Interrelationships among heart rate, activity, and body temperature in the rat

Abstract

The range and temporal characteristics of heart rate (HR) and the interrelationships among HR, activity, and deep body temperature (DBT) were studied in unrestrained rats using biotelemetry techniques to measure HR and DBT. In Experiment 1, 1-min samples of HR and activity were recorded every 10 min over 21 hr on three separate days. A measure of DBT was added in Experiment 2 and all three variables were sampled every other min for 72 consecutive hr. The maximum HR averaged over hourly periods was consistently about 100 BPM above the daily minimum HR, with the daily average HR falling almost exactly at the midpoint of the range. The circadian components of HR and DBT were much more pronounced than that of activity, suggesting that the former are generated, at least in part, by factors other than somatic activity. Phasic (within-hour) changes in HR were highly correlated with activity but not with DBT. In contrast, tonic (between-hour) changes in HR were highly correlated with both activity and DBT. HR and DBT remained highly correlated when the effects of activity were statistically removed, again indicating that tonic HR is importantly regulated by factors other than somatic activity.