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First published online on May 12, 2005.
 Copyright © 2005 by The Physiological Society
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jphysiol.2005.087320v1
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Received March 23, 2005
Revised April 11, 2005
Accepted after revision May 11, 2005

DIFFERENTIAL RESPONSES TO CO2 AND SYMPATHETIC STIMULATION IN THE CEREBRAL AND FEMORAL CIRCULATIONS IN HUMANS

Philip N Ainslie1, Jon C Ashmead1, Kojiro Ide1, Barbara J Morgan2, and Marc J Poulin1*

1 University of Calgary
2 University of Wisconsin-Madison

* To whom correspondence should be addressed. E-mail: poulin{at}ucalgary.ca.

The relative importance of CO2 and sympathetic stimulation in the regulation of cerebral and peripheral vasculatures has not been previously studied in humans. We investigated the effect of sympathetic activation, produced by isometric handgrip (HG) exercise, on cerebral and femoral vasculatures during periods of isocapnia and hypercapnia. In 14 healthy males [28.1 ± 3.7 (SD) yrs], we measured flow velocity (VP; transcranial Doppler ultrasound) in the middle cerebral artery during euoxic isocapnia (ISO, +1 Torr above rest) and two levels of euoxic hypercapnia (HC5, end-tidal PCO2, PETCO2, = +5 Torr above ISO; HC10, PETCO2 = +10 above ISO). Each PETCO2 level was maintained for 10 min using the dynamic end-tidal forcing technique during which increases in sympathetic activity were elicited by a 2-min HG at 30% of maximal voluntary contraction. Femoral blood flow (FBF; Doppler ultrasound), muscle sympathetic nerve activity (MSNA; microneurography), and mean arterial pressure (MAP; Portapres) were also measured. Hypercapnia increased VP and FBF by 5.0% and 0.6% Torr-1, respectively, and MSNA by 20-220%. Isometric HG increased MSNA by 50% and MAP by 20%, with no differences between ISO, HC5 and HC10. During the ISO HG there was an increase in cerebral vascular resistance (CVR; 20 ± 11%), whilst VP remained unchanged. During HC5 and HC10 HG, VP increased (13% and 14%, respectively), but CVR was unchanged. In contrast, HG-induced sympathetic stimulation increased femoral vascular resistance (FVR) during ISO, HC5 and HC10 (17-41%), whilst there was a general decrease in FBF below ISO. The HG-induced increases in MSNA were associated with increases in FVR in all conditions (r = 0.76-0.87), whereas increases in MSNA were associated with increases in CVR only during ISO (r = 0.91). In summary, in the absence of hypercapnia, HG exercise caused cerebral vasoconstriction, myogenically and/or neurally, which was reflected by increases in CVR and a maintained VP. In contrast, HG increased FVR during conditions of ISO, HC5 and HC10. Therefore, the cerebral circulation is more responsive to alterations in PCO2 and less responsive to sympathetic stimulation than the femoral circulation.


Key words: Cerebral circulation • Hypercapnia • Sympathetic nerve activity




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