Submitted on February 10, 2007
Accepted on July 3, 2007
Sex differences in forebrain and cardiovagal responses at the onset of isometric handgrip exercise: a retrospective fMRI study
Savio W. Wong1, Derek S. Kimmerly1, Nicholas Masse1, Ravi S. Menon2, David F. Cechetto3, and J. Kevin Shoemaker4*
1 School of Kinesiology, The University of Western Ontario, London, Canada
2 Centre for Functional & Metabolic Mapping, Robarts Research Institute, London, Canada
3 Department of Anatomy and Cell Biology, The University of Western Ontario, London, Canada
4 School of Kinesiology, The University of Western Ontario, London, Canada; Department of Physiology and Pharmacology, The University of Western Ontario, London, Canada
* To whom correspondence should be addressed. E-mail: kshoemak{at}uwo.ca.
In general, cardiac regulation is dominated by the sympathetic and parasympathetic nervous systems in men and women respectively. Our recent study had revealed sex differences in the forebrain network associated with sympathoexcitatory response to baroreceptor unloading. The present study further examined the sex differences in forebrain modulation of cardiovagal response at the onset of isometric exercise. Forebrain activity in healthy men (n = 8) and women (n = 9) was measured using functional magnetic resonance imaging (fMRI) during four trials of 5% and 35% maximal voluntary contraction (MVC) handgrip exercise. Heart rate (HR), mean arterial pressure (MAP) and muscle sympathetic nerve activity (MSNA) were collected in a separate recording session. During the exercise, HR and MAP increased progressively while MSNA was suppressed (p<0.05). Relative to men, women demonstrated smaller HR (8 ± 2 bpm vs.18 ± 3 bpm) and MAP (3 ± 2 mmHg vs. 11 ± 2 mmHg) responses to the 35% MVC trials (p<0.05). Although a similar forebrain network was activated in both groups, the smaller cardiovascular response in women was reflected in a weaker insular cortex (IC) activation. Nevertheless, men did not show a stronger deactivation at the ventral medial prefrontal cortex (vMPFC) that has been associated with modulating cardiovagal activity. In contrast, the smaller cardiovascular response in women related to their stronger suppression of the dorsal anterior cingulate cortex (dACC) activity that has been associated with sympathetic control of the heart. Our findings revealed sex differences in both the physiologic and forebrain responses to isometric exercise.
