Differential regulation of the oscillations in sympathetic nerve activity and renal blood flow following volume expansion

Renal sympathetic nerve activity (RSNA) and renal blood flow (RBF) both show oscillations at various frequencies but the functional significance and regulation of these oscillations is not well understood. To establish whether the strength of these oscillations is under differential control we measured the frequency spectrum of RSNA and RBF following volume expansion in conscious rabbits. Seven days prior to experiment animals underwent surgery to implant an electrode for recording renal nerve activity and a flow probe for recording RBF. Volume expansion (Haemaccel, 1.5 ml min−1 kg−1 for 15 min) resulted in a 25±5% decrease in mean RSNA, paralleled by an increase in RBF to 60±12 ml min−1 from resting levels of 51±11 ml min−1. Renal denervated rabbits did not show an increase in RBF with volume expansion. Arterial baroreflexes were unaltered by volume expansion. Spectral analysis of the different frequencies in RSNA showed oscillations in RSNA between 0.2 and 0.4 Hz were selectively decreased following volume expansion (14±3 to 6±1% of total power in RSNA at <3 Hz). A corresponding decrease in the strength of oscillations in RBF at this frequency was also seen (20±6 to 8±2%). In contrast, the strength of respiratory (0.8–2.0 Hz) and cardiac (3–6 Hz) related rhythms did not change with volume expansion. These results show that selective changes in the different frequency components of RSNA can occur. We suggest that input from cardiopulmonary receptors and/or other vascular beds, and/or altered vascular resistance after volume expansion can reduce the strength of the 0.3 Hz oscillation independent of changes in arterial baroreflex control of RSNA.