Β-adrenoceptor Function Research Articles

Effects of extremely low frequency electromagnetic fields on turkeys

Several studies have examined the potential biological effects of electromagnetic fields (EMF) on birds; however, little attention has been paid to the extremely low frequency (ELF; 0–300 Hz; 0–50 μT) radiation found in an urbanized environment. For monitoring the effects of ELF EMF, we used a turkey (Meleagris gallopavo) model, because the nucleated erythrocytes of turkeys contain β-adrenoceptors, and norepinephrine- (NE-) activated β-adrenoceptors have an important role in physiological and behavioral processes. Our aims were the following: 1) to investigate the intracellular mechanisms; 2) to compare the intracellular mechanisms in the treated and control groups over time, considering inter-individual differences and intra-subject correlations; 3) and to study the reversible nature of the response. The turkeys in the treatment group were treated in vivo with ELF EMF (50 Hz; 10 μT) for 3 wk after a 1-wk-long adaptation period. The animals were not exposed to ELF EMF during the regeneration period (5 wk following the exposure). The NE-activated β-adrenoceptor function was detected by measuring the amount of 3′5′-cyclic-adenosine-monophosphate (cAMP), and the biochemical enzyme parameters were defined. Repeated measurements of cAMP levels were analyzed using marginal models and a piecewise linear mixed model to compare treatment and control groups over time. According to our results, NE-activated β-adrenoceptor function was decreased in the treated birds in a time-dependent manner, while there were no differences between toxicological parameters in the serum, compared to the normal ranges. The decreased NE-dependent β-adrenoceptor function could be compensated by the homeostatic complex during the 5-wk regeneration period. Extended experimental periods and more sophisticated analysis methods may help prevent harmful environmental effects on birds; furthermore, these findings could affect public health and the economy.

Dual regulation of β2-adrenoceptor messenger RNA expression in human lung fibroblasts by β2-cAMP signaling; delayed upregulated inhibitors oppose a rapid in onset, direct stimulation of gene expression.

Based on their bronchodilatory effect, β2-adrenoceptor agonists constitute essential elements in the treatment of bronchial asthma and COPD. As treatment with β2-adrenoceptor agonists has been associated with worsening of airway hyper-reactivity, possibly because of loss of β-adrenoceptor function, molecular mechanism of the regulation of β2-adrenoceptor expression were studied. MRC-5 human lung fibroblasts were cultured in absence or presence of test substances followed by β2-adrenoceptor messenger RNA (mRNA) determination by qPCR. After inhibition of mRNA synthesis by actinomycin D, β2-adrenoceptor mRNA decreased with a half-life of 23 min, whereas inhibition of protein synthesis by cycloheximide caused an about 5- and 6-fold increase within 1.5 and 4 h, respectively. β2-Adrenoceptor mRNA was increased by about 100 % after 1 h exposure to formoterol or olodaterol but decreased by about 60 % after 4 h agonist exposure. Both effects of β2-adrenoceptor agonists were mimicked by forskolin, a direct activator of adenylyl cyclase and cholera toxin, which stimulates adenylyl cyclase by permanent activation of Gs. β2-Adrenoceptor agonist-induced upregulation of β2-adrenoceptor mRNA was blocked by the β2-adrenoceptor antagonist ICI 118551 and prevented by actinomycin D, but not by cycloheximide. Moreover, in presence of cycloheximide, β2-adrenoceptor agonist-induced reduction in β2-adrenoceptor mRNA was converted into stimulation, resulting in a more than 10-fold increase. In conclusion, expression of β2-adrenoceptors in human lung fibroblasts is highly regulated at transcriptional level. The β2-adrenoceptor gene is under strong inhibitory control of short-living suppressor proteins. β2-Adrenoceptor activation induces via adenylyl cyclase - cyclic adenosine monophosphate (cAMP) signaling a rapid in onset direct stimulation of the β2-adrenoceptor gene transcription, an effect opposed by a delayed upregulation of inhibitory factors.

β-Adrenoceptor Modulation in Chronic Obstructive Pulmonary Disease: Present and Future Perspectives

The common coexistence of chronic obstructive pulmonary disease (COPD) and cardiovascular disease (CVD) presents several therapeutic constraints that have not been comprehensively investigated. Pharmacologic modulation of β-adrenoceptor (β-AR) function is one of the critical issues in the treatment of these patients because inhaled β(2)-AR agonists may induce adverse events in patients with COPD, mainly in those with coexisting CVD. Moreover, the use of β-AR blockers has traditionally been contraindicated in COPD, mainly because of the potential for acute bronchospasm and increased airway hyperresponsiveness after their administration. However, there now appears to be good evidence that β-AR blockers are not only safe but may have benefits in COPD that extend beyond a reduction in cardiovascular mortality. This article starts with a succinct outline of the evolution in our understanding of β-AR modulation in COPD, touching on treatment of COPD with β-AR agonists and the issues of β-AR blockade and cardioselectivity in patients with comorbid CVD. We then summarize the current evidence for a COPD benefit from β-AR blockers and hypothesize on the mode of action. Finally, we provide a view of the future landscape in terms of therapeutic possibilities and what still needs to be resolved, based on our opinion.

P38 Hydrogen sulfide regulates cardiac function via S-sulfhydration of adenylyl cyclase

We previously reported that hydrogen sulfide (H2S) may negatively regulate heart function via suppression of β-adrenoceptor function. In the present study, we found that NaHS (an H2S donor) significantly attenuated myocyte contractility, cAMP accumulation and adenyly cyclase (AC) activity in the cardiac myocytes stimulated by forskolin, a selective adenylyl cyclase activator. These data suggest that the regulatory effect of H2S involves suppression of AC/cAMP pathway. To study the molecular mechanisms, we transfected HEK293 cells with the cDNAs of AC2, AC5 and AC6, the main AC isoforms expressed in the heart. It was found that overexpression of these isoforms largely increased intracelluar cAMP level in HEK293 cells. This effect was abolished by pretreatment with NaHS and iodoacetamide, a thiol blocking agent, implying that the thiol group in AC is important to the vascular function of H2S. Western blotting analysis shows that NaHS induced S-sulfhydration of the thiol group of the cysteine residues in AC. Since cytosolic domains of AC are implicated in catalysis, we cloned the C1a domain from AC1 (AC1C1a, aa236-aa471) and C2 domain from AC2 (AC2C2D3, aa855-aa1090). We found that forskolin significantly stimulated cAMP production in the mixture of our recombinant soluble proteins of AC1C1a and AC2C2D3. These data confirm that these two segments alone may produce the same function of the full length of AC. Treatment with NaHS significantly attenuated the effect of forskolin on cAMP production in the mixture of these two recombinant proteins. In addition, NaHS also induced obvious S-sulfhydration in either of these proteins. We therefore propose that S-sulfhydration of the key cysteine residues (e.g. cys355 and/or Cys358) may block the binding of ATP to the catalytic site of AC and therefore inhibit its activity. In conclusion, we demonstrated for the first time that H2S may regulate cardiac function by suppression of AC/cAMP pathway through S-sulfhydration of AC.

Chronic treatment in vivo with β‐adrenoceptor agonists induces dysfunction of airway β2‐adrenoceptors and exacerbates lung inflammation in mice

Inhalation of a β-adrenoceptor agonist (β-agonist) is first-line asthma therapy, used for both prophylaxis against, and acute relief of, bronchoconstriction. However, repeated clinical use of β-agonists leads to impaired bronchoprotection and, in some cases, adverse patient outcomes. Mechanisms underlying this β(2) -adrenoceptor dysfunction are not well understood, due largely to the lack of a comprehensive animal model and the uncertainty as to whether or not bronchorelaxation in mice is mediated by β(2) -adrenoceptors. Thus, we aimed to develop a mouse model that demonstrated functional β-agonist-induced β(2) -adrenoceptor desensitization in the context of allergic inflammatory airway disease. We combined chronic allergen exposure with repeated β-agonist inhalation in allergen-treated BALB/C mice and examined the contribution of β(2) -adrenoceptors to albuterol-induced bronchoprotection using FVB/NJ mice with genetic deletion of β(2) -adrenoceptors (KO). Associated inflammatory changes - cytokines (ELISA), cells in bronchoalevolar lavage and airway remodelling (histology) and β(2) -adrenoceptor density (radioligand binding) - were also measured. KEY RESULTS β(2) -Adrenoceptors mediated albuterol-induced bronchoprotection in mice. Chronic treatment with albuterol induced loss of bronchoprotection, associated with exacerbation of the inflammatory components of the asthma phenotype. This animal model reproduced salient features of human asthma and linked loss of bronchoprotection with airway pathobiology. Accordingly, the model offers an advanced tool for understanding the mechanisms of the effects of chronic β- agonist treatment on β-adrenoceptor function in asthma. Such information may guide the clinical use of β-agonists and provide insight into development of novel β-adrenoceptor ligands for the treatment of asthma.

Role of Sphingosine-1-Phosphate in β-adrenoceptor Desensitization via Ca2+ Sensitization in Airway Smooth Muscle

The correlation between inflammatory cells and airway smooth muscle plays fundamental roles in the pathophysiology of asthma. This study was designed to determine whether pre-exposure of airway smooth muscle to sphingosine-1-phosphate (S1P), which is released from mast cells by allergic reactions, causes a deterioration of β-adrenoceptor function. Isometric tension and the ratio of fluorescence intensities at 340 and 380 nm (F(340)/F(380)), an indicator of intracellular Ca2+ levels, were simultaneously measured using fura-2 loaded guinea-pig tracheal tissues. Intracellular cAMP levels were also measured. Pre-exposure to S1P caused a reduction in the inhibitory effects of 0.3μM isoprenaline, a β-adrenoceptor agonist, and 10μM forskolin, a direct activator of adenylyl cyclase, against 1μM methacholine-induced contraction in concentration- and time- dependent manners. In contrast, the values of F(340)/F(380) were not augmented under this experimental condition. After incubation with S1P in the presence of 0.001-1μM Y-27632, a Rho-kinase inhibitor, the reduced responsiveness to forskolin induced by S1P was reversed in a concentration-dependent manner. Moreover, pre-treatment with pertussis toxin (PTX), an inhibitor of G(i), suppressed the loss of forskolin-induced relaxation induced by S1P. Pre-exposure to S1P markedly inhibited the augmentation of cAMP accumulation induced by forskolin. However, addition of Y-27632 and pre-exposure to PTX returned forsokin-induced cAMP accumulation to the control level. Pre-exposure to S1P causes heterologus desensitization of β-adrenoceptors by increasing the sensitivity of airway smooth muscle to intracellular Ca2+. Ca2+ sensitization regulated by G(i) and Rho-kinase is involved in this phenomenon.

Limb-specific differences in the skin vascular responsiveness to adrenergic agonists

In this study, to test the hypothesis that adrenergic vasoconstrictor responses of the legs are greater compared with the arms in human skin, cutaneous vascular conductance (CVC) in the forearm and calf were compared during the infusion of adrenergic agonists in healthy young volunteers. Under normothermic conditions, norepinephrine (NE, α- and β-agonist, 1 × 10(-8) to 1 × 10(-2) M), phenylephrine (PHE, α(1)-agonist, 1 × 10(-8) to 1 × 10(-2) M), dexmedetomidine (DEX, α(2)-agonist, 1 × 10(-9) to 1 × 10(-4) M), and isoproterenol (ISO, β-agonist, 1 × 10(-8) to 1 × 10(-3) M) were administered by intradermal microdialysis. Skin blood flow (SkBF) was measured by laser-Doppler flowmetry, and the local temperature at SkBF-measuring sites was maintained at 34°C throughout the experiments. CVC was calculated as the ratio of SkBF to blood pressure and expressed relative to the baseline value before drug infusion. The dose of NE at the onset of vasoconstriction and the effective dose (ED(50)) resulting in 50% of the maximal vasoconstrictor response for NE were lower (P < 0.001) in the calf than forearm. The ED(50) for PHE and DEX was also lower (P < 0.05) in the calf than forearm. Increases in CVC in response to ISO were potentially smaller in the calf, but the statistical differences in the responses were dependent on the expressions of CVC. These findings suggest that the cutaneous vasoconstrictor responsiveness to exogenous NE is greater in the legs than in the arms due to a higher α(1)- and α(2)-adrenoceptor reactivity, while the β-adrenoceptor function plays a minor role in regional differences in adrenergic vasoconstriction in normothermic humans.

Role of Ca 2+ mobilization in desensitization of β-adrenoceptors by platelet-derived growth factor in airway smooth muscle

Platelet-derived growth factor (PDGF), which is released from eosinophils and fibroblasts, may be implicated in the pathophysiology of bronchial asthma. To examine the involvement of airway inflammation in β-adrenergic desensitization, the present study was designed to determine whether pre-exposure to PDGF deteriorates β-adrenoceptor function in airway smooth muscle. We focused on Ca 2+ signaling as an intracellular mechanism involved in this phenomenon. Isometric tension and F 340/F 380 (an indicator of intracellular Ca 2+ concentration) induced by isoprenaline and other cAMP-related agents were simultaneously measured before and after exposure to PDGF in fura-2-loaded guinea-pig tracheal smooth muscle. Indomethacin was applied throughout the experiments to abolish prostaglandin synthesis by PDGF. After exposure of the tissues to 10 ng/ml PDGF for 15 min, the effects of isoprenaline, a β-adrenoceptor agonist, and forskolin, a direct inhibitor of adenylyl cyclase, against methacholine-induced contraction were markedly reduced with increasing F 340/F 380. However, in the presence of verapamil, an inhibitor of voltage-dependent Ca 2+ channels, the reduced responsiveness to isoprenaline and forskolin induced by pre-exposure to PDGF was reversed with reducing F 340/F 380. Reduced responsiveness to isoprenaline by PDGF was also not observed in the presence of Ca 2+-free solution. The inhibitory effects of db-cAMP, an analogue of cAMP, and theophylline, a nonselective inhibitor of phosphodiesterase, were not attenuated by PDGF. In conclusion, pre-exposure to PDGF causes impairment of the β-adrenoceptors/adenylyl cyclase processes in airway smooth muscle that is independent of cyclooxygenase synthesis by PDGF. Ca 2+ mobilization by Ca 2+ influx through voltage-dependent Ca 2+ channels is involved in this heterologous desensitization of β-adrenoceptors.

Negative regulation of β-adrenergic function by hydrogen sulphide in the rat hearts

β-Adrenoceptor is over-stimulated during myocardial ischemia, in which hydrogen sulphide (H 2S) concentration was found to be lowered. The present study attempted to investigate if H 2S modulates β-adrenoceptor function and the underlying mechanism. We examined the effect of NaHS (a H 2S donor) on myocyte contraction and electrically-induced (EI) intracellular calcium ([Ca 2+] i) transients upon β-adrenergic stimulation in rat ventricular myocytes with a video edge tracker method and a spectrofluorometric method using fura-2/AM as a calcium indicator, respectively. We found that isoproterenol (ISO, 10 − 9 –10 − 6 M), a β-adrenoceptor agonist, concentration-dependently increased the twitch amplitude of ventricular myocytes, which was attenuated by NaHS (10 − 5 –10 − 3 M) in a dose-dependent manner. The amplitudes and maximal velocities (± dl/dt) of myocyte twitch and EI-[Ca 2+] i transient amplitudes were enhanced by ISO, forskolin (an adenylyl cyclase activator), 8-bromoadenosine-3′,5′-cyclic monophosphate (an activator of protein kinase A) and Bay K-8644 (a selective L-type Ca 2+ channel agonist). Administration of NaHS (100 μM) only significantly attenuated the effects of ISO and forskolin. Moreover, NaHS reversed ISO-induced cAMP elevation and forskolin-stimulated adenylyl cyclase activity. In addition, stimulation of β-adrenoceptor by ISO significantly decreased endogenous H 2S production in rat ventricular myocytes. In conclusion, H 2S may negatively modulate β-adrenoceptor function via inhibiting adenylyl cyclase activity. Impairment of this negative modulation during ischemia may induce cardiac arrhythmias. Our study may provide a novel mechanism for ischemia-induced cardiac injury.

β-adrenoceptor affinity as a biological predictor of treatment response to paroxetine in patients with acute panic disorder

Background Few studies have reported on the functional differences of the β-adrenoceptor between treatment responders and non-responders in panic disorder (PD). The aim of this study was to compare the nature of the β-adrenoceptor function and clinical variables between treatment responders and non-responders to paroxetine treatment in acute PD patients. Method Paroxetine was administered to all of the panic patients for 12 weeks. The lymphocyte β-adrenoceptor density (Bmax), affinity (1/Kd), and sensitivity (cAMP ratio) were measured in 22 untreated outpatients with acute PD and 22 age, sex and BMI matched control subjects. Psychological assessments were conducted using the HAM-A, and HAM-D, STAI-S and STAI-T, Anxiety sensitivity index (ASI), and Acute panic inventory (API). Results A significantly higher Kd was observed in the panic patients before treatment as compared with the control subjects, but there was no significant difference in Kd between the panic patients and control subjects after the treatment. Among the 22 patients, the 11 treatment responders (50%) showed a significantly higher Kd and lower mean scores of HAM-D, STAI-S, STAI-T, and ASI at baseline, compared with the non-responders. Logistic regression revealed that the pretreatment Kd and HAM-D were significantly reliable predictors for treatment response ( p < 0.05). Conclusion The β-adrenoceptor affinity (1/Kd) was decreased and adaptively normalized after treatment with paroxetine in the acute panic patients. In addition, a low pretreatment β-adrenoceptor affinity (1/Kd) was found to predict the treatment response and can be suggested as a biological predictor of treatment response in acute PD.

Histamine augments β 2-adrenoceptor-induced cyclic AMP accumulation in human prostate cancer cells DU-145 independently of known histamine receptors

Androgen-independent prostate cancer cells DU-145 express a number of G protein-coupled receptors, including histamine H 1 receptors. There is evidence for the presence of β-adrenoceptors in the human prostate, and in this work we set out to characterise the expression of β-adrenoceptors by DU-145 cells, their linking to cyclic AMP (cAMP) formation and the possible modulation by histamine H 1 receptors of β-adrenoceptor function. Saturation [ 3H]-dihydroalprenolol binding indicated that DU-145 cells express moderate levels of β-adrenoceptors (22.7 ± 2.5 fmol/mg protein), which belong to the β 2-subtype as assessed by inhibition by the antagonists ICI-118,551 and CGP-20712A. Inhibition of [ 3H]-dihydroalprenolol binding by agonists (noradrenaline, adrenaline and isoproterenol) showed the presence of both high-(53–59%) and low-affinity binding sites. β-Adrenoceptor stimulation with isoproterenol resulted in robust [ 3H]-cAMP accumulation (10–30-fold of basal, EC 50 142 nM; pEC 50 6.85 ± 0.05). While not having effect of its own on basal [ 3H]-cAMP accumulation, histamine significantly augmented the β 2-adrenoceptor-induced response (overall effect 152 ± 6% of isoproterenol alone) with EC 50 1.35 μM (pEC 50 5.87 ± 0.06). This effect was independent of extracellular Ca 2+, insensitive to antagonists/agonists at H 1, H 2 or H 3/H 4 receptors and mimicked by drugs containing an imidazole ring in their chemical structure and by imidazole itself. Taken together, our results show that in DU-145 cells histamine augments β 2-adrenoceptor-induced cAMP independently of the activation of known histamine receptors. The effect may involve other mechanisms such as allosteric modulation of β 2-adrenoceptors by the imidazole moiety of histamine.

The Failing Cardiomyocyte

This article presents an integrated account of the subcellular alterations underlying each of the main observed defects in function of the ventricular cardiomyocytes from failing human heart: poor contraction, slow relaxation, β-adrenoceptor desensitization, and tendency to arrhythmia. The key role of the β-adrenoceptor system is highlighted: loss of tonic support from cyclic AMP-stimulated pathways contributes to the contraction and relaxation defects observed, but it is residual β-adrenoceptor function that exacerbates arrhythmias. Most of the alterations are “acquired” during the process of adaptation of the heart to cell loss, and are common to various etiologies of heart failure. Information gained from studies on genetic causes of cardiomyocyte malfunction, however, has been instructive in the interpretation of the functional effects of subcellular alterations.

Potentiation of β-adrenoceptor function in bovine tracheal smooth muscle by inhibition of protein kinase C

To examine the role of contractile agonist-induced activation of protein kinase C (PKC) in functional antagonism of airway smooth muscle contraction by β-adrenoceptor agonists, we examined the effects of the specific PKC-inhibitor GF 109203X (2-[1-(3-dimethylaminopropyl)-1 H-indol-3-yl]-3-(1 H-indol-3-yl) maleimide) on isoprenaline-induced relaxation of bovine tracheal smooth muscle contracted by various concentrations of methacholine and histamine. In the absence of GF 109203X, the potency of isoprenaline ( pD 2) was gradually reduced at increasing methacholine- and histamine-induced smooth muscle tones, but the maximal relaxation ( E max) was decreased only at higher concentrations of methacholine. In the presence of GF 109203X, pD 2 values were significantly increased for both methacholine- and histamine-induced contractions. Moreover, isoprenaline E max values in the presence of high concentrations of methacholine were also increased. Although both methacholine- and histamine-induced contractions were slightly reduced by GF 109203X, the changes in isoprenaline pD 2 could only partially be explained by reduced contractile tone. In contrast to isoprenaline, forskolin-induced relaxations were not affected by GF 109203X. The results indicate that PKC activation contributes to the reduced β-adrenergic responsiveness induced by methacholine and histamine, which may involve uncoupling of the β-adrenoceptor from the effector system. Since many mediators and neurotransmitters in allergic airway inflammation can activate PKC, this cross talk may be important in the reduced bronchodilator response of patients with severe asthma.

Effects of chronic lesions of the anteroventral region of the third ventricle on cardiac β-adrenoceptor function in conscious rats

This study examined whether cardiac β-adrenoceptor (β-AR) function was altered in conscious rats with lesions surrounding the anteroventral third ventricle (AV3V). The findings were: (1) β 1,2-AR-mediated tachycardia was similar in sham and AV3V-lesion rats, (2) β 3- and/or atypical β-AR-mediated tachycardia elicited by isoproterenol (10 μg/kg, i.v.; ISO) was diminished in AV3V-lesion rats treated with β 1,2-AR antagonists, but was not in similarly-treated sham-lesion rats, and (3) the tachycardia elicited by the membrane permeable cAMP-analogue, 8-(4-chlorophenylthiol)-cAMP (10 μmol/kg, i.v.), was similar in AV3V- and sham-lesion rats. The possibility that increased plasma sodium/osmolality in AV3V-lesion rats down-regulated cardiac β 3- and/or atypical β-ARs, but not β 1,2-ARs or intracellular cAMP signaling is discussed.

Impact of off-pump coronary artery surgery on myocardial performance and β-adrenoceptor function

Objective: To determine the hemodynamic changes during beating heart revascularization of the left anterior descending artery, the circumflex artery, and the right coronary artery as well as cardiovascular β-adrenoceptor function before and after off-pump coronary artery bypass surgery. Design: Prospective study. Setting: University department of cardiothoracic anesthesia. Participants: Twenty patients scheduled for off-pump coronary artery bypass surgery using the Octopus 2 stabilizer system. Interventions: Isoproterenol, 4 μg, was administered intravenously after induction of anesthesia and again after surgery to monitor cardiac β-receptor function. Measurements and Main Results: The hemodynamic responses to isoproterenol and cardiovascular variables were monitored before, during, and after immobilization of the target coronary artery with catheters in the radial and pulmonary arteries. During surgery on the left anterior descending artery (n = 23), stroke volume and cardiac index decreased 17 mL (21%) and 400 mL (17%). During revascularization of the circumflex artery (n = 9), stroke volume and cardiac index decreased 19 mL (28%) and 300 mL (17%). During surgery on the posterior aspect of the heart (n = 13), stroke volume and cardiac index decreased 22 mL (29%) and 400 mL (17%). All the cardiovascular variables had returned to baseline values 5 minutes after releasing the heart. The hemodynamic responses to isoproterenol were equal before and after surgery. Conclusion: This study provides evidence that the hemodynamic changes associated with off-pump surgery on the 3 major coronary arteries are similar and of short duration. No desensitization of cardiovascular β-adrenoceptors was found. This finding is in contrast to the deterioration in β-adrenoceptor function seen after surgery with cardiopulmonary bypass. Copyright © 2001 by W.B. Saunders Company

Altered β-adrenoceptor function associated to protein kinase C activation in hyperproliferative T lymphocytes

β-Adrenoceptor (βAR) expression and function as well as its modulation via intracellular transduction signals, were analyzed on the T cell lymphoma BW5147. Independently to the kinetic of proliferation and relative to the number of receptors displayed in normal T lymphocytes, BW5147 cells displayed a decreased number of βAR, uncoupled to adenylate cyclase, but coupled to protein kinase C stimulation. This last effect was impaired by a β-antagonist and by blockers of the enzymatic pathways involved in T lymphocyte proliferation, inducing a recovery of βAR sites. Down-regulation of βAR would implicate the loss of a negative neuroimmune control mechanism for lymphocyte proliferation. The coupling of the remaining sites to a positive signal for cellular activation, would contribute to establish an hyperproliferative state.

Vascular β-adrenoceptor function in hypertension and in ageing

Functional beta-adrenoceptors (beta-AR) have been identified and characterized in blood vessels under in vivo conditions as well as in vascular smooth muscle cells (SMC) grown in culture. Agonist occupancy of beta-AR activates adenylyl cyclase (AC) via the stimulatory guanine nucleotide-binding protein (Gs) and leads to elevations in intracellular adenosine 3'5'-cyclic monophosphate levels (cAMP). Increased cAMP activates the cAMP-dependent protein kinase (PKA), with subsequent phosphorylation of various target proteins. This beta-AR pathway interacts with several other intracellular signalling pathways via cross-talk, so that activation by beta-AR agonists may also modulate other second messengers and protein kinases. SMC beta-AR play an important role in SMC function. In intact blood vessels they mediate SMC relaxation by various intracellular mechanisms, ultimately causing a decrease in intracellular Ca2+ levels. In cultured SMC, activation of the beta-AR pathway results in inhibition of cellular proliferation, the development of SMC polyploidy, and SMC apoptosis. Blood vessels from hypertensive animals are characterized by an increase in SMC cell mass, a greater incidence of SMC polyploidy in the aorta, and an impairment in the beta-agonist-mediated SMC relaxation. Some of these changes may result from an attenuation of beta-AR function due to agonist-induced receptor desensitization caused by the uncoupling of receptors from the Gs-AC system. The phosphorylated beta-AR may in turn trigger new signals and activate different intracellular pathways. However, the details of these mechanisms are still unresolved. Since functional beta-AR play such a prominent and multi-faceted role in SMC function, it is important to understand how these diverse physiological effects are mediated by this receptor system, and how they contribute to the development of hypertension. With ageing, a decrease in beta-AR-Gs-AC coupling is observed, and this is implicated in the reduced responsiveness of SMC. The similarities in SMC beta-AR functional changes in hypertension and in ageing suggest that the underlying mechanisms are also analogous.

Vascular β-adrenoceptor function in hypertension and in ageing

Functional beta-adrenoceptors (beta-AR) have been identified and characterized in blood vessels under in vivo conditions as well as in vascular smooth muscle cells (SMC) grown in culture. Agonist occupancy of beta-AR activates adenylyl cyclase (AC) via the stimulatory guanine nucleotide-binding protein (Gs) and leads to elevations in intracellular adenosine 3'5'-cyclic monophosphate levels (cAMP). Increased cAMP activates the cAMP-dependent protein kinase (PKA), with subsequent phosphorylation of various target proteins. This beta-AR pathway interacts with several other intracellular signalling pathways via cross-talk, so that activation by beta-AR agonists may also modulate other second messengers and protein kinases. SMC beta-AR play an important role in SMC function. In intact blood vessels they mediate SMC relaxation by various intracellular mechanisms, ultimately causing a decrease in intracellular Ca2+ levels. In cultured SMC, activation of the beta-AR pathway results in inhibition of cellular proliferation, the development of SMC polyploidy, and SMC apoptosis. Blood vessels from hypertensive animals are characterized by an increase in SMC cell mass, a greater incidence of SMC polyploidy in the aorta, and an impairment in the beta-agonist-mediated SMC relaxation. Some of these changes may result from an attenuation of beta-AR function due to agonist-induced receptor desensitization caused by the uncoupling of receptors from the Gs-AC system. The phosphorylated beta-AR may in turn trigger new signals and activate different intracellular pathways. However, the details of these mechanisms are still unresolved. Since functional beta-AR play such a prominent and multi-faceted role in SMC function, it is important to understand how these diverse physiological effects are mediated by this receptor system, and how they contribute to the development of hypertension. With ageing, a decrease in beta-AR-Gs-AC coupling is observed, and this is implicated in the reduced responsiveness of SMC. The similarities in SMC beta-AR functional changes in hypertension and in ageing suggest that the underlying mechanisms are also analogous.

The influence of animal age on beta-adrenoceptor density and function in tracheal airway smooth muscle.

We examined the influence of animal age on the functional response of guinea-pig (0-156 weeks) and rat (4-136 weeks) isolated tracheal tissue to beta-adrenoceptor agonists. In addition, the binding density and affinity of [125I]iodocyanopindolol ([125I]CYP) binding to tracheal tissue was examined with respect to animal age. Significant age-related changes in isoprenaline potency were observed in tracheal ring preparations taken from animals during the early maturation phase of animal growth in the guinea-pig and rat. In addition, in rat isolated tracheal tissue, age-related decreases in fenoterol potency were observed during senescence, but not maturation. The changes in the functional responsiveness of tracheal tissue were not reflected by changes in the binding density or affinity for [125I]cyanopindolol ([125I]CYP) of beta-adrenoceptors, or in changes in specific autoradiographic grain density over smooth muscle tissue. In both guinea-pig and rat, no significant age-related changes in the influence of catechol-O-methyl transferase (COMT) or of extraneuronal uptake inhibition were detected. This study has demonstrated significant age-related changes in the responsiveness of guinea-pig and rat isolated tracheal tissue to beta-adrenoceptor agonists that were not related to changes in the density or affinity of the beta-adrenoceptor population or in the activity of COMT or extraneuronal uptake. The possibility of age-related changes in receptor-signal transduction coupling should be explored.

Monitoring of β-receptor sensitivity in cardiac surgery

Objective: To determine the repeatability of the hemodynamic response to repeated isoproterenol challenge doses to validate the standardized isoproterenol sensitivity test as an index of cardiovascular β-receptor function. Design: Prospective, single-blind, nonrandomized clinical trial. Setting: University department of cardiothoracic anesthesia. Participants: Twenty middle-aged men scheduled for primary elective coronary artery bypass surgery, 10 of whom had been treated with cardioselective β 1-antagonists for more than 3 months. Interventions: After induction of anesthesia and baseline hemodynamic evaluation, cardiac β-receptor sensitivity was estimated from the chronotropic/inotropic responses to four intravenous 4-pg isoproterenol bolus doses. Measurements and Main Results: Baseline cardiovascular function and pharmacodynamic response to the four isoproterenol challenge doses were monitored with catheters in the radial and pulmonary arteries (thermodilution). Heart rate was continuously recorded and calculated from the electrocardiogram. Baseline hemodynamic status and response to the first 4 μg of isoproterenol were similar in the 10 patients treated with β 1-antagonists and the rest of the patients. In all 20 patients, heart rate response to the three subsequent isoproterenol challenge doses decreased progressively by 28%. Conclusion: The standardized isoproterenol sensitivity test is unreliable for clinical monitoring of cardiac β-adrenoceptor function.