Problems in assessing beta-adrenoceptor antagonism in man

Differential cardiovascular effects of propranolol, atenolol, and pindolol measured by impedance cardiography

We have evaluated Sramek's method of impedance cardiography as a non-invasive way of detecting the cardiovascular effects of drugs. We made cardiovascular measurements using the method during passive tilting and exercise 2 h after the oral administration of atenolol (50 and 100 mg), propranolol (40 and 80 mg), pindolol (5 and 10 mg), and placebo in seven separate studies involving eight healthy male volunteers. Equivalent doses of the pure antagonists atenolol (beta 1) and propranolol (beta 1, beta 2) produced similar reductions in heart rate, systolic blood pressure, and cardiac index, and increases in stroke volume and total peripheral resistance, particularly during exercise. In contrast the partial agonist pindolol produced increases in heart rate and cardiac index, and reductions in peripheral resistance at rest. During passive tilting and exercise pindolol reduced heart rate, but cardiac output and total peripheral resistance were unchanged except at the highest levels of exercise. The similar cardiovascular effects of atenolol and propranolol, but differing effects of pindolol, are consistent with reports using other methods of measurement. This suggests that impedance cardiography may have a place in the non-invasive assessment of the cardiovascular effects of drugs.

A comparative pharmacokinetic and pharmacodynamic study of conventional and sustained-release preparations of acebutolol in healthy volunteers

Pharmacokinetics and the degree of beta-blockade of sustained release (SR) acebutolol (500 mg/day) and conventional acebutolol (200 mg tid) were examined after the first oral dose and after 10 days of treatment in ten healthy volunteers. After the first dose, acebutolol Cmax did not significantly differ between the two formulations; however, on day 10 acebutolol Cmax was significantly higher after SR formulation. Cmax of diacetolol, the major metabolite, did not differ between SR and conventional acebutolol neither on day 1 nor on day 10. The dose-corrected relative bio-availability of acebutolol was not different from 100% on day 1 and day 10; however the dose-corrected diacetolol AUC, SR/conventional ratio, was significantly lower than 100% on days 1 and 10. Both acebutolol preparations exerted a significant reduction in exercise tachycardia over 24 h on day 1 and day 10; however, conventional acebutolol exhibited a greater reduction 24 h after the first dose. Exercise-induced increase in systolic blood pressure was similarly inhibited by both treatments except for 24 h after the first dose when systolic blood pressure was significantly higher with SR than with conventional acebutolol. The percent reduction in heart rate during exercise was linearly correlated with log acebutolol plasma concentrations for each treatment regimen. These results suggest that beta-blockade exerted by SR acebutolol in healthy volunteers is equivalent to that of conventional acebutolol.

Clinical pharmacokinetics of atenolol--a review

Atenolol is a hydrophilic betareceptor blocking drug, which is predominantly eliminated via the kidneys, only about 5% of the atenolol is metabolised by the liver. After oral administration atenolol is incompletely absorbed from the intestine, so about 50% of the beta blocker are finally biovailable. In plasma only 3% of atenolol are protein-bound. There exists a linear relationship between the atenolol plasma levels and the degree of beta blocking effect measured by inhibition of the exercise-induced tachycardia. No correlation was found between plasma levels of atenolol and blood pressure lowering activity of the drug. After oral administration elimination half life of atenolol is calculated from 6 to 9 h by different authors. In patients with impaired renal function elimination half life of atenolol gradually increases to values of 36 h in uraemic patients (glomerular filtration rate (GFR) less than 10 ml/min). Between GFR and atenolol plasma clearance as well as renal clearance a close significant correlation is described. Prolongation of elimination half life requires a dosage adjustment of atenolol in patients with renal failure. A marked interaction of atenolol is found when calcium or aluminium hydroxide are concurrently administered with the beta blocker whereas cimetidine does not influence atenolol kinetics.

Cardiovascular and metabolic effects of terbutaline

Terbutaline is a selective beta 2 agonist used predominantly in the treatment of asthma. Since beta-mediated responses increase heart rate, dilate peripheral arteries, modify carbohydrate metabolism and the uptake of electrolytes into cells, the administration of terbutaline might be expected to produce widespread effects. In this study the intravenous administration of 0.5 mg terbutaline over 60 mn has been shown to produce marked changes without upsetting the volunteers. Heart rate, systolic blood pressure and plasma glucose all increase; diastolic pressure and serum potassium decrease. The data suggests that the terbutaline infusion may be a useful tool for the investigator. The results also quantitate some of the side effects which may result from the intravenous administration of a therapeutic dose of terbutaline given to asthmatics or to pregnant women to reduce uterine activity and delay childbirth.

Onset and duration of beta-adrenergic receptor blockade following single oral dose acebutolol hydrochloride (Sectral)

1. The onset and duration of action of once daily acebutolol (400 mg) on resting and exercise heart rate and blood pressure were studied in normal subjects in a double-blind, cross-over trial against placebo. Subjects were studied 1.5, 3, 8 and 24 h after the first dose and 24 h after the fifth dose. 2. Resting and exercise heart rate and blood pressure were significantly reduced within 90 min of the first dose. 3. A significant reduction in these variables persisted throughout 24 h after the fifth daily dose. However, at 24 h the effects were considerably attenuated, the falls in resting and exercise systolic blood pressure being only 5%. 4. A linear relationship was noted between log serum drug level and percentage reduction of exercise heart rate.

Clinical pharmacokinetics of beta-adrenoreceptor blocking drugs

All beta-adrenoreceptor blocking drugs seem to be fairly rapidly and completely absorbed from the gastro-intestinal tract. The rate of absorption, however, appears to be lower in elderly patients and possibly also in patients with renal failure than in younger patients. The extent of bioavailability varies considerably between different beta-blockers. Some of these drugs(e.g. alprenolol and propranolol) have a low extent of bioavailability due to a high first-pass elimination effect, while pindolol and practolol for example are in influenced very little by this effect. However, as some beta-blockers from active metabolites, the bioavailability calculated as the ratio between the area under the plasma concentration time curve of unchanged drug after oral and intravenous administration does not give an accurate estimation of the fraction of the biologically active dose reaching the systemic circulation. The beta-blockers so far studied are rapidly distributed in the body. The t1/2 of distribution ranges between 5 to 30 minutes. The apparent volume of distribution varies 3- to 4-fold between the compounds but in all cases the apparent volume of distribution exceeds the physiological body space. In patients with impaired liver function an increase of the volume of distrubution of propranolol has been found. The beta-blockers are relatively rapidly eliminated from the body and most of them have an elimination half-life between 2 to 4 hours. For atenolol, practolol and sotalol higher values have been reported. The most lipophilic beta-blockers are almost completely metabolised in the liver, wheras those of lower lipophilicity are mainly excreted via the kidneys. Impraired liver and kidney function have been found to significantly influence the rate of elimination of those beta-blockers eliminated via the insufficient organ of elimination. Numerous investigators have shown that the beta-blocking effect is linearly related to the logarithm of the plasma concentration. In spite of this relationship, it is difficult from mean data to predict the individual plasma concentration which is necessary for a certain degree of beta-blockade. This might be due to variations in the quantitative formation of active metablolites, individual differences in the plasma protein binding and rather flat plasma level-response curves. Also with respect to the therapeutic effect, the plasma levels vary considerably between individuals. This limits the value of determination of plasma concentrations in order to adjust the therapeutic dose. Our recommendation is that these facilities should be utilised in selected patient groups, eg. those who have a poor therapeutic response to a beta-blocker although the dose is high, and those patient with impaired renal or liver function. The duration of beta-blockade is dose-dependent since the pharmacological effect declines with a constant rate (zero-order kinetics) within relatively wide dosage intervals...