Resonance in the renal vasculature evoked by activation of the sympatheticnerves

We examined the ability of different frequencies in sympathetic nerve activ ity (SNA) to induce oscillations in renal blood flow (RBF). In anesthetized rabbits the renal nerves were stimulated using modulated sine patterns (ba se 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 indu ced oscillations in RBF were calculated using spectral analysis. Although f aster 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 freq uency, indicating a low-pass filtering/integrating characteristic of the va sculature. Frequencies <0.6 Hz were associated with increasing ability to i nduce 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 wa s a distinct peak in the frequency response curve, indicating the vasculatu re was more sensitive in this frequency band to sympathetic stimulation. Bl ockade of endogenous nitric oxide by N-G-nitro-L-arginine methyl ester (L-N AME; 20 mg/kg) did not alter resting RBF levels nor was the low-pass filter ing/integrating characteristic of the vasculature to nerve stimulation chan ged (i.e., the curve was not shifted left or right); however, there was a s elective increase in the sensitivity to stimulation at 0.16 Hz, i.e., large r oscillations in RBF were evoked. These results indicate an ability of SNA to induce resonant oscillations in the renal vasculature and that there ma y 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 glom erular filtration.