Heart rates of male and female Sprague-Dawley and spontaneously hypertensive rats housed singly or in groups.

This study was conducted to confirm our previous reports that group housing lowered basal heart rate and various evoked heart-rate responses in Sprague-Dawley male and female rats and to extend these observations to spontaneously hypertensive rats. Heart rate data were collected by using radiotelemetry. Initially, group- and single-housed rats were evaluated in the same animal room at the same time. Under these conditions, group-housing did not decrease heart rate in undisturbed male and female rats of either strain compared with single-housed rats. Separate studies then were conducted to examine single-housed rats living in the room with only single-housed rats. When group-housed rats were compared with these single-housed rats, undisturbed heart rates were reduced significantly, confirming our previous reports for Sprague-Dawley rats. However, evoked heart rate responses to acute procedures were not reduced universally in group-housed rats compared with either condition of single housing. Responses to some procedures were reduced, but others were not affected or were significantly enhanced by group housing compared with one or both of the single-housing conditions. This difference may have been due, in part, to different sensory stimuli being evoked by the various procedures. In addition, the variables of sex and strain interacted with housing condition. Additional studies are needed to resolve the mechanisms by which evoked cardiovascular responses are affected by housing, sex, and strain.