Relationship between bronchial effects and plasma practolol concentration in man

Assessment of airways, tremor and chronotropic responses to inhaled salbutamol in the quantification of beta 2-adrenoceptor blockade

1. The purpose of the study was to assess and compare the effects of inhaled salbutamol on heart rate (HR), finger tremor (Tr) and specific airways conductance (sGaw) in the measurement of beta 2-adrenoceptor blockade in normal subjects. 2. Five healthy volunteers were given oral doses of atenolol 50 mg, 100 mg, 200 mg (A50, A100, A200), propranolol 40 mg (P40) or identical placebo (P1) in a single-blind crossover design. 3. Three hours after drug ingestion, dose-response curves were constructed using cumulative doses of inhaled salbutamol: 200 micrograms, 700 micrograms, 1700 micrograms, 3200 micrograms, 6200 micrograms. HR, Tr and sGaw were measured at each dose increment, made every 20 min. 4. Increasing doses of atenolol were associated with progressive reduction in salbutamol induced beta-adrenoceptor responses. The greatest attenuation occurred with propranolol. These effects on beta-adrenoceptor responses were similar for HR, Tr and sGaw. Geometric mean dose ratios (compared with placebo) for A50, A100, A200 and P40 were as follows HR: 1.98, 2.75, 4.29; Tr: 1.60, 3.78, 6.34, 80.50; sGaw: 1.08, 4.35, 12.30, 66.0 (no dose ratio was obtained for HR with P40). 5. These results showed that atenolol and propranolol attenuated the effects of salbutamol on HR to a similar degree as Tr and sGaw. Furthermore, the variability was least in the measurement of chronotropic responses, suggesting that this may be used to quantify beta 2-adrenoceptor antagonism. The beta 1-adrenoceptor selectivity of atenolol was a dose-dependent phenomenon, although the beta 2-adrenoceptor blockade of A200 was much less than with P40.

Beta-adrenoceptor-blocking agents: are pharmacologic differences relevant?

beta-Adrenoceptor-blocking agents constitute a heterogeneous group of compounds. Membrane- stabilizing (quinidine-like) effects can be demonstrated pharmacologically with most compounds, but only at relatively high concentrations. There is no evidence to suggest that this property is of clinical relevance. Some compounds have a certain selectivity for receptors of the beta 1-type, whereas others possess beta-adrenoceptor stimulant activity (partial agonism). The clinical importance of these latter properties remains controversial. The selectivity for beta 1-adrenoceptors, which can be demonstrated pharmacologically for atenolol, metoprolol, and practolol, appears quite broad. Nevertheless a clear advantage over nonselective compounds with respect to their effects on lung function and vascular resistance in patients has not been established. There are two possible explanations. The first is that the doses used therapeutically may lie outside the selective range; the second is that most tissues appear to possess and mixed population of beta 1- and beta 2-adrenoceptors. According to our present understanding, even absolute specificity for a given subtype cannot provide organ or tissue specificity. Partial agonists provide a constant stimulation of beta-adrenoceptors while at the same time preventing access of catecholamines to the receptor they occupy. With some compounds (e.g., pindolol), stimulant activity may be sufficient to counterbalance the myocardial depression normally resulting from blockade of basal sympathetic tone. Heart rate and cardiac output are thus maintained within normal limits and compensatory increases in vasomotor tone (seen with antagonists lacking intrinsic activity) do not occur. Pindolol has been shown to dilate blood vessels at very low doses and to produce significant relaxation of isolated tracheal smooth muscle at concentrations within the range of therapeutic plasma levels found in humans. These effects may underly the relatively low incidence of bronchopulmonary and vascular side effects reported for this compound.

Influence of environmental temperature and humidity on bronchial responses during assessment of selectivity of beta-adrenoceptor antagonists in man

1 The effects of beta-adrenoceptor antagonists given intravenously in single doses were examined in a double-blind, placebo controlled study performed in six healthy volunteers. Heart rate and peak expiratory flow rate (PEFR) were measured at rest and during standardised exercise. 2 Atenolol 0.2 mg/kg, betaxolol 0.15 mg/kg, practolol 1 mg/kg and propranolol 0.2 mg/kg all reduced heart rate to a similar extent during exercise at 2 and 4 h after administration; betaxolol 0.6 mg/kg had a significantly greater effect than the other treatments at all times. Only betaxolol 0.15 and 0.6 mg/kg significantly inhibited exercise tachycardia at 24 h. 3 None of the treatments studied had any effect on PEFR at rest or during exercise. 4 A second study was performed to determine whether this lack of effect of propranolol on exercise PEFR could have been due to the warm and humid conditions prevailing during the experiment. Eight healthy men underwent a standardised exercise test under alternately 'warm and humid' and 'cool and dry' conditions before and after propranolol 0.2 mg/kg or saline placebo intravenously. 5 Propranolol treatment did not influence resting or exercise PEFR in either environment, but did reduce FEV1 immediately after exercise under the cool and dry conditions but not under the warm and humid conditions. 6 Comparison of the effects of beta-adrenoceptor antagonists on exercise heart rate and PEFR is not a reliable or sensitive method of measuring the cardioselectivity of these drugs. 7 Environmental temperature and humidity should ideally be controlled when the action of any drug on airflow resistance is being studied.