Limitation of cardiac output by a coronary [alpha]1-constrictor
tone during exercise in dogs.
Kim, Song-Jung, Geoffrey Kline, and Patricia A. Gwirtz.
Department of Physiology, University of North Texas Health Science
Center, Fort Worth, Texas 76107-2699
APStracts 3:0105H, 1996.
This study was performed to examine whether an [alpha]1-constrictor
tone, which limits coronary functional hyperemia during exercise,
imposes a significant limitation on global cardiac performance as
determined by cardiac output. Seven dogs were chronically
instrumented to measure left ventricular pressure (LVP), dP/dtmax,
heart rate (HR), mean aortic pressure (MAP), circumflex blood flow
velocity (CFV), and cardiac output (CO) at rest and during submaximal
exercise. Either the selective [alpha]1-adrenergic antagonist
prazosin (0.5 mg) or the vasodilator adenosine was administered into
the circumflex artery during exercise at 6.4 kph/16% treadmill
incline. Exercise caused significant increase in MAP, HR, LVP,
dP/dtmax, CFV, stroke volume (SV), and CO, while systemic vascular
resistance (SVR) was significantly reduced. After intracoronary
[alpha]1-blockade with prazosin, CFV, dP/dtmax, SV and CO increased
further (17 +/- 2%, 19 +/- 3%, 16 +/- 2%, and 17 +/- 2%,
respectively) without changing MAP, HR, or SVR. Comparable increases
were observed when CFV was increased by a similar degree using the
direct vasodilator adenosine. These results indicate that increasing
coronary flow by removing a coronary [alpha]1-constrictor tone with
prazosin or by direct vasodilation with adenosine during submaximal
exercise leads to an increase in myocardial oxygen supply and, as a
result, cardiac pump performance (stroke volume and cardiac output).
Received 22 August 1994; accepted in final form 20 February 1996.
APS Manuscript Number H753-4.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 20 March 96