Factors affecting the breathing rate of mice as used for studies of radiation damage to lungs

Abstract

Breathing rate is a useful parameter for evaluating lung function in mice after various treatment procedures which may damage the lung, including radiation doses relevant to radiotherapy. It can be measured by non-invasive, non-destructive methods, thus allowing quantitative assessment of lung changes in the same mouse over a period of time (Travis et al., 1979). The method is being used regularly to assay early and late changes after irradiation of the whole lung as well as drug-induced lung injury (Travis et al., 1979; 1980; Collis et al., 1980). A wide range of measured values of normal breathing rates in undisturbed mice has been reported. Rates as low as 109 breaths per minute (BPM) (Crosfill and Widdicombe, 1961) and as high as 230 BPM have been reported (Guyton, 1947; Spector, 1956). In a study of changes in breathing rate after X-irradiation of both lungs, we measured a mean rate of 330 BPM in control, unirradiated male CBA mice during quiet respiration. Because this rate was higher than the previously reported values in mice, we carried out a series of experiments in unirradiated mice to answer the following questions. (1) Was the apparently high breathing rate observed by us in CBA mice an artefact of our testing system? (2) How much did breathing rate depend on the strain and sex of the mice? (3) Was there a correlation between breathing rate and relative lung weight in different strains of mice? A whole-body mouse plethysmograph was used for measuring breathing rate (Travis et al., 1979).