The arterial baroreceptor reflex of the rat exhibits positive feedback properties at the frequency of Mayer waves

1. Modelling studies have led to the proposal that Mayer waves (similar to 0.4 Hz in rats) could result from a resonance phenomenon in a feedback cont rol loop. In this study, we investigated the presence of a resonance freque ncy in the arterial baroreceptor reflex loop, i.e. a particular frequency a t which arterial pressure feeds back positively to the baroreceptors. 2. Frequency responses of mean arterial pressure (MAP) to aortic depressor nerve (ADN) stimulation were studied in fifteen urethane anaesthetized, ven tilated rats with cardiac autonomic blockade. The ADN was stimulated using rectangular trains of impulses (2 ms, 100 Hz) delivered at frequencies rang ing from 0.1 to I Hz. Phase angles between impulses and MAP were calculated using cross-spectral analysis based on a fast Fourier transform algorithm. 3. Rhythmic ADN stimulation induced regular MAP oscillations at the expecte d frequencies that were attenuated by ol-adrenoceptor blockade and abolishe d after ganglionic blockade. The relationship between impulse and MAP oscil lations was characterized by a strong coherence and a positive phase shift at low frequencies, indicating that impulses led MAP with respect to the ou t-of-phase pattern. Deviation of the phase from the out-of-phase behaviour was mainly due to the presence of a fixed time delay (similar to 0.8 s) bet ween ADN stimuli and MAP changes. Phase angles fell to zero at 0.42 +/- 0.0 2 Hz. 4. In rats, the arterial baroreceptor reflex exhibits a resonance frequency close to the frequency of spontaneously occurring Mayer waves. The reflex therefore seems the most likely origin for the Mayer waves.