Spectral analysis of blood pressure and heart rate in conscious rats: effects of autonomic blockers.

We investigated the fluctuations which underly the spontaneous variability of blood pressure and heart rate in conscious rats. Intrafemoral blood pressure was computed to generate evenly spaced signals (systolic, diastolic, mean blood pressure, heart rate) at 200 ms intervals. This equidistant sampling allowed a direct spectral analysis using a Fast Fourier Transform algorithm. Systolic blood pressure and heart rate exhibited low-frequency oscillations (Mayer waves, 20-605 mHz) and a high- frequency oscillation related to respiration (1855 mHz). The respiratory fluctuations in heart rate were almost abolished by vagal blockade (atropine). Heart rate fluctuations in the low-frequency regime were diminished by vagal blockade or cardiac sympathetic blockade (atenolol). The respiratory frequency fluctuations in systolic blood pressure were markedly increased by alpha-sympathetic blockade (prazosin). In contrast, the low-frequency oscillations in systolic blood pressure were reduced by alpha-sympathetic blockade. These data indicate that in conscious rats: (1) the heart rate oscillation with respiration is vagally mediated; (2) the heart rate fluctuation in the low-frequency range is jointly mediated by beta-sympathetic and parasympathetic activities; (3) the respiratory oscillation in systolic blood pressure depends on fluctuations in cardiac output and is normally counteracted by the sympathetic tone; (4) the low-frequency oscillations in systolic blood pressure reflect the sympathetic activity to the resistance vessels.