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Effects of ventilation, humidity and temperature on airway responsiveness to methacholine in rats

Dept of Medicine, School of Medicine, University of Sao Paulo, Sao Paulo, Brazil

CORRESPONDENCE: M.A. Martins, Departamento de Clínica Médica, Faculdade de Medicina da Universidade de São Paulo, Av. Dr. Arnaldo 455 - Sala 1216, 01246-903 -, São Paulo, SP, Brazil. Fax: 55 1130850992. E-mail: mmartins@usp.br

Keywords: airway smooth muscle, alveolar ventilation, experimental asthma, hyperventilation-induced bronchospasm, pulmonary responsiveness

Received: March 29, 2001
Accepted January 16, 2002

This study was supported by the following Brazilian Scientific Agencies: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Programa de Núcleos de Excencia (PRONEX-MCT).

Exercise-induced bronchoconstriction is associated with heat and water loss from the airways. It is not known whether these conditions can influence the response to bronchoactive agonists. The effects of different degrees of alveolar ventilation on the pulmonary response to methacholine and the role of humidity and temperature in this response were evaluated.

Wistar rats were anaesthetized, tracheostomized and mechanically ventilated. Increasing doses of methacholine were infused intravenously and respiratory system resistance (R_rs) and elastance (E_rs) were measured. The rats were ventilated with dry air at 13°C, dry air at 37°C, humid air at 13°C and humid air at 37°C. These four groups were further divided into three subgroups with a respiratory frequency adjusted to reach a carbon dioxide tension in arterial blood of 30, 40 and 50 mmHg.

Temperature, humidity and level of alveolar ventilation did not influence the position of the dose/response curve to methacholine. However, the maximal changes in E_rs were significantly lower in the rats ventilated with humid air. In addition, maximal changes in E_rs were significantly higher in the rats with lower alveolar ventilation. These differences were not observed for maximal values of R_rs.

The pulmonary response to methacholine in normal rats is significantly affected by the humidity of inspired air and the level of alveolar ventilation. This influence is more intense in the small airways and/or distal airspaces. This suggests that exercise or hyperventilation can change the behaviour of airway smooth muscle.

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