Transfer function analysis from arterial baroreceptor afferent activity to renal nerve activity in rabbits

In vagotomized and anesthetized rabbits, aortic pressure (AP), aortic depressor nerve activity (ANA), and renal sympathetic nerve activity (RNA) were simultaneously measured while perturbing AP randomly. To quantitatively characterize the role of the arterial baroreflex system in generating RNA, we determined the transfer function (TF) of the central baroreflex arc from ANA to RNA in the frequency domain (0.02-5 Hz). The magnitude of squared coherence was > 0.5, the phase was close to -180 degrees, and the gain of TF was flat over 0.02-0.3 Hz, indicating that changes in RNA were linearly and instantaneously but inversely related to changes in ANA over this frequency range. Above 0.3 Hz, the coherence was low, suggesting that RNA unrelated to ANA existed in the frequency range. In animals without AP perturbations, power spectrum of RNA resided over 0.2-5 Hz with a broad peak at 1 Hz, which may represent central activity. Our results suggest that over 0.02-0.3 Hz the relationship between arterial baroreceptor afferent nerve activity and RNA is linear and instantaneous but above 0.3 Hz it is not linear possibly due to an interaction between central activity and arterial baroreflex.