Scientific Research -1988 January to June

Summary                                  

In asthma, the salutary effect of bronchodilators derives from their decreasing pulmonary resistance which is invariably, accompanied by an increase in pulmonary compliance. In an anaesthetized cat a state of experimentally-induced broncho-constriction, bronchodilators cause a decrease in pulmonary resistance in an increase in pulmonary compliance. The effects of bronchodilators are accompanied by changes in systemic and pulmonary arterial pressure. Drug-induced changes in the diastolic pulmonary pressure of such a cat were found to be associated with opposite changes in pulmonary compliance. However, no clear-cut inter-relationship could be drawn between changes in pulmonary compliance and concomitant alterations in pulmonary vascular pressure. This suggests that additional factors may play a part in drug-related changes in pulmonary compliance.

Introduction

The bronchodilator effects of the sympathomimetic amines-manifested in their reduction in pulmonary resistance and increase in pulmonary compliance in an asthmatic condition is attributable to the mediation of their pharmacological action through B-Adrenoceptors. Whereas there is evidence for the involvement of B₂ adrenoceptors in reduction in airway resistance, the involvement of B-adrenoceptors in the increase in pulmonary compliance is not clearly established. Airways resistance is the change in driving pressure per unit change in airflow or the degree of obstruction offered by the conducting airways to airflow; pulmonary compliance, on the other hand, involves the distensibility of the peripheral airways, that is, it is the change in lung volume per unit of pressure change.

This experiment aims to investigate the relationship that may exist between the effects of various drugs on pulmonary compliance and pulmonary arterial pressure and, to determine to what extent the adrenoceptor types in the pulmonary vessels may affect their alterations in pulmonary compliance. Evidence so far accumulated has indicated the presence of a predominantly alpha-adrenoceptor population in the pulmonary circulation in the dog, rabbit, calf, guinea-pig and man (1, 2, 3, 4) which mediate vasso-constriction. More yet is to be known about the adrenoceptor population in the pulmonary vessels of the cat. Although much pharmacological information on extra pulmonary blood vessels has recently become available from which some generalizations may be made, it is worthy to note that blood vessels are highly heterogeneous in their response to drugs (5, 6). Thus Williams (7), Bevan (8), Buffolo and Waddel (9), and Misu, Kaiho, Ogawa and Kubo (10) also noted the heterogeneity of various mammalian vascular vessels in their response to sympathomimetic a mines. Experimental results from in vitro work could permit only limited prediction of pharmacological characteristics of a short segment of a blood vessel, much less of an entire vascular system.

An increase in pulmonary compliance is known to decrease pulmonary vascular pressure and vice versa (11). If the pulmonary vessels in the cat have any appreciable B-adrenoceptor population, then it is conceivable that the B-adrenoceptor agonists and antagonists would have an indirect effect on pulmonary compliance by their action on pulmonary vascular pressure.

© The Author(s) 1988. Published by the Pharmaceutical Society of Ghana (PSGH). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.