Intracoronary injections of salbutamol demonstrate the presence of functional beta 2-adrenoceptors in the human heart

Beta 1- and beta 2-adrenoceptor-mediated relaxation in human internal mammary artery and saphenous vein: unchanged beta- and alpha-adrenoceptor responsiveness after chronic beta 1-adrenoceptor blockade

1. We have recently reported that patients taking beta 1-adrenoceptor-selective antagonists exhibit marked sensitization of beta 2-adrenoceptor responses but unaltered beta 1-adrenoceptor responses in the heart, both in vitro and in vivo. We therefore investigated beta 1- and beta 2-adrenoceptor-mediated relaxant responses in rings of human internal mammary artery and saphenous vein without endothelium, taken from beta 1-blocked and non-beta-blocked patients undergoing coronary artery bypass graft surgery, for comparison. We also examined alpha 1-adrenoceptor-mediated contraction in these vessels, to determine whether beta 1-blockade had any cross-regulatory effect. 2. Following alpha-blockade with 10 microM phenoxybenzamine, both noradrenaline adrenaline produced concentration-dependent relaxations in both blood vessels, their effects being mediated predominantly through beta 2-adrenoceptors; a lesser beta 1-adrenoceptor component to relaxation was also found in internal mammary artery and a minor beta 1-adrenoceptor component was present in saphenous vein. No differences were found in beta 1- or in beta 2-adrenoceptor-mediated vasorelaxation between beta 1-blocked and non-beta-blocked patients. 3. Methoxamine produced concentration-dependent contractions in both blood vessels, and the potency and efficacy were not significantly different between vessels from beta 1-blocked and from non-beta-blocked patients. 4. These findings indicate that, in these tissues, which possess a relatively minor beta 1-adrenoceptor component in contrast to myocardial tissue, chronic beta 1-blocker treatment does not alter either beta 1- or beta 2-adrenoceptor responses. Likewise, in such tissues, alpha 1-adrenoceptor responses are unaffected by prior beta 1-blockade.

Single dosing comparison of the relative cardiac beta 1/beta 2 activity of inhaled fenoterol and salbutamol in normal subjects

BACKGROUND

The aim of the present study was to compare the dose related effects of fenoterol and salbutamol on cardiac beta 1 and beta 2 receptors using the beta 1 selective antagonist atenolol, in order to dissect out relative beta 1/beta 2 mediated responses.

METHODS

Fourteen normal volunteers were randomised to receive pretreatment with either atenolol 25 mg or placebo, followed by inhaled fenoterol or salbutamol in equal doses by weight (cumulative doses of 1 mg and 4 mg). Measurements were made 30 minutes after inhaling each dose of beta 2 agonist. Values (mean and 95% CI) were expressed as a change from baseline.

RESULTS

At 4 mg fenoterol produced equivalent falls in serum potassium and increases in tremor to salbutamol. The mean (95% CI) increase in heart rate (beats/min) with fenoterol at 4 mg after placebo was 47 (41-53) and after atenolol was 34 (28-40), with values for salbutamol being 46 (40-52) after placebo and 30 (24-36) after atenolol. The inotropic response (stroke distance) after atenolol at the 4 mg dose was 5.0 (3.9-6.1) cm for fenoterol and 4.7 (3.5-5.9) cm for salbutamol. There were no significant differences in heart rate or stroke distance response between the two drugs after either placebo or atenolol. Furthermore, ECG effects (Q-Tc and T wave) of fenoterol and salbutamol were comparable at both doses.

CONCLUSIONS

These results show that there is no difference in the respective chronotropic or inotropic activities of fenoterol and salbutamol on cardiac beta 1 or beta 2 receptors when given at higher than conventional doses.

Beta adrenoreceptor subtype cross regulation in the human heart

OBJECTIVES

To find out in a prospective study whether beta 1 blocker treatment causes selective beta 2 adrenoreceptor sensitisation, and to find whether such sensitisation is confined to the heart.

DESIGN

A placebo controlled cross over study of two weeks of selective beta 1 blocker treatment with 10 mg of bisoprolol daily.

SUBJECTS

Six healthy volunteers.

OUTCOME MEASURES

Three days after stopping the 10 mg of bisoprolol or placebo, subjects underwent treadmill exercise (to measure cardiac beta 1 receptor responsiveness) and were given salbutamol injections (to measure cardiac beta 2 receptor responsiveness). Secondary end points were the responses of serum potassium, glucose, and insulin to beta 2 stimulation.

RESULTS

There was no difference in exercise induced increases in heart rate, but after treatment with bisoprolol the dose of salbutamol required to increase heart rate by 40 beats/min was 1.9 micrograms/kg compared with 2.9 micrograms/kg after placebo (p < 0.005). The fall in diastolic blood pressure was not significantly different on the two occasions. Hypokalaemia induced by salbutamol, but not hyperglycaemia or hyperinsulinaemia, was enhanced after bisoprolol.

CONCLUSION

This study shows that treatment with a beta 1 blocker in vivo leads to sensitisation of cardiac beta 2 adrenoreceptors but not cardiac beta 1 adrenoreceptors or vascular beta 2 receptors. This previously unrecognised form of receptor cross sensitisation in the heart may noticeably diminish the efficacy of selective beta 1 blockade in preventing arrhythmias in patients with ischaemic heart disease. These findings reopen the question of which type of beta blocker is more appropriate for such patients.

Selectivity of antagonist and partial agonist activity of celiprolol in normal subjects

1. The aims of this study were to assess the relative beta 1/beta 2 selectivity of the antagonist and partial agonist activity (PAA) of celiprolol in man. 2. Eight normal males received single oral doses of celiprolol 200 mg (C200), 400 mg (C400) and 800 mg (C800); atenolol 50 mg (A50), 100 mg (A100) and 200 mg (A200); nadolol 40 mg (N40) and placebo (PL), administered in a single-blind, randomised crossover design. 3. At rest, in the presence of low levels of circulating adrenaline and noradrenergic tone, a low dose of celiprolol (C200) showed evidence of beta 1-PAA by significant increases in systolic blood pressure and resting heart rate. At higher doses (C400, C800), beta 2-PAA became evident by a significant increase in postural finger tremor, whereas C200 had no effect. 4. In the presence of a beta 1-adrenoceptor agonist, as assessed by reduction of exercise tachycardia, increasing doses of celiprolol produced significantly less beta 1-adrenoceptor blockade compared with atenolol. Furthermore, there was no increase in beta 1-adrenoceptor blockade beyond C400. 5. In the presence of a beta 2-adrenoceptor agonist, as assessed by blunting of terbutaline-induced chronotropic, hypokalaemic and finger tremor responses, celiprolol exhibited less beta 2-adrenoceptor blockade than comparable doses of atenolol used in clinical practice. 6. Exercise hyperkalaemia was blunted significantly by C400 and C800 in comparison with all doses of atenolol and nadolol.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathetic nerve stimulation activates both beta 1- and beta 2-adrenoceptors of SA and AV nodes in anesthetized dog hearts

We investigated blocking effects of the selective beta 1-adrenoceptor blocker atenolol (0.1-100 micrograms/kg, i.v.), the selective beta 2-adrenoceptor blocker ICI 118,551 (1-1000 micrograms/kg, i.v.) and the combination of the two drugs on positive chronotropic and dromotropic responses to norepinephrine (NE) released by stimulation of the sympathetic nerves in anesthetized, neurally decentralized, open-chest dogs after atropine was given. Stimulation of the intracardiac sympathetic nerves to the SA nodal region or to the AV nodal region selectively increased heart rate or decreased AV conduction time, respectively. ICI 118,551 inhibited the chronotropic or dromotropic response to each stimulation in a dose-dependent manner, but its inhibition of the dromotropic response was less than that of the chronotropic response. Atenolol similarly inhibited either the positive chronotropic or dromotropic response to each stimulation in a dose-related manner. The combination of atenolol and ICI 118,551 attenuated the responses to each stimulation more than atenolol alone. These data indicate that sympathetic nerve stimulation activates both beta 1- and beta 2-adrenoceptors of the SA and AV nodes and that the proportion of beta 2-adrenoceptor-mediated effects on the AV node is less than that on the SA node. These results suggest that neurally released NE in part controls physiological functional cardiac responses mediated through beta 2-adrenoceptors, in addition to the responses predominantly mediated through beta 1-adrenoceptors.

Metabolic effects of beta 2-agonists

Catecholamines produce a number of biochemical changes most of which result from stimulation of beta 2-receptors. Interest in these metabolic effects has increased recently as a consequence of the concern over the relatively high mortality from acute asthma attacks. In this review the data on the impact of beta 2-agonists on glucose production, insulin release and lipolysis are presented. Thereafter the subject of hypokalaemia, the mechanism for its production by beta 2-agonists and its relevance to cardiac arrhythmias are considered in detail. Finally the fall in plasma magnesium and the possible role of beta 2-agonists in the production of lactic acidosis are discussed.

Cardiac electrical responses to catecholamines are differentially mediated by beta 2-adrenoceptors in anesthetized dogs

We investigated the beta 2-adrenoceptor-mediated effects of atrial and ventricular effective refractory period (ERP), SA node pacemaker activity, and AV conductivity induced by sympathetic nerve stimulation or epinephrine infusion in anesthetized dogs. A beta 2-adrenoceptor antagonist, ICI 118,551 up to 100 micrograms/kg, i.v., inhibited the positive chronotropic and dromotropic responses to sympathetic stimulation but did not shorten the atrial or ventricular ERP, ICI 118,551 also attenuated the positive chronotropic and dromotropic responses and the shortening of atrial ERP in response to epinephrine but not the shortening of ventricular ERP. A selective beta 1-adrenoceptor antagonist, atenolol, inhibited each electrical cardiac response to sympathetic stimulation and epinephrine infusion in a similar manner. These results suggest that beta 2-adrenoceptor-mediated electrical cardiac responses to endogenous catecholamines also exist in addition to the predominant beta 1-adrenoceptor-mediated responses, and that the order of the proportion of beta 2-adrenoceptor-mediated cardiac effects was SA node pacemaker activity much greater than AV conductivity = atrial ERP much greater than ventricular ERP in the dog heart.

The effects of enalapril and spironolactone on terbutaline-induced hypokalemia

OBJECTIVE

To investigate whether enalapril (E) 10 mg and spironolactone (S) 100 mg attenuate the hypokalemic effect of inhaled terbutaline (T).

DESIGN

Randomized single-blind crossover. Subjects received the following treatment combinations: (a) placebo (P), (b) T alone, (c) T + E, or (d) T + S.

SETTING

University Department of Clinical Pharmacology.

PARTICIPANTS

Twenty healthy volunteers (ten male, ten female) of mean age 22.8 +/- 3.1 years.

MAIN OUTCOME MEASURES

Serum potassium, magnesium, ECG changes (R-R interval, T wave, and QTc interval) for 4 h after terbutaline inhalation.

MAIN RESULTS

Baseline serum potassium levels were higher following prior treatment with E and S; P, 3.78 mmol/L (3.67 to 3.88); T + E, 3.93 mmol-1 (3.82 to 4.03); and T + S, 4.03 mmol/L (3.93 to 4.14) (p less than 0.05). Mean potassium concentrations over 4 h were also higher following prior treatment with E and S; T, 3.58 mmol/L (3.54 to 3.63); T + E, 3.68 mmol/L (3.64 to 3.72) (p less than 0.05); and T + S, 3.73 mmol/L (3.68 to 3.78) (p less than 0.01).

CONCLUSIONS

Enalapril and spironolactone protect against the fall in serum potassium over 4 h by elevating baseline potassium concentration. These potassium-sparing drugs, however, should not be used to prevent the hypokalemic and electrocardiographic sequelae of inhaled beta 2-agonists.

Evaluation of in vivo partial beta 1/beta 2-agonist activity: a dose-ranging study with carteolol

1. The aims of this study were to investigate the partial agonist profile of carteolol and evaluate methodology for differentiating relative beta 1 and beta 2 partial agonist activity (PAA) in vivo. 2. Eight normal subjects received single oral doses of carteolol 10 mg, 30 mg and 60 mg; nadolol 40 mg; pindolol 30 mg and placebo, given in a single-blind, randomised crossover design. 3. beta 1-PAA was demonstrated with carteolol by dose-related increases in resting heart rate and systolic blood pressure, and a plateau in the dose-response curve for attenuation of exercise tachycardia. beta 2-PAA with carteolol was evidenced by a dose-related increase in resting finger tremor and progressive attenuation of exercise-induced hyperkalaemia. beta 2-adrenoceptor antagonism was shown by attenuation of terbutaline induced hypokalaemic, chronotropic and finger tremor responses. 4. Carteolol behaved as a non-selective beta-adrenoceptor antagonist with both beta 1 and beta 2-PAA components. In the standard clinical dose range of 10-30 mg, its in vivo PAA effects were relatively beta 1-selective. Thus at low doses, there appeared to be a dissociation between selectivity of antagonist and partial agonist activity. 5. Attenuation of exercise hyperkalaemia appears to be a novel and sensitive method for the evaluation of beta 2-PAA in vivo.

Dose-response study with high-dose inhaled salmeterol in healthy subjects

A double-blind, placebo-controlled, cross-over study in 10 healthy male subjects has been carried out to investigate the non-pulmonary effects of single inhaled doses of salmeterol 100 micrograms, 200 micrograms and 400 micrograms and salbutamol 400 micrograms from a metered-dose inhaler. At all doses tested, salmeterol produced statistically significant changes in pulse rate, tremor, blood glucose and plasma potassium concentrations, compared with placebo. All changes were dosed related. A number of dose-related adverse events including tremor, awareness of heart beat and headache were reported after salmeterol administration.

Inhaled beta 2-adrenoceptor agonists in asthma: help or hindrance?

Conventional low doses of inhaled beta 2-adrenoceptor agonists produce effective bronchodilation without systemic effects. Higher doses of inhaled beta 2-adrenoceptor agonists may produce substantial improvements in bronchodilator response, which may be helpful to patients with more severe airway obstruction. At higher than recommended doses, in asthmatic patients, fenoterol appears to cause greater dose-related systemic beta 2-responses compared with salbutamol or terbutaline, although there is no evidence to suggest that fenoterol is any less beta 2-selective in vivo. Furthermore, tolerance develops to systemic but not to bronchodilator effects during chronic treatment with inhaled beta 2-adrenoceptor agonists. The link between asthma mortality and systemic adverse effects of inhaled beta 2-adrenoceptor agonists at present remains unproven. A critical reappraisal of the regular use of inhaled beta 2-adrenoceptor agonists including long-acting drugs is now indicated in the light of their possible adverse effects on disease control. Patients requiring regular use of inhaled beta 2-adrenoceptor agonists should be given additional anti-inflammatory therapy with inhaled corticosteroids.

Risks versus benefits of inhaled beta 2-agonists in the management of asthma

The therapeutic goal for the treatment of asthma should be to suppress bronchial mucosal inflammation with preventive drugs such as inhaled corticosteroids, and to relieve symptoms of wheezing and breathlessness with bronchodilator drugs. The lower recommended doses of inhaled beta 2-agonists produce rapid effective bronchodilatation without systemic adverse effects; higher doses may produce substantial improvements in airway response which may help patients with more severe airflow obstruction. Higher doses of inhaled beta 2-agonists also cause dose-related systemic adverse beta 2 effects including tremor, tachycardia, hypokalaemia and associated electrocardiographic sequelae. In this respect, although fenoterol appears to cause greater extrapulmonary beta 2-mediated adverse effects at higher doses, there is no evidence to suggest that it is any less beta 2-selective. There is also some evidence to suggest that use of regular inhaled beta 2-agonists may cause increased bronchial hyperreactivity and possibly deterioration in disease control. Patients who require such regular use should therefore be given additional anti-inflammatory therapy with inhaled corticosteroids. The recent availability of novel, longer-acting inhaled beta 2-agonists such as salmeterol and formoterol will also make necessary a careful reappraisal of their long term use in patients with asthma.

Some aspects of heart beta adrenoceptor function

Heart rate and force can be increased by noradrenaline and adrenaline through an interaction with both beta 1-adrenoceptors (beta 1AR) and beta 2-adrenoceptors (beta 2 AR). Several ionic currents (I) can flow upon beta AR activation: ICa (through either beta 1AR or beta 2AR), INa, IK, and ICl. Calcium currents (ICa) can be increased directly by the alpha s unit of a GTP binding protein, Gs, or by coupling of Gs to adenylyl cyclase with subsequent formation of cyclic AMP, release of the catalytic unit of cyclic AMP-dependent protein kinase, and phosphorylation of calcium channels and other proteins. Chronic exposure (days or months), but not acute exposure (hours), to a catecholamine downregulates human heart beta 1AR. Acute desensitization partially uncouples human heart beta AR from the adenylyl cyclase. Both acute and chronic desensitization reduce positive inotropic responses to catecholamines. In human heart, catecholamine-induced activation of one beta 2AR causes the production of at least four times more cyclic AMP than activation of one beta 1AR. Chronic treatment of patients with beta 1AR-selective blockers paradoxically induces selective inotropic beta 2AR hyperresponsiveness, presumably by increasing coupling of beta 2AR to Gs. Several partial agonists with high affinity for heart beta 1AR and beta 2AR cause stimulant effects that are resistant to blockade of beta 1AR and beta 2AR. Such nonconventional partial agonists could perhaps interact with beta AR that resemble beta 3 adrenoceptors.

Comparison of hypokalaemic, electrocardiographic and haemodynamic responses to inhaled isoprenaline and salbutamol in young and elderly subjects

The purpose of the study was to compare beta-adrenoceptor responsiveness to salbutamol (beta-2 selective agonist) and isoprenaline (non-selective) in young (n = 10, age 23 y) and elderly (n = 7, age 71 y) subjects. Subjects were given cumulative doubling doses of inhaled isoprenaline or salbutamol (500-4000 micrograms), and placebo, in a single-blind randomised cross-over design. Plasma potassium, electrocardiographic (R-R, T-wave, Q-Tc) and blood pressure responses were measured at baseline and at each dose step. There were no difference between baseline values for each of the three study days within each group of subjects. Hypokalaemia was significantly greater in response to salbutamol compared with isoprenaline in both the young (as change from baseline): -0.61 versus -0.10 mmol.l-1: and in the elderly: -0.68 versus -0.20 mmol.l-1. There were no differences between young and elderly responses. T-wave amplitude fell significantly in response to isoprenaline and salbutamol, although this effect was progressively attenuated with increasing doses of isoprenaline. Maximum T-wave response (change from baseline) was greater with salbutamol than isoprenaline in the young: -0.22 versus -0.11 mV: and in the elderly: -0.17 versus -0.08 mV, and there were no differences between the two groups. There were no differences between the effects of isoprenaline and salbutamol on Q-Tc prolongation or heart rate. Chronotropic responses to salbutamol were greater in the elderly: 39 versus 24 beats.min-1. There were larger increases in SBP with isoprenaline in both groups. Falls in DBP in response to isoprenaline and salbutamol were significantly greater in the elderly.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypokalemic and ECG sequelae of combined beta-agonist/diuretic therapy. Protection by conventional doses of spironolactone but not triamterene

Salbutamol (Albuterol) and diuretics are commonly prescribed together in patients with airflow obstruction and are associated with electrocardiographic effects. We have now investigated whether the use of potassium-sparing drugs might prevent the ECG sequelae of such combined therapy. Ten healthy subjects received seven days of randomized treatments with: placebo, bendrofluazide (5 mg), bendrofluazide plus triamterene 50 mg (conventional dose), or triamterene 200 mg (high dose), and bendrofluazide plus spironolactone (100 mg). Potassium and ECG responses to inhaled salbutamol, 2 mg, were measured after each treatment period. The T-wave flattening in response to bendrofluazide and salbutamol (0.24[CI, 0.19 to 0.29]mV) was attenuated by the addition of triamterene, 200 mg (0.33[CI, 0.28 to 0.37]mV; p less than 0.05) and spironolactone 100 mg (0.42[CI, 0.37 to 0.47]mV; p less than 0.01), but not by triamterene 50 mg (0.25[CI, 0.20 to 0.30]mV). Spironolactone and high dose triamterene also diminished the frequency of U waves and ST depression. The ECG effects mirrored hypokalemic responses which were also blunted by high dose (p less than 0.01) but not low dose triamterene, as well as by spironolactone (p less than 0.001). Thus, the use of high dose triamterene and spironolactone protected against the hypokalemic and ECG sequelae of combined beta-agonist/diuretic therapy, whereas a conventional dose of triamterene had no effect. These findings may be important in the prevention of a potentially dangerous interaction in susceptible patients taking this combination of drugs.