Clonidine reduces blood pressure and heart rate oscillations in the conscious rat

We investigated the effects of clonidine on the fluctuations that underlie the spontaneous variability of blood pressure (BP) and heart rate (HR) in conscious rats. Analog-to-digital conversion of the intrafemoral BP was used to determine systolic, diastolic, and mean BP and HR every 200 ms. The equidistant sampling allowed a direct spectral analysis using a fast Fourier transform algorithm. An i.v. dose of 10 micrograms/kg of clonidine markedly reduced the variability of BP and HR after 20 min as indicated by a reduction in the variances by approximately one-half of the control value for BP and to one-third of the control value for HR. At this time, clonidine had not significantly altered BP or HR. Spectral profiles of systolic BP and HR illustrated the alterations in the spontaneous oscillations underlying these variance changes. Clonidine dramatically reduced the amplitude of BP and HR oscillations in the frequency region of 195-605 mHZ, which depends on the activity of the autonomic nervous system. We suggest that an increased sensitivity of the baroreflex is responsible for the apparent better control of BP and HR with clonidine.