Submitted on November 17, 2008
Revised on January 23, 2009
Accepted on February 5, 2009
Whole body heat stress attenuates baroreflex control of muscle sympathetic nerve activity during post exercise muscle ischemia
Jian Cui1, Manabu Shibasaki2, Scott L. Davis3, David A. Low4, David Melvin Keller2, and Craig G. Crandall2*
1 Penn State University, College of Medicine
2 Presbyterian Hospital of Dallas
3 University of Texas Southwestern Medical Center at Dallas
4 Institute for Exercise and Environmental Medicine
* To whom correspondence should be addressed. E-mail: craigcrandall{at}texashealth.org.
Both whole-body heat stress and stimulation of muscle metabolic receptors activate muscle sympathetic nerve activity (MSNA) through non-baroreflex pathways. In addition to stimulating muscle metaboreceptors, exercise has the potential to increase internal temperature. Although we and others report that passive whole-body heating does not alter the gain of the arterial baroreflex, it is unknown whether increased body temperature, often accompanying exercise, affects baroreflex function when muscle metaboreceptors are stimulated. This project tested the hypothesis that whole-body heating alters the gain of baroreflex control of muscle sympathetic nerve activity (MSNA) and heart rate during muscle metaboreceptor stimulation engaged via post-exercise muscle ischemia (PEMI). MSNA, blood pressure (BP, Finometer), and heart rate were recorded from 11 healthy volunteers. The volunteers performed isometric handgrip exercise until fatigue, followed by 2.5 min of PEMI. During PEMI, BP was acutely reduced and then raised pharmacologically using the modified Oxford technique. This protocol was repeated 2 - 3 times when volunteers were normothermic, and again during heat stress (increase core temperature ~ 0.7 °C) conditions. The slope of the relationship between MSNA and BP during PEMI was less negative (i.e., decreased baroreflex gain) during whole-body heating when compared with the normothermic condition (-4.34 ± 0.40 to -3.57 ± 0.31 units/beat/mmHg, respectively, p=0.015). The gain of baroreflex control of heart rate during PEMI was also decreased during WBH (p<0.001). These findings indicate that whole-body heat stress reduces baroreflex control of MSNA and heart rate during muscle metaboreceptor stimulation.
