THE EFFECTS OF RESTRAINT ON VENTILATORY RESPONSES TO HYPERCAPNIA AND HYPOXIA IN ADULT MICE

The aim of this experiment was to determine whether ventilatory measur ements in adult restrained mice provide a valid assessment of chemosen sitivity. We used whole-body plethysmography to compare breathing patt erns in eight restrained and eight unrestrained outbred Swiss mice dur ing air breathing, hypercapnia, and hypoxia. The mice in the restraine d group were each placed in a loosely restraining wire-mesh cage. The unrestrained mice could move freely inside the plethysmograph. All the mice received three hypercapnic stimuli (8.5% CO2) and three hypoxic isocapnic stimuli (10% O-2, 3.5% CO2). As compared to unrestrained mic e, restrained mice had significantly lower breath durations (TT, 445 /- 110 ms vs. 323 +/- 32 ms) and higher ventilation ((V)over dotE) lev els (15.7 +/- 2.6 mu l/(sec.g) vs. 22.2 +/- 4.5 mu l/(sec.g)), whereas no difference was observed for tidal volume (VT). The increases in fr equency and ventilation from baseline to hypercapnia were not signific antly different in restrained and unrestrained mice. The (V)over dotE response to hypoxia was marginally higher in restrained mice. We concl ude that chemosensitivity to hypercapnia, and to a lesser extent to hy poxia, can be measured in restrained adult mice, but that the baseline breathing pattern cannot. (C) 1998 Elsevier Science B.V. All rights r eserved.