CIRCADIAN-RHYTHMS OF HEART-RATE AND BLOOD-PRESSURE IN 4 STRAINS OF RAT - DIFFERENCES DUE TO, AND SEPARATE FROM, LOCOMOTOR-ACTIVITY

Five or six rats from four rat strains (two normotensive, two hyperten sive) have been studied for seven consecutive days. All four strains w ere entrained to a 12:12 L:D cycle. While freely mobile, the blood pre ssure, heart rate and locomotor activity have been measured throughout and mean values obtained over 15-min intervals have been analysed. In all strains there was a marked circadian rhythm of locomotor activity peaking in the dark phase. In both normotensive strains (Wistar-Kyoto and Sprague-Dawley) the circadian rhythms of heart rate, systolic and diastolic blood pressures paralleled those of activity. In spontaneou sly hypertensive rats the mean heart rate was lower than in the normot ensive strains, but its circadian rhythm was similarly phased; blood p ressures were higher and, though the values were higher in the active (dark) phase, the profiles did not closely parallel that of heart rate . In a hypertensive, transgenic strain, heart rate showed an altered c ircadian profile, though still showing higher values in the active pha se. By contrast, the circadian profiles of blood pressure were almost the inverse of those in the other strains and of the profile of heart rate in the transgenic strain. Purification of the heart rare and bloo d pressure data showed that these differences could not be ascribed to masking effects caused by locomotor activity; that is, they are endog enous in origin. These results are discussed in terms of whether the t wo hypertensive strains can be considered to act as models for contrib uting to our understanding of primary and secondary hypertension.