Circadian rhythms in blood pressure and heart rate have been described in animals and humans. The question whether or not these rhythms are of endogenous origin has not been addressed so far. In the present study we, therefore, evaluated rhythms in blood pressure, heart rate, and motor activity in freely moving Sprague-Dawley rats under conditions of alternating light and darkness (LD), constant light (LL), and constant darkness (DD). Blood pressure, heart rate, and motor activity were measured by telemetry. Rhythm analysis was performed by fitting of a single cosine curve to data from subsequent four-day periods. In LD circadian rhythms were observed in activity and in all cardiovascular parameters. In DD these rhythms were free-running as indicated by a continuous shift in the phase position of the acrophases. In LL circadian rhythmicity was greatly disturbed in either function investigated. Furthermore, the circadian amplitude in heart rate but not in blood pressure was markedly reduced when compared to DD. From these observations we conclude that the circadian rhythm in blood pressure has an endogenous component, which is more prominent than that in heart rate.
Tijdschrift beschrijving