Journal of Neurophysiology, Vol 70, Issue 6 2596-2605, Copyright © 1993 by APS
Flow and thermal responses of aortic baroreceptors
P. A. Munch
Department of Internal Medicine, University of California, Davis 95616.
1. The aims of this study were 1) to determine whether the impulse activity
in rabbit aortic baroreceptors (BRs) was influenced by changes in
nonpulsatile flow through the aortic lumen and 2) to examine the BR and
aortic arch responses to changes in temperature. 2. An in vitro aortic
arch-aortic nerve preparation was used to record suprathreshold
steady-state discharge in a total of 29 single-unit BRs from 12 New Zealand
white rabbits. Changes in BR frequency were measured relative to control
and were recorded simultaneously with aortic arch pressure, flow,
temperature, diameter, and calculated wall shear stress (Sw). 3. With
pressure held constant, stair-step increases in flow (3-18 ml/min)
constricted the arch and evoked two types of BR responses: activation in
most units (15 of 17 BRs tested) and inhibition in 2 units. The activation
response appeared closely related to the changes in flow and Sw, but also
appeared related to uncontrolled changes in perfusate temperature. 4. When
the effects of temperature were examined more closely with pressure and
flow held constant, controlled step increases in temperature (between 32
and 42 degrees C) constricted the arch and again evoked two BR responses:
activation in 11 of 14 BRs tested and inhibition in 3 units. The Q10 for
the activation response was 1.55 +/- 0.08 (mean +/- SE), which was not
significantly different from the Q10 for activation when temperature varied
with flow (1.65 +/- 0.1, P < 0.05). Thus the effect of temperature on
most BRs was directionally and quantitatively similar in the presence and
absence of changes in flow. 5. Last, when flow was examined over a wide
range (15-515 ml/min) with temperature and pressure held constant,
stair-step increases in flow failed to produce any change in BR frequency
in each of 15 fibers tested (10 arches). The lack of response was not due
to a functionally damaged preparation because the usual BR and aortic arch
responses to pressure and to drug-evoked vasoconstriction (norepinephrine)
and endothelial-mediated vasodilation (acetylcholine) were intact. 6. These
results demonstrate that aortic BRs in rabbits are not sensitive to flow
and thus are not likely influenced by fluctuations in cardiac output apart
from associated changes in pressure. The aortic BRs are affected directly
by variations in temperature and in some cases indirectly by
temperature-related vasoconstriction. The effects of temperature may have
important implications for neural control of the circulation when core
temperature varies because of physiological and environmental stress.
