Atypical beta-adrenoceptor- (beta 3-adrenoceptor) mediated relaxation of canine isolated bronchial smooth muscle

Chronic treatment in vivo with β-adrenoceptor agonists induces dysfunction of airway β(2) -adrenoceptors and exacerbates lung inflammation in mice

BACKGROUND AND PURPOSE

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.

EXPERIMENTAL APPROACH

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.

CONCLUSIONS AND IMPLICATIONS

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.

Beta-Adrenergic Agonists

Inhaled β₂-adrenoceptor (β₂-AR) agonists are considered essential bronchodilator drugs in the treatment of bronchial asthma, both as symptoms-relievers and, in combination with inhaled corticosteroids, as disease-controllers. In this article, we first review the basic mechanisms by which the β₂-adrenergic system contributes to the control of airway smooth muscle tone. Then, we go on describing the structural characteristics of β₂-AR and the molecular basis of G-protein-coupled receptor signaling and mechanisms of its desensitization/ dysfunction. In particular, phosphorylation mediated by protein kinase A and β-adrenergic receptor kinase are examined in detail. Finally, we discuss the pivotal role of inhaled β₂-AR agonists in the treatment of asthma and the concerns about their safety that have been recently raised.

Characterisation of the acute and reversible airway inflammation induced by cadmium chloride inhalation in healthy dogs and evaluation of the effects of salbutamol and prednisolone

The aims of this study were firstly to characterise a model of subclinical and reversible bronchial inflammation induced by cadmium chloride inhalation in healthy dogs and then to examine the effect of prednisolone or salbutamol treatment on the resulting bronchitis. The model characterisation and the effects of treatment were studied using clinical symptoms, haematology, thoracic radiography, bronchoscopy and bronchoalveolar lavage, barometric whole-body plethysmography and histamine broncho-provocation tests. In addition, the activity of matrix metalloproteinases (MMP)-2 and -9 were determined in serum and bronchoalveolar lavage fluid (BALF). Cadmium inhalation induced: (1) a transient bronchial inflammation, dominated by neutrophils; (2) a neutrophilia of the blood that persisted for up to 4 weeks; (3) a transient increased bronchial reactivity, and (4) a significant increase in MMP-9 activity in the BALF. Prednisolone treatment reduced the influx of inflammatory cells into the BALF, but not significantly, had no effect on pulmonary function, and did not reduce of airway hypersensitivity. Salbutamol had almost no effect on any of the parameters investigated.

Effects of formoterol and BRL 37344 on human umbilical arteries in vitro in normotensive and pre-eclamptic pregnancy

Alterations in vascular responses to beta-adrenoceptor agonists in normotensive pregnancy and pre-eclampsia are not fully understood. Thus, we studied changes in vasodilator responses to beta(2)-adrenoceptor agonist formoterol and beta(3)-adrenoceptor agonist BRL 37344 on umbilical arteries isolated from normotensive (n=12) and pre-eclamptic (n=12) pregnant women. Changes in the relaxant effect of formoterol and BRL 37344 were investigated by measuring isometric tensions in endothelium-denuded strips of umbilical arteries in the presence or absence of metoprolol, ICI 118.551 and SR 59230A (beta(1), beta(2), beta(3)-adrenoceptor antagonists, respectively, 10(-6) mol/L). Effects of formoterol and BRL 37344 on cAMP levels of umbilical arteries were evaluated by radioimmunoassay kits. Formoterol (10(-10)-10(-4) mol/L) and BRL 37344 (10(-10)-10(-4) mol/L) caused concentration-dependent relaxation of the contraction induced by phenylephrine (10(-5) mol/L) in umbilical artery strips isolated from both groups. E(max) values of formoterol and BRL 37344 (for normotensive pregnant women: 87.33+/-0.87 and 53.25+/-1.17 vs. for pre-eclampsia: 73.68+/-1.58 and 43.64+/-1.19, n=12, P>0.05, respectively) were significantly smaller in strips from pre-eclamptic women (P<0.05), with no significant change in pD(2) values. E(max) values of formoterol were significantly higher than those of BRL 37344 in both tissue (P<0.05). ICI 118.551 and SR 59230A, but not metoprolol, antagonized the relaxant effects of formoterol and of BRL 37344 on umbilical artery strips isolated from normotensive and pre-eclamptic pregnant women. Formoterol and BRL 37344 increased cAMP levels in both groups, but less significant in pre-eclamptic strips (P<0.05). These results suggest that the relaxation caused in human umbilical arteries by formoterol and BRL 37344 is mediated by a mixed population of beta(2)- and beta(3)-adrenoceptor subtypes, with contribution of cAMP. Umbilical arteries from subjects with pre-eclampsia showed a weaker beta(2)- and beta(3)-receptor-mediated relaxation to formoterol and BRL 37344, suggesting that the reduced action of formoterol and BRL 37344 may be partly due to a decreased effect of cAMP.

Isoproterenol reduces ischemia-reperfusion lung injury despite beta-blockade

BACKGROUND

If lungs could be retrieved from non-heart-beating donors (NHBDs), the shortage of lungs for transplantation could be alleviated. The use of lungs from NHBDs is associated with a mandatory warm ischemic interval, which results in ischemia-reperfusion injury upon reperfusion. In an earlier study, rat lungs retrieved 2-h postmortem from NHBDs had reduced capillary leak measured by filtration coefficient (Kfc) when reperfused with isoproterenol (iso), associated with an increase in lung tissue levels of cyclic AMP (cAMP). The objective was to determine if this decrease in Kfc was because of beta-stimulation, or would persist despite beta-blockade.

MATERIALS AND METHODS

Donor rats were treated intraperitoneally with beta-blockade (propranolol or pindolol) or carrier, sacrificed, and lungs were retrieved immediately or 2 h postmortem. The lungs were reperfused with or without iso and the beta-blockers in the reperfusate. Outcome measures were Kfc, wet:dry weight ratio (W/D), lung levels of adenine nucleotides and cAMP.

RESULTS

Lungs retrieved immediately after death had normal Kfc and W/D. After 2 h of ischemia, Kfc and W/D were markedly elevated in controls (no drug) and lungs reperfused with beta-blockers alone. Isoproterenol-reperfusion decreased Kfc and W/D significantly (P < 0.01) even in the presence of beta-blockade. Lung cAMP levels were increased only with iso in the absence of beta-blockade.

CONCLUSIONS

The attenuation of ischemia-reperfusion injury because of iso occurs even in the presence of beta-blockade, and may not be a result of beta-stimulated increased cAMP.

Cardiovascular effects of beta 3-adrenoceptor stimulation in perinephritic hypertension

BACKGROUND

A new beta 3-adrenoceptor (beta3-AR) has been shown to mediate peripheral vasodilation. This study was conducted to evaluate effects of the beta3-AR agonist, SR58611 in normal and hypertensive dogs.

MATERIALS AND METHODS

In protocol 1, SR58611 was infused in normal dogs after placebo, after beta1/beta2 blockade with nadolol, after beta1/beta2/beta3 blockade with bupranolol and after combined autonomic blockade (CAB). In protocol 2, perinephritic hypertension was produced in dogs, which received SR58611 at 3 and 6 weeks of hypertension. Effects of SR58611 were evaluated at 7 weeks of hypertension after CAB.

RESULTS

In normal dogs, SR58611 produced a dose-dependent decrease in mean aortic pressure (AOP) (from 116 +/- 19 to 100 +/- 19 mmHg, - 14%; P < 0.05) that was accompanied by baroreflex activation (heart rate increased by 70%; P < 0.01). This hypotensive effect resulting from peripheral vasodilation persisted after nadolol or CAB while baroreflex activation was blunted or abolished. A biphasic response of cardiac output, characterized by a rise and a decline (P < 0.05) reflected a reduction in after- and pre-load. After CAB, SR58611 did not modify cardiac contractility. SR58611 stimulated lipolysis as reflected by a 4-fold increase in blood free fatty acids (FFA) (P < 0.0005). Under CAB, the rise of FFA was reduced (P < 0.01). In hypertensive dogs, SR58611 produced a dose-dependent decrease in mean AOP (from 168 +/- 32 to 125 +/- 35 mmHg; - 26%, P < 0.0001), that was greater than in normal dogs (P < 0.05). Reflex-mediated tachycardia also occurred but at higher blood pressure values. Blood FFA rose similarly (P < 0.0001). Under CAB, heart rate remained unchanged but SR58611 still induced a decrease (P < 0.0001) in mean AOP concomitantly with a rise of (dP/dt)/DP40 (P < 0.005), an effect not observed in normal dogs.

CONCLUSIONS

Beta3-AR stimulation exerts hypotensive effects, increases cardiac contractility and stimulates lipolysis in hypertensive dogs.

Beta(3)-adrenoceptors control Cl(-) conductance in rabbit nasal epithelium

We have investigated the effects of beta(3)-adrenoceptor stimulation in vivo on nasal epithelium. We have recorded the transepithelial potential difference in New Zealand white rabbit nostrils. Superfusion of the nasal epithelial surface with a Cl(-)-free medium supplemented with amiloride, hyperpolarized the nasal potential difference. Isoprenaline produced a hyperpolarization of the nasal potential difference that was not prevented by nadolol, a potent beta(1)-/beta(2)-adrenoceptor antagonist, but was abolished by bupranolol, a nonselective beta(1-3)-adrenoceptor antagonist. SR 58611 ((RS)-N-[(25)-7-ethoxycarbonylmethoxy-1,2,3,4-tetrahydronapht-2-yl]-(2R)-2-(3-chlorophenyl)-2 hydroethanamine hydrochloride) and CGP 12177 (4-[3-t-butylamino-2-hydroxypropoxy]benzimidazol-2-1), a preferential and a partial beta(3)-adrenoceptor agonists, respectively, also produced hyperpolarization of the nasal potential difference. SR 59230 (3-(2-ethylphenoxy)-1-[(1S)1,2,3,4-tetrahydronaphth-1-ylaminol]-(2S)-2-propanol oxalate), a selective beta(3)-adrenoceptor antagonist, abolished the effects of CGP 12177. We conclude that beta(3)-adrenoceptor stimulation resulted in modifications in the nasal potential difference. These findings strengthen the view that beta(3)-adrenoceptors are implicated in controlling water and salt transport in the normal respiratory epithelium.

Genomic cloning and species-specific properties of the recombinant canine beta3-adrenoceptor

A molecular clone encoding a beta3-adrenoceptor was isolated from a canine genomic library. The cloned receptor exhibited a pharmacological profile similar to that of other species: in particular, high efficiency of the two selective beta3-adrenoceptor agonists, CL 316,243 (disodium(R,R)-5[2[[2-(chlorophenyl)-2hydroxyethyl]-amino]propyl]- 1,3-benzodioxole-2,2-dicarboxylate) and ICI 201651 ((R)4-(2-hydroxy-3-phenoxypropylaminoethoxy)-N-(2-methoxyethyl)phe noxy acetic acid) and a low affinity for the radioligand (-)-[3-(125)I]-iodocyanopindolol. Interestingly, CGP 12177A ((+/-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one), which is described as a partial agonist for the human receptor, was a full agonist for the canine receptor. After expression and stimulation of the canine beta3-adrenoceptor in stably transfected Chinese hamster ovary cells there was a very low accumulation of cAMP, suggesting weak coupling to Gs-protein and adenylyl cyclase. However, the response was much better in human embryonal kidney cells transfected with the canine beta3-adrenoceptor gene. The cloning of the canine beta3-adrenoceptor and the insights gained from its pharmacological characterization may allow the development of selective compounds for use in the treatment of obese dogs.

Expression of beta3-adrenoceptors in rat detrusor smooth muscle

PURPOSE

To investigate the expression of beta-adrenoceptor (AR) subtypes responsible for detrusor smooth muscle relaxation.

MATERIALS AND METHODS

Isolated rat detrusor smooth muscle was examined by tension measurement and reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

Norepinephrine (NE), epinephrine (EP) and isoproterenol (ISO) were found to relax the detrusor muscle pre-contracted by 6 x 10(-7) M carbachol in the presence of 10(-6) M phentolamine. NE relaxed the detrusor muscle as potently as EP. This potency order (NE=EP) thus indicated the beta-ARs of the rat detrusor muscle to possibly be a beta1 subtype. However, in the presence of 10(-6) M propranolol, beta1- and beta2- but not beta3-AR antagonist, NE showed a more potent relaxation than EP. This observation indicated that the rat detrusor muscle also possesses beta3-AR. RT-PCR revealed all three subtypes of beta-AR mRNA, namely beta1-, beta2- and beta3-AR mRNA, to be expressed in rat detrusor smooth muscle cells.

CONCLUSION

We concluded that beta3-ARs exist in rat detrusor smooth muscle based on both pharmacological and molecular biological studies. Based on these findings, beta-ARs of rat detrusor smooth muscle are considered to be mixed populations consisting of three subtypes which play an important role in relaxing smooth muscle in response to catecholamines.

Effect of beta 3-adrenergic agonist (Zeneca D7114) on thermoregulation in near-term lambs delivered by cesarean section

We investigated the effect of a beta 3-adrenergic agonist, Zeneca D7114, on thermoregulation in near-term lambs delivered by cesarean section. Lambs were delivered into a cool ambient temperature of 15 degrees C, and between 60 and 80 min of life were given an oral dose of Zeneca D7114 (10 mg.kg body weight-1) dissolved in 20 mL of milk, or milk alone. During the first 0.5 h of life colonic temperature decreased in all lambs, and then increased to plateau levels (39.6-40.4 degrees C) after 120-150 min of life, in 19 out of 23 lambs studied. In the remaining lambs, colonic temperature failed to return to normothermic values, plateauing at 34.3 degrees C. All control lambs were observed to shiver throughout the study, but after Zeneca D7114 treatment 7 out of 10 normothermic lambs stopped shivering, and plateau colonic temperature was 0.8 degree C higher. Hypothermic beta 3-agonist-treated lambs had significantly lower rates of heat production, breathing frequency, and plasma triiodothyronine and cortisol concentrations than normothermic lambs. the level of GDP binding and norepinephrine content of brown adipose tissue (BAT) sampled from hypothermic beta 3-agonist-treated lambs was significantly lower than in normothermic lambs. There was no difference in GDP binding in BAT between control and Zeneca D7114-treated groups, but the Hb content was higher in the latter group. It is concluded that administration of Zeneca D7114 to euthyroid lambs enhances their ability to thermoregulate and restore colonic temperature without altering the thermogenic activity of BAT. This response may be mediated by increasing blood flow to BAT and/or an improvement in the animal's thermal efficiency (i.e. decreased heat loss) due to a reduced reliance on shivering thermogenesis.

Localisation and expression of beta-adrenoceptor subtype mRNAs in human lung

The cellular localisation and distribution of mRNAs encoding beta-adrenoceptor subtypes in human lung were studied by in situ hybridisation and Northern blot analysis. The 851-bp SmaI/PvuII fragment of human beta 1-adrenoceptor cDNA, the 439-bp SmaI fragment of human beta 2-adrenoceptor cDNA and the 975-bp SmaI fragment of human beta 3-adrenoceptor cDNA bound to single mRNA species of approximately 3.2 kb, 2.2 kb and 2.3 kb in size, respectively. Human lung and heart and rabbit lung expressed both beta 1- and beta 2-adrenoceptor mRNAs with no detectable level of beta 3-adrenoceptor mRNA, while rabbit perirenal adipose tissue expressed beta 1-, beta 2- and beta 3-adrenoceptor mRNAs. Cultured human airway epithelial cells and airway smooth muscle cells expressed only beta 2-adrenoceptor mRNA. In situ hybridisation in human lung, using 35S-labelled antisense RNA probes revealed a high level of expression of beta 1- and beta 2-adrenoceptor mRNAs in the pulmonary blood vessels, high level of expression of beta 2-adrenoceptor mRNA in the alveolar walls with less expression of beta 1-adrenoceptor mRNA. There was a moderate expression of beta 2-adrenoceptor but not beta 1-adrenoceptor mRNA in airway epithelium and smooth muscle of peripheral airways and no detectable beta 3-adrenoceptor mRNA in any lung structures.

Regulation of airway cholinergic neurotransmission by Ca(2+)-activated K+ channel and Na(+)-K+ adenosinetriphosphatase

Stimulation of Ca(2+)-activated K+ channel and Na(+)-K(+)-ATPase may play an important role in the relaxant responses of airway smooth muscle to certain bronchodilators. To test whether cholinergic neuroeffector transmission can be modulated by Ca(2+)-activated K+ channel and Na(+)-K(+)-ATPase, canine airway smooth muscle was studied under isometric conditions in vitro. Addition of charybdotoxin (10(-7) M) did not alter the contractile responses to acetylcholine but augmented electrical field stimulation-induced contractions at 1-10 Hz (p < .01), whereas apamin and glibenclamide were without effect. This effect of charybdotoxin was dose dependent, with the maximal increase being 36.8 +/- 5.3% (p < .001). Ouabain (10(-7) M) increased contractions induced by both electrical field stimulation and acetylcholine. The magnitude of the increase in contractile responses to electrical field stimulation was similar to that of acetylcholine at an ouabain concentration of up to 3 x 10(-7) M, but the former was significantly greater at 10(-6) M ouabain (p < .05). These results suggest that both Ca(2+)-activated K+ channel and Na(+)-K(+)-ATPase may be operative in the regulation of cholinergic neurotransmission by inhibiting the exocytotic release of acetylcholine from the vagal nerve terminals.

Effects of macrolide antibiotics on neurally mediated contraction of human isolated bronchus

BACKGROUND

Long-term administration of macrolide antibiotic substances is an alternative therapy used in the treatment of asthma and airway hyperresponsiveness, but neither its mechanism of action nor whether this substance exerts an immediate action in the airways is known.

METHODS

Contractile responses of human isolated bronchial strips to electrical field stimulation (EFS) and acetylcholine were assessed under isometric conditions in the absence and presence of erythromycin, roxithromycin, or clarithromycin.

RESULTS

Incubation of tissues with erythromycin (3 x 10(-5) mol/L) attenuated the contractile responses to EFS so that the stimulus frequency required to produce 50% of the maximal contraction increased from 4.1 +/- 0.5 to 10.1 +/- 0.7 Hz (mean +/- SE; p < 0.001). In contrast, contractile responses to acetylcholine were not changed. Erythromycin reduced the EFS-induced contraction in a concentration-dependent fashion; the maximal decrease from the baseline response was 92.8% +/- 3.6% (p < 0.001). This inhibitory effect was not altered by propranolol, indomethacin, ouabain, charybdotoxin, or mechanical removal of the epithelium. Roxithromycin and clarithromycin likewise inhibited neurally mediated contraction.

CONCLUSIONS

These results suggest that macrolides may inhibit cholinergic neuroeffector transmission in the human airway smooth muscle, probably by reducing exocytotic release of acetylcholine from the nerve terminals.

Beta 3-adrenoceptors and airways

beta 3-adrenoceptors have been identified in a variety of tissues from humans and animals: adipose tissue, gastrointestinal smooth muscle, rat skeletal muscle, bovine skeletal muscle, and human and canine heart. In the airways, the investigation of the beta 3-adrenoceptors came from studies with a series of novel selective agonists. Stimulation of the "atypical" beta-adrenoceptor increases the active transport of albumin across the ferret tracheal epithelium and the ciliary beat frequency of canine bronchial epithelium. Furthermore, it has been demonstrated that beta 3-adrenoceptors agonists selectively inhibited nonadrenergic noncholinergic contractions of guinea-pig bronchi induced by electrical field stimulation or capsaicin. The presence of functional beta 3-adrenoceptors in the bronchial smooth muscle is disputed and seems to be species-related. In isolated canine bronchi, selective agonists induced a relaxation whereas they had no or slight effect in isolated human, guinea-pig and sheep bronchi. Likewise in man, a fall in airway resistance measured by plethysmography, was mediated by beta 2-adrenoceptors, but not beta 3-adrenoceptors. To conclude, an "atypical" or beta 3-adrenoceptor-mediated modulation of bronchomotricity exists, nevertheless strong species specific differences have been reported.

Pertussis toxin-sensitive airway beta-adrenergic dysfunction by somatostatin

To elucidate the effect of somatostatin and its mechanism of action on airway beta-adrenergic function, we studied canine bronchial smooth muscle under isometric conditions in vitro. Somatostatin (10(-6) M) inhibited the salbutamol-induced relaxation, so that the salbutamol concentration-response curves were displaced to higher concentrations (P < 0.01). This inhibition was dose dependent, the concentration of somatostatin required to produce a half-maximal effect being 10(-8) M. The relaxant responses to forskolin were likewise inhibited by somatostatin, but those to dibutyryl 3',5'--adenosine cyclic monophosphate (DB-cAMP), verapamil and nitroprusside were not. Somatostatin inhibited the salbutamol-induced accumulation of intracellular cAMP. These effects were abolished by the somatostatin antagonist cyclo [7-aminoheptanoyl-Phe-D-Trp-Lys-Thr (Bz)] or pertussis toxin. These observations suggest that somatostatin down-regulates beta-adrenergic function of airway smooth muscle through activation of an inhibitory guanine nucleotide (GTP)-binding regulatory protein, Gi, coupled to adenylate cyclase.