Effects of no on the baroreflex control of heart rate and renal
nerve activity in conscious rabbits.
Liu, Jun-Li, Hiroshi Murakami, and Irving H. Zucker.
Department of Physiology and Biophysics, University of Nebraska
College of Medicine, Omaha, Ne. 68198-4575
APStracts 3:0022R, 1996.
Recent data suggests that nitric oxide (NO) plays a role in the
modulation of sympathetic nerve activity and baroreflex sensitivity.
Most of these studies have been carried out in anesthetized
preparations and little, if any comparison has been made on the
relative role of NO on the baroreflex control of heart rate and
sympathetic nerve activity. In the present studies, the effect of the
NO synthase inhibitor N-nitro-L-arginine (L-NNA) on the baroreflex
control of heart rate (HR) and renal sympathetic nerve activity
(RSNA) were investigated in conscious, instrumented rabbits.
Intravenous bolus injections of 13 mg/kg of L-NNA decreased baseline
HR (205.0+/-6.0 to 145.5+/-8.2 bpm; p<.05) without significant
changes in mean arterial pressure (MAP) and RSNA. L-NNA significantly
reduced the lower plateau of the HR-MAP curves and increased the
sensitivities of baroreflex control of HR and RSNA. L-arginine (600
mg/kg, i.v.) but not D-arginine reversed the above effects. The
effects of L-NNA on baseline HR were not completely blocked by
metoprolol (2 mg/kg) or by atropine (0.2 mg/kg). After pretreatment
with metoprolol, baroreflex sensitivity was reduced and L-NNA
increased baroreflex sensitivity back to the control level. After
pretreatment with atropine, L-NNA still reduced the lower plateau but
did not significantly affect baroreflex sensitivity. L-NNA increased
the HR responses but not the RSNA response to electrical stimulation
of the aortic nerve in chloralose anesthetized, sino-aortic
denervated (SAD) rabbits. L-NNA had no effect on the HR response to
right vagal stimulation. In both conscious intact and SAD rabbits, L
-NNA did not increase baseline RSNA. These results suggest that
endogenous NO decreases baroreflex control of HR and RSNA. Both
sympathetic and parasympathetic components play a role in the effects
of NO on the baroreflex control of HR. The effects of NO in the
central nervous system plays a more important role in the baroreflex
control of HR than of RSNA.
Received 18 August 1995; accepted in final form 3 January 1996.
APS Manuscript Number R514-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 29 January 96