Comparison of the chronotropic effect and the cyclic AMP accumulation induced by beta 2-agonists in rat heart cell culture

Effects of β2-adrenergic agonists on house dust mite-induced adhesion, superoxide anion generation, and degranulation of human eosinophils

Background

Even in subjects who are not sensitized to house dust mite (HDM), allergic symptoms can be clinically aggravated by exposure to dust. We previously reported that Dermatophagoides farinae (Df), an important HDM, or Der f 1, a major allergen of Df, induced the effector functions of eosinophils, which may be an important mechanism for HDM-induced symptoms in nonsensitized patients. In a clinical setting, β2-adrenergic agonists, such as salbutamol and formoterol, are used for the treatment of asthma attacks or exacerbation to release the airway obstruction. Several reports have suggested that some β2-adrenergic agonists have an anti-inflammatory capacity.

Objective

In this study, we investigated whether β2-adrenergic agonist could modify the Df- or Der f 1-induced activation of eosinophils.

Methods

Blood eosinophils obtained from healthy donors were preincubated with either formoterol (1 μM), salbutamol (1 μM), or buffer control and then stimulated with Df extract (1 μg/mL) or Der f 1 (100 pg/mL). Eosinophil adhesion to intercellular adhesion molecule (ICAM)-1 was measured using eosinophil peroxidase assays. Generation of superoxide anion (O₂ ^-) was examined based on the superoxide dismutase-inhibitable reduction of cytochrome C. Eosinophil-derived neurotoxin (EDN) concentrations in cell media were measured by enzyme-linked immunosorbent assay.

Results

Formoterol, but not salbutamol, suppressed the Df- or Der f 1-induced eosinophil adhesion to ICAM-1. Furthermore, formoterol, but not salbutamol, suppressed Df-induced O₂ ^- generation or EDN release. Neither formoterol nor salbutamol suppressed spontaneous eosinophil adhesion, O₂ ^- generation, or EDN release.

Conclusion

These findings suggested that formoterol, but not salbutamol, suppressed Df- or Der f 1-induced eosinophil activation when used at the same concentration. Therefore, formoterol could potentially be used for the treatment of bronchial asthma via both bronchodilation and anti-inflammatory effect.

Effect of beta2-adrenergic agonists on eosinophil adhesion, superoxide anion generation, and degranulation

BACKGROUND

Eosinophils play important roles in the development of asthma exacerbation. Viral infection is a major cause of asthma exacerbation, and the expression of IFN-γ-inducible protein of 10 kDa (IP-10) and cysteinyl leukotrienes (cysLTs) is up-regulated in virus-induced asthma. As β2-adrenergic agonists, such as formoterol or salbutamol, are used to treat asthma exacerbation, we examined whether formoterol or salbutamol could modify eosinophil functions such as adhesiveness, particularly those activated by cysLTs or IP-10.

METHODS

Eosinophils were isolated from the blood of healthy subjects and were pre-incubated with either formoterol or salbutamol, and subsequently stimulated with IL-5, LTD4, or IP-10. Adhesion of eosinophils to intercellular cell adhesion molecule (ICAM)-1 was measured using eosinophil peroxidase assays. The generation of eosinophil superoxide anion (O2(-)) was examined based on the superoxide dismutase-inhibitable reduction of cytochrome C. Eosinophil-derived neurotoxin (EDN) release was evaluated by ELISA as a marker of degranulation.

RESULTS

Neither formoterol nor salbutamol suppressed the spontaneous adhesion of eosinophils to ICAM-1. However, when eosinophils were activated by IL-5, LTD4, or IP-10, formoterol, but not salbutamol, suppressed the adhesion to ICAM-1. Formoterol also suppressed IL-5, LTD4, or IP-10 induced eosinophil O2(-) generation or EDN release.

CONCLUSIONS

These findings suggest that formoterol, but not salbutamol, suppresses eosinophil functions enhanced by IL-5, LTD4, or IP-10. As these factors are involved in the development of asthma exacerbation, our results strongly support the hypothesis that administration of formoterol is a novel strategy for treating asthma exacerbation.

PDE2 activity differs in right and left rat ventricular myocardium and differentially regulates β2 adrenoceptor-mediated effects

The important regulator of cardiac function, cAMP, is hydrolyzed by different cyclic nucleotide phosphodiesterases (PDEs), whose expression and activity are not uniform throughout the heart. Of these enzymes, PDE2 shapes β1 adrenoceptor-dependent cardiac cAMP signaling, both in the right and left ventricular myocardium, but its role in regulating β2 adrenoceptor-mediated responses is less well known. Our aim was to investigate possible differences in PDE2 transcription and activity between right (RV) and left (LV) rat ventricular myocardium, as well as its role in regulating β2 adrenoceptor effects. The free walls of the RV and the LV were obtained from Sprague-Dawley rat hearts. Relative mRNA for PDE2 (quantified by qPCR) and PDE2 activity (evaluated by a colorimetric procedure and using the PDE2 inhibitor EHNA) were determined in RV and LV. Also, β2 adrenoceptor-mediated effects (β2-adrenoceptor agonist salbutamol + β1 adrenoceptor antagonist CGP-20712A) on contractility and cAMP concentrations, in the absence or presence of EHNA, were studied in the RV and LV. PDE2 transcript levels were less abundant in RV than in LV and the contribution of PDE2 to the total PDE activity was around 25% lower in the microsomal fraction of the RV compared with the LV. β2 adrenoceptor activation increased inotropy and cAMP levels in the LV when measured in the presence of EHNA, but no such effects were observed in the RV, either in the presence or absence of EHNA. These results indicate interventricular differences in PDE2 transcript and activity levels, which may distinctly regulate β2 adrenoceptor-mediated contractility and cAMP concentrations in the RV and in the LV of the rat heart.

Pathophysiological relevance of the cardiac β2-adrenergic receptor and its potential as a therapeutic target to improve cardiac function

β-adrenoceptors are members of the G protein-coupled receptor superfamily which play a key role in the regulation of myocardial function. Their activation increases cardiac performance but can also induce deleterious effects such as cardiac arrhythmias or myocardial apoptosis. In fact, inhibition of β-adrenoceptors exerts a protective effect in patients with sympathetic over-stimulation during heart failure. Although β(2)-adrenoceptor is not the predominant subtype in the heart, it seems to importantly contribute to the cardiac effects of adrenergic stimulation; however, the mechanism by which this occurs is not fully understood. This review summarizes the current knowledge on the role of β(2)-adrenoceptors in the regulation of cardiac contractility, metabolism, cardiomyocyte survival and cardiac arrhythmias. In addition, therapeutic considerations relating to stimulation of the β(2)-adrenoceptor such as an increase in cardiac contractility with low arrythmogenic effect, protection of the myocardium again apoptosis or positive regulation of heart metabolism are discussed.

In vivo and in vitro assessment of cardiac beta-adrenergic receptors in larval zebrafish (Danio rerio)

β-Adrenergic receptors (βARs) are crucial for maintaining the rate and force of cardiac muscle contraction in vertebrates. Zebrafish (Danio rerio) have one β1AR gene and two β2AR genes (β2aAR and β2bAR). We examined the roles of these receptors in larval zebrafish in vivo by assessing the impact of translational gene knockdown on cardiac function. Zebrafish larvae lacking β1AR expression by morpholino knockdown displayed lower heart rates than control fish, whereas larvae deficient in both β2aAR and β2bAR expression exhibited significantly higher heart rates than controls. These results suggested a potential inhibitory role for one or both β2AR genes. By using cultured HEK293 cells transfected with zebrafish βARs, we demonstrated that stimulation with adrenaline or procaterol (a β2AR agonist) resulted in an increase in intracellular cAMP levels in cells expressing any of the three zebrafish βARs. In comparison with its human βAR counterpart, zebrafish β2aAR expressed in HEK293 cells appeared to exhibit a unique binding affinity profile for adrenergic ligands. Specifically, zebrafish β2aAR had a high binding affinity for phenylephrine, a classical α-adrenergic receptor agonist. The zebrafish receptors also had distinct ligand binding affinities for adrenergic agonists when compared with human βARs in culture, with zebrafish β2aAR being distinct from human β2AR and zebrafish β2bAR. Overall, this study provides insight into the function and evolution of both fish and mammalian β-adrenergic receptors.

The increase in rat ventricular automaticity induced by salbutamol is mediated through β(1)- but not β(2)-adrenoceptors: role of phosphodiesterases

AIMS

While β(2)-adrenoceptor (AR) agonists are useful bronchodilators, they also produce cardiac arrhythmias. These agents are not fully selective and also activate β(1)-AR, but the involvement of β(1)-AR and β(2)-AR in the observed pro-arrhythmic effect has not been established. We studied the effect of β(1)-AR and β(2)-AR activation on ventricular automaticity and the role of phosphodiesterases (PDE) in regulating this effect.

MAIN METHODS

Experiments were performed in the spontaneously beating isolated right ventricle of the rat heart. We also measured cAMP production in this tissue.

KEY FINDINGS

The β(2)-AR agonist salbutamol (1-100 μM) produced a concentration-dependent increase in ventricular automaticity that was not affected by 50nM of the β(2)-AR antagonist ICI 118551. This effect was enhanced by the non-selective PDE inhibitor theophylline (100 μM) and by the selective PDE4 inhibitors rolipram (1 μM) and Ro 201724 (2 μM), but not modified by the selective PDE3 inhibitors cilostamide (0.3 μM) or milrinone (0.2 μM). The effects of salbutamol alone and in the presence of either theophylline or rolipram were virtually abolished by 0.1 μM β(1)-AR antagonist CGP 20712A. Salbutamol (10 μM) increased the cAMP concentration, and this effect was abolished by CGP 20712A (0.1 μM) but enhanced by theophylline (100 μM) or rolipram (1 μM). Cilostamide (0.3 μM) failed to modify the effect of salbutamol on cAMP concentration.

SIGNIFICANCE

These results indicate that the increase of ventricular automaticity elicited by salbutamol was exclusively mediated through β(1)-AR and enhanced by non-selective PDE inhibition with theophylline or selective PDE4 inhibition. However, PDE3 did not appear to regulate this effect.

Phosphodiesterases inhibition unmask a positive inotropic effect mediated by beta2-adrenoceptors in rat ventricular myocardium

The effects of salbutamol on contractility and cAMP levels were investigated in rat right ventricular myocardium. Salbutamol (1-300 microM), produced a concentration-dependent positive inotropic effect which was not affected by ICI 118551 (50 nM), a beta2-adrenoceptor antagonist but was abolished by CGP 20712A (1 microM) a beta1-adrenoceptor antagonist. However, in rats pretreated with pertussis toxin (30 microg/kg intraperitoneal injection) salbutamol increases contractility (Emax = 9.8 +/- 1.8%, - log EC50 = 6.25 +/- 0.07, n = 5). The combination of salbutamol + CGP 20712A, also produces a concentration-dependent enhancement of contractility (Emax = 43.0 +/- 7.5%, - log EC50 = 6.3 +/- 0.04, n = 6), in the presence of 30 microM of the non selective phosphodiesterase (PDE) inhibitor 3-isobutylmethylxantine (IBMX) which was prevented by ICI 118551 (50 nM). Also, salbutamol + CGP 20712A fail to increase cAMP tissue levels but enhance them in the presence of IBMX. This effect was also prevented by ICI 118551. These results indicate that PDEs blunt contractility and cAMP production mediated by beta2-adrenoceptors in rat ventricular myocardium. Gi protein, although less efficiently than PDEs, also limits inotropic effects of salbutamol mediated by beta2-adrenoceptors in this tissue.

Effect of orthovanadate on platelet aggregation induced by platelet-activating factor

Orthovanadate (vanadate) inhibited the platelet aggregation induced by platelet-activating factor (PAF) in a dose-dependent manner. Propranolol, a nonspecific beta-adrenergic receptor antagonist, and H-8, a selective inhibitor of cAMP-dependent protein kinase (PKA), suppressed the inhibition of the PAF-induced platelet aggregation by vanadate. Vanadate increased the cAMP content in platelets accompanied by the activation of PKA. The beta-adrenergic receptors of platelets have been reported to be abundant in the beta(2) isoform, coupled to adenylyl cyclases (R. Kerry and M. C. Scrutton, Br. J. Pharmacol., 79, 681-691 (1983)). When the washed platelets were preincubated with vanadate, salbutamol, a selective beta(2)-adrenergic receptor agonist, or 8-Br-cAMP, the latter two mimicked the vanadate-induced anti-platelet aggregation and prolongation of clotting time of plasma, suggesting involvement of the increased intracellular cAMP content in both actions of vanadate. Butoxamine, a selective beta(2)-adrenergic receptor antagonist, suppressed both actions of vanadate. The vanadate-induced increase in cAMP content was inhibited in part by butoxamine or genistein. These results suggest that vanadate inhibits the PAF-induced platelet aggregation by the stimulation of a cAMP/PKA-dependent process via the beta(2)-adrenergic receptor and receptor tyrosine kinases, and that the anti-platelet aggregation is involved in part in mechanisms of the anticoagulant action of vanadate.

Do beta 2-adrenergic receptors modulate Ca2+ in adult rat ventricular myocytes?

We examined the role of beta 2-adrenergic receptors (ARs) in modulating calcium homeostasis in rat ventricular myocytes. Zinterol (10 microM), an agonist with a 25-fold greater affinity for beta 2-ARs over beta 1-ARs, modestly enhanced L-type calcium current (ICa) magnitude by approximately 30% and modestly accelerated the rate of Ca2+ concentration ([Ca2+]) decline (approximately 35%) but had little effect on the magnitude of the [Ca2+] transient (a nonsignificant 6% increase). However, 1 microM of the highly selective beta 1-AR antagonist CGP-20712A completely blocked the ICa increase induced by 10 microM zinterol. Pretreatment of cells with pertussis toxin (PTX) did not alter ICa enhancement by 10 microM zinterol, although it did abolish the ability of acetylcholine to block the forskolin-induced enhancement of ICa. Zinterol (10 microM) approximately doubled adenosine 3',5'-cyclic monophosphate (cAMP) accumulation, although one-half of this increase was blocked by CGP-20712A. In contrast, 1 microM of the nonselective beta-agonist isoproterenol increased cAMP production 15-fold. Thus we found no evidence that activation of beta 2-ARs modulates calcium homeostasis in rat ventricular myocytes, even after treatment with PTX.

Role of beta1- and beta2-adrenergic receptors in regulation of Cl- and Ca2+ channels in guinea pig ventricular myocytes

The role of beta1- and beta2-adrenergic receptor stimulation in modulating adenosine 3',5'-cyclic monophosphate (cAMP)-regulated Cl- and Ca2+ currents was investigated with use of guinea pig ventricular myocytes. Activation of the Cl- current by the nonselective beta-receptor agonist isoproterenol (Iso) was not affected by the beta2-receptor antagonist ICI-118,551 (ICI), but it was blocked by the beta1-receptor antagonist atenolol. The inability of beta2-receptor stimulation to activate the Cl- current was confirmed by the lack of response to the selective beta2-receptor agonists salbutamol and zinterol. Responses to beta2-adrenergic receptor stimulation were also looked for in pertussis toxin (PTX)-treated myocytes because PTX increases the sensitivity of responses to Iso, and PTX has been reported to increase the responsiveness to beta2- but not beta1-receptor stimulation. PTX treatment increased the sensitivity of the Cl- current to activation by Iso in the presence of ICI, indicating that PTX increases beta1-receptor responsiveness. PTX treatment also resulted in the ability of salbutamol to activate the Cl- current. However, the response to salbutamol was blocked by atenolol but not by appropriate concentrations of ICI, suggesting that salbutamol was activating beta1-receptors. These results indicate that PTX treatment increases the sensitivity to beta1-receptor stimulation, without affecting beta2-responsiveness. To verify that the lack of response to beta2-receptor stimulation was not unique to the Cl- current, the effects of beta2-receptor agonists on the L-type Ca2+ current were also examined. The Ca2+ current was only affected by high concentrations of zinterol or salbutamol, and such responses were blocked by atenolol, but not by ICI, suggesting that activation of beta1-receptors was involved. These results indicate that beta1- but not beta2-adrenergic receptor stimulation plays an important role in modulating the cAMP-regulated Cl- and Ca2+ currents in guinea pig ventricular myocytes.

Evidence for a cooperation between adenosine A2 receptors and beta 1-adrenoceptors on cardiac automaticity in the isolated right ventricle of the rat

1. The effects of the adenosine receptor agonists, 5'-N-ethyl-carboxamidoadenosine (NECA) and 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosin e (CGS-21680) on ventricular automaticity induced by a local injury in the isolated right ventricle of the rat were studied. 2. In concentrations ranging from 0.1 to 100 nM, NECA significantly increased ventricular automaticity. This effect was more reproducible when the adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) was present at 5 nM, a concentration that blocks A1 adenosine receptors. 3. The excitatory effect of NECA was not observed when DPCPX was present at a concentration of 10 microM, which antagonizes both A1 and A2 adenosine receptors, as well as when rats were reserpinized. 4. In reserpinized rats, NECA increased ventricular automaticity in the presence of isoprenaline and the beta 2-adrenoceptor antagonist, ICI-118,551, but not in the presence of the beta 1-adrenoceptor antagonists, bisoprolol or atenolol. 5. The A 2s-selective adenosine receptor agonist, CGS-21680 (0.1 nM-10 microM) was devoid of excitatory effect on ventricular automaticity. Binding studies of this compound to the rat ventricular membranes revealed that in the preparation there was no specific binding. 6. These results suggest that the excitatory effect of NECA on ectopic ventricular automaticity is dependent on endogenous catecholamines and is mediated by an A2 adenosine receptor which is in some way 'linked' to the beta 1-adrenoceptor. These A2 receptors do not appear to be of the A2a adenosine receptor subtype.

Supersensitivity to beta-adrenoceptor stimulation evoked in cultured neonatal rat heart myocytes by L(+)-lactate and pyruvate

1. Cells from the ventricles of newborn rats were cultured for 8 days in flasks attached to a rocker apparatus to ensure an adequate oxygen supply. 2. The rocked cultures, which had previously been found to be low in lactate and subsensitive to the positive chronotropic action of the beta-adrenergic agonist isoprenaline (ISO; EC50 of (+/-)-ISO around 7 x 10(-7) M), became resensitized and even highly supersensitive to the catecholamine upon treatment with 3 mM L(+)-lactate or 1 mM pyruvate. 3. The resulting concentration-response curves were anomalous in that they extended over 8 log units, with a threshold at about 10(-13) M, but with little or no change in the height and the position of the maximum (at 10(-5) M). The EC50 values and 95% confidence intervals were 2.4 (1.9-3.0) and 5.4 (4.9-6.0) x 10(-11) M, respectively, for the lactate- and pyruvate-induced components of the chronotropic response to (+/-)-ISO. 4. The supersensitive portion of the ISO concentration-response curve was abolished by (-)-propranolol (10(-6) M), indicating that it was due to beta-adrenoceptor stimulation. 5. The cultured heart cells had to be incubated with L(+)-lactate or pyruvate for a minimum of 45 min before an increase in sensitivity to ISO became apparent. This latency was not due to a requirement for protein synthesis. 6. The adenosine-3',5'-monophosphate (cAMP) response to ISO was not noticeably altered by lactate, but (+/-)-ISO, which at 10(-8) M had no effect on the activation state of cAMP-dependent protein kinase (PKA), caused a significant increase in the activity of the enzyme following a 2-h exposure of the cells to 3 mM L(+)-lactate. 7. alpha-cyanocinnamate, an inhibitor of transmembrane transport of lactate and pyruvate, severely inhibited the utilization of L-[U-14C] lactate by the cultured cells at a concentration (5 microM) that eliminated the lactate-evoked potentiation of the chronotropic action of ISO without significantly affecting its unpotentiated action. 8. The beat-accelerating action of the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine (IBMX) and the lipophilic N6,2'-O-dibutyryl derivative of cAMP (dbcAMP), both of which are capable of elevating myocardial cAMP levels, was not potentiated by 1 mM pyruvate. 9. The question is raised, whether accumulation of lactate, a biochemical hallmark of anaerobiosis, might be a factor in some of the catecholamine-triggered events occurring in acute myocardial ischaemia and infarction.

Formoterol. A review of its pharmacological properties and therapeutic potential in reversible obstructive airways disease

Formoterol, a long-acting beta 2-selective adrenoceptor agonist, produces dose-proportional bronchodilation in patients with obstructive airways disease with a reversible component. A significant effect occurs within minutes of inhalation of a therapeutic formoterol dose and persists for approximately 12 hours. Oral formoterol has a slower onset of action than the inhaled formulations, but also produces prolonged bronchodilatory effects. Inhaled formoterol has shown a therapeutic efficacy equivalent to or better than comparable dosages of the conventional beta 2-agonists salbutamol, fenoterol and terbutaline in short and long term trials, in both adults and children with asthma. Its prolonged duration of action permits a twice-daily dosage regimen and results in improved control of nocturnal symptoms by reducing the 'morning dip'. Formoterol also compares well with oral slow release theophylline. In addition, significantly more patients with chronic obstructive airways disease (COAD) had an improvement in symptoms when treated with formoterol compared with salbutamol or fenoterol. Noncomparative studies indicate formoterol also provides effective prophylaxis of exercise-induced asthma. Development of tachyphylaxis has not been observed. Formoterol is generally well tolerated. Adverse effects observed represent predictable extensions of its pharmacology. Tremor and palpitations are most frequently reported. The incidence of adverse events is dose-proportional and therefore related to the route of administration, being more frequent following oral than inhalation therapy. The long-acting beta 2-agonists, including formoterol, represent a significant advance over current maintenance or prophylactic bronchodilator therapy with intermediate-acting beta 2-agonists such as salbutamol, fenoterol and terbutaline, predominantly because of the twice daily administration regimen. However, comparisons with other long-acting beta 2-agonists, such as salmeterol, evaluation of its role in improving symptom control in patients failing to respond to prophylactic therapy, and clarification of the optimal role of beta 2-agonists in asthma maintenance therapy are required to fully determine the value of formoterol in the management of obstructive airways disease.

Effect of isoproterenol on lipid metabolism and prostaglandin production in cultures of newborn rat heart cells, under normoxic and hypoxic conditions

Catecholamines are known to exert deleterious effects on heart cells and to provoke biochemical alterations similar to those observed during myocardial infarction. In order to investigate the mechanisms of these effects, we have studied in cultures of muscle (M) and fibroblast-like (F) cells derived from newborn rat hearts, the action of isoproterenol on membrane lipid metabolism and on prostaglandin production. We showed in F cells that beta-agonist stimulation produced a striking loss of membrane phospholipids and a moderate hydrolysis of cell triglycerides. In addition, isoproterenol treatment induced a significant stimulation of the secretion of prostacyclin but not of prostaglandin E2 by F cells. None of these effects were potentiated by oxygen deprivation. In contrast, M cells, which are sensitive to ischemia, failed to respond to isoproterenol treatment. These results suggest that catecholamines and hypoxia may exert combined deleterious effects on heart tissue by acting separately on the different target cells in vivo.

Increased beta 2-adrenoreceptor density in heart, kidney and lung of spontaneously hypertensive rats

The development of beta-adrenoreceptor density and beta 1- and beta 2-adrenoreceptor distribution has been investigated in heart, kidney and lung of spontaneously hypertensive (SHR) and normotensive (WKY) rats by (-)125I-iodocyanopinolol binding at the age of 5-6, 9-10 and 19-21 weeks. At all ages beta-adrenoreceptor density was similar in hearts of both strains, while it was increased in kidney and lung of SHR compared to WKY. The beta 2-adrenoreceptor density was higher in all three tissues of SHR at all ages investigated. On the other hand, beta 1-adrenoreceptor density was decreased in heart, unchanged in lung and increased in kidney of SHR compared to WKY. Destruction of presynaptic nerve terminals by treatment of WKY with 6-hydroxydopamine produced a 24% loss of cardiac beta 2-adrenoreceptors, whereas the beta 1-adrenoreceptor density remained unchanged, suggesting that at least part of the cardiac beta 2-adrenoreceptor population is localized prejunctionally. It is suggested that beta 2-adrenoreceptors are involved in the development or maintenance of high blood pressure in SHR, possibly by facilitating noradrenaline release.

Beta-adrenoceptor antagonists (non-selective as well as beta 1-selective) with partial agonistic activity decrease beta 2-adrenoceptor density in human lymphocytes. Evidence for a beta 2-agonistic component of the partial agonistic activity

In the present study the effects of pindolol [non-selective beta-adrenoceptor antagonist with strong partial agonistic activity (PAA)] on beta 2-adrenoceptor density in lymphocytes (assessed by (-)-[125I]iodocyanopindolol (ICYP) binding) were compared with those of the beta 1-selective antagonists celiprolol (with PAA) and bisoprolol (no PAA) in normotensive young volunteers to get further insights into the nature of PAA. Administration of pindolol (2 X 5 mg/day) caused an about 25% decrease in lymphocyte beta 2-adrenoceptor density after 2 days; during treatment beta 2-adrenoceptor density declined further (maximum decrease after 7 days: 50%). After withdrawal of pindolol lymphocyte beta 2-adrenoceptor density recovered very slowly being still after 4 days significantly reduced, although no pindolol was detectable in plasma after 36 h. The KD-values for ICYP, however, did not change during or after pindolol treatment. The decrease in lymphocyte beta 2-adrenoceptor density induced by pindolol could be completely prevented by simultaneous administration of propranolol (3 X 40 mg/day) indicating that the PAA of pindolol is the cause of its beta-adrenoceptor decreasing effect. Administration of the non-selective beta-adrenoceptor antagonist bopindolol (1 X 2 mg/day) with PAA caused decreases in lymphocyte beta 2-adrenoceptor density (maximum decrease after 7 days: 40%); concomitantly the 10 mumol/l (-)-isoprenaline evoked increases in the intracellular level of lymphocyte cyclic AMP were attenuated to a similar extent indicating that the beta-adrenoceptor antagonist-induced decrease in beta-adrenoceptor density is accompanied by a loss in beta-adrenoceptor function.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta 1- and beta 2-adrenoceptor binding and functional response in right and left atria of rat heart

The properties of beta 1- and beta 2-adrenoceptors in right and left atria of rat heart, and their roles in mediating chronotropic and inotropic responses to beta-adrenoceptor agonists were examined. [125I](-)pindolol (125IPIN) bound saturably and specifically to a single class of high affinity sites in homogenates of both right and left atria. The k1's for association in right and left atria were 6.5 X 10(9) l/mol-min and 2.3 X 10(9) l/mol-min respectively, while the k-1's for dissociation were 0.20 min-1 and 0.17 min-1. The kinetically determined KD's were 75 pmol/l in right and 30 pmol/l in left atria and were similar to the equilibrium KD's determined from Scatchard analysis of saturation isotherms of specific 125IPIN binding. Inhibition of 125IPIN binding by beta-adrenoceptor antagonists was stereoselective and the order of potency was timolol greater than l-propranolol greater than d-propranolol greater than sotalol. Inhibition by beta 1- and beta 2-adrenoceptor subtype selective antagonists yielded flat displacement curves with low Hill coefficients. Nonlinear regression analysis of displacement by beta 1-selective (practolol, atenolol and metoprolol) and beta 2-selective (ICI 118,551) antagonists gave estimates of the proportion of beta 1- and beta 2-adrenoceptors present in rat atria. Right atria contained 67 +/- 4.2% beta 1- and 33 +/- 4.2% beta 2-adrenoceptors, while left atria contained 67 +/- 2.8% beta 1- and 33 +/- 2.8% beta 2-adrenoceptors. Increases in the rate of spontaneously beating right atria and the force of electrically driven left atria caused by beta-adrenoceptor agonists were also measured. pA2 values for non-subtype selective beta-adrenoceptor antagonists in inhibiting isoprenaline-induced increases in rate and force were highly correlated with KD values determined for specific 125IPIN binding. pA2 values for beta 1- and beta 2-selective antagonists in inhibiting isoprenaline-induced increases in rate and force correlated well with the pKD values of these drugs in binding to beta 1-adrenoceptors, but not with the pKD values in binding to beta 2-adrenoceptors. Dose-response curves for stimulation of both rate and force by the beta 2-selective agonists procaterol and zinterol were shifted to a much greater extent by selective blockade of beta 1-adrenoceptors with metoprolol than by selective blockade of beta 2-adrenoceptors with ICI 118,551, suggesting that these compounds caused their effects by activating beta 1-adrenoceptors.(ABSTRACT TRUNCATED AT 400 WORDS)