Β-adrenoceptor Density Research Articles

Gangliosides enhance behavioral and neurochemical effects induced by chronic desipramine (DMI) treatment

Rats pretreated with gangliosides showed a significant enhancement of the anti-immobility effect of desipramine (DMI) in the forced swimming test. Accordingly, an associated treatment of gangliosides and DMI for 7 days significantly enhanced β-adrenergic down-regulation in the frontal cortex as compared with the effect of DMI alone. Gangliosides exerted an accelerating effect on the decrease of β-adrenoceptor density induced by DMI, since the down-regulation phenomenon appeared after 3 days of treatment. Gangliosides did not affect the pharmacokinetics of DMI, since associated acute or prolonged treatments did not modify the brain levels of DMI as compared to the levels of animals that had received DMI alone. These results evidence a stimulating effect of gangliosides on the development of adaptative receptor changes induced by chronic DMI treatment.

Subsensitivity of the Failing Human Heart to Isoprenaline and Milrinoneo is Related to β-Adrenoceptor Downregulation

The number of cardiac beta-adrenoceptors and the positive inotropic effect of isoprenaline and milrinone were measured in cardiac membranes and isolated, electrically driven muscle strips from nonfailing donor hearts and from patients with mitral valve disease (NYHA II-III), ischemic heart disease, and dilated cardiomyopathy (NYHA IV). In nonfailing hearts, the number of beta-adrenoceptors were 41.5 fmol/mg protein (mean, n = 3). In ischemic heart disease and NYHA II-III, there was a loss of cardiac beta-adrenoceptors (22.1 fmol/mg protein, mean, n = 3; 23.2 +/- 2.7 fmol/mg protein, n = 30), respectively. In NYHA IV, there was a pronounced reduction of the number of cardiac beta-adrenoceptors to 12.1 +/- 1.5 fmol/mg protein (n = 15). The Kd value did not differ in either group. Correspondingly, the positive inotropic effect of isoprenaline was more pronounced in nonfailing myocardium, reduced in NYHA II-III and ischemic heart disease and almost blunted in NYHA IV. Similar results were observed with the phosphodiesterase inhibitor milrinone. A good correlation of the beta-adrenoceptor density to the maximal positive inotropic effect of isoprenaline and milrinone was observed. Neither the number of cardiac beta-adrenoceptors nor the positive inotropic effect of isoprenaline correlated with the age of the patients. We conclude that the number of cardiac beta-adrenoceptors and the positive inotropic effect of beta-adrenoceptor agonists are reduced in the failing human heart depending on the severity of heart failure. Furthermore, the positive inotropic effect of milrinone is also reduced and related to the reduction of beta-adrenoceptors. The lack of correlation with the age of the patients provides evidence for a predominant role of heart disease rather than aging in the reduction of beta-adrenoceptors and subsensitivity to cyclic AMP-increasing positive inotropic agents in the failing human heart.

β-Adrenoceptor desensitization in guinea-pig isolated trachea

Exposure to (−)-isoprenaline (25 μM, 1 h) caused a stereoselective, time and concentration-related decrease in smooth muscle β 2-adrenoceptor function in guinea-pig trachea. Furthermore, tracheal relaxant responsiveness to the β-adrenoceptor agonists (±)-fenoterol and (−)-noradrenaline was reduced, while that to theophylline and nitroprusside was unaffected. Responsiveness to forskolin was marginally but significantly reduced. Indomethacin, a cyclooxygenase inhibitor and mepacrine, an inhibitor of phospholipid turnover, had no significant effect on the extent of isoprenaline-induced desensitization. Conversely, cortisol (25 μM) significantly reduced desensitization and enhanced the rate of spontaneous recovery of responsiveness to isoprenaline. Desensitization was not accompanied by a reduction in the density of β-adrenoceptors in the trachea, as assessed by binding and light microscopic autoradiography using [ 125I]iodocyanopindolol ([ 125I]CYP). Thus, desensitization was probably caused primarily by β-adrenoceptor/adenyl cyclase uncoupling. This model may be useful in investigations of the effect of glucocorticoids on the β-adrenoceptor dysfunction recognized in severe asthma.

Adenylate cyclase activity coupled to the stimulatory guanine nucleotide binding protein in patients having electrophysiologic studies and either structurally normal hearts or idiopathic myocardial disease

There has been much recent interest in transmembrane signaling involving adrenergic and other receptors both in animals and humans with congestive heart failure (CHF). In patients with idiopathic dilated cardiomyopathy and severe CHF, β-adrenoceptor density and maximal adenylate cyclase activity stimulated by (−)-isoproterenol in the presence of guanosine triphosphate (GTP) are reduced. 1 Other studies in animal models of CHF preceded by myocardial hypertrophy have revealed an increase in β-adrenoceptor density accompanied by reduced receptor affinity for agonist and a decrease in the stimulatory guanine nucleotide binding protein (Gs). 2,3 To date, there are no data reporting functional alterations in Gs in humans.

Coupling of receptors in brain membranes by fusion to the adenylate cyclase system of a foreign effector cell

Receptor-effector coupling in the adenylate cyclase (AC) system was studied using fusion transfer of rat or monkey frontal cortex membranes to Friend erythroleukemia (Fc) cells. The indigenous AC activity of cortex membranes had previously been inactivated with N-ethylmaleimide. In the fusates, the AC activity could be stimulated through β-adrenoceptors using noradrenaline (NA), or through specific receptors fpor vasoactive intestinal polypeptide (VIP), or by fluoride which activates the effector components of the AC-system directly, bypassing receptors. There was a critical stoichiometric relationship between the receptor-stimulated cAMP output and the number of recipient cells of the fusion system, i.e. the total AC capacity as measured by fluoride stimulation. In fusates with rat brain membranes, the β-adrenoceptor coupling increased as the availability of recipient cells increased; on the other hand, the excess of recipient cells did not change the VIP receptor coupling capacity. In fusates with monkey brain membranes, the situation was the opposite: VIP receptor coupling increased as larger amounts of recipient cells were made available, but the β-adrenoceptor coupling capacity remained unchanged. The differences in coupling capacities were related to differences in receptor binding with higher β-adrenoceptor density in rat than in monkey frontal cortex membranes, as opposed to higher VIP receptor density in monkey than in rat frontal cortex membranes. Neuropeptide tyrosine (NPY) attenuated both NA-and VIP-induced activation of AC in the fusates; it was equally potent against both agents. In rat brain membrane fusates, the NA-induced AC activity was attenuated in a dose-dependent and apparently non-competitive manner. Pretreatment with pertussis toxin abolished the effect indicating that the NPY receptor couples to a mechanism involving G i and G o protein. In conclusion, the fusion procedure may allow the determination of the coupling capacity of receptors, activating or inhibiting adenylate cyclase.

Role of serotonergic input in the down-regulation of β-adrenoceptors following long-term clorgyline treatment

Administration of the selective monoamine oxidase (MAO) type A-inhibiting antidepressant clorgyline (1 mg/kg per day) to rats for 21 days caused a significant decrease in cortical [ 3H]dihydroalprenolol binding. Selective lesioning of central serotonergic axons by 5,7-dihydroxytryptamine (5,7-DHT; confirmed by the presence of the serotonin syndrome in response to a 40 mg/kg dose of 5-hydroxytryptophan (5-HTP) or inhibition of 5-HT synthesis by parachlorophenylalanine (PCPA) caused significant 5-HT and 5-HIAA depletions in the cortex without much effect on NE and DA concentrations, but did not have any significant effect on β-adrenoceptor density, and furthermore failed to attenuate clorgyline-induced decreases in β-adrenoceptor density. Clorgyline treatment partially antagonized 5-HT depletion by the 5,7-DHT lesion or PCPA treatment. These findings suggest that due to their ability to raise 5-HT concentrations, MAO-inhibiting antidepressants may be a better alternative than the tricyclics in treating depressed patients with reduced 5-HT if down-regulation of β-adrenoceptors is critical for antidepressant efficacy.

Rapid down regulation of β-adrenoceptors by co-administration of desipramine and fluoxetine

Co-administration of desipramine and fluoxetine resulted in a 27% decline in cerebral cortical β-adrenoceptor density after four days — a time point at which neither agent alone was effective. After 14 days, desipramine- and desipramine + fluoxetine-treated rats showed decreased receptor levels, with a greater decrement seen with the combined treatment. Fluoxetine, alone, had no effect on β-adrenoceptor density at any time point examined. These effects are attributable of central serotonergic action since they were prevented by prior treatment with p-chlorophenylalanine. Cyproheptadine, a 5-HT 2 antagonist, did not block these effects. Independent administration of fluoxetine and desipramine produced ∼20% decrement in isoproterenol-stimulated cyclic AMP accumulation after four days of treatment. Co-administration of desipramine and fluoxetine resulted in a 35% decrement in cyclic AMP accumulation which was nearly additive with that produced by either drug alone. Consequently, the combination of a norepinephrine and serotonin uptake inhibitor may be an advantageous and rapid treatment for the alleviation of certain forms of depression.

Determination of beta adrenoceptor density on rabbit mononuclear leukocytes

The purpose of the present study was to devise a technique for the isolation of a relatively homogeneous mononuclear leukocyte (MNL) preparation from rabbit whole blood and determine the density and affinity of β-adrenoceptors on MNL. A modified method based upon that by Böyum was developed to maximize isolation of MNL from red blood cells (RBC), granulocytes, and platelets. The method involved an initial centrifugation to remove platelets and two centrifugations with a Ficoll solution to eliminate RBC and granulocytes. β-Adrenoceptor density as determined with [ 125I]cyanopindolol and MNL membrane or whole cell preparations ranged between 317 and 360 sites per cell. Affinity for the binding sites was dependent upon whether membrane or whole cell preparations were studied, being 56.3 ± 9.9 and 11.4 ± 1.4 pM, respectively. Binding sites were found to be saturable and noncooperative. In addition, the binding sites demonstrated selectivity and stereospecificity for β-adrenoceptor ligands. It is concluded that the modified method of harvesting MNL from rabbit whole blood provides a relatively homogeneous cell suspension that can be used to study the β-adrenoceptor system.

Genetic influences on agonist binding to cardiac β-receptors

Regulation of β-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<0.02) lower EC 50 for guanyl nucleotide inhibition of isoproterenol competition for ICYP binding than the MNRA/Har. Confirming previous experimental results, a significantly (P<0.04) greater density of ventricular β-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 β-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 β-adrenoceptor for the MR/Har and MNRA/Har rat strains were similar [the IC 50 for displacement of ICYP binding by isoproterenol, the accompanying Hill coefficients in the Gpp(NH)p present and absent condition, the K d of the high- and low-affinity states in the absence of Gpp(NH)p, and the K d 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.

Acute and Long-Term Effects of Acebutolol on Systemic and Renal Hemodynamics, Body Fluid Volumes, Catecholamines, Active Renin, Aldosterone, and Lymphocyte β-Adrenoceptor Density

Acebutolol is a relatively new beta-adrenoceptor blocking antagonist, possessing both beta 1-adrenoceptor selectivity and partial agonist activity (PAA). Its acute (24 h, 400 mg, twice daily) and long-term effects (3 weeks) on systemic and renal hemodynamics, body fluid volumes, hormones, and beta-adrenoceptor density on lymphocytes were studied in a single-blind placebo-controlled trial, in 10 hypertensive patients. The initial response to acebutolol (1-2 h) was a fall in heart rate (HR) (-9.6 +/- 2.7%), cardiac output (-16.0 +/- 3%), and stroke volume (SV) (-10.7 +/- 0.2%), and an increase in systemic vascular resistance (SVR) (18.0 +/- 3.9%). Mean arterial pressure (MAP) began to fall 2-3 h after dosing in parallel with a decrease in SVR. At the end of the acute study, MAP and SVR were decreased by 18.1 +/- 2.7% and 15.6 +/- 5.6%, respectively. By that time, HR and SV had returned to control values despite blockade of beta-adrenoceptors. After 3 weeks of treatment (mean dose of acebutolol 480 mg twice daily), the fall in MAP was 10.1 +/- 2.7% and HR was decreased by 13.0 +/- 2.3%. Renal blood flow and glomerular filtration rate did not change. Acute and long-term treatment had no effect on the density of lymphocyte-membrane beta-adrenoceptors. This could be explained by acebutolol's beta 1 selectivity or, alternatively, this could be due to the drug's PAA.

Characteristics of cardiac .BETA.-adrenoceptors in Suncus murinus.

Characteristics of cardiac β-adrenergic responses in Suncus murinus (suncus) were examined in comparison with those of the rat. In both species, isoproterenol produced positive inotropic and chronotropic responses in spontaneously beating right atria and positive inotropic response in electrically driven papillary muscles. However, the effects of β-adrenoceptor stimulation in suncus papillary muscles were considerably smaller than in the rat. Tyramine, an indirect sympathomimetic amine, did not affect the contraction of ventricular muscle in suncus. The affinity and density of specific [3H] dihydroalprenolol binding sites in suncus atrium were close to those in rat atrium. On the other hand, specific [3H] dihydroalprenolol binding sites in suncus ventricle were distinctly lower both in affinity and in density than those of rat ventricle. These results suggest that ventricular β-adrenergic responses in the suncus are markedly smaller than those in the rat, probably due to lower density and affinity of β-adrenoceptors.

Effects of choline and inositol on 3H-dihydroalprenolol and 125I-iodocyanopindolol bindings to beta-adrenergic receptors of the guinea pig cerebral cortical membranes.

This experiment was designed to examine the effects of choline and inositol in Eagle's minimal essential medium (MEM), because the values of specific binding were increased when the 3H-dihydroalprenolol (3H-DHA) binding assay of β-adrenoceptors in the guinea pig and rat cerebral cortical membranes was carried out in Eagle's MEM instead of 60 mM Tris-HCl, 20 mM MgCl2 buffer (pH 7.4). All chemicals contained in the MEM were added to the 3H-DHA binding mixture and the values of the specific binding were compared with the control values obtained in the absence of the chemicals. The presence of choline (0.1-10 μM) or inositol (0.1-10 μM) induced an increase of the values of the specific binding. These effects of choline and inositol were observed after preinclubation for 20 and 1 min, respectively. After 30 min of incubation, no changes were observed upon removal of these chemicals. Furthermore, choline and inositol significantly increased the β-adrenoceptor density (β ) when 3H-DHA and I-iodocyanopindolol were used as the radioligands, as determined by Scatchard analysis, but no significant change was observed in the value of the dissociation constant (Kd) in 3H-DHA binding. These results suggest that choline and inositol in the membranes could have a crucial role in drug-receptor interaction.

Lymphocyte β2-Adrenoceptor Density in Pre-Eclamptic Patients: No Difference Compared with Controls

Lymphocyte β2-adrenoceptor density was measured by (-)[125I] cyanopindolol in 13 pre-eclamptic and 13 control patients at the last trimester of pregnancy. No significant difference was detected between the groups in lymphocyte β2-adrenoceptor density or dissociation constant of equilibrium. The lack of compensatory mechanism, i.e. increased number of β2-adrenoceptor, may be one cause of vasoconstriction in pre-eclampsia

The effect of ischemia and recirculation, hypoxia and recovery on anti-oxidant factors and β-adrenoceptor density: Is the damage in the erythrocytes a reflection of brain damage caused by complete cerebral ischemia and by hypoxia?

This investigation was focussed on the gravity of tissue injury caused by complete ischemia (for five min) and hypoxia (for three weeks) in the cerebral cortex (homogenate) and the erythrocyte lysate or the erythrocyte membrane of the rat in order to investigate if the changes that occur in brain tissue are reflected in the erythrocyte. To this end, glutathione (GSH), superoxide dismutase (SOD) and catalase were measured, also alterations in β-adrenoceptor density under these two conditions were examined. It was found that in ischemia partial parallellism in changes that occur in the central nervous system (cerebral cortex) and the erythrocyte exists. The SOD activity became higher and the β-adrenoceptor density (measured as specific (−)-[ 125I] iodocyanopindolol binding) was decreased in both tissues. However after the hypoxic condition we established a decrease in the number of β-adrenoceptors in the cerebral cortex but an increase in β-adrenoceptor density in the erythrocyte.

IMPAIRED β-ADRENERGIC FUNCTION IN LYMPHOCYTES OF CHILDREN WITH PERTUSSIS (P)

There is indirect evidence from clinical studies in human as well as in animal studies that infection and vaccination with B. pertussis might affect the β-adrenoceptor/cyclic AMP system. In order to study whether this view warks, the adrenergic system was investigated on lymphocytes in children within the paroxysmal stage of P, in children after vaccination (V.) with P-mono-vaccine, and in an age-matched control group.Methods: β-adrenoceptor (β-R) density was estimated by means of binding studies using 125-J-cyanopindolol. In children with P β-E number of lymphocytes was 2-3 fold lower compared to controls and after V. High and low affinity states of the β-R, indicating the intrinsic activity, were obtained by Hofstee-plot-transformation of salbutanol displacement curves. Basal levels, as well as maximum cAMP accumulation after specific (isoprenaline = IPN) and unspecific (forskoline = FO) β-R stimulation, were determined using a protein binding assay.Results: In children with P β-R number of lymphocytes was 2 to 3fold lower compared to controls. In addition, an increased number of low affinity state β-R indicated impaired function. Decreased cAMP levels in both, the vaccination children and those suffering from P were observed. However, the latter showed distinct reduction of IPN induced maximal cAMP accumulation.It is concluded that P-infection, as well as V., strongly affects the adrenergic system.

A single dose of FG 7142 causes long-term increases in mouse cortical β-adrenoceptors

There is evidence that central monoamines are involved in the actions of benzodiazepines. We have investigated the effects of a single dose of the benzodiazepine partial inverse agonist, FG 7142, on radioligand binding to α 2- and β-adrenoceptors in mouse cerebral cortex. We found that seven days after a single injection of FG 7142 there was a large increase in the density of β-adrenoceptors. This rise was not detectable either 15–30 min, or 24 h after the injection and no statistically significant changes in α 2-adrenoceptor binding were found at any of these times. Administration of the benzodiazepine antagonist Ro 15-1788 at the same time as FG 7142 prevented the rise in β-adrenoceptor density. We discuss the possibility that the β-adrenoceptor unregulation to the behavioural effects of FG 7142.

Treatment of rats with thyrotropin (TSH) reduces the adrenoceptor sensitivity of adenylate cyclase from cerebral cortex

Prolonged treatment of rats with TSH attenuates the noradrenaline (NA) sensitivity of the cerebral cortical cAMP generating system. This effect can be measured in slices from brain cortex and with a membraneous, cellfree adenylate cyclase preparation from the same brain region. The development of this downregulation is time (7 d treatment required) and dose dependent (EC(50) = 3.5 mU/kg for 9 d). After discontinuation of treatment, about 12 d were required for reversal of this effect. Density of ?-adrenoceptors in cerebral cortex as measured by (?) [ (3)H]dihydroalprenolol (DHA) binding was reduced by TSH treatment (22% reduction). The catalytic activity of adenylate cyclase as tested with guanosinemonophosphate phosphoimidophosphate (GMPPNP) was unaffected by TSH treatment. It is suggested that TSH-mediated feedback mechanisms on TRH levels and receptors in brain are responsible for the changes of the central adrenoceptor system. The data are discussed with respect to the blunted TSH-response to TRH observed in certain psychiatric illnesses.

Age-related changes of β-adrenoceptors in spleen lymphocytes and cerebral cortex of NZB/BIN mice

The density (B max) of β-adrenoceptors in splenic lymphocytes of NZB/BIN mice decreased up to an age of about 40 weeks and then levelled out. The B max in cerebral cortex, on the other hand, increased in the first half of life and then changed relatively little. The dissociation constant of the ligand ( K d) was larger in the cortex than the spleen and showed relatively little age-dependent change.

β-Adrenoceptors in the rat parotid gland enlarged by salivariectomy and bulk diet. Effects of denervation

The numbers of β-adrenergic receptors and level of cyclic AMP (cAMP) of the parotid gland of adult female rats were determined 4 weeks after introduction of a regimen that induced a 2-fold increase in gland weight. This regimen consisted of ablation of the submandibular-sublingual glands and substitution of the normal chow diet with a bulk diet consisting of 50% inert cellulose and 50% ground solid chow. There was a 2.4-fold increase in number (density) of β-adrenoceptors in the enlarged parotid gland when comparison was made with parotid glands of control rats. The β-adrenoceptor present in the enlarged and normal glands was of the β 1 subtype. Removal of either autonomic pathway at the time of partial salivariectomy and dietary substitution was followed by a small reduction in number of β-adrenoceptors (4–9% with either denervation), but when both nerves were removed the reduction was 25%; in magnitude, these changes were generally similar to those observed with denervated parotid glands of chow-fed rats. The norepinephrine concentration of the enlarged gland was much less than that of normal glands (reduced 38%); sympathectomy of normal or enlarged parotid glands resulted in a marked lowering of norepinephrine concentrations (to 1–5% of control levels): parasympathectomy had no effect on norepinephrine concentration of enlarged parotid glands but caused a decrease in that of the parotid of normal size. Apparently, the number of β-adrenoceptors depends on the degree of activity of both the parasympathetic and sympathetic nerves to the parotid. The level of cAMP was elevated in the enlarged parotid gland and was more than 6 times as high as that of control parotid glands.

Regional distribution of myocardial β-adrenoceptors in the cat

The purpose of this study was to delineate the distribution of β-adrenoceptor density in the cat heart, with an emphasis on areas within the left ventricle. β-Adrenoceptor densities, determined for hearts obtained from five cats, were not significantly different in the left and rights atria, i.e. 47.6 ± 7.2 and 32.8 ± 7.5 fmol/mg protein, respectively. β-Adrenoceptor densities for the septum and right ventricle were 105.4 ± 15.0 and 65.0 ± 14.0 fmol/mg protein, respectively. The β-adrenoceptor density for the proximal distribution of the left anterior descending artery LV1, distal distribution of the left anterior descending artery LV2 and posterior wall of the left ventricle LV3 were: 81.3 ± 11.5, 145.1 ± 20.8 and 165.4 ± 35.8 fmol/mg protein, respectively. Thus, the distribution of the β-adrenoceptor densities was greatest in the apex of the left ventricle. The data suggest that there are regional differences in the β-adrenoceptor densities among the areas of the heart and within the left ventricle. These differences may be related to functional differences.