Differential responses of frequency components of renal sympathetic nerve activity to arterial pressure changes in conscious rats

Abstract

The present study examined the effects of baroreceptor loading and unloading on the various rhythms present in the renal sympathetic nerve activity (RSNA) of 10 conscious rats. Short-lasting (4-5 min), steady-state decreases (from -10 to -40 mmHg) and increases (from 5 to 30 mmHg) in arterial pressure (AP) were induced by the intravenous infusion of sodium nitroprusside and phenylephrine, respectively. The relationship between changes in AP level and RSNA total power (fast Fourier transform analysis; 0-25 Hz) was characterized by an inverse sigmoid function. Basal AP was located 6.3 mmHg above AP at the midrange of the curve, that is, near the lower plateau. Sigmoid relationships were also observed for spectral powers in the low (LF, 0.030-0.244 Hz), respiratory (0.79-2.5 Hz) and high-frequency (HF, 2.5-25 Hz) bands. In contrast, in the MF band (0.27-0.76 Hz) containing oscillations associated with Mayer waves, the AP-RSNA power relationship showed a bell curve shape with a maximum at 21 mmHg below basal AP. Similarly, changes in RSNA power at the frequency of the heart beat were well characterized by a bell curve reaching a maximum at 22 mmHg below basal AP. Under baseline conditions, LF, MF, respiratory and HF powers contributed approximately 3, 10, 18, and 69% of the total RSNA power, respectively. The pulse-synchronous oscillation of RSNA accounted for only 11 +/- 1% of HF power. The contribution of HF power to total power did not change consistently with AP changes. Therefore, most of the baroreflex-induced changes in RSNA are mediated by changes in the amplitude of fast, irregular fluctuations.

Analysis of sympathetic neural discharge in rats and humans

“…During druginduced baroreceptor loading and unloading, the amplitude of LF oscillations of RSNA shows biphasic changes: reaching a maximum at approximately K20 mmHg and then decreasing at larger AP reductions (Bertram et al 2005). These data point to a complex relationship between the mean level of RSNA and the amplitude of LF oscillations of RSNA.…”

“…In conscious rats, the amplitude of the cardiac-related oscillation of RSNA follows a bell-shaped curve in response to drug-induced changes in arterial pressure. Such behaviour is well predicted by a simple linear model where pulse-synchronous baroreceptor activity periodically inhibits a steady level of SNA, with the only assumption being that baroreceptor activity occurs above a threshold value of arterial pressure (Bertram et al 2005). Regarding the function of these oscillations, it has been proposed that they might serve to set the mean level of vasoconstrictor tone.…”

“…Finally, a common limitation to all three SNAs is that, when the nerve is not cut distal from the recording electrode, a contribution from the activity of afferent nerves cannot be ruled out. In the case of the renal nerve, this contribution is probably negligible as RSNA recorded after acute ganglionic blockade is not consistently higher than post-mortem activity (Bertram et al 2005).…”

“…Chronic sinoaortic baroreceptor denervation has been reported either to diminish (Kunitake & Kannan 2000) or to leave unchanged (Julien et al 2003) the respiratory oscillations of RSNA in conscious rats. During infusion of vasoactive drugs, the amplitude of these oscillations is reflexly modulated (Bertram et al 2005). As spectral power in the respiratory band contributes approximately 20 per cent of total spectral power (when computed from 0.03 to 25 Hz) of RSNA in conscious rats, baroreflex-mediated changes in the amplitude of respiratory oscillations of RSNA significantly contribute to baroreflex-mediated changes in total spectral power and mean RSNA.…”

Patients With Continuous-Flow Left Ventricular Assist Devices Provide Insight in Human Baroreflex Physiology

“…9,20,32 One limitation of our study is the relatively low number of patients. Furthermore, because we did not include unstable patients or patients with a complicated postoperative course, our findings have to be interpreted with caution.…”