Beta-adrenoceptor Affinity Research Articles

Alterations of myocardial and vascular adrenergic receptor-mediated responses in Escherichia coli-induced septic shock in the rat

To investigate responsiveness to exogenous catecholamines in rat bacteremic shock by studying both myocardial and vascular functional parameters; to determine in the same study the relationship of these parameters with other relevant biological parameters of the adrenergic pathway, such as myocardial beta-adrenergic receptors and cyclic adenosine monophosphate (cAMP); and to indirectly approach the roles of tumor necrosis factor-alpha (TNF-alpha) and nitric oxide. Experimental, comparative study. Laboratory in a university hospital. Male Sprague-Dawley rats, weighing 270 to 320 g. Intravenous injection of live Escherichia coli DH5 alpha (2 x 10(10) organisms/kg) or saline (0.6 mL) and comparison of the two groups. Mean arterial pressure and heart rate (HR) were recorded, and circulating TNF-alpha concentrations were measured, during the first 3 hrs after E. coli administration. Myocardial and vascular functional parameters were obtained, respectively, from Langendorff-perfused hearts and isolated aortic rings. Adrenergic biochemical parameters (catecholamines, density and affinity of beta-receptors, and isoproterenol-stimulated myocardial cAMP) were determined 3 hrs after E. coli injection. Mean arterial pressure decreased within 5 to 60 mins after bacteria injection and returned to basal levels in the last 2 hrs; HR was unchanged. Serum TNF-alpha concentrations peaked at 120 mins (7333 +/- 672 pg/mL) and were still increased at 3 hrs. Plasma concentrations of epinephrine and norepinephrine were significantly (p < .05) increased. Baseline values for differential left ventricular pressure and coronary flow were significantly (p < .0001, p < .001, respectively) reduced; HR remained unchanged. Isoproterenol induced a similar increase in differential left ventricular pressure and in HR. There was no decrease in the functional myocardial response to adrenergic stimulation. beta-adrenergic receptors were similar in density and in affinity in the two groups. Isoproterenol-stimulated myocardial cAMP was significantly (p < .01) reduced compared with the control group. In aortic rings, bacteria administration significantly (p < .01) shifted the dose-response curve to norepinephrine to the right, both in the presence and absence of endothelium. NG-monomethyl-L-arginine significantly increased the contractions to attain the control level: p < .001 in presence of endothelium; p < .05 in absence of endothelium. In ex vivo experiments, 3 hrs after E. coli injection, vascular responsiveness was sharply decreased. This impaired response was improved by inhibition of nitric oxide. The heart, nevertheless, was still able to modulate its inotropic and chronotropic response to isoproterenol, even though an impaired beta-adrenergic-receptor stimulation of cAMP was already present.

Effects of in vitro mast cell degranulation on human lung beta-receptor binding parameters.

Numerous studies have demonstrated that subjects with allergic asthma have beta-adrenergic hyporesponsiveness, predisposing these individuals toward bronchospasm, mucous production, and mast cell degranulation. Since sympathetic innervation of the human respiratory tract is sparse, reduced beta-responsiveness probably results from alterations at or beyond the receptor level. We therefore examined whether anaphylaxis of human lung tissue acutely modulated the human lung beta-receptor system in ways that might lead to decreased beta-adrenergic responsiveness. Fresh thoracotomy peripheral lung samples from 26 patients were incubated with (anaphylaxis) or without (control) anti-IgE (1:100) for up to 90 minutes and histamine release was documented. Lung fragments were quick frozen at various times after anti-IgE for analyses of beta-receptor binding parameters. Antagonist Kd (dissociation constant) and receptor concentration values were determined using (-)[125I]pindolol and agonist IC50 values were determined using isoproterenol. In comparison with time O, neither anaphylaxis nor control samples had differences in receptor binding parameters with time. There were also no differences between anaphylaxis and control lung samples at any time point, and ratios of log control binding parameter/log anaphylaxis binding parameter ranged from 0.96 to 1.01. Anaphylaxis of lung does not lead to acute changes in antagonist or agonist affinity for beta-receptors or changes in receptor concentration. Under the conditions studied, lung mast cell degranulation does not acutely alter the human lung beta-receptor system in ways that might account for the beta-adrenergic hyporesponsiveness found in allergic asthma.

Impaired activation of adenylyl cyclase in lung of the Basenji-greyhound model of airway hyperresponsiveness: decreased numbers of high affinity beta-adrenoceptors.

1. To evaluate mechanisms involved in the impaired beta-adrenoceptor stimulation of adenylyl cyclase in tissues from the Basenji-greyhound (BG) dog model of airway hyperresponsiveness, we compared agonist and antagonist binding affinity of beta-adrenoceptors, beta-adrenoceptor subtypes, percentage of beta-adrenoceptors sequestered, and coupling of the beta-adrenoceptor to Gs alpha in lung membranes from BG and control mongrel dogs. We found that lung membranes from the BG dog had higher total numbers of beta-adrenoceptors with a greater percentage of receptors of the beta 2 subtype as compared to mongrel lung membranes. 2. Agonist and antagonist binding affinity and the percentage of beta-adrenoceptors sequestered were not different in BG and mongrel dog lung membranes. However, the percentage of beta-adrenoceptors in the high affinity state for agonist was decreased in BG lung membranes suggesting an uncoupling of the receptor from Gs alpha. 3. Impaired coupling between the beta-adrenoceptor and G protein documented by the decreased numbers of beta-adrenoceptors in the high affinity state in BG lung membranes, is a plausible explanation for the reduced stimulation of adenylyl cyclase and the resultant reduction in airway smooth muscle relaxation in this model.

In vitro pharmacological profile of YM-43611, a novel D2-like receptor antagonist with high affinity and selectivity for dopamine D3 and D4 receptors.

1. We investigated some neurochemical properties of a novel benzamide, YM-43611, [(S)-N-(1-benzyl-3-pyrrolidinyl)-5-chloro-4-cyclopropylcarbonylamino+ ++-2- methoxybenzamide] in comparison with putative D2-like receptor antagonists using both rat and human cloned dopamine D2-like receptors in vitro. 2. Receptor binding studies revealed that YM-43611 had appropriately potent affinities for both rat and human D2-like receptors, with moderate selectivity for D3 receptors and high selectivity for D4 receptors over D2 receptors (Ki values (nM) for rat receptors: D2, 165; D3, 35.5; D4, 1.85, and for human receptors: D2, 42.9; D3, 11.2; D4, 2.10). 3. YM-43611 displayed weak or negligible affinity for other neurotransmitter receptors, namely D1, D5, alpha(1), alpha(2), beta, 5-HT1A, 5-HT2A, 5-HT3, H1, M1 and M2 receptors. 4. Dopamine stimulated low-Km GTPase activity on membranes from Chinese hamster ovary (CHO) cells expressing the human D2-like receptor subtype. This response to dopamine of low-Km GTPase activity was inhibited by use of putative D2-like receptor antagonists. YM-43611 showed a moderate selectivity for D3 receptors (Ki = 45.5 nM) and a high selectivity for D4 receptors (Ki = 3.28 nM) over D2 receptors (Ki = 70.6 nM). 5. Dopamine inhibited forskolin-stimulated adenylate cyclase in intact CHO cells expressing the human D2-like receptor subtype. YM-43611 shifted the inhibition curve of dopamine on respective D2-like receptor subtype-mediated cyclic AMP formation to the right in a parallel fashion, showing a pA2 value of 7.42 (38.1 nM) for D2 receptors, a pKB value of 8.06 (8.68 nM) for D3 receptors, and a pA2 value of 8.42 (3.77 nM) for D4 receptors. 6. YM-43611 but not the other D2-like receptor antagonists exhibited good selectivity with respect to dual antagonism for D3 and D4 receptors in both receptor binding and functional assays. 7. These results indicate that YM-43611 is a novel D2-like receptor antagonist with high potency and selectivity for both D3 and D4 receptors. YM-43611 is therefore expected to be valuable in exploration of the physiological role of D3 and D4 receptors.

Desipramine administration in the olfactory bulbectomized rat: changes in brain β‐adrenoceptor and 5‐HT2A binding sites and their relationship to behaviour

1. The effects of repeated administration of the tricyclic antidepressant drug, desipramine (DMI), on behaviour (locomotor activity and rearing) and the number and affinity of brain beta-adrenoceptor and 5-HT2A receptor binding sites were examined in olfactory bulbectomized (OB) and sham-operated control rats. 2. Locomotor activity and rearing were increased in OB rats compared to sham-operated controls. The effect of various doses of DMI (administered orally twice daily for 21 days) on these behavioural measures was examined. A dose of 7.5 mg kg-1 provided optimal reversal of hyperlocomotion and increased rearing in OB rats, without changing these measures in sham-operated controls. 3. The time course of DMI (7.5 mg kg-1) on behavioural and neurochemical measures was examined. locomotion and rearing in OB rats were not significantly altered after 7 days, were significantly attenuated after 14 days and were normalized after 21 days. 4. After 7 days of DMI administration the number of beta-adrenoceptors was lower in frontal and occipital cortex and hippocampus. This reduction was largely restricted to the beta 1-adrenoceptor subtype. Administration of DMI for 14 or 21 days did not further reduce the number of beta-adrenoceptors. The DMI induced reduction in beta-adrenoceptors did not differ in OB and sham-operated control rats. 5. DMI administration for up to 21 days produced a progressive reduction in the number of 5-HT2A receptors in frontal cortex, without significant alterations in occipital cortex. 6. The time course of the reduction in the number of 5-HT2A receptors was similar to that of the DMI-induced behavioural changes whereas that for the reduction in beta-adrenoceptors was clearly different. 7. The present results suggest that the action of DMI in this animal model is unlikely to be directly related to a reduction in beta-adrenoceptors but may be related to a reduction in frontal cortical 5-HT2A receptors.

TUMOR NECROSIS FACTOR-α ADMINISTRATION INCREASES KUPFFER CELL CYCLIC ADENOSINE MONOPHOSPHATE LEVELS

Although studies have indicated that both Kupffer cell cyclic adenosine monophosphate (cAMP) and circulating TNF levels increase following trauma-hemorrhage and resuscitation, it remains unknown whether the elevated TNF levels are responsible for the increased Kupffer cell cAMP levels. To determine this, recombinant murine TNF-alpha (1.2 x 10(7) U/mg) was infused intravenously (.25 mg/kg body wt) over 30 min in normal rats. At 1 h after TNF-alpha or vehicle infusion, Kupffer cells and hepatocytes were isolated and cAMP levels were determined by radioimmunoassay. The levels of cAMP in the spleen and kidney were also measured. In addition, the maximal binding capacity and affinity of beta-adrenergic receptors were determined in Kupffer cells and hepatocytes by using [125I]iodopindolol. To determine whether there is any correlation between Kupffer cell cAMP and prostaglandin E2 (PGE2) or epinephrine, plasma levels of catecholamines and PGE2 were measured. The results indicated that TNF-alpha infusion significantly increased Kupffer cell cAMP levels while hepatocyte cAMP levels were not altered. Moreover, cAMP levels also increased in the macrophage/lymphocyte-rich spleen but were not altered in the kidney. Kupffer cell beta-receptor binding characteristics were not significantly affected by TNF-alpha infusion. In contrast, TNF-alpha administration markedly increased plasma levels of PGE2 and epinephrine. Thus, the elevated Kupffer cell cAMP levels induced by TNF-alpha are not due to upregulation of beta-adrenergic receptors, but may be associated with the elevated levels of circulating PGE2 and/or epinephrine.

-Adrenoceptor Changes in Mononuclear Leukocytes During Pregnancy

Leukocyte beta 2-adrenoceptors mirror similar receptors in the uterus. This study examined changes in beta 2-adrenoceptors and in cAMP production during pregnancy using peripheral mononuclear leukocytes isolated from 17 pregnant women and 5 nonpregnant controls. beta 2-Adrenoceptor density was determined by 125I-pindolol binding. cAMP production under basal and stimulated (10 microM isoproterenol) conditions was determined by radioimmunoassay. Groups were compared by unpaired t test. There was a nonsignificant decrease in the density of beta-adrenoceptors during pregnancy. While basal cAMP production was unchanged during pregnancy, stimulated cAMP production was decreased (44% of control, P < 0.001; 95% CI 33%-56%). Stimulated cAMP production per receptor was lower in leukocytes from pregnant women and was not a constant relationship, but was increased markedly in leukocytes having fewer than 400 beta-adrenoceptor sites per cell. There are significant changes in the coupling of beta-adrenoceptors to cAMP production during pregnancy without changes in beta-receptor density, affinity, or basal cAMP production.

Decrease in myometrial beta-adrenergic receptors with prenatal cocaine use

To determine if cocaine use during pregnancy is associated with a reduction in the number or affinity of beta-adrenergic receptors in human myometrium. Myometrium was obtained at cesarean delivery of five women who reported using cocaine during pregnancy and from ten controls. Saturation binding assays were performed on the myometrial membrane fractions using [125I]-cyanopindolol to determine beta-adrenergic receptor concentration and affinity. The percentages of beta 1- and beta 2-adrenergic receptors were determined in three cocaine users and four control patients by performing competition binding assays using the beta 2 antagonist ICI 118,551. Results were compared using unpaired Student t tests. Women who reported using cocaine during pregnancy had a significantly lower mean (+/- standard deviation) concentration of myometrial beta-adrenergic receptors than did controls (22 +/- 8 versus 52 +/- 23 fmol/mg protein, respectively). There was no difference in the receptor affinity constants between cocaine users and controls (16 +/- 2 pmol/L for both groups). The percentages of beta 1- and beta 2-adrenergic receptors in the myometrium of the cocaine-use group and control group were similar: 86 +/- 1% beta 2 in the cocaine-use group and 83 +/- 7% beta 2 in the control group. Cocaine use during pregnancy may be associated with a down-regulation of beta-adrenergic receptors in human myometrium. This could result in a decreased capacity for uterine relaxation and, consequently, a predisposition to preterm labor.

Cardiac beta-adrenoceptor-effector coupling in portal vein-stenosed rats.

Cardiac beta-adrenergic signal transduction was examined in chronic portal vein-stenosed rats. Basal tension and maximum rate of tension development were significantly depressed in left ventricular papillary muscles (0.21 +/- 0.03 N/cm2 and 8.2 +/- 1.7 N.s-1.cm-2, respectively) compared with sham-operated controls (0.51 +/- 0.05 N/cm2 and 19.9 +/- 4.4 N.s-1.cm-2, respectively). The positive inotropic response to isoproterenol was also attenuated. Adenosine 3',5'-cyclic monophosphate formation was decreased significantly when GTP (-41.9%), isoproterenol with GTP (-45.3%), or guanosine 5'-O-(3-thiotriphosphate) (-52.4%) was used to stimulate adenylyl cyclase, but not when Mn2+ or forskolin was used. Beta-Adrenoceptor density (sham operated 24.6 +/- 2.0 fmol/mg; portal vein stenosed 26.4 +/- 2.1 fmol/mg) and the apparent dissociation constant (sham operated 0.26 +/- 0.04 nM; portal vein stenosed 0.29 +/- 0.04 nM) were unaffected. Portal venous hypertension did not alter beta-adrenergic receptor affinity for isoproterenol. However, it was necessary for isoproterenol to occupy three times the number of receptors in papillary muscles from stenosed animals to produce an equal increase in force generation. These data suggest that although portal vein stenosis does not alter cardiac beta-adrenoceptor density or affinity for ligands, transduction of the signal between the receptor and adenylyl cyclase is adversely influenced and may be responsible for the diminished responsiveness of beta-adrenoceptors in the myocardium.

Participation of Tubulin in the Stimulatory Regulation of Adenylyl Cyclase in Rat Cerebral Cortex Membranes

This study examined effects of tubulin on the activation of adenylyl cyclase in rat cerebral cortex membranes. Tubulin, prepared from rat brain by polymerization with the hydrolysis-resistant GTP analogue 5'-guanylylimidodiphosphate (GppNHp) caused significant activation of the enzyme by approximately 156% under conditions in which stimulation rather than inhibition of the enzyme was favored. Tubulin-GppNHp activated isoproterenol-sensitive adenylyl cyclase, potentiated forskolin-stimulated activity of the enzyme, and reduced agonist binding affinity for beta-adrenergic receptors. When tubulin, polymerized with the hydrolysis-resistant photoaffinity GTP analogue [32P]P3(4-azidoanilido)-P1-5'-GTP ([32P]AAGTP), was incubated with cerebral cortex membranes, AAGTP was transferred from tubulin to Gs alpha as well as Gi alpha. These results suggest that, in rat cerebral cortex membranes, the tubulin dimer participates in the stimulatory regulation of adenylyl cyclase by transferring guanine nucleotide to Gs alpha, as well as affecting the Gi-mediated inhibitory pathway.

Effects of chemical denervation with 6-hydroxydopamine on myocardial responsiveness to isoproterenol in rabbits.

This study examined the hypothesis that chemical denervation with 6-hydroxydopamine (6-OHDA) would increase myocardial responsiveness to isoproterenol. Five days previously, 15 New Zealand white rabbits were given 60 mg/kg 6-OHDA intravenously. Fifteen control rabbits received vehicle. Hemodynamic, coronary blood flow (CBF), and cardiac output measurements were obtained before and during isoproterenol infusion (0.5 microgram/kg/min for 15 min). Norepinephrine tissue content, beta-adrenoceptor number (Bmax) and affinity (Kd), cyclic AMP content and cyclic AMP-phosphodiesterase (PDE) activity were measured in the subepicardium (EPI) and subendocardium (ENDO). Myocardial norepinephrine content was significantly decreased from 1263 +/- 292 (EPI) and 874 +/- 221 ng/g tissue (ENDO) in the control to 148 +/- 33 (EPI) and 90 +/- 45 ng/g tissue (ENDO) in the denervated group. There were no significant changes in cyclic AMP-PDE activity or Bmax and Kd of beta-adrenoceptors. Cyclic AMP content was similar at baseline, but controls had a significantly larger increase (123-155%) during isoproterenol infusion when compared to the denervated group (27-37%). The denervated animals showed a smaller increase in cardiac output during isoproterenol infusion (from 203 +/- 30 to 235 +/- 26 ml/min), when compared to the control animals (from 135 +/- 18 to 216 +/- 42 ml/min). Baseline CBF was significantly higher in the EPI but not ENDO of the denervated group (185 +/- 20 ml/100 g/min in EPI and 150 +/- 8 in ENDO) compared to the control group (108 +/- 13 in EPI and 133 +/- 17 in ENDO). The relative increase in CBF during isoproterenol infusion was smaller in the denervated group (44-45%) than the control group (107-109%). Isoproterenol infusions of 0.1 and 2.5 micrograms/kg/min showed similarly depressed coronary blood flow responses in denervated animals. Thus, the chemically denervated animals did not have beta-adrenoceptor upregulation, exhibited a lesser increase in cyclic AMP with isoproterenol, and had a reduced functional and coronary blood flow response to isoproterenol. This occurred without any significant change in beta-adrenoceptor number or affinity, or in cyclic AMP-phosphodiesterase activity, indicating there may be receptor uncoupling or other changes in the signal transduction pathway.

Deficient Activity of Stimulatory Nucleotide-Binding Regulatory Protein in Lymphocytes From Patients With Essential Hypertension

Lymphocytes are widely used as a model for the cardiovascular beta-adrenoceptor-adenylate cyclase system. We evaluated the role of this system in the pathogenesis of hypertension by studying lymphocytes obtained from patients with essential hypertension. Untreated hypertensive patients and normotensive control subjects were studied. The number and affinity of the beta-adrenoceptors were measured by a radioligand binding method with 125I-cyanopindolol. The responses of cyclic adenosine monophosphate (cAMP) to isoproterenol, cholera toxin, and forskolin were also determined. The concentration and affinity of beta-adrenoceptors did not differ significantly in the two groups, nor was a significant difference found in the basal level of cAMP. The effects of isoproterenol on the accumulation of cAMP were reduced in the lymphocytes from the hypertensive compared with the normotensive subjects. There was no significant difference in the effect of forskolin on cAMP accumulation in the two groups. These results indicate that the activity of the stimulatory nucleotide binding regulatory protein (Gs-protein) is reduced in lymphocytes from patients with essential hypertension. This defect of Gs-protein in the lymphocytes may represent a defect of Gs-protein in the cardiovascular system in such patients.

Cellular and molecular alterations in the beta adrenergic system with cardiomyopathy induced by tachycardia.

The aim was to examine the relationship between changes in myocyte function to changes in protein and mRNA content of components of the beta adrenergic system with tachycardia induced cardiomyopathy. Contractile function and beta adrenergic responsiveness were measured in isolated myocytes from control pigs (n = 6) and in pigs subjected to three weeks of pacing induced supraventricular tachycardia (n = 6). beta Receptor density and affinity, the relative content of the stimulatory (Gs) and inhibitory (Gi) subunits of the G protein complex, and adenylate cyclase activity were determined from sarcolemmal preparations. In order to determine whether these changes were accompanied by alterations in steady state mRNA levels for specific components of the beta adrenergic system, mRNA content for the beta 1 adrenergic receptor and the G alpha s and G alpha i2 subunits of the G protein complex was measured. Chronic supraventricular tachycardia caused a 36% increase in left ventricular end diastolic dimension and a 61% decrease in left ventricular fractional shortening compared to controls. The velocity of isolated myocyte shortening was 50% lower in myocytes from hearts with tachycardia cardiomyopathy than in control myocytes. In the presence of 50 nM isoprenaline or 2 microM forskolin, the velocity of myocyte shortening was 65% lower in the myopathic myocytes than in the controls. With the development of tachycardic cardiomyopathy, beta adrenergic receptor density fell by 25% with no change in affinity, Gs decreased by 35%, and Gi increased by over 50% compared to controls. Basal adenylate cyclase activity and isoprenaline and forskolin stimulated adenylate cyclase activity fell by over 50% with supraventricular tachycardia compared to controls. The relative content of G alpha i2 mRNA increased threefold with the development of tachycardic cardiomyopathy with no change in the relative abundance of mRNA for the beta 1 receptor or G alpha s when compared with controls. The changes in myocyte beta adrenergic responsiveness with the development of tachycardic cardiomyopathy are due to alterations in cellular mechanisms (decreased beta receptor and Gs density, increased Gi) and in molecular mechanisms (increased Gi mRNA content).

Uterine beta-adrenergic receptors in allogeneic and syngeneic pregnancy.

Two different mechanisms of regulation in uterine beta-adrenergic receptors in allogeneic pregnancy (AP) and syngeneic pregnancy (SP) are described in this work. Firstly we noted changes in beta-adrenergic sensitivity to isoproterenol in AP, while in SP differences in uterine reactivity to isoproterenol were found. In binding assays with the beta-antagonist [3H]dihydroalprenolol, changes in beta-receptor affinity were seen in AP, while in SP the maximal number of binding sites is altered. In addition, there are differences in uterine concentrations of cyclic AMP in both types of pregnancies as well as in the nucleotide response to isoproterenol. The differences in the reactivity and expression of uterine beta-adrenoceptors in both types of pregnancies may be due to a distinctive immunological role of the semiallogeneic fetus in AP.

Long-term exposure to norepinephrine results in down-regulation and reduced mRNA expression of pulmonary beta-adrenergic receptors in guinea pigs.

We studied the down-regulation of beta 2-adrenoceptors in guinea pig lung after chronic exposure to a beta-agonist. Guinea pigs were exposed to either norepinephrine (NE) or vehicle for 7 days. The density and affinity of beta-adrenoceptors were determined by Scatchard analysis of [125I]cyanopindolol binding in a lung membrane preparation and beta 1- and beta 2-adrenoceptor subtypes were studied in the presence of 0.1 microM ICI 118,551, a selective beta 2-adrenoceptor antagonist, or 0.1 microM CGP 20712 A, a selective beta 1-adrenoceptor antagonist, respectively. beta 2-Adrenoceptor mRNA was examined by Northern blot analysis. The tissue distribution of beta 2-adrenoceptors and of beta 2-adrenoceptor mRNA in lung after NE infusion were determined using receptor autoradiography and in situ hybridization, respectively. The functional significance of the decrease in beta 2-adrenoceptor was tested by measuring the relaxation response to a beta 2-agonist in isolated parenchymal strips. NE treatment resulted in decreases of 75 +/- 9%, 84 +/- 4%, and 66 +/- 9% of total beta-, beta 1-, and beta 2-adrenoceptors in the lung, compared with control animals. The administration of NE had only minimal effects on the dissociation constant (Kd) of the receptor for the radioligand. beta 2-Adrenoceptors mRNA was decreased by 46 +/- 13% after NE treatment. There was a correspondence between the distribution of beta 2-adrenoceptors and beta 2-adrenoceptor mRNA localization in both control and NE-treated guinea pigs. NE treatment reduced alveolar beta 2-adrenoceptors by 70 +/- 3% and its mRNA expression by 78 +/- 5%.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of beta-adrenoceptors on circulating mononuclear cells in hypertensives and normotensives before and after reduction of central sympathetic outflow by clonidine.

We have studied beta-adrenoceptor number and affinity on peripheral blood mononuclear cells (PBMCs) in normotensives (NT) and hypertensives (HT), before and after intravenous administration of clonidine, an alpha 2-adrenoceptor agonist which lowers blood pressure predominantly by reducing central nervous system sympathetic outflow. After clonidine, there was a decrease in blood pressure and plasma noradrenaline (NA) and adrenaline (Ad) levels, with an increase in growth hormone (GH) levels, in both NT and HT. There was no difference in basal beta-adrenoceptor densities on PBMCs between NT and HT. After clonidine at 30 and 60 min, there was an increase in beta-adrenoceptor density associated with a low affinity in NT. In HT, no changes were observed. The increased beta-adrenoceptor densities on PBMCs in NT after clonidine, returned to baseline values after 2 h. Short term up-regulation of beta-adrenoceptors on PBMCs in NT after clonidine is accompanied by a fall in blood pressure (BP) and plasma levels of catecholamines. The changes may represent a compensatory mechanism reflecting a rapid externalization-activation of adrenoceptors residing on the internal surface of the membranes with a change of the coupling ability between the receptor and the catalytic component. In HT, although the haemodynamic and neurohormonal response to clonidine was similar to NT, short term upregulation of receptors did not occur. The lack of such response may mirror a form of regulatory dysfunction of beta-adrenoceptors in HT.

Parasympathetic muscarinic stimulation limits noradrenaline induced myocardial creatine kinase release: a study in the isolated perfused working rat heart.

It has long been known that high concentrations of catecholamines may induce myocardial damage, and aggravate ischaemic injury. It has also been shown that beta-blockade may protect the myocardium from ischaemic damage. Stimulation of muscarinic receptors modulates beta-adrenergic receptor affinity for isoproterenol and attenuates isoproterenol induced adenylyl cyclase activation. Effects of muscarinic receptor stimulation were therefore investigated in isolated anterogradely perfused rat hearts under different experimental conditions. One group of hearts was perfused with noradrenaline, 10(-6) mol l-1 for 45 min, and another group was perfused with different carbachol concentrations (3 x 10(-7)-10(-5) mol l-1) with or without noradrenaline 10(-6) mol l-1, for 45 min. Release of creatine kinase to the perfusion buffer was taken as a sign of cell damage. Heart rate, left ventricular maxdP/dt and left ventricular pressure were measured throughout the perfusion time by insertion of a 20 gauge cannula through the left ventricular wall near the base. Carbachol (3 x 10(-7) mol l-1) alone induced a decrease of heart rate by 25% and maxdP/dt by 13%. Noradrenaline produced a 20% increase in heart rate, whereas the combination of noradrenaline plus carbachol induced a minor decrease in heart rate. Muscarinic receptor stimulation alone decreased myocardial contractility. However, when combined with noradrenaline no decrease in contractility was seen. Also, the release of creatine kinase to the perfusion buffer containing the combination of carbachol plus noradrenaline was reduced. Thus, muscarinic receptor stimulation protected the myocardium from catecholamine induced damage at concentrations where no change in contractility was seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac function and responsiveness to beta-adrenoceptor agonists in rats with obstructive jaundice.

The function and role of the heart and the contribution of cardiac beta-adrenoceptors in the pathogenesis of circulatory failure in obstructive jaundice were studied in the 3-day bile duct-ligated (BDL) rat using three different techniques to measure cardiac function and beta-adrenoceptor activity, number, and affinity. The techniques were the pithed rat preparation, the isolated working heart preparation, and a competitive radioligand binding assay for beta-adrenoceptors. The results of these experiments were compared with those obtained in 3-day bile duct-manipulated (sham operated; SO) rats. Impaired indexes of basal cardiac contractility were observed in the BDL pithed rats and isolated working hearts. In these two preparations, responsiveness to norepinephrine and the beta-adrenoceptor agonists, isoproterenol and dobutamine, was unaffected by bile duct ligation. The affinity and number of cardiac beta-adrenoceptors in membranes from the hearts of SO and BDL rats were not significantly different from each other. These experiments have established for the first time that the 3-day BDL rat has a cardiac myopathy associated with intact responsiveness to beta-adrenoceptor agonists, a normal unchanged affinity and number of cardiac beta-adrenoceptors.

Beta-adrenoceptor expression on circulating mononuclear cells of idiopathic Parkinson's disease and autonomic failure patients before and after reduction of central sympathetic outflow by clonidine.

There is a short-term up-regulation of beta-adrenoceptors on peripheral blood mononuclear cells (PBMC) after reduction of central sympathetic outflow by clonidine in normal individuals. We have studied beta-adrenoceptor number and affinity on PBMC in idiopathic Parkinson's disease (PD), pure autonomic failure (PAF), and multiple system atrophy (MSA; Shy-Drager syndrome) patients and age- and sex-matched normal controls (NC) before and after intravenous administration of clonidine, an alpha 2-adrenoceptor agonist which lowers blood pressure predominantly by reducing CNS sympathetic outflow. Basal beta-adrenoceptor density was high in PAF but within the normal range in PD and MSA patients. After clonidine there was a decrease in plasma levels of noradrenaline (NA) and adrenaline (Ad) in PD, MSA, and NC, and an increase in growth hormone (GH) in PD, PAF, and NC. NC. In PAF, NA and Ad remained unchanged. In MSA, there was no increase in GH levels. There was an up-regulation of beta-adrenoceptors on PBMC at 30 and 60 minutes after clonidine administration, which returned to baseline values after 2 hours, and the affinity of the receptors was decreased in NC and PD patients. Intracellular production of cAMP after isoproterenol stimulation demonstrated that the up-regulation was not functional. Up-regulation after clonidine did not occur in PAF and MSA patients. The observed correlation of plasma NA and sympathetic defect with basal and clonidine-induced up-regulation of beta-adrenoceptors on PBMC may provide insight into beta-adrenoceptor changes in other tissues and also help in differentiating subgroups of autonomic failure patients.

Dexamethasone reverses the labor-associated myometrial desensitization to βadrenergic agonists in the rat

The purpose of our study was to determine the susceptibility of the myometrium to beta-adrenergic agonist-induced desensitization and to determine the ability of dexamethasone to prevent this homologous desensitization. Myometrial tissue was collected from nonpregnant, pregnant, and postpartum rats (n > or = 6) 12 hours after injection of either dexamethasone (0.25 mg subcutaneously) or vehicle (corn oil). The ability of isoproterenol to induce and maintain inhibition of contractile activity was determined in vitro. In addition, the density and affinity of beta-adrenergic receptors was estimated by ligand binding and Scatchard analysis. Dexamethasone significantly increased both the duration of myometrial responsiveness to isoproterenol and the beta-adrenergic receptor number at parturition, but not at other time points. Regardless of pretreatment, the susceptibility of the myometrium to beta-agonist-induced desensitization was greatest around the time of delivery. In the rat, beta-adrenergic agonists are least effective in maintaining myometrial inhibition peripartum, although their effectiveness at this time only can be increased by dexamethasone.