Evidence showing that beta-adrenoceptor subtype responsible for the relaxation induced by isoprenaline is principally beta2 but not beta1 in guinea-pig tracheal smooth muscle

1. The present study was carried out to pharmacologically identify the beta-adrenoceptor subtype that mediates isoprenaline-elicited relaxation in the isolated guinea-pig tracheal smooth muscle, to answer the question whether it is beta(1)- or beta(2)-subtype? 2. Isoprenaline as well as salbutamol, a well-known beta(2)-selective adrenoceptor agonist, produced a concentration-dependent relaxation with a pD(2) value of 8.12 vs. 7.54 for salbutamol. 3. Isoprenaline-elicited relaxation was not affected by beta(1)-selective antagonists, atenolol and CGP-20,712A, within the concentration ranges supposed to antagonize beta(1)-subtype: atenolol, < or =10(-6) M; CGP-20,712A, < or =10(-8) M. 4. By contrast, the concentration-response curves for isoprenaline as well as salbutamol were shifted rightwards in a competitive fashion by atenolol at the concentrations > or =3 x 10(-6) M. However, pA(2) values of atenolol against isoprenaline (5.86) and salbutamol (5.71) were consistent with the value corresponding to beta(2)- but not to beta(1)-subtype (around 7.00), and these values were not significantly different from each other. 5. Competitive antagonism of the relaxations to isoprenaline and salbutamol were also obtained with beta(2)-selective antagonists, butoxamine and ICI-118,551. Against isoprenaline and salbutamol, the pA(2) values of butoxamine (6.51 vs. 6.81) and ICI-118,551 (8.83 vs. 8.90) were substantially identical. Thus the primary mediation of beta(2)-receptor in the relaxations was strongly supported. 6. The present findings provide evidence that the beta-adrenoceptor which mediates isoprenaline-elicited relaxation of guinea-pig tracheal smooth muscle is essentially beta(2)- but not beta(1)-subtype. The present study also indicates the importance of using multiple receptor antagonists with different pA(2) values to pharmacologically identify the responsible receptor subtype in smooth muscle mechanical responses.

Nitric oxide and cardiac muscarinic control in humans

Cardiac parasympathetic activity reduces susceptibility to potentially lethal ventricular arrhythmias in heart failure and ischemic heart disease. This influence is mediated in large part by antagonism of the adverse cardiac effects of sympathetic overactivity ("indirect" parasympathetic activity) in addition to the "direct" effects of muscarinic stimulation. Nitric oxide modulates parasympathetic cardiac signaling in some animal models, but human data are lacking. We have investigated the influence of endogenous nitric oxide on cardiac responses to parasympathetic stimulation in healthy humans. In 18 volunteers, we studied chronotropic and inotropic responses to muscarinic stimulation, both before and after prestimulation with isoproterenol. Cardiac muscarinic stimulation was achieved using an intravenous bolus of the short-acting cholinesterase inhibitor, edrophonium. Responses were assessed during a background infusion of a nitric oxide synthase inhibitor (N(G)-monomethyl-L-arginine [L-NMMA]), placebo (saline), or phenylephrine (vasoconstrictor control) in a single-blind, random order, crossover protocol. L-NMMA did not affect chronotropic responses to edrophonium alone (direct parasympathetic activity). The decrease in heart rate attributable to "indirect" parasympathetic activity (derived by comparison with the effect of edrophonium during concurrent adrenergic stimulation) was substantially attenuated by L-NMMA in comparison to both control infusions. No modification of muscarinic inotropic responses by L-NMMA was apparent in comparison to the vasoconstrictor control. Nitric oxide exerts a powerful facilitating influence on indirect (antiadrenergic) but not direct human cardiac parasympathetic control. Stimulation of the endogenous nitric oxide pathway might enhance parasympathetic protection against the adverse influences of cardiac sympathetic overactivity.

Adipocyte differentiation of multipotent cells established from human adipose tissue

In this study multipotent adipose-derived stem cells isolated from human adipose tissue (hMADS cells) were shown to differentiate into adipose cells in serum-free, chemically defined medium. During the differentiation process, hMADS cells exhibited a gene expression pattern similar to that described for rodent clonal preadipocytes and human primary preadipocytes. Differentiated cells displayed the key features of human adipocytes, i.e., expression of specific molecular markers, lipolytic response to agonists of beta-adrenoreceptors (beta2-AR agonist > beta1-AR agonist >> beta3-AR agonist) and to the atrial natriuretic peptide, insulin-stimulated glucose transport, and secretion of leptin and adiponectin. hMADS cells were able to respond to drugs as inhibition of adipocyte differentiation was observed in the presence of prostaglandin F2alpha, tumour necrosis factor-alpha, and nordihydroguaiaretic acid, a natural polyhydroxyphenolic antioxidant. Thus, for the first time, human adipose cells with normal karyotype and indefinite life span have been established. They represent a novel and valuable tool for studies of fat tissue development and metabolism.

Overexpression of beta 1-adrenoceptors in adult rat ventricular myocytes enhances CGP 12177A cardiostimulation: implications for 'putative' beta 4-adrenoceptor pharmacology

1. CGP 12177A mediates cardiostimulation by activation of the 'putative' beta(4)-adrenoceptor; however, it has recently been reported that disruption of the beta(1)-adrenoceptor gene abolishes this effect. We have adenovirally overexpressed beta(1)-adrenoceptors in isolated, cultured adult rat ventricular cardiomyocytes and observed the inotropic potency of isoprenaline and CGP 12177A (in the presence of 1 microm propranolol). 2. Isoprenaline was a full inotropic agonist at rat ventricular myocytes (pD(2) 7.69+/-0.12). CGP 12177A was a nonconventional partial agonist (pD(2) 6.34+/-0.09), increasing inotropy and lusitropy, with an intrinsic activity of 0.34 and antagonised by bupranolol. 3. beta(1)-adrenoceptor overexpression enhanced the inotropic potency of isoprenaline by 11.7-fold (pD(2) 8.76+/-0.14) and CGP 12177A by 5.9-fold (7.11+/-0.10), respectively. Green fluorescent protein (GFP) overexpression did not alter the potency of isoprenaline or CGP 12177A (pD(2) 7.41+/-0.24 and pD(2) 6.60+/-0.50, respectively). 4. The cardiostimulant effects of CGP 12177A were enhanced by IBMX (phosphodiesterase inhibitor) and decreased by Rp-cAMPS (cAMP antagonist). CGP 12177A also increased cAMP levels. CGP 12177A but not isoprenaline initiated arrhythmias at lower concentrations following beta(1)-adrenoceptor overexpression. 5. (125)I-Cyanopindolol saturation binding in Adv.beta(1) myocytes demonstrated approximately 18-fold increase in beta(1)-adrenoceptors. (3)H-CGP 12177A saturation binding, in the presence of propranolol, increased approximately 5-fold following overexpression of beta(1)-adrenoceptors. 6. This study demonstrates enhanced cardiostimulation by CGP 12177A (in the presence of propranolol) in rat ventricular myocytes overexpressing beta(1)-adrenoceptors, mediated by a Gs/cAMP signalling pathway. 'Putative' beta(4)-adrenoceptor pharmacology appears to be mediated by activation of a novel affinity state of the beta(1)-adrenoceptor.

Blockade of β1- and desensitization of β2-adrenoceptors reduce isoprenaline-induced cardiac fibrosis

The aim of the present study was to analyse the role of beta(1)- and beta(2)-adrenoceptors in the catecholamine-induced myocardial remodeling, especially the interstitial fibrosis. Wistar rats were subjected to a 2-week chronic isoprenaline administration (30 microg/kg/h). Rats received a concomitant treatment with the selective beta(1)-adrenoceptor antagonist, bisoprolol (50 mg/kg/day p.o.) or were chronically pretreated with the selective beta(2)-adrenoceptor agonist salbutamol (40 microg/kg/h) for 1 week to induce beta(2)-adrenoceptor desensitization. The pretreatment with salbutamol induced a 59% down-regulation of left ventricular beta(2)-adrenoceptors compared to control. The extent of the isoprenaline-induced left ventricular fibrosis was significantly reduced in both the bisoprolol and salbutamol groups compared with the control isoprenaline-treated group especially in the apical region (1.7+/-0.6% and 1.4+/-0.3% versus 6.0+/-1.3%, respectively, P<0.005). beta(1)-adrenoceptor blockade and beta(2)-adrenoceptors down-regulation provided similar protection against isoprenaline-induced cardiac interstitial fibrosis suggesting that both beta-adrenoceptors are involved in such cardiac remodeling process.

Noradrenaline inhibits pacemaker currents through stimulation of beta 1-adrenoceptors in cultured interstitial cells of Cajal from murine small intestine

1. Interstitial cells of Cajal (ICCs) are pacemaker cells that activate the periodic spontaneous inward currents (pacemaker currents) responsible for the production of slow waves in gastrointestinal smooth muscle. The effects of noradrenaline on the pacemaker currents in cultured ICCs from murine small intestine were investigated by using whole-cell patch-clamp techniques at 30 degrees C. 2. Under current clamping, ICCs had a mean resting membrane potential of -58+/-5 mV and produced electrical slow waves. Under voltage clamping, ICCs produced pacemaker currents with a mean amplitude of -410+/-57 pA and a mean frequency of 16+/-2 cycles min(-1). 3. Under voltage clamping, noradrenaline inhibited the amplitude and frequency of pacemaker currents and increased resting currents in the outward direction in a dose-dependent manner. These effects were reduced by intracellular GDP beta S. 4. Noradrenaline-induced effects were blocked by propranolol (beta-adrenoceptor antagonist). However, neither prazosin (alpha(1)-adrenoceptor antagonist) nor yohimbine (alpha(2)-adrenoceptor antagonist) blocked the noradrenaline-induced effects. Phenylephrine (alpha(1)-adrenoceptor agonist) had no effect on the pacemaker currents, whereas isoprenaline (beta-adrenoceptor agonist) mimicked the effect of noradrenaline. Atenolol (beta(1)-adrenoceptor antagonist) blocked the noradrenaline-induced effects, but butoxamine (beta(2)-adrenoceptor antagonist) did not. In addition, BRL37344 (beta(3)-adrenoceptor agonist) had no effect on pacemaker currents. 5. 9-(Tetrahydro-2-furanyl)-9H-purine-6-amine (SQ-22536; adenylate cyclase inhibitor) and a myristoylated protein kinase A inhibitor did not inhibit the noradrenaline-induced effects and 8-bromo-cAMP had no effects on pacemaker currents. 8-Bromo-cGMP and SNAP inhibited pacemaker currents and these effects of SNAP were blocked by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; a guanylate cyclase inhibitor). However, ODQ did not block the noradrenaline-induced effects. 6. Neither tetraethylammonium (a voltage-dependent K(+) channel blocker), apamin (a Ca(2+)-dependent K(+) channel blocker) nor glibenclamide (an ATP-sensitive K(+) channel blocker) blocked the noradrenaline-induced effects. 7. The results suggest that noradrenaline-induced stimulation of beta(1)-adrenoceptors in the ICCs inhibits pacemaker currents, and that this is mediated by the activation of G-protein. Neither adenylate cyclase, guanylate cyclase nor a K(+) channel-dependent pathway are involved in this effect of noradrenaline.

Differences in endosomal targeting of human (beta)1- and (beta)2-adrenergic receptors following clathrin-mediated endocytosis

The beta(2)-adrenergic receptor (beta(2)AR) undergoes agonist-mediated endocytosis via clathrin-coated pits by a process dependent on both arrestins and dynamin. Internalization of some G protein-coupled receptors, however, is independent of arrestins and/or dynamin and through other membrane microdomains such as caveolae or lipid rafts. The human beta(1)AR is less susceptible to agonist-mediated internalization than the beta(2)-subtype, and its endocytic route, which is unknown, may be different. We have found that (i) co-expression of arrestin-2 or -3 enhanced the internalization of both subtypes whereas co-expression of dominant-negative mutants of arrestin-2 or dynamin impaired their internalization, as did inhibitors of clathrin-mediated endocytosis. (ii) Agonist stimulation increased the phosphorylation of beta(2)AR but not beta(1)AR. (iii) In response to agonist, each subtype redistributed from the cell surface to a distinct population of cytoplasmic vesicles; those containing beta(1)AR were smaller and closer to the plasma membrane whereas those containing beta(2)AR were larger and more perinuclear. (iv) When subcellular fractions from agonist-treated cells were separated by sucrose density gradient centrifugation, all of the internalized beta(2)AR appeared in the lighter endosomal-containing fractions whereas some of the internalized beta(1)AR remained in the denser plasma membrane-containing fractions. (v) Both subtypes recycled with similar kinetics back to the cell surface upon removal of agonist; however, recycling of beta(2)AR but not beta(1)AR was inhibited by monensin. Based on these results, we propose that the internalization of beta(1)AR is both arrestin- and dynamin-dependent and follows the same clathrin-mediated endocytic pathway as beta(2)AR. But during or after endocytosis, beta(1)AR and beta(2)AR are sorted into different endosomal compartments.

Airway hyperresponsiveness to bradykinin induced by allergen challenge in actively sensitised Brown Norway rats

The mechanism(s) of bradykinin-induced bronchoconstriction was investigated in the Brown Norway (BN) rat model of allergic asthma. Bronchoconstrictor responses to i.v. bradykinin in BN rats were maximally augmented 24 h following challenge with allergen and declined at later time points. Histological evaluation of the inflammatory status of the lungs after ovalbumin (OA) challenge showed a marked inflammatory response, which was maximal at 24 h and declined thereafter. However, pretreatment with budesonide did not inhibit the augmented bronchoconstrictor response to bradykinin 24 h after allergen challenge. The selective B1 receptor agonist, Lys-[desArg9]-BK had no bronchoconstrictor effects, whereas the selective B2 receptor antagonist, HOE 140, abolished the response to bradykinin in OA-challenged animals. The augmented response to bradykinin was not affected by methysergide, indomethacin, disodium cromoglycate, iralukast, the 5-lipoxygenase inhibitor, CGS8515, or the NK2 receptor antagonist, SR48968. It was, however, partially inhibited by atropine both in saline- and OA-challenged animals. Pretreatment with captopril and thiorphan markedly potentiated responses to bradykinin both in saline- and OA-challenged animals. Thus, augmentation of the bronchoconstrictor response to bradykinin occurs in actively sensitised BN rats 24 h after challenge with OA and is associated with marked pulmonary inflammation. The response is entirely B2 receptor mediated and approximately 50% of the response is cholinergic. However, mast cell activation, the products of the cyclooxygenase or 5-lipoxygenase pathways and tachykinins are not involved. Peptidase inhibition mimics the effect of allergen challenge on the bronchoconstrictor response to bradykinin and it remains possible that the mechanism of the augmented response to bradykinin following allergen challenge involves downregulation of peptidase activity as a consequence of the inflammatory response.

Impaired Gene Expression of .BETA.1-Adrenergic Receptor, but Not Stimulatory G-Protein Gs.ALPHA., in Rat Ventricular Myocardium Treated with Isoproterenol

We investigated the gene expression of beta(1)-adrenergic receptor (beta(1)AR) and stimulatory G-protein Gsalpha, important signal transduction elements for regulating heart rate and contractility, in ventricle after chronic treatment with isoproterenol (ISO) in rat. Rats were treated with ISO (4 mg/kg, intraperitoneal) twice a day for 4 d. Ventricle weight of the heart and ventricle weight/body weight ratio were increased by 23% and 25% compared with control, respectively. Positive inotropic responses to ISO in left atrial muscle preparations isolated from ISO-treated rats were markedly decreased. Northern blot hybridization showed that the mRNA transcript of beta(1)AR was significantly decreased in ventricle of ISO-treated rats, whereas Gsalpha mRNA level was unchanged. Present results demonstrate that the gene expression of myocardial beta(1)AR, but not Gsalpha, was decreased in rat myocardium of ISO-induced cardiac hypertrophy, and suggesting that decrease in the gene expression of beta(1)AR may be one of the mechanisms responsible for the diminished cardiac function.

Protein kinase C regulates functional coupling of beta1-adrenergic receptors to Gi/o-mediated responses in cardiac myocytes

The effect of protein kinase C (PKC) activation on beta1-adrenergic receptor (beta1-AR) regulation of the cardiac L-type Ca2+ current (ICa,L) was studied using the whole-cell patch clamp technique. Treatment of guinea pig ventricular myocytes with phorbol-12,13-dibutyrate (PDBu) caused a significant decrease in ICa,L sensitivity to stimulation by submaximal beta1-AR activation using isoproterenol (Iso). This decrease in sensitivity was also associated with the ability of higher concentrations of Iso to directly inhibit the stimulatory response. PDBu treatment produced similar effects on H2 histamine receptor-mediated ICa,L responses. In the presence of PDBu, higher concentrations of Iso inhibited the histamine stimulated ICa,L, and this effect was blocked by a selective beta1-AR antagonist. Higher concentrations of histamine also inhibited the Iso stimulated ICa,L, and this effect was blocked by a selective H2 receptor antagonist. The effects of PDBu were blocked by the PKC inhibitor bisindolylmaleimide I, and they were not mimicked by the inactive phorbol ester 4alpha-phorbol-12,13-didecanoate. The inhibitory effects of Iso and histamine were significantly reduced when Gi/o mediated responses were blocked with pertussis toxin. These results suggest that PKC promotes coupling of cardiac beta1-adrenergic and H2 histamine receptors to Gi/o mediated inhibitory responses.

Atypical beta-adrenoceptors, different from beta 3-adrenoceptors and probably from the low-affinity state of beta 1-adrenoceptors, relax the rat isolated mesenteric artery

(1) We examined whether beta3- and/or atypical beta-adrenoceptors relax the rat isolated mesenteric artery. (2) Mesenteric arteries precontracted with phenylephrine were relaxed by beta-agonists with the following potencies (pD2): nonselective agonist isoprenaline (6.00)>nonconventional partial agonist cyanopindolol (5.45)>beta2-agonist fenoterol (4.98)>nonconventional partial agonist CGP 12177 (4.19)>beta3-agonist ZD 2079 (3.72). The beta3-agonist CL 316243 1 mm relaxed the vessel only marginally. (3) The concentration-response curves (CRCs) for cyanopindolol, CGP 12177 and ZD 2079 were not affected by the nonselective beta-antagonist propranolol 0.3 microm, the beta2-antagonist ICI 118551 1 microm and by CL 316243 60 microm, but shifted to the right by bupranolol (pA2 5.3-5.7), CGP 20712 (5.4) and SR 59230A (6.5-6.7) (the latter three drugs block atypical and/or beta3-adrenoceptors at high concentrations). (4) The CRC for isoprenaline was shifted to the right by propranolol (pA2 7.0) but, in the presence of propranolol 0.3 microm, not affected by SR 59230A 1 microm. The CRC for fenoterol was shifted to the right by propranolol (pA2 6.9) and ICI 118551 (6.8). (5) Removal of endothelium diminished the vasorelaxant effects of cyanopindolol, CGP 12177 and ZD 2079. (6) Fenoterol and cyanopindolol also relaxed (endothelium-intact) mesenteric arteries precontracted with serotonin. The relaxant effect of cyanopindolol was antagonized by bupranolol to about the same degree as in phenylephrine-contracted vessels. (7) In conclusion, beta2- and atypical beta-adrenoceptors (but not beta3-adrenoceptors) relax the rat mesenteric artery. The atypical beta-adrenoceptor, which is partially located endothelially, may differ from the low-affinity state of the beta1-adrenoceptor.

Lack of inverse agonistic activity of nebivolol, its D- and L-enantiomers and of in vivo metabolized nebivolol in human myocardium

The pharmacological profile of nebivolol may be mediated by its enantiomers and/or its hydroxylated metabolites. Therefore, the cardiac effects of the nebivolol enantiomers as well as of serum specimens containing hydroxylated nebivolol metabolites were studied in human myocardium. For control, the beta1-adrenoceptor selective antagonist metoprolol was used. After pre-stimulation of force of contraction with forskolin (0.3 microM) or isoprenaline (0.01 microM), force developement was decreased only at high concentrations (> or =300 nM) of nebivolol or its enantiomers in isolated trabeculae. Nebivolol and its enantiomers, in contrast to metoprolol (0.4 microM: -72% basal force), produced only minor negative intropic effects in isolated trabeculae under basal conditions. Basal force of contraction was not decreased by in vivo metabolized nebivolol in pharmacological concentrations. Neither D- nor L-nebivolol (30 microM) influenced myofibrillar Ca2+ responsiveness. Nebivolol and the D-enantiomer, but not the L-enantiomer (all 0.5 microM), improved the frequency-dependent force generation. D-Nebivolol, in contrast to L-nebivolol, was a beta1-adrenoceptor selective compound in membrane preparations from non-failing donor hearts. In conclusion, nebivolol and its enantiomers as well as in vivo metabolized nebivolol produce only minor negative inotropic effects. This and the finding that nebivolol and its D-enantiomer improve the frequency-dependent force generation may be of particular advantage when treating patients with already compromised cardiac function.

Partial agonist activity of bucindolol is dependent on the activation state of the human beta1-adrenergic receptor

BACKGROUND

In contrast to other beta-blockers, bucindolol has failed to reduce mortality in patients with chronic heart failure. It is currently debated whether this is due to partial agonist activity of this agent. We investigated whether conflicting results previously reported concerning the intrinsic activity of bucindolol can be explained by species differences or by different activation states of beta-adrenergic receptors (beta-ARs) in the respective tissues.

METHODS AND RESULTS

On isolated right atria from transgenic mice with cardiac overexpression of human beta1-ARs, bucindolol led to a greater increase in beating frequency (P<0.05) compared with wild-type mice. The increase amounted to 47% of the effect of xamoterol and was blocked by propranolol. On isolated, electrically stimulated, left ventricular muscle-strip preparations from failing human myocardium, bucindolol did not change the force of contraction under control conditions. In myocardial preparations pretreated with metoprolol (30 micromol/L, 90 minutes, subsequent washout), bucindolol significantly increased the force of contraction (P<0.001 vs control). In nonfailing atrial myocardium, isoproterenol pretreatment (1 micromol/L, 60 minutes) abolished the positive inotropic effect of xamoterol that was present under control conditions (P<0.05 vs control). The inotropic effects of bucindolol or xamoterol were inversely correlated to the inotropic response to forskolin in the respective specimens (r=-0.75 and -0.74, respectively; P<0.005).

CONCLUSIONS

We conclude that bucindolol is a partial agonist at the human beta1-AR. In human failing myocardium, its partial agonist activity is masked by increased activation states of beta-ARs and is unmasked after in vitro pretreatment with metoprolol. Thus, the partial agonist activity of bucindolol is dependent on the activation state of the human beta1-AR.

Role of beta 1- and beta 2-adrenoceptors in hypertrophic and apoptotic effects of noradrenaline and adrenaline in adult rat ventricular cardiomyocytes

In adult rat ventricular cardiomyocytes alpha1-adrenoceptor (AR) stimulation causes increases in protein synthesis. On the other hand beta1-AR stimulation inhibits protein synthesis, and evokes apoptotic cell death. We studied, in adult rat ventricular cardiomyocytes, effects of noradrenaline (NA), adrenaline (ADR) and phenylephrine (PE) on protein synthesis (assessed by [3H]-phenylalanine incorporation into the cardiomyocytes) in relation to effects on early apoptosis (measured by Annexin V/propidium iodide staining). PE (10(-9)-10(-5) M) induced protein synthesis was not affected by the beta1-AR blocker CGP 20712A (CGP, 300 nM) or beta2-AR blocker ICI 118,551 (ICI, 55 nM). ADR (10(-9)-10(-5) M) induced protein synthesis was enhanced by CGP and decreased by ICI. Pretreatment of the cardiomyocytes with pertussis toxin (PTX) decreased NA- and ADR- induced protein synthesis, but did not affect PE-effects. NA (10(-5) M) and ADR (10(-5) M) caused a significant increase in the number of apoptotic cells; these effects were enhanced by PTX-treatment, abolished by CGP, but not significantly affected by ICI. Furthermore, there was a significant negative correlation between catecholamine-evoked apoptosis and catecholamine-induced hypertrophic effects. We conclude that, in ventricular cardiomyocytes of adult rats, growth-promoting effects of NA and ADR are composed of alpha1A-AR mediated increase in protein synthesis and beta1-AR mediated apoptosis that counteracts increases in protein synthesis. The role of beta2-adrenoceptor appears to be a balance of antiapoptotic effects via a PTX-sensitive pathway and proapoptotic effects via a GS-adenylyl cyclase pathway.

Effects of atropine on human cardiac beta 1- and/or beta 2-adrenoceptor stimulation

The aim of this study was to find out whether, in humans, the increase in vagal tone accompanying cardiac beta-adrenoceptor (beta-AR) stimulation might be different dependent on beta1- or beta2-AR stimulation. For this purpose we studied, in six male healthy volunteers (aged 28+/-1 years), the effects of atropine infusion (0.15 microg/kg/min continuously) on increase in heart rate (HR) and contractility (determined as shortening of HR-corrected duration of electromechanical systole-QS2c) evoked by infusion of isoprenaline (3.5-35 ng/kg/min, increasing HR and QS2c via beta1-and beta2-AR), terbutaline (25-150 ng/kg/min, increasing HR and QS2c via beta2-AR), adrenaline (20-160 ng/kg/min, increasing HR via beta2-and QS2c via beta1-AR) and bicycle exercise in supine position (increasing HR and QS2c via beta1-AR). The three beta-AR agonists and exercise increased HR and shortened QS2c in a dose- or work-load-dependent manner, respectively. Atropine enhanced HR-increasing effects of all three beta-AR agonists and exercise; increases were larger for beta2-AR (terbutaline, adrenaline) mediated effects than for beta1-AR (exercise) mediated effects. Moreover, atropine enhanced beta-AR agonists-induced QS2c shortening; however, atropine effects on QS2c were markedly less pronounced than on HR. From the results we conclude that, in humans, beta1-and beta2-AR mediated stimulation evoked HR-increases are composed of two components: increases via direct beta-AR stimulation and simultaneously decreases via increase in vagal tone. In addition, beta-AR mediated increases in contractility are also dampened by simultaneous activation of vagal tone but to a lesser extent possibly because human ventricular myocardium is only sparsely parasympathetically innervated.

The beta1-adrenoceptor site activated by CGP12177 varies in behavior according to the estrous cycle phase and stress

The aim of this work was to assess whether stress and estrous cycle phases affected the beta1-adrenoceptor (beta1-AR) site activated by CGP12177 in the right atria of rats. The chronotropic response to CGP12177 in the absence or presence of antagonists was determined in atria from rats submitted to one daily foot-shock session for 3 consecutive days. Blood was collected for hormonal assays. The pD2 for CGP12177 in atria from females was lower than in atria from males and was unaltered by stress or the estrous cycle. Propranolol (200 nM) or CGP20712A (3 microM) shifted the concentration-response curves to CGP12177 to the right in control and stressed estrus or control diestrus rats. Atria from stressed diestrus rats were resistant to blockade by propranolol or CGP20712A, indicating that the effect of beta-adrenoceptor antagonists on the response to CGP12177 is influenced by estrous cycle phases. The stress-induced increase in serum corticosterone levels was independent of the estrous cycle or gender, but the estradiol/progesterone ratio was affected differently in the two groups of female rats. In the diestrus group, serum estradiol levels decreased after the first foot-shock session and remained low until the day of sacrifice, whereas in the estrus group the serum levels of estradiol did not decrease after stress and peaked on the second day, which corresponded to proestrus. These data do not indicate whether there is a direct or indirect effect of stress hormones and (or) sex steroids on cardiac beta1-AR sensitivity. However, they do show that the classic and low-affinity binding sites of the beta1-AR are independently regulated and that the beta1-AR atypical site affinity for antagonists depends on the estrous cycle.

Effects of two beta3-agonists, CGP 12177A and BRL 37344, on coronary flow and contractility in isolated guinea pig heart

The functional role of beta(3)-adrenergic receptors in the heart is still not clear. The actions of two widely used beta(3)-adrenoceptor agonists, such as BRL 37344 and CGP 12177, were studied in the isolated guinea pig heart, perfused at constant pressure according to the Langendorff technique. Heart contractility (dP/dt, first derivative of pressure measured over time) and coronary flow (CF) were assessed simultaneously. BRL 37344 and CGP 12177A at a concentration range of 10-8-10-5 M increased dP/dt and CF. The selective beta(3)-antagonist L-748337 (10-6 M) did not significantly influence either BRL 37344 or CGP 12177A-induced responses. However, both dP/dt and CF responses to BRL 37344 and CGP 12177A at a concentration of 10-7 M were abolished in the presence of the beta(1)/beta(2)-antagonist nadolol (10-5 M). In contrast, cardiovascular responses to CGP 12177A at a higher concentration of 10-5 M were hardly inhibited by nadolol (10-5 M). In addition, BRL 37344 and CGP 12177A at concentrations as low as 10-8 M almost completely abolished an isoprenaline-induced increase in contractility, suggesting that both BRL 37344 and CGP 12177A display beta(1)-antagonistic properties. These data suggest that the stimulatory cardiovascular responses to BRL 37344 at a full range of concentrations, and CGP 12177A at a low concentration of 10-7 M, are not mediated by beta(3)-adrenergic receptors, but rather by activation of beta(1)- or beta(2)-adrenergic receptors. Cardiovascular effects of CGP 12177A at a high concentration of 10-5 M are independent of beta(1)/beta(2)/beta(3)-adrenergic receptors. Summing up, it seems that in the isolated guinea pig heart the functional role of beta(3)-adrenoceptors is not significant. Nonetheless, BRL 37344 and CGP 12177A are not ideal tools for investigation of beta(3)-adrenergic receptor-dependent effects, because these compounds interact with other types of beta-adrenergic receptors.

Differences in the bioenergetic response of the isolated perfused rat heart to selective beta1- and beta2-adrenergic receptor stimulation

BACKGROUND

In the heart, striking functional differences exist after stimulation of the beta1- and beta2-adrenergic receptor (AR) subtypes. These may be linked to differences in metabolic response during beta1- and beta2-AR stimulation.

METHODS AND RESULTS

The relation between work and metabolism was examined during selective beta1- and beta2-AR stimulation (beta1 and beta2 groups, respectively) in the isolated perfused rat heart. Measurements were made of rate-pressure product (RPP=LV developed pressure x heart rate), phosphorus-containing metabolites, and pH by 31P nuclear magnetic resonance spectroscopy and of O2 consumption by fiber-optic oximetry. Experiments were performed under high constant flow (HCF) and under flow-limiting conditions (constant pressure, CP). Despite substantially greater RPP increases relative to baseline during beta1-AR (HCF, 475%; CP, 150%) than beta2-AR (HCF, 90%; CP, 72%) stimulation, the relative decrease in the intracellular energy charge relative to baseline was similar for the beta1 (HCF, 49%; CP, 64%) and beta2 (HCF, 59%; CP, 55%) groups. For each group, an increase in oxygen consumption (MVO2) occurred commensurate with workload during HCF (beta1, 141%; beta2, 30%). During CP, however, the MVO2 increase was similar (beta1, 39%; beta2, 34%), despite the large RPP difference between the groups. During both protocols, there was greater acidosis during beta1-AR than during beta2-AR stimulation. Thus, at a given workload, intracellular energy charge decreased, and MVO2 (CP) increased to a greater extent during beta2 than beta1-AR stimulation.

CONCLUSIONS

The bioenergetic differences are consistent with access to an additional substrate pool during beta1-AR stimulation. This may occur via increased glycogenolysis during beta1-AR stimulation, facilitating increased energy production by oxidative phosphorylation, and under flow-limiting conditions, anaerobic glycolysis.

Role of beta 1- and beta 2-adrenoceptor subtypes in preconditioning against myocardial dysfunction after ischemia and reperfusion

Using an isolated nonworking rat heart model, this study investigated the role of beta-adrenergic preconditioning (beta-PC) to attenuate myocardial dysfunction after an ischemia/reperfusion injury. After a 20-min stabilization period, the noradrenaline depleted hearts were perfused for 5 min with isoproterenol (ISO) before 40-min global ischemia (I) followed by 30-min reperfusion (R). ISO 0.02 microM provided significant protection versus unconditioned in vivo reserpinized IR control, causing a decrease of creatine kinase (CK) release (mIU/min/g wet weight) on reperfusion in coronary effluent, a preservation of the mean coronary flow (MCF) and preservation of left ventricular function assessed by the rate-pressure product (RPP). These beneficial effects were similar to those of ischemic preconditioning (I-PC) in both nonreserpinized and reserpinized rats. Propranolol (1 microM) and atenolol (10 microM) completely suppressed the ISO preconditioning. In contrast, ICI 118551 (2 microM) a highly selective beta -blocker, did not blunt the salutary effects of ISO on CK release and MCF preservation. These results indicate that ISO pretreatment provides a significant cardioprotection against prolonged ischemic myocardial injury. Although endogenous catecholamines are not necessary for I-PC in isolated rat hearts, cardioprotection provided by beta-adrenergic stimulation is quite similar to I-PC. This significant cardioprotection is mediated less by beta -adrenoceptor than by beta -adrenoceptor activation, which seems to play a crucial role in the beta-PC mechanism.

Greater inotropic and cyclic AMP responses evoked by noradrenaline through Arg389 beta 1-adrenoceptors versus Gly389 beta 1-adrenoceptors in isolated human atrial myocardium

1. We studied the biochemical and contractile responses of isolated human myocardial tissue expressing native receptor variants of the 389G>R beta(1)-adrenoceptor polymorphism. 2. Right atrial appendage was obtained from homozygous RR patients (n=37) and homozygous GG patients (n=17) undergoing elective cardiac surgery. The positive inotropic effect of noradrenaline in these tissues, mediated through beta(1)-adrenoceptors, was studied using electrically stimulated (1 Hz) atrial strips, as well as the effects of noradrenaline on cyclic AMP levels and cyclic AMP-dependent protein kinase. 3. Tissue from RR homozygotes (n=14) showed significantly increased inotropic potency to noradrenaline (-log EC(50), M=6.92+/-0.12) compared to GG homozygotes (n=8, -log EC(50), M=6.36+/-0.11, P<0.005). This difference was not dependent on tissue basal force. 4. Tissue cyclic AMP levels (pmol mg(-1)) were also greater in RR homozygotes (basal 34.8+/-3.7 n=12, 300 nM noradrenaline 41.4+/-7.6 n=9, 30 micro M noradrenaline 45.2+/-3.2 n=22, 0.2 mM isoprenaline 48.3+/-4.2 n=16) compared to GG homozygotes (basal 30.7+/-4.4 n=5, 300 nM noradrenaline 32.6+/-6.92 n=5, 30 micro M noradrenaline 38.1+/-3.1 n=8, 0.2 mM isoprenaline 42.6+/-5.2 n=6, P=0.007). There were no differences between the variants in terms of cyclic AMP-dependent protein kinase activity. 5. These data provide the first evidence that enhanced G-protein coupling of the R389 beta(1)-adrenoceptor variant reported in rodent fibroblast expression systems is also present in native human receptors. The functional consequence of this is to significantly alter the inotropic potency of beta(1)-adrenoceptor activation depending on its genotype at the 389 position.

(-)-CGP 12177 increases contractile force and hastens relaxation of human myocardial preparations through a propranolol-resistant state of the beta 1-adrenoceptor

Two forms of the activated beta1-adrenoceptor exist, one that is stabilized by (-)-noradrenaline and is sensitive to blockade by (-)-propranolol and another which is stabilized by partial agonists such as (-)-pindolol and (-)-CGP 12177 but is relatively insensitive to (-)-propranolol. We investigated the effects of stimulation of the propranolol-resistant beta1-adrenoceptor in the human heart. Myocardium from non-failing and failing human hearts were set up to contract at 1 Hz. In right atrium from non-failing hearts in the presence of 200 nM (-)-propranolol, (-)-CGP 12177 caused concentration-dependent increases in contractile force (-logEC50[M] 7.3+/-0.1, E(max) 23+/-1% relative to maximal (-)-isoprenaline stimulation of beta1- and beta2-adrenoceptors, n=86 patients), shortening of the time to reach peak force (-logEC50[M] 7.4+/-0.1, E(max) 37+/-5%, n=61 patients) and shortening of the time to reach 50% relaxation ( t(50%), -logEC50[M] 7.3+/-0.1, E(max) 33+/-2%, n=61 patients). The potency and maxima of the positive inotropic effects were independent of Ser49Gly- and Gly389Arg-beta1-adrenoceptor polymorphisms but were potentiated by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (-logEC50[M] 7.7+/-0.1, E(max) 68+/-6%, n=6 patients, P<0.0001). In the presence of (-)-propranolol and 3-isobutyl-1-methylxanthine, the potency (-logEC50[M] 7.4+/-0.1, P=0.0013, n=9 patients) but not the maximal effect of (-)-CGP 12177 was reduced in right atrium from failing hearts, which was associated with 64% and 52% reductions in the densities of low-affinity and high-affinity (-)-[3H]CGP 12177 binding sites. In the presence of (-)-propanolol and 3-isobutyl-1-methylxanthine, (-)-CGP 12177 increased atrial cyclic AMP levels and activated cyclic AMP-dependent protein kinase in right atrium from non-failing hearts. In right ventricle from failing hearts (-)-CGP 12177 increased contractile force (-logEC50[M] 7.4+/-0.1, E(max) 34+/-3%, n=13 patients) and hastened the time to peak force (-logEC50[M] 7.6+/-0.1) and time to reach 50% relaxation (-logEC50[M] 7.4+/-0.1) in the presence of (-)-propranolol and 3-isobutyl-1-methylxanthine. Our results show that (-)-CGP 12177 increases contractility and hastens relaxation through a cyclic AMP pathway in human myocardium, consistent with mediation through a (-)-propranolol-resistant state of the beta1-adrenoceptor. The reduction in heart failure of atrial inotropic potency of (-)-CGP 12177, as well as of the high-affinity and low-affinity binding sites for (-)-[3H]CGP 12177, is consistent with the beta1-adrenoceptor nature of these sites.

Evidence against beta 3-adrenoceptors or low affinity state of beta 1-adrenoceptors mediating relaxation in rat isolated aorta

1 The presence of beta(3)-adrenoceptors and the low affinity state of the beta(1)-adrenoceptor (formerly "putative beta(4)-adrenoceptor") was investigated in ring preparations of rat isolated aorta preconstricted with phenylephrine or prostaglandin F(2alpha) (PGF(2alpha)). Relaxant responses to isoprenaline, selective beta(3)-adrenoceptor agonists (BRL 37344, SR 58611A, CL 316243) and non-conventional partial agonists (CGP 12177A, cyanopindolol, pindolol) were obtained. 2 In phenylephrine-constricted, but not PGF(2alpha)-constricted rings, relaxations to isoprenaline showed a propranolol-resistant component. 3 In phenylephrine-constricted rings, relaxations to BRL 37344 (pEC(50), 4.64) and SR 58611A (pEC(50), 4.94) were not antagonized by the selective beta(3)-adrenoceptor antagonist SR 59230A (< or =1 microM). CL 316243 (< or =100 microM) failed to produce relaxation. In PGF(2alpha)-constricted rings only SR 58611A produced relaxation, which was not affected by SR 59230A (< or =3 microM). 4 Non-conventional partial agonists produced relaxation in phenylephrine-constricted but not PGF(2alpha)-constricted rings. The relaxation to CGP 12177A was unaffected by SR 59230A (< or =1 microM) or by CGP 20712A (10 microM), reported to block the low affinity state of the beta(1)-adrenoceptor. 5 beta-adrenoceptor antagonists also produced relaxation in phenylephrine-constricted rings with an order of potency of (pEC(50) values): bupranolol (5.5) approximately 38;SR 59230A (5.47) approximately 38;cyanopindolol (5.47)>pindolol (5.30)>alprenolol (5.10)>propranolol (4.83)>ICI 118551 (4.60)>CGP 12177A (4.38) approximately 38;CGP 20712A (4.35). Bupranolol (100 microM), alprenolol (30 microM), propranolol (100 microM) and SR 59230A (10 microM) produced no relaxation in PGF(2alpha)-constricted rings. 6 These results provide no evidence for the presence of functional beta(3)-adrenoceptors or the low affinity state of the beta(1)-adrenoceptor in rat aorta.

Existence of functional beta1- and beta2-adrenergic receptors on microglia

We examined the expression and function of beta-adrenergic receptor (beta-AR) subtypes in both isolated primary rat microglia and a rat microglial cell line. RT-PCR analyses revealed that microglia expressed beta(1)- and beta(2)-ARs but not beta(3)-ARs, whereas rat primary peritoneal macrophages expressed only beta(2)-ARs. Stimulation of beta-ARs on microglia by norepinephrine (NE) resulted in an increase in the level of intracellular cAMP and the subsequent expression of interleukin-1beta mRNA. These effects were prevented by propranolol. Similar results were obtained with other selective beta(1)-AR agonists and antagonists. beta(2)-ARs on microglia were also functional. It is possible that noradrenergic innervations participate in the control of microglial functions via beta(1)-ARs on microglia in the brain, because NE has high affinity for beta(1)- and beta(3)-ARs but little or no affinity for beta(2)-ARs. It seems physiologically significant that microglia can be controlled by NE, which predominates over epinephrine in the brain, whereas macrophages in peripheral tissues can be controlled by epinephrine, which is at higher levels in peripheral tissues.

Beta-adrenoceptor control of G protein function in the neonate: determinant of desensitization or sensitization

Neonatal beta-adrenoceptors (beta-ARs) are resistant to agonist-induced desensitization. We examined the functioning of G(i) and G(s) after repeated administration of beta-AR agonists to newborn rats. Isoproterenol (beta(1)/beta(2) agonist) obtunded G(i) function in the heart but not the liver; in contrast, terbutaline, a beta(2)-selective agonist, enhanced G(i) function. Isoproterenol, but not terbutaline, increased membrane-associated G((s)alpha), which would enhance receptor function. In addition, isoproterenol increased and terbutaline maintained the proportion of the short-splice (S) variant of G((s)alpha) in the membrane fraction; G((s)alpha)S is functionally more active than the long-splice variant. Either isoproterenol or terbutaline treatment increased G((s)alpha) in the cytosolic fraction, a characteristic usually associated with desensitization in the adult. Decreased G(i) activity, coupled with increased membrane-associated G((s)alpha) concentrations and maintenance or increases in membrane G((s)alpha)S, provide strong evidence that unique effects on G protein function underlie the ability of the immature organism to sustain beta-AR cell signaling in the face of excessive or prolonged stimulation; these mechanisms also contribute to tissue selectivity of the effects of beta-agonists with divergent potencies toward different beta-AR subtypes.

Mg2+ efflux from the isolated perfused rabbit heart is mediated by two states of the beta1-adrenergic receptor

The non-selective beta-adrenergic receptor agonist isoproterenol stimulates Mg(2+) efflux from the perfused heart. The beta-adrenergic receptor subtype governing Mg(2+) efflux was determined in rabbit hearts perfused by the method of Langendorff with Mg(2+)-free Krebs Henseleit buffer. Magnesium efflux was examined during infusion of isoproterenol (a non-selective beta-adrenergic agonist), dobutamine (beta(1)-selective), salbutamol (beta(2)-selective), BRL37344 in the presence of 200 nM propranolol (beta(3)-selective conditions) or CGP12177 (beta(3)/low affinity state beta(1)-selective). Isoproterenol increased Mg(2+) efflux in a dose-dependent manner, and was the most potent and efficacious agent used. Dobutamine and CGP12177 each significantly increased Mg(2+) efflux, but with markedly different time characteristics. Dobutamine induced significantly less Mg(2+) release than isoproterenol. Although the maximal effect of CGP12177 on Mg(2+) release was 30% less than that of isoproterenol, the difference was not statistically significant. Neither salbutamol nor BRL37344 had any effect on Mg(2+) efflux. These results suggest that isoproterenol-induced Mg(2+) efflux is mediated by both the high and low affinity states of the beta(1)AR, with the low affinity state making the larger contribution.