Guild, Sarah-Jane, Paul C. Austin, Michael Navakatikyan, John V. Ringwood, and Simon C. Malpas. Dynamic relationship between sympathetic nerve activity and renal blood flow: a frequency domain approach. Am J Physiol Regulatory Integrative Comp Physiol 281: R206-R212, 2001.-Blood pressure displays an oscillation at 0.1 Hz in humans that is well established to be due to oscillations in sympathetic nerve activity (SNA). However, the mechanisms that control the strength or frequency of this oscillation are poorly understood. The aim of the present study was to define the dynamic relationship between SNA and the vasculature. The sympathetic nerves to the kidney were electrically stimulated in six pentobarbital-sodium anesthetized rabbits, and the renal blood flow response was recorded. A pseudo-random binary sequence (PRBS) was applied to the renal nerves, which contains equal spectral power at frequencies in the range of interest (Ͻ1 Hz). Transfer function analysis revealed a complex system composed of low-pass filter characteristics but also with regions of constant gain. A model was developed that accounted for this relationship composed of a 2 zero/4 pole transfer function. Although the position of the poles and zeros varied among animals, the model structure was consistent. We also found the time delay between the stimulus and the RBF responses to be consistent among animals (mean 672 Ϯ 22 ms). We propose that the identification of the precise relationship between SNA and renal blood flow (RBF) is a fundamental and necessary step toward understanding the interaction between SNA and other physiological mediators of RBF. modeling; pseudo-random binary sequence SYMPATHETIC NERVE ACTIVITY (SNA) has been proposed to play an important role in the regulation of renal blood flow (RBF) (12). Whereas much previous research has focused on how the mean level of SNA regulates the mean level of RBF in response to a range of afferent stimuli (10), there has been little consideration given to fact that SNA is a signal made up of multiple frequency bands, ranging from 10 to 0.1 Hz (9). Surprisingly little is known about how these frequency components impact on the renal vasculature. Mathematical models have been developed to describe the effect blood pressure has on RBF (4-6). These models have proven useful in understanding the dynamics of autoregulation and tubuloglomerular feedback; however, there is a paucity of information on the dynamics of the neural control of RBF. Information on how the various frequencies in SNA regulate RBF is likely to be valuable in understanding the origin of oscillations present in blood pressure (2) and in predicting how diseases or therapeutic treatments that alter SNA could affect the control of RBF and potentially blood pressure.Previous work by our lab has determined that the renal vasculature appears only able to follow frequencies in SNA below 0.7 Hz, with frequencies above this level producing steady vascular tone (11). The corresponding frequency response characteristics suggested that a ...

We examined the ability of different frequencies in sympathetic nerve activity (SNA) to induce oscillations in renal blood flow (RBF). In anesthetized rabbits the renal nerves were stimulated using modulated sine patterns (base frequency 5 Hz, 5-ms duration pulses) that varied in amplitude between 0 and 10 V at a frequency between 0.04 and 1.0 Hz. The strengths of the induced oscillations in RBF were calculated using spectral analysis. Although faster rhythms in simulated SNA >0.6 Hz contributed to the level of vascular tone, 95% of the power in the frequency response curve was below this frequency, indicating a low-pass filtering/integrating characteristic of the vasculature. Frequencies <0.6 Hz were associated with increasing ability to induce oscillations in RBF. The ability of an SNA rhythm at 0.6 Hz to induce a rhythm in RBF was 21 times less than that at 0.25 Hz. At 0.16 Hz there was a distinct peak in the frequency response curve, indicating the vasculature was more sensitive in this frequency band to sympathetic stimulation. Blockade of endogenous nitric oxide by N G-nitro-l-arginine methyl ester (l-NAME; 20 mg/kg) did not alter resting RBF levels nor was the low-pass filtering/integrating characteristic of the vasculature to nerve stimulation changed (i.e., the curve was not shifted left or right); however, there was a selective increase in the sensitivity to stimulation at 0.16 Hz, i.e., larger oscillations in RBF were evoked. These results indicate an ability of SNA to induce resonant oscillations in the renal vasculature and that there may be active and passive modulators of these responses. Naturally occurring oscillations in SNA <0.6 Hz are likely to contribute to the dynamic control of RBF, ensuring it responds rapidly and with high gain to the stimuli of daily life, while filtering out the faster oscillations ensures stable glomerular filtration.

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