Assessment of selective beta-adrenoceptor blockade in man

Haemodynamic and metabolic effects of xamoterol in exercising dogs

The effects of xamoterol on the haemodynamic adaptation to graded treadmill exercise were evaluated during five subsequent cycles in chronically instrumented dogs. At rest xamoterol, 0.2 mg/kg i.v., preferentially showed a positive inotropic effect, whereas 1 mg/kg i.v. also exhibited a marked chronotropic effect. The cardiac output and left ventricular power increased dose dependently. The mean left atrial pressure and total peripheral resistance decreased concomitantly. Xamoterol did not produce a noteworthy decrease in heart rate or positive dp/dtmax during exercise, even at a dosage of 1 mg/kg. A beta-adrenoceptor blocking effect could only be seen from the diminution of the exercise-induced changes in heart rate, dp/dtmax, cardiac output, left ventricular power and total peripheral resistance. Determination of the blood glucose, lactate and pyruvate levels before the start of each exercise cycle revealed that the drug induced a decrease in blood glucose and an increase in blood pyruvate. Thus, xamoterol exerted a dose-dependent sympathomimetic effect in dogs at rest. However, there was little evidence for a beta-adrenoceptor blocking action even at higher work loads, although preliminary experiments in conscious dogs showed that xamoterol shifted the isoprenaline dose-response curve to the right by a factor of 1.31 (0.2 mg/kg) and 3.05 (1 mg/kg).

Effect of CGP 17/582, a selective beta-adrenoceptor antagonist, on the haemodynamic and hypokalaemic response to adrenaline

1. CGP 17/582B is a new beta-adrenoceptor antagonist which on experimental studies appears to combine selective beta 1-adrenoceptor blockade with partial agonist activity (ISA). Assessing beta-adrenoceptor selectivity and the degree of partial agonist activity in vivo can be difficult. 2. In a double-blind placebo controlled crossover study we have compared the effect of oral pretreatment for 7 days with CGP (100 mg twice daily), with propranolol (non-selective beta-adrenoceptor blocker with no ISA) and metoprolol (selective beta-adrenoceptor blocker with no ISA) on resting heart rate and heart rate response to submaximal exercise on a bicycle ergometer to assess the degree of beta-adrenoceptor blockade and also the changes in blood pressure, heart rate and potassium during the intravenous infusion of (-)-adrenaline to determine the degree of beta 2-adrenoceptor blockade. 3. Subjects underwent submaximal exercise testing on the second and fifth day of each treatment period and on the seventh day received a 2 h infusion of (-)-adrenaline (0.06 microgram kg-1 min-1). Heart rate, blood pressure, plasma potassium and catecholamines were measured throughout the study period. 4. All three active treatments significantly reduced exercise induced tachycardia. The (-)-adrenaline infusion significantly reduced plasma noradrenaline levels following propranolol and metoprolol and to a lesser extent with placebo but were unaltered on CGP. Baseline heart rate was unaltered by CGP but was significantly reduced by metoprolol and propranolol. Adrenaline significantly reduced plasma potassium levels following placebo and CGP pretreatment but plasma potassium was unaltered by adrenaline with metoprolol and propranolol pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacologic aspects of intrinsic sympathomimetic activity in beta-blocking drugs

The possession of intrinsic sympathomimetic activity (ISA) by a beta-adrenoceptor blocking drug results in a number of different pharmacologic properties. Most profound are the central hemodynamic effects. A drug with a significant degree of ISA results in less of a decrease in heart rate at rest and cardiac output, and, at least partly because of this, less of a decrease in peripheral blood flow. If prevailing sympathetic tone is low enough (e.g., during sleep) and the degree of ISA is sufficient, an increase in heart rate may be seen from an ISA-possessing drug. If the drug possesses beta 2 ISA, then a peripheral vasodilation action from stimulation of beta 2 vasodilator receptors may also be relevant. If high levels of exercise and full dosages of the drugs are used, a beta-blocking drug with ISA produces less of a decrease in heart rate. In asthmatic subjects, the modest beta-stimulant action on bronchial smooth muscle is not important, as these patients are potentially sensitive to any receptor blockade. Isoprenaline responses are inhibited to a similar degree compared with inhibition of exercise tachycardia, by nonselective drugs with and without ISA, whereas beta 1 selective agents produce much less inhibition of isoprenaline-induced tachycardia. A drug with ISA "down regulates" beta receptors; thus, when the drug is withdrawn there is no post-beta-blocking drug hypersensitivity in contrast to agents without ISA. There is evidence that ISA results in less of a disturbance in certain metabolic processes, particularly lipid metabolism and the metabolism of liver-metabolized drugs.

Immediate central hemodynamic effects of five different beta-adrenoceptor-blocking agents, acebutolol, atenolol, pindolol, practolol, and propranolol, in patients with ischemic heart disease

The hemodynamic effects of acebutolol were studied in six patients with ischemic heart disease. The changes in heart rate, cardiac output, and arterial blood pressure were determined after intravenous administration of six increasing doses of acebutolol to a cumulative dose of 0.64 mg/kg. After the sixth dose of acebutolol, cardiac output and heart rate were reduced 15% and 8%, respectively. Pulmonary artery pressure was increased by 4 mm Hg. Arterial blood pressure was not changed significantly. The effects of graded doses of acebutolol on heart rate and cardiac output were compared with earlier obtained results after atenolol (0.19 mg/kg), pindolol (0.025 mg/kg), practolol (0.64 mg/kg), and propranolol (0.19 mg/kg). The effects of increasing doses of acebutolol and practolol were very similar and significantly different from the effects of the other three drugs in spite having been administered at equipotent doses. The hemodynamic effects of acebutolol support the hypothesis that the hemodynamic response to beta-adrenoceptor antagonist drugs at rest is determined primarily by the degree of intrinsic sympathomimetic activity, whereas beta-1 selectivity does not modify the central hemodynamic response.

Effects of the beta 2-adrenoceptor antagonist ICI 118,551 on exercise tachycardia and isoprenaline-induced beta-adrenoceptor responses in man

ICI 118,551, 5 to 80 mg orally, did not significantly alter resting heart rate or blood pressure. In doses less than 40 mg the reduction in exercise tachycardia was under 10 beats/min. ICI 118,551, 10 to 40 mg, did not appear to reduce the maximum rise in systolic pressure with isoprenaline but did attenuate the changes in diastolic pressure, forearm blood flow and finger tremor. It also attenuated the isoprenaline-induced changes in serum glucose, insulin and potassium. On these observed changes, the effect of ICI 118,551 20 mg was similar to that of 40 mg and of propranolol 10 mg, but greater than that of atenolol 25 mg. An isoprenaline tachycardia was attenuated by all doses of ICI 118,551 studied. After atropine (0.04 mg/kg) ICI 118,551 20 mg still significantly reduced the effects of isoprenaline suggesting that functional beta 2-adrenoceptors may be present in the human heart. In doses less than 40 mg, ICI 118,551 appears to be a selective and competitive antagonist of beta 2-adrenoceptors in man.

Ancillary pharmacologic properties of acebutolol: cardioselectivity, partial agonist activity, and membrane-stabilizing activity

Acebutolol, a new beta-blocking agent, possesses the ancillary pharmacologic properties of cardioselectivity and partial agonist and membrane-stabilizing activities. Compared to propranolol at equipotent doses, acebutolol produces less bronchoconstriction and preserves the bronchodilator response to isoprenaline. Similarly, acebutolol has less of an effect on peripheral vascular hemodynamics than does propranolol. Because of partial agonist activity, acebutolol produces a lesser reduction in heart rate and cardiac output than do propranolol and atenolol and has been found to have minimal effects on lipoprotein metabolism. Acebutolol may be the only beta-blocking agent that demonstrates some membrane-stabilizing activity at clinically achievable plasma concentrations. The ancillary pharmacologic properties of cardioselectivity and partial agonist activity are distinct and offer definite advantages to selected patients, particularly patients with respiratory disease, in whom cardioselective acebutolol, particularly at low doses, can minimize patient risk. The ancillary property of membrane-stabilizing activity may also guide therapy in selected patients.

Vagal activity is increased during intravenous isoprenaline infusion in man

Eight healthy normal subjects (19-23 years) received graded intravenous isoprenaline infusions PRE- and POST-atropine, 0.04 mg/kg i.v. The adequacy of atropinization was tested with bolus intravenous injections of phenylephrine. PRE-atropine, isoprenaline caused an increase in heart rate and systolic pressure, but a decrease in diastolic pressure with little change in mean pressure (+32.8 +/- 3.0 beats/min, +18.9 +/- 3.0 mm Hg, -13.5 +/- 1.9 mm Hg, -2.7 +/- 1.9 mg Hg respectively at isoprenaline 2 micrograms/min). POST-atropine, the increase in heart rate was enhanced, the rise in systolic pressure abolished and the falls in diastolic and mean pressures exaggerated (+47.0 +/- 2.8 beats/min, -8.9 +/- 2.9 mm Hg, -27.3 +/- 2.1 mm Hg, -21.1 +/- 1.9 mm Hg, respectively at isoprenaline 2 micrograms/min). During an isoprenaline infusion, when the heart rate and blood pressure changes are stable, there is an increase rather than a decrease in cardiac vagal tone.

Acebutolol pharmacokinetics in renal failure

Acebutolol was administered orally in a single dose of 200 mg to 17 individuals whose renal function varied markedly. The plasma half-life and elimination rate constant for acebutolol showed a four-fold variation but these did not correlate with the degree of renal impairment. However, there was a good correlation between the renal clearance of creatinine and that of acebutolol (P less than 0.001). The half-life and elimination rate of the acetyl metabolite, diacetolol, were subject to 10-fold inter-individual variability which correlated significantly with the creatinine clearance and serum creatinine concentration. The AUC for the acetyl metabolite showed a 40-fold individual variation which also correlated with renal function. It is concluded that renal elimination is the principal route of excretion for diacetolol but not the parent compound, acebutolol.

An assessment of physiological finger tremor as an indicator of beta-adrenoceptor function

Physiological finger tremor has been assessed as an indicator of beta-adrenoceptor function. Tremor was not correlated with the sex, age, weight or height of the subjects and was stable over 5 min when the hand and fingers were held horizontally. It was not increased by mental arithmetic, the Valsalva manoeuvre or 3 min exercise. Satisfactory dose-response curves could be constructed for the isoprenaline enhanced increases in finger tremor. In six subjects, practolol 120 mg produced a small shift to the right of the isoprenaline dose-response curve for finger tremor (dose ratio 2.1) but propranolol 40 mg was seven times more effective (dose ratio 17.1). Physiological finger tremor appears to be a stable parameter which may be useful in the investigation of the selectivity of beta-adrenoceptor blocking drugs.

Standardised isoprenaline sensitivity tests--a comparison of existent methods

1 Six healthy subjects (four male, two female, aged 19-24 years) received intravenous boluses of isoprenaline sulphate by two standardised methods of injection (Cleaveland et al., 1972; George et al., 1972). Heart rate changes were analysed by three different methods. 2 The I25 (dose of isoprenaline required to increase the heart rate by 25 beats/min) was not significantly altered by either the method of injection or the method of heart rate measurement. 3 The slope of the heart rate dose response curve was significantly steeper when isoprenaline was injected in a fixed 2 ml volume through a three-way tap, and when the heart rate was measured from the shortest three R-R intervals. 4 However the differences in method of injection and heart rate measurement between the two techniques do not appear to produce major alterations in the results obtained, particularly the I25.

Comparison of the beta 1 and beta 2 adrenoceptor blocking properties of acebutolol and propranolol

The purposes of this study were to evaluate the beta 1 and beta 2 adrenoceptor blocking properties of acebutolol and propranolol and measure the plasma levels of acebutolol, its acetylated metabolite and propranolol. Ten patients with reversible obstructive airways disease and hypertension received two separate dose levels of acebutolol and propranolol for five days each. Cardioselective properties were assessed by determining the beta 1 and beta 2 adrenergic-stimulating effects of terbutaline 5 mg before and at the end of each five-day treatment period. Both acebutolol and propranolol were clinically well tolerated. Following study drug there was a 100 percent inhibition of the beta 1 terbutaline effect, and an approximate 83 percent inhibition of the beta 2 terbutaline effect. There were no clinically significant differences between acebutolol and propranolol. The acetylated acebutolol metabolite accumulated levels two to three times higher than the parent compound, and its effects may have destroyed the cardioselectivity of acebutolol. Thus, acebutolol did not demonstrate clinically relevant cardioselectivity.

Beta-adrenoceptors in human lung, bronchus and lymphocytes

1 The ability of propranolol, metoprolol, acebutolol, atenolol and practolol to reverse the stimulant effect of 10^-5 M isoprenaline on lymphocyte cyclic AMP levels was determined. The ratios of doses required to produce 50% inhibition as compared with propranolol were 1:316 for metoprolol 1:1780 for acebutolol and 1:2820 for atenolol. No ratio could be calculated for practolol as it never produced more than 35% inhibition possibly because of its intrinsic sympathomimetic activity. This rank order of antagonist potencies suggest the lymphocyte β-receptor has β₂ selectivity.

2 For peripheral airway smooth muscle isoprenaline, adrenaline and noradrenaline had relative EC₅₀s of 1:25:9290 respectively in the absence of uptake blockade. However, the EC₅₀s changed to 1:15:82 in the presence of neuronal uptake blockade and x-adrenoceptor blockade. These observations are consistent with β₂ selectivity. Salbutamol behaved as a partial agonist on peripheral airway smooth muscle. It was possible to demonstrate competitive antagonism to isoprenaline. Estimates of the dissociation constant of salbutamol were obtained (6.6 ± 0.02) × 10^-7 M, n=4, mean ± s.e. mean).

3 A novel technique for determining the pA₂ is described which is particularly useful on peripheral airway smooth muscle. Values obtained for propranolol and practolol using this technique were:

propranolol pA₂ 7.83 ± 0.14; slope=0.97 ± 0.05 (n=4),

practolol pA₂ 5.62 ± 0.15; slope=1.17 ± 0.17 (n=4)

These values support the β₂ selectivity suggested by the agonist studies.

4 The cyclic AMP response of lung parenchyma to β-adrenoceptor agonists and antagonists (in the presence of isoprenaline demonstrated β₂ selectivity. Salbutamolol appeared to be a partial agonist when compared with the cyclic AMP response to isoprenaline.

5 The β-adrenoceptors of the lymphocyte, bronchial smooth muscle and lung parenchyma have β₂ selectivity although they differ somewhat quantitatively from each other.

Effective treatment of supraventricular arrhythmias with acebutolol

To evaluate the antiarrhythmic efficacy of the new beta adrenergic blocking agent acebutolol, 15 monitored patients with supraventricular arrhythmias received, in double-blind fashion, an intravenous infusion of either acebutolol or saline solution after a control period. Patients treated with saline solution demonstrated no change (P greater than 0.05) in heart rate or arterial blood pressure or conversion to sinus rhythm. After administration of acebutolol, significant (P less than 0.05) reductions in heart rate were noted at 5 minutes. Peak reduction occurred at 10 to 30 minutes and correlated with maximal acebutolol plasma concentrations, antiarrhythmic activity persisted for 24 hours. Mild reductions in systolic blood pressure were observed in the majority of patients. Two patients with atrial fibrillation and one with multifocal atrial tachycardia had conversion to sinus rhythm. Frequent premature atrial complexes noted in one patient were greatly suppressed after administration of the drug. In the nine patients with clinical evidence of chronic obstructive lung disease acebutolol was well tolerated. Adverse reactions were limited to transient dyspnea in one patient with prior heart failure and a decrease in systolic blood pressure to less than 90 mm Hg in three patients who remained asymptomatic. In the patients studied, acebutolol was an effective agent for the treatment of supraventricular arrhythmias and appeared to be of special value in those with chronic obstructive lung disease.

Quantitative assessment of bronchial beta-adrenoceptor blockade in man

1. We describe a method for assessing bronchial beta-adrenoceptor blockade quantitatively in man. Specific airway conductance is measured after increasing doses of inhaled salbutamol and the extent to which the dose-response curve is displaced to the right after beta-adrenoceptor blocking drugs is used to assess bronchial beta-adrenoceptor blockade. 2. Salbutamol dose-response curves were plotted for six normal subjects by measuring sGaw 15 min after increasing doses of inhaled salbutamol. Salbutamol produced a 30-70% increase in sGaw. 3. Salbutamol dose response curves were obtained 2 h after oral practolol (100 mg and 200 mg) and oral propranolol (40 mg and 80 mg) on separate days and were displaced to the right. 4. The mean dose ratios for practolol 100 mg and 200 mg were 1.2 and 2.1 and for propranolol 40 mg and 80 mg they were 21 and 61 respectively.

Exercise responses of healthy subjects in the evaluation of cardioselectivity of beta-blockers

The effects of intravenous propranolol, metoprolol, acebutolol and placebo on exercise-induced changes in heart rate and peak flow rate (PFR) have been studied in a group of healthy subjects. The three beta-blockers produced significant and comparable reductions in exercise-induced tachycardia and the magnitude of the reduction was related to the log plasma concentration of each drug. Significant cardiac beta-blockade was detectable for three hours after giving propranolol and for four hours after metoprolol and acebutolol. The exercise-induced changes in PFR were small and variable and were not significantly affected by any of the drugs. We conclude that, contrary to published reports, exercise-induced changes in heart rate and PFR in healthy subjects do not provide a satisfactory test system for assessing the selectivity of beta-blockers.

Catecholamine sensitivity in hyperthyroidism and hypothyroidism

1. Catecholamine sensitivity in hyper- and hypothyroidism has been studied using a standardised isoprenaline sensitivity test. 2. Seven patients with hyperthyroidism and seven with hypothyroidism were tested both when showing evidence of thyroid dysfunction and again when euthyroid. 3. No significant differences were seen in heart rate responses to isoprenaline when patients became euthyroid compared to their response when either hyperthyroid or hypothyroid. 4. These results indicate that sensitivity to catecholamines is not altered in thyroid dysfunction and, in particular, that hypersensitivity does not occur in spontaneous hyperthyroidism.

Pharmacokinetics of tolamolol in the treatment of hypertension

Tolamolol was administered in a "double-blind" study to fifteen hypertensive patients by dose-titration against arterial blood pressure. Mean steady-state plasma tolamolol concentrations (Css) were determined for each patient from the area under the plasma concentration--time curve during a dosage interval whilst patients were receiving optimal tolamolol doses. No significant correlation was observed between daily tolamolol dose and Css; the relationship between fall in lying mean arterial pressure and Css also failed to reach conventional levels of statistical significance, but Css was observed to be correlated with the fall in standing pressure. The results suggest that plasma concentrations in excess of 200 ng/ml may be required to achieve an effective hypothensive response with the drug.

Dose-titrated, double-blind, cross-over comparison of a selective beta-blocker and methyldopa in the treatment of hypertension

The efficacy and toxicity of tolamolol and methyldopa in hypertensive patients has been compared by a dose-titrated, double-blind, cross-over study. Thirteen patients completed the trial. Within the dose ranges investigated (tolamolol - 300 mg/day - 900 mg/day; methyldopa - 750 mg/day - 2250 mg/day)both drugs produced significant falls in laying and standing, systolic and diastolic blood pressures. Although the hypotensive effects of methyldopa were more marked than tolamolol, these only achieved conventional (P less than 0.05) levels of significance for lying blood pressure. There were no objective changes in haematological or biochemical indices during treatment with either drug, but patients complained of tiredness, weak limbs and mouth dryness significantly more during methyldopa treatment, than during either placebo or tolamolol therapy.

Ventricular electrical instability in the conscious dog: effects of psychologic stress and beta adrenergic blockade

The effect of psychologic stress on cardiac vulnerability was examined in 10 conscious dogs. The repetitive extrasystole threshold was employed as a measure of susceptibility to ventricular fibrillation. Instrumental aversive conditioning constituted a stressful environment. The repetitive extrasystole threshold decreased by nearly 50 percent during 3 days in which the animals were exposed to the stressful environment. When Tolamolol hydrochloride, a cardioselective beta adrenoceptor blocking agent, was administered before a stress session, the repetitive extrasystole threshold was unaltered from the control value. Thus, stress-evoked changes in cardiac vulnerability are mediated through the sympathetic nervous system.

The effects of propranolol, practolol and metoprolol on exercise-induced tachycardia in relation to plasma levels in man

1. The effects of single oral doses of propranolol, practolol and a new cardioselective beta-adrenoceptor blocking drug, metoprolol, on exercise-induced tachycardia in relation to plasma levels were studied in six normal volunteers. 2. Exercise undertaken on treadmill was submaximal which, under control conditions, increased the heart rate from 74-3 (s.e.m. = 6-8) to 153-8 (s.e.m. = 9.8) beats/min. 3. Plasma concentrations of propranolol and practolol were assayed fluorometrically and of metoprolol by electron-capture gas liquid chromatography, the details of which are described. 4. Between 1-5 and 2 h after drug ingestion 80 mg of propranolol associated with plasma level of 50-60 ng/ml (half-life 2-75 h), reduced the exercise-induced tachycardia by 27%, 250 mg of practolol with plasma levels of 1050-1100 ng/ml reduced it by 28% and 100 mg of metoprolol with plasma concentrations of 140-150 ng/ml (half-life 1-7 h), reduced it by 30%. 5. The resting heart rates were reduced significantly by propranolol and metoprolol but not by practolol. 6. Metoprolol is a potent short-acting beta-adrenoceptor antagonist; its advantages as a cardioselective agent over practolol in therapeutic use are discussed.

Efficacy of cardioselective beta adrenergic blockade with intravenously administered tolamolol in the treatment of cardiac arrhythmias

The efficacy of tolamolol, a cardioselective beta adrenergic blocking agent, was evaluated in the treatment of cardiac arrhythmias in 27 patients. Nineteen patients had supraventricular arrhythmias and eight had ventricular arrhythmias. Evaluation was by doulbe-blind randomized trial in 23 patients. Tolamolol was effective in reducing ventricular rate in 17 (85 percent) of 19 patients with supraventricular arrhythmias and resulted in conversion to sinus rhythm in 2 of the 17. The mean ventricular rate in 17 patients decreased from 135 to 102/min 10 minutes after initiation of administration of tolamolol and gradually decreased further to 93/min after 60 minutes. Reduction in ventricular rate was sustained for 2 hours of monitoring undergone by all patients and for 4 and 6 hours monitoring in two subgroups. Among the eight patients with ventricular ectopic beats, tolamolol reduced their frequency in four patients and had no effect in four. Six patients had chronic obstructive pulmonary disease and experienced no adverse clinical effects on respiratory function in association with administration to tolamolol. Untoward effects occurred in 10 patients, including hypotension in 3, 1 of whom required vasopressor therapy. Other side effects were sedation, nausea, dyspnea and warmth in the chest, all of which were mild and transient, requiring no treatment. Cardioselective beta adrenergic blockade with tolamolol was highly effective in controlling ventricular rate in supraventricular arrhythmias and reduced the frequency of ventricular ectopic beats in half of the small group of patients with this arrhythmia. It is particularly applicable in patients with obstructive pulmonary disease in whom cardiac beta adrenergic blockade is indicated. Hypotension is an important potential side effect.

A thin-layer chromatographic method for the determination of acebutolol and its major metabolite in serum

A sensitive and specific thin-layer chromatographic method for the simultaneous determination of acebutolol [DL-1-(2-acetyl-4-n-butyramidophenoxy)-2-hydroxy-3-isopropylaminopropane] and its major metabolite [DL-1-(2-acetyl-4-acetamidophenoxy)-2-hydroxy-3-isopropylaminopropane] is described. A 2-ml volume of serum with 350 ng of quinidine as internal standard was extracted at pH 10, the solvent was evaporated off and the residue was dissolved in 50 mul of methanol. A 10-mul volume of the solution was spotted on a thin-layer plate and after elution (ethyl acetate-methanol-ammonia, 75:20:5) the plate was dried at 90 for 15 min and, after cooling, dipped in a 10% paraffin wax solution. The fluorescence was measured using a spectrofluorimeter with a thin-layer scanning attachment. The peak-height ratios of acebutolol to internal standard and metabolite to internal standard were used to quantitate acebutolol and the metabolite, respectively.

Bioavailability of tolamolol

Bioavailability of capsule and tablet formulations of tolamolol were compared by measuring plasma concentration of tolamolol and reduction in maximum exercise heart rate over a period of twelve hours in eight healthy subjects in a two-way cross-over study. Tolamolol was absorbed more rapidly from capsules than from tablets; this did not result in any significant difference in the reduction in maximum exercise heart rate between the two formulations. There was no significant difference between area under curve of reduction in exercise tachycardia and area under-curve of plasma concentration of tolamolol for the two formulations. Reduction in maximum exercise heart rate was related to logarithm of plasma concentration of tolamolol between two and twelve hours after both formulations.

Electrophysiologic effects of tolamolol on atrioventricular conduction in man

The electrophysiologic effects of tolamolol (UK-6558-01), a beta-adrenergic blocking agent, were studied in 13 patients by means of intracardiac electrograms and the extrastimulus method. Tolamolol (4 to 30 mg. intravenously) resulted in : (1) prolongation of sinus cycle length (SCL) in all patients (p less than 0.01); (2) prolongation of sinus escape time (SET) in 11 of 13 patients (p less than 0.001); (3) prolongation of A-V nodal conduction time during sinus rhythm in 1i of 13 patients (p less than 0.001); (4) onset of A-V nodal Wenckebach block at longer paced cycle lengths in 10 of 11 patients (p less than 0.001); (5) prolongation of the functional refractory period (FRP) of the A-V node in 11 of 11 patients (p less than 0.001); and (6) prolongation of the effective refractory period (ERP) of the A-V node in 10 of 10 patients (P less than 0.001). Tolamolol had no effect on His-Purkinje system (HPS) conduction time in any patient, including 3 patients with abnormal H-V intervals. Because of the marked increase in A-V nodal conduction time encountered by premature atrial depolarizations, the relative and effective refractory periods of the HPS could not be determined in any patient after tolamolol. Atropine (0.5 or 1.0 mg. intravenously) significantly reversed the effects of tolamolol on: sinus cycle length (4 of 5 patients); sinus escape time (3 of 3 patients); A-V nodal conduction time (4 of 5 patients); and A-V nodal refractioriness (5 of 5 patients).

Pharmacokinetic and pharmacological studies with tolamolol in man

Pharmacokinetic and physiological variables were measured in six healthy subjects after intravenous and oral administration of tolamolol. 2. After intravenous injection of tolamolol (20 mg), there was a biphasic decline both in plasma concentration and attenuation of maximum exercise tachycardia. First and second phase half-lives of plasma concentration were 7 min and 2.5 h respectively. WReduction of maximum exercise tachycardia declined from 32 beats/min at 2 h to 19 beats/min at 8 hours. Clearance of tolamolol from blood ranged from 0.8-1.41 min-1. 3. After the oral administration of tolamolol (100 mg), the average volume of distribution was 220.1 and plasma concentration half-life 1.8 hours. After ten eight-hourly doses of 100 mg there was no accumulation of tolamolol and the half-life of plasma clearance was unchanged. 4. Hydroxytolamolol was detected in plasma in two of six subjects after oral tolamolol. 5. There was a significant positive correlation between reduction in maximum exercise heart rate and logarithm of plasma concentration of tolamolol after both oral and intravenous administration.

Cardiac and pulmonary effects of acebutolol

In a double-blind randomised study, single intravenous doses of propranolol (0-1 mg. per kg.), practolol (1 mg. per kg.), acebutolol (1 mg. per kg.), or placebo were each administered at weekly intervals to six healthy volunteers. Forced expiratory volume in 1 second (F.E.V.1), resting and exercise heart-rate, and resting and exercise peak flow-rate (P.F.R.) were determined before and at 2, 3, 4, and 6 hours after each treatment. Venous blood-samples were also obtained at these times. Compared with placebo, resting heart-rate was reduced after all three drugs, but the corresponding differences in exercise heart-rate were much greater, more consistent, and of greater statistical significance. At 2, 3, and 4 hours when acebutolol and propranolol produced equivalent cardiac beta-blocking activity (judged by reductions in exercise heart-rate), their mean plasma concentratios were in the ratio of about 8/1; and at 2 hours when practolol and acebutolol gave rise to almost equivalent cardiac beta blockade, their mean plasma concentratio ration was 3/1. At times, reductions in F.E.V.1 and resting P.F.R. after propranolol (but not after practolol or acebutolol) were significantly greater than the corresponding changes after placebo. The reductions in exercies P.F.R. after propranolol (6 hours) and acebutolol (4 hours) (but not after practolol) were significantly greater than the changes after placebo. Changes in F.E.V.1, resting and exercise P.F.R. after propranolol, and the corresponding changes after practolol, were significantly different, all of which confirmed that practolol was more cardioselective than propranolo. In general, the reductions in F.E.V.1 and resting P.F.R. after acebutolol were slightly smaller than after propranolol, excepting at 6 hours when the difference between them was significant. The reductions in exercise P;F.R. after acebutolol and propranolol were of the same order, there being no significant differences between the two, whereas the reductions after acebutolol were clearly greater than the corresponding changes after practolol, the differences being significant at 2, 3, and 4 hours.

Beta-adrenoceptors in the human dorsal hand vein, and the effects of propranolol and practolol on venous sensitivity to noradrenaline

1 Infusion of isoprenaline into dorsal hand veins preconstricted with noradrenaline produced vasodilatation in four subjects out of five which was reversed by addition of propranolol. It is confirmed that beta-adrenoceptors are present in the hand veins of most subjects. 2 Addition of either propranolol or practolol to infusions of vasoconstrictor doses of noradrenaline potentiated the vasoconstriction in the hand vein. 3 After systemic administration of propranolol (0.3 mg/kg) or of practolol (1.0 mg/kg) intravenously, the sensitivity of the hand vein to the vasoconstrictor activity of noradrenaline was increased by 7.5 times and 2.9 times respectively. Significant reductions in hand skin temperature and resting pulse rate occurred after propranolol only. 4 It is suggested that this may be a useful method for the quantitative estimation of peripheral vascular beta-adrenoceptor blocking activity in man. At the doses used, significant beta-adrenoceptor blockade appears to occur at this site with practolol as well as propranolol.

Double-blind comparison of tolamolol, propranolol, practolol, and placebo in the treatment of angina pectoris

Forty-two patients with angina pectoris have completed a randomized, double-blind trial comparing tolamolol 100 mg and 200 mg with propranolol 80 mg, practolol 100 mg, and placebo, all given three times a day. Tolamolol 200 mg thrice daily was found to be equivalent to propranolol 80 mg thrice daily in anti-anginal efficacy. Anginal attack rates and trinitrin consumption were significantly reduced by all active treatments as compared with the placebo but tolamolol and propranolol were the most effective. Tolamolol 200 mg thrice daily was most effective in reducing blood pressure, while propranolol was most effective in reducing the resting heart rate. All treatments except the placebo significantly increased the amount of exercise which could be performed before angina appeared (exercise work), while tolamolol 200 mg thrice daily significantly reduced Robinson's index when compared with all other active agents. The degree of S-T segment depression induced by exercise was significantly lessened by both tolamolol and propranolol but not by practolol or placebo. There was no difference in patient preference between tolamolol and propranolol but tolamolol at both dose levels was preferred to practolol. Both tolamolol and propranolol are potent adrenergic beta-receptor antagonists and equal in anti-anginal efficacy but tolamolol has the advantage of being cardioselective. It is superior to practolol.

The metabolism of tolamolol in the mouse, rat, guinea-pig, rabbit and dog

1. [3H, 14C]Tolamolol was well absorbed after oral administration to mice, rats, guinea-pigs, rabbits and dogs. 2. The major route for excretion of radioactivity by mice, rats and guinea-pigs was the faeces; in rabbits the major route was the urine. Dogs excreted similar amounts of radioactivity by both routes. Biliary excretion of radioactivity by the rat and guinea-pig was demonstrated. 3. Tolamolol was extensively metabolized by all five species. The major metabolite in mice, rats, guinea-pigs and rabbits was the product of hydroxylation of the tolyl ring, which was excreted as such as the glucuronide and sulphate conjugates. 4. In the dog the major metabolite was the acid resulting from hydrolysis of the carbamoyl group. This acid was also excreted by the rabbit, but was only a minor metabolite in the other species studied.

The selectivity of beta-adrenoceptor antagonists on cardiovascular and bronchodilator responses to isoprenaline in the anaesthetized dog

1 The actions of five beta-adrenoceptor antagonists, chosen because of reported differences in their selectivities, were compared using the positive chronotropic, vasodepressor and bronchodilator responses to isoprenaline in anesthetized dogs. 2 Propranolol was a potent antagonist of the isoprenaline responses in all three systems. 3 Practolol and acebutolol (M & B 17,803) blocked the positive chronotropic responses to isoprenaline to a greater extent than the vasodepressor or bronchodilator responses. 4 Butoxamine and alpha-methyl dichloroisoprenaline showed the opposite selectivity, blocking the vasodepressor and bronchodilator responses to isoprenaline to a greater extent than positive chronotropic responses. However, both drugs were considerably less potent than the other antagonists studied and their selectivities were less clear-cut than those of practolol or acebutolol. 5 All the antagonists lowered the resting heart rate and to a lesser extent the diastolic blood pressure. The effects of propranolol, practolol and acebutolol on heart rate probably result from cardiac beta-adrenoceptor blockade. With butoxamine and alpha-methyl dichloro isoprenaline, however, the effects on heart rate probably result from a direct cardiodepressant action. 6 The relevance of the results to the problem of the sub-classification of beta-adrenoceptors is discussed.

Isoprenaline- and exercise- induced tachycardia in the assessment of beta-adrenoceptor blocking drugs; a comparison between tolamolol, practolol and propranolol

The effects of tolamolol, propranolol and practolol on both isoprenaline- and exercise-induced tachycardia have been studied in conscious dogs. Tolamolol was approximately equipotent to propranolol and 50 times more potent than practolol in antagonizing exercise-induced tachycardias, but was approximately 12 times less potent than propranolol and 8 times more potent than practolol in blocking isoprenaline-induced tachycardia. It is suggested that antagonism of the tachycardia induced by exercise affords a more meaningful assessment of the possible therapeutic potential of beta-adrenoceptor blocking drugs than does that induced by isoprenaline.