Long-term control of renal blood flow: what is the role of the renal nerves?

We have developed a system for long-term continuous monitoring of cardiovascular parameters in rabbits living in their home cage to assess what role renal sympathetic nerve activity (RSNA) has in regulating renal blood flow (RBF) in daily life. Blood pressure, heart rate, locomotor activity, RSNA, and RBF were recorded continuously for 4 wk. Beginning 4-5 days after surgery a circadian rhythm, dependent on feeding time, was observed. When averaged over all days RBF to the innervated and denervated kidneys was not significantly different. However, control of RBF around these mean levels was dependent on the presence of the renal sympathetic nerves. In particular we observed episodic elevations in heart rate and other parameters associated with activity. In the denervated kidney, during these episodic elevations, the increase in renal resistance was closely related to the increase in arterial pressure. In the innervated kidney the renal resistance response was significantly more variable, indicating an interaction of the sympathetic nervous system. These results indicate that whereas overall levels of RSNA do not set the mean level of RBF the renal vasculature is sensitive to episodic increases in sympathetic nerve activity.

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.

Differential neural control of intrarenal blood flow.

To test whether renal sympathetic nerve activity (RSNA) can differentially regulate blood flow in the renal medulla (MBF) and cortex (CBF) of pentobarbital sodium-anesthetized rabbits, we electrically stimulated the renal nerves while recording total renal blood flow (RBF), CBF, and MBF. Three stimulation sequences were applied 1) varying amplitude (0.5-8 V), 2) varying frequency (0.5-8 Hz), and 3) a modulated sinusoidal pattern of varying frequency (0. 04-0.72 Hz). Increasing amplitude or frequency of stimulation progressively decreased all flow variables. RBF and CBF responded similarly, but MBF responded less. For example, 0.5-V stimulation decreased CBF by 20 +/- 9%, but MBF fell by only 4 +/- 6%. The amplitude of oscillations in all flow variables was progressively reduced as the frequency of sinusoidal stimulation was increased. An increased amplitude of oscillation was observed at 0.12 and 0.32 Hz in MBF and to a lesser extent RBF, but not CBF. MBF therefore appears to be less sensitive than CBF to the magnitude of RSNA, but it is more able to respond to these higher frequencies of neural stimulation.

Modelling the neural control of intrarenal blood flow.

1. The aim of the present study was to produce a mathematical model that describes the way dynamic changes in renal sympathetic nerve activity affect renal, cortical and medullary blood flow. 2. Cortical blood flow (CBF) and medullary blood flow (MBF) were measured using laser-Doppler flowmetry and (total) renal blood flow (RBF) was measured by transit-time flowmetry in six pentobarbitone-anaesthetized rabbits. The renal nerves were stimulated with rectangular pulses of 2 msec width and constant voltage at frequencies of 0.5, 1, 1.5, 2 and 3 Hz. 3. An exponential function with two parameters was applied; steady state gain and a dynamic constant for the blood flow reduction with stimulation. The steady state gain coefficients were similar for RBF and CBF, but significantly less for MBF. The time taken to reach minimum flow was less for MBF than for RBF and CBF. 4. The model parameters indicate that there is differential neural control of CBF and MBF.