Abstract
Ambient temperature was lowered from 30°C to 15°C in unrestrained mice with and without continuous presence of autonomic blockers, and the autonomic response to cold stress was analyzed. Time series of RR intervals were obtained from ECG recordings sampled continuously for 40 seconds every 2 minutes, and the mean RR interval and the mean respiratory interval were calculated for each time series. Frequency domain area of the amplitude Fourier spectrum of the variation of RR-intervals, LF (0.025-0.300 Hz) and HF (0.3-4.0 Hz), were calculated. After lowering the temperature to 15°C, the mean RR interval had decreased gradually and stabled at the minimum steady level with small variations in RR intervals in both control and pharmacologically treated mice. This minimum steady level of the mean RR-interval was not significantly different between the control mice and the atropine treated mice. These results suggested that the sympathetic activity was enhanced and the parasympathetic activity was eliminated by the cold stress. The tachycardia in cold accompanied the tachypnea. It was suggested that the increased metabolism was underlying the autonomic heart rate response to cold. The reason for inadequacy of the LF/HF ratio as an index for the autonomic balance in mice will be specified based on the present results. [J Physiol Sci. 2006;56 Suppl:S199]
