Is active skeletal muscle functionally vasoconstricted during
dynamic exercise in conscious dogs?.
O'leary, Donal S., Evelyn D. Robinson, and John L. Butler.
Department of Physiology, Wayne State University School of Medici
ne, Detroit, MI 48201, (313) 577-1540, (313) 577-5494 FAX, email -
doleary@med.wayne.edu
APStracts 3:0299R, 1996.
We investigated whether the increase in hindlimb blood flow and
vascular conductance in conscious dogs during graded dynamic exercise
is functionally restrained by the sympathetic nervous system. Dogs
were chronically instrumented to monitor terminal aortic blood flow
(TAQ) as an index of hindlimb skeletal muscle blood flow and mean
arterial pressure (MAP). The extent of functional sympathetic tone
was assessed by measuring the increase in TAQ and terminal aortic
vascular conductance (TAC, calculated as TAQ/MAP) in response to i.a.
infusion of the [alpha] - adrenergic antagonist prazosin (PZ, 50
[mu]g/kg) into the hindlimbs at rest and during steady state dynamic
(treadmill) exercise ranging from mild (3.2 kph, 0% grade) to
moderately-heavy (8 kph, 15% grade) workloads. This dose of PZ
completely abolished the large hindlimb vasoconstrictor response to
i.a. phenylephrine (1 [mu]g/kg). At rest PZ increased TAQ by 0.10 +/-
0.02 l min-1 and TAC by 1.85 +/- 0.53 ml min-1 mmHg-1. During
exercise, as workload increased and the control levels of TAQ and TAC
rose progressively, _TAQ and _TAC with PZ infusion also increased. At
the highest workload, PZ increased TAQ by 0.41 +/- 0.07 l min-1 and
TAC by 4.81 +/- 0.38 ml min-1 mmHg-1. The increases in TAQ and TAC
with PZ were linearly related to the control level of TAQ indicating
that as workload increases, progressively greater restraint of muscle
vasodilation by the sympathetic nervous system occurs. We conclude
that during dynamic exercise in conscious dogs the sympathetic
nervous system progressively restrains the normal vasodilation in
active skeletal muscle thereby limiting skeletal muscle perfusion.
Received 18 March 1996; accepted in final form 24 July 1996.
APS Manuscript Number R166-6.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 29 August 1996