A transfer function method for the continuous assessment of baroreflex control of renal sympathetic nerve activity in rats

Abstract

The present study examined whether the gain of the transfer function relating cardiac-related rhythm of renal sympathetic nerve activity (RSNA) to arterial pressure (AP) pulse might serve as a spontaneous index of sympathetic baroreflex sensitivity (BRS). AP and RSNA were simultaneously recorded in conscious rats, either baroreceptor-intact (control, n = 11) or with partial denervation of baroreflex afferents [aortic baroreceptor denervated (ABD; n = 10)] during 1-h periods of spontaneous activity. Transfer gain was calculated over 58 adjacent 61.4-s periods (segmented into 10.2-s periods). Coherence between AP and RSNA was statistically (P < 0.05) significant in 90 +/- 3% and 56 +/- 10% of cases in control and ABD rats, respectively. Transfer gain was higher (P = 0.0049) in control [2.39 +/- 0.13 normalized units (NU)/mmHg] than in ABD (1.48 +/- 0.22 NU/mmHg) rats. In the pooled study sample, transfer gain correlated with sympathetic BRS estimated by the vasoactive drug injection technique (R = 0.75; P < 0.0001) and was inversely related to both time- (standard deviation; R = -0.74; P = 0.0001) and frequency-domain [total spectral power (0.00028-2.5 Hz); R = -0.82; P < 0.0001] indices of AP variability. In control rats, transfer gain exhibited large fluctuations (coefficient of variation: 34 +/- 3%) that were not consistently related to changes in the mean level of AP, heart rate, or RSNA. In conclusion, the transfer function method provides a continuous, functionally relevant index of sympathetic BRS and reveals that the latter fluctuates widely over time.

Baroreflex control of lumbar and renal sympathetic nerve activity in conscious rats

“…The abdominal wall was then closed in layers and the rat was turned onto its right side. Through a flank incision, a branch of the left renal nerve was dissected, and a recording electrode was implanted as previously described in detail (7,13,14). The electrode cable was also exteriorized at the back of the neck.…”

“…One major advantage of the method is that its temporal resolution is about 1 min. Therefore, it provides a continuous estimate of sympathetic BRS and has already been used to show that the renal BRS exhibits minute-to-minute spontaneous variations (14).The objective of the present study was to compare the baroreflex control of RSNA and LSNA in the same conscious rats. The lumbar nerve was chosen because it innervates mainly the skeletal muscle circulation of the hindlimb (1, 32).…”

“…It is based on the computation of the gain of the transfer function relating RSNA and AP at heart rate (HR) frequency (24). The method has been validated in partially (aortic) baroreceptor denervated rats (14). One major advantage of the method is that its temporal resolution is about 1 min.…”

“…One major advantage of the method is that its temporal resolution is about 1 min. Therefore, it provides a continuous estimate of sympathetic BRS and has already been used to show that the renal BRS exhibits minute-to-minute spontaneous variations (14).…”

“…Pharmacological estimates of the sympathetic BRS were obtained by means of the sequential administration of sodium nitroprusside and phenylephrine (7,13). Spontaneous renal and lumbar BRS were assessed with the transfer function method (14).…”