Ultradian oscillations in blood pressure and heart rate in free-running dogs.

A long-term (14-24 h) telemetry recording of mean arterial pressure (MAP) and heart rate (HR) was obtained from conscious unrestrained dogs (n = 9) maintained in a closed environment. MAP and HR records were analyzed for their harmonic content by the fast Fourier transform method. The power spectral density functions of MAP and HR clearly revealed the existence of coherent ultradian rhythms of MAP and HR with a period of 1-2 h. HR and MAP are in phase and sustained during a 24-h period. To elucidate the nature of these rhythms, two additional studies were conducted (n = 4) in which clonidine (200 micrograms/kg) was administered systemically. In series I, a 7-h control period preceded the administration of clonidine, whereas in series II the order was reversed. Control (series I) and recovery (series II) periods were compared with a 0- to 7-h postinjection period. The results show that clonidine drastically and significantly reduces the amplitude of both MAP and HR oscillations for an average period of 7 h. This response was very consistent in both series and in all four dogs. These results implicate the central nervous system as the origin of these rhythms.