Modulation of the frequency and amplitude of sympathetic discharges
by baroreflexes during normoxia and hypoxia in conscious rabbits.
Malpas, Simon C., Robert D. Bendle, Geoffrey A. Head, and James H.
Ricketts.
Baker Medical Research Institute, Prahran, Victoria, Australia
APStracts 3:0359H, 1996.
Sympathetic nerve activity (SNA) from multifiber preparations exhibits
two distinct components, the frequency at which discharges occur and
their relative amplitude (reflecting the number of activated nerve
fibers within each burst). These two components may respond
independently to various afferent inputs indicating separate central
controlling processes. We examined the response in the frequency and
amplitude of renal SNA to changes in baroreceptor activity and the
effect of two forms of hypoxia on this relationship in 9 conscious
rabbits. Rabbits breathed either room air or one of two hypoxic gas
mixtures (10% O2 or 10% O2+3% CO2 balance nitrogen) for 20 min,
during which baroreflexes were stimulated by ramp increases and then
decreases in arterial pressure using i.v. phenylephrine (0.5mg/ml)
and nitroprusside (1mg/ml) (total arterial pressure range induced
80mmHg). Hypoxia with 10% O2 significantly increased the resting
frequency of SNA, prior to baroreflex modifications, from 2.15+0.18
to 2.82+0.25 discharges per second and to 3.20+21 during hypoxia with
10% O2+3% CO2. The amplitude of sympathetic discharges was increased
44+5% over control levels during 10% O2 but not further increased by
the addition of 3% CO2. The baroreflex curve for total SNA (one
second averages of the integrated neurogram) showed a graded response
to the two hypoxic stimuli with significant increases in the upper
plateau, gain and resting point on the curves. However the baroreflex
curve for the frequency or amplitude of sympathetic discharges did
not show graded responses to each hypoxic treatment. The frequency
baroreflex curve was sigmoidal and not changed from air during 10%
O2. During 10% O2+3% CO2 the gain (responsiveness) of the curve was
increased although the range of frequencies occurring was unaltered.
The baroreflex curve for the amplitude showed similar responses to
the two hypoxic stimuli, namely an increase in the upper plateau,
gain and resting point of the curve. We conclude that the frequency
and amplitude of sympathetic discharges are able to respond
differentially to changes in afferent stimuli. Given that alterations
in the frequency and the recruitment of sympathetic fibers
(amplitude) to the kidney may have differing effects, this phenomenon
may provide a previously unknown level of renal haemodynamic control
through the interaction of specific afferent inputs to the central
nervous system.
Received 30 August 1995; accepted in final form 22 May 1996.
APS Manuscript Number H825-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 29 August 1996