Proceedings of The Physiological Society

University College Cork (2004) J Physiol 560P, C2



Ainslie,Philip N.; Ashmead,Jon C.; Ide,Kojiro ; Morgan,Barbara J.; Poulin,Marc J.;

1. Physiology & Biophysics, University of Calgary, Calgary, AB, Canada. 2. Orthopedics and Rehabilitation, University of Wisconsin, Madison, WI, USA.

  • Table 1. Responses during isocapnia hypercapnia and HG.

    * significantly different from * ISO a HC5 and b HC10 at P < 0.05. VP (cm.sec-1); CVR(mmHg/ cm.sec-1); MAP mmHg; MSNA %; FBF (ml.min-1); FVR (mmHg/ ml.min-1).

The relative importance of CO2 and sympathetic stimulation in cerebral and peripheral vascular regulation has not been previously studied in humans. We hypothesized that: 1) increases in sympathetic outflow would elicit cerebral vasoconstriction during isocapnia but not during hypercapnia; 2) sympathetic vasoconstriction would be evident in the peripheral circulation under both conditions. In 14 healthy males [28.1±3.7 (SD) yrs], we measured blood flow velocity (VP, n=14; transcranial Doppler ultrasound) in the middle cerebral artery during euoxic isocapnia (ISO, end-tidal PCO2, PETCO2, = +1 Torr above rest) and two levels of euoxic hypercapnia (HC5, PETCO2 = +5 Torr above ISO; HC10, PETCO2 = +10 above ISO). Each PETCO2 level was maintained for 10 min using the technique of dynamic end-tidal forcing, during which increases in sympathetic activity were elicited by a 2-min isometric handgrip (HG) at 30% of maximal voluntary contraction. Femoral blood flow (FBF, n=11; Doppler ultrasound), muscle sympathetic nerve activity (MSNA, n=7; microneurography), and mean arterial pressure (MAP; Portapres) were also measured. Hypercapnia increased VP and FBF by 5% and 0.6 %.Torr-1, respectively, and MSNA by (20-220%). Isometric HG increased MSNA by 50% and MAP by 20% during all conditions. During the ISO HG there was an increase in cerebral vascular resistance (CVR; 12±10%), whilst VP remained unchanged. During hypercapnia, VP increased [9.2±3.3 and 12.6±5.2 cm.sec-1 in HC5 and HC10, respectively] but CVR was unchanged. In contrast, HG-induced sympathetic stimulation increased femoral vascular resistance (FVR) during ISO, HC5 and HC10 (21-34%). HG elicited cerebral vasoconstriction under isocapnic, but not hypercapnic, conditions. In contrast, HG increased FVR during both conditions. Therefore, the cerebral circulation is less responsive to sympathetic stimulation and more responsive to alterations in PCO2 than the peripheral circulation.

Where applicable, experiments conform with Society ethical requirements