The present study compared the spontaneous fluctuations of renal sympathetic nerve activity (SNA) and lumbar SNA in conscious rats. Renal and lumbar SNAs were simultaneously recorded in 6 freely behaving rats during approximately 4 h. The relation between spontaneous fluctuations of SNAs was assessed by computing regression lines relating 1-min average values of SNAs. Although significant (P = 0.0017 ± 0.0015), the coefficients of determination revealed a rather low level of shared variance between SNAs (r2 = 0.107 ± 0.029). Similarly, coherence (a measure of linear coupling in the frequency domain) did not reach significance for fluctuations lasting from ~5 s to 54.6 min. However, for oscillations associated with arterial pressure Mayer waves (oscillations generated by the baroreflex with period near 2.5 s), coherence peaked at 0.78 ± 0.04, thus revealing a strong linear coupling. Coherence was also significant (0.54 ± 0.05) in the band containing respiratory frequencies (usually between 1 and 2 Hz). Mathematically removing the influence of arterial pressure eliminated the relationship between SNAs at Mayer wave’s frequency, thus pointing to a common baroreflex origin for oscillations of both SNAs. In addition, both SNAs were tightly coupled during drug-induced changes in arterial pressure. We conclude that spontaneous, slow fluctuations of renal and lumbar SNAs are mostly dissociated, although they vary in a coordinated way during both spontaneous and forced baroreflex solicitation. These findings argue for caution in extrapolations from one regional SNA to overall sympathetic outflow.
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