Frequency-dependent modulation of renal blood flow by renal nerve activity in conscious rabbits

To examine the influence of the various frequency components of renal sympathetic nerve activity (RSNA) on renal blood flow (RBF) dynamics, a Doppler flow probe and renal nerve electrode were implanted on the left renal artery of 10 rabbits. Experiments were performed 4-9 days after surgery. Physiological changes in RSNA were induced by subjecting the rabbits to periods of breathing hypoxic gas mixtures. Signals were sampled at 1 kHz and analyzed by spectral analysis. During moderate hypoxia (arterial PO2 = 44 +/- 1 mmHg), arterial pressure and heart rate did not change, averaged RSNA increased by 90 +/- 7%, and RBF fell by 18 +/- 3%. In a separate group of renal-denervated rabbits (n = 6), no changes in RBF occurred during hypoxia. In intact rabbits, 53 +/- 4% of spectral density power of RSNA was found at the cardiac frequency and the remainder was predominantly coupled to respiration (approximately 0.9 Hz). During moderate hypoxia the amplitude of the RSNA oscillations increased 17 +/- 6 times at the cardiac frequency and 10 +/- 3 times at the respiration-related frequency. Modulation of RBF variability by the fluctuations of RSNA at the cardiac- and respiration-related frequency was, however, small. The normalized transfer gain between RSNA and RBF was approximately 0.1 at > 0.5 Hz. This means that, at > 0.5 Hz, maximally 10% of the amplitude of the RSNA oscillations is transmitted to corresponding RBF fluctuations. These transfer properties did not change during hypoxia. At < 0.5 Hz the transfer gain between RSNA and RBF increased. During moderate hypoxia, 0.3-Hz coherent oscillations of RSNA and RBF were found. In renal-denervated rabbits, 0.3-Hz oscillations in RBF were absent. Thus the renal vasculature was able to follow relatively low-frequency (< 0.5-Hz) fluctuations of RSNA and responded with corresponding oscillations in RBF. In contrast, the renal vasculature responded with increased constriction at the high-frequency (> 0.5-Hz) fluctuations of RSNA. These findings suggest that, in conscious rabbits, high-frequency oscillations of RSNA contribute to the vasoconstrictor tone, whereas the lower frequencies of RSNA contribute to the variability of RBF.