The influence of body temperature on the ventilatory response to CO 2 in anaesthetized rats

The breathing frequency, tidal volume and partial pressure of CO2 in arterial blood (PaCO2) were measured continuously in urethan anaesthetized rats at different body temperatures. In urethan anaesthesia the colonic temperature of our rats averaged 33.5 plus or minus 1.29 degree Celsius, when the ambient temperature was 22-24 degree Celsius. At this temperature their PaCO2 was the highest, 42 plus or minus 1.2 mm Hg on the average. With decreasing or increasing body temperature the PaCO2 declined. The breathing frequency increased with increasing body temperature. The ventilatory CO2 response curve in warmed rats was shifted towards higher ventilation. Its relative slope was parallel, in absolute units it was steeper than that of the cooled animals. After vagotomy the rate of breathing averaged 42-48 percent of the control values, at all body temperatures. The calculated inspirio-inhibitory index (V/VT) shows a relatively identical decrease (37-39 percent) after vagotomy in warmed, same as in cooled rats. The results indicate that in urethan anaesthetized rats the sensitivity to CO2 remains the same in the range of body temperatures from 28 to 38 degree Celsius. Also the relative role of the vagus nerves does not change within the range of these temperatures.