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1 Baker Medical Research
Institute,
We have
examined the role of the renal sympathetic nerves in the renal blood
flow (RBF) response to hemorrhage in seven conscious rabbits.
Hemorrhage was produced by blood withdrawal at 1.35 ml · min
1 · kg1
for 20 min while RBF and renal sympathetic nerve activity (RSNA) were
simultaneously measured. Hemorrhage was associated with a gradual
increase in RSNA and decrease in RBF from the 4th min. In seven
denervated animals, the resting RBF before hemorrhage was significantly
greater (48 ± 1 vs. 31 ± 1 ml/min intact), and the decrease in
RBF did not occur until arterial pressure also began to fall (8th min);
however, the overall percentage change in RBF by 20 min of blood
withdrawal was similar. Spectral analysis was used to identify the
nature of the oscillations in each variable. Before hemorrhage, a
rhythm at ~0.3 Hz was observed in RSNA, although not in RBF, whose
spectrogram was composed mostly of lower-frequency (<0.25 Hz)
components. The denervated group of rabbits had similar frequency
spectrums for RBF before hemorrhage. RSNA played a role in dampening
the effect of oscillations in arterial pressure on RBF as the transfer
gain between mean arterial pressure (MAP) and RBF for frequencies
>0.25 Hz was significantly less in intact than denervated rabbits
(0.83 ± 0.12 vs. 1.19 ± 0.10 ml · min1 · mmHg1).
Furthermore, the coherence between MAP and RBF was also significantly higher in denervated rabbits, suggesting tighter coupling between the
two variables in the absence of RSNA. Before the onset of significant
decreases in arterial pressure (up to 10 min), there was an increase in
the strength of oscillations centered around 0.3 Hz in RSNA. These were
accompanied by increases in the spectral power of RBF at the same
frequency. As arterial pressure fell in both groups of animals, the
dominant rhythm to emerge in RBF was centered between 0.15 and 0.20 Hz
and was present in intact and denervated rabbits. It is speculated that
this is myogenic in origin. We conclude that RSNA can induce
oscillations in RBF at 0.3 Hz, plays a significant role in altering the
effect of oscillations in arterial pressure on RBF, and mediates a
proportion of renal vasoconstriction during hemorrhage in conscious
rabbits.
conscious rabbit; sympathetic nervous system; spectral analysis; renal denervation
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