindolol, [3H](-)dihydroalprenolol and [3H](-)CGP12177, with different degrees of lipophilicity were used in the binding experiments. In membrane preparations of rat ventricular myocardium and uterus, the number of specific binding sites was similar when comparing experiments performed with the different radioligands. The percentage of the beta 1- and beta 2-adrenoceptor subpopulations in the tissues studied was not dependent on the radioligand or displacing compound used. Furthermore, the affinity of metoprolol and atenolol for beta 1- and beta 2-adrenoceptors was independent of the radioligand used or the tissue studied. The beta 1-adrenoceptor affinity of metoprolol was about 6-7 times higher than that of atenolol, while the beta 1-adrenoceptor selectivity was similar (about 30-fold) for the two beta-blockers. It is concluded that the physical-chemical properties of the radioactive ligands and beta-blockers studied do not affect the results obtained from beta-adrenoceptor-binding experiments in cellular membrane fractions. The beta 1- and beta 2-adrenoceptor affinities did not change in any experiments performed in tissues from the rat, the guinea pig and man for either atenolol or metoprolol.

Independent regulation of beta 1- and beta 2-adrenoceptors

1 The down-regulation of beta-adrenoceptors has been postulated as a biochemical marker of antidepressant efficacy. Here we demonstrate that chronic treatment with desipramine down-regulates beta 1-adrenoceptors in rat cerebral cortex and that beta-adrenoceptor subtypes can be independently regulated by treatment with different beta-adrenoceptor agonists. 2 Desipramine, (+/-)-clenbuterol, prenalterol, corwin (20 mg kg-1 daily) and corwin (10 mg kg-1 daily) were administered to male, Sprague-Dawley rats, over eight days, by means of osmotic Alzet pumps placed subcutaneously and removed 24 h before analysis. Control rats received vehicle only. The beta 1- and beta 2-adrenoceptor populations were measured in cerebral cortex by a modified (-)-[125I]-pindolol receptor binding assay. 3 The conventional antidepressant, desipramine, preferentially down-regulated beta 1-adrenoceptors whereas the non-selective beta-adrenoceptor agonist (+/-)-clenbuterol preferentially down-regulated beta 2-adrenoceptors. The beta 1-selective partial agonist, prenalterol, up-regulated beta 1-adrenoceptors perhaps acting more as an antagonist than as an agonist. Finally, neither dose of corwin had any significant effect on beta-adrenoceptor number.

Beta 1- and beta 2-adrenoceptor affinity and stimulatory effects of (S)-pindolol and iodinated (S)-pindolol

The beta 1- and beta 2-adrenoceptor affinity and stimulatory effects of iodinated (S)-pindolol (IPIN) and (S)-pindolol were investigated in vitro using beta-adrenoceptor binding technique and isolated right atrium (rate increase, beta 1) and uterus (relaxation, beta 2) of the rat. IPIN had a higher affinity towards beta-adrenoceptors compared to (S)-pindolol, with some beta 2-adrenoceptor selectivity. In the rat uterus, IPIN produced only marginal stimulatory effects, while (S)-pindolol caused a concentration-dependent relaxation with a maximal effect that was 55% of that generated by isoprenaline. In the right atrium IPIN caused an increase in the atrial rate similar to that caused by (S)-pindolol. The concentration of IPIN required in the right atrium for a half-maximal response (pD2 = 7.81) was markedly greater than that required for occupation of half the beta-adrenoceptor population (pKB = 9.81). The beta 1-selective blocker metoprolol antagonized the effect of (S)-pindolol and IPIN on the atrial rate but a greater concentration of metoprolol (5 X 10(-6) M compared with 5 X 10(-7) M) was required to antagonize the effect of IPIN significantly. It is concluded that iodination of (S)-pindolol increased its affinity and decreased its efficacy towards beta-adrenoceptors.

Beta-adrenoceptor antagonists (non-selective as well as beta 1-selective) with partial agonistic activity decrease beta 2-adrenoceptor density in human lymphocytes. Evidence for a beta 2-agonistic component of the partial agonistic activity

In the present study the effects of pindolol [non-selective beta-adrenoceptor antagonist with strong partial agonistic activity (PAA)] on beta 2-adrenoceptor density in lymphocytes (assessed by (-)-[125I]iodocyanopindolol (ICYP) binding) were compared with those of the beta 1-selective antagonists celiprolol (with PAA) and bisoprolol (no PAA) in normotensive young volunteers to get further insights into the nature of PAA. Administration of pindolol (2 X 5 mg/day) caused an about 25% decrease in lymphocyte beta 2-adrenoceptor density after 2 days; during treatment beta 2-adrenoceptor density declined further (maximum decrease after 7 days: 50%). After withdrawal of pindolol lymphocyte beta 2-adrenoceptor density recovered very slowly being still after 4 days significantly reduced, although no pindolol was detectable in plasma after 36 h. The KD-values for ICYP, however, did not change during or after pindolol treatment. The decrease in lymphocyte beta 2-adrenoceptor density induced by pindolol could be completely prevented by simultaneous administration of propranolol (3 X 40 mg/day) indicating that the PAA of pindolol is the cause of its beta-adrenoceptor decreasing effect. Administration of the non-selective beta-adrenoceptor antagonist bopindolol (1 X 2 mg/day) with PAA caused decreases in lymphocyte beta 2-adrenoceptor density (maximum decrease after 7 days: 40%); concomitantly the 10 mumol/l (-)-isoprenaline evoked increases in the intracellular level of lymphocyte cyclic AMP were attenuated to a similar extent indicating that the beta-adrenoceptor antagonist-induced decrease in beta-adrenoceptor density is accompanied by a loss in beta-adrenoceptor function.(ABSTRACT TRUNCATED AT 250 WORDS)