Influence of muscle metaboreceptor stimulation on middle cerebral artery blood velocity in humans

Abstract

New Findings: What is the central question of this study? Is the ability of metabolically sensitive skeletal muscle afferents (muscle metaboreceptors) to increase cerebral perfusion masked by a hyperventilation-mediated reduction in the arterial PCO2 (indexed using end-tidal PCO2,P ET ,CO2), hence cerebral vasoconstriction, during post exercise muscle ischaemia? What is the main finding and its importance? We found that cerebral perfusion is elevated during post exercise muscle ischaemia when P ET ,CO2 is clamped at baseline, but when P ET ,CO2 is permitted to fluctuate no such elevation occurs. These findings indicate that muscle metaboreceptors increase cerebral perfusion when the confounding effects of changes in P ET ,CO2 are obviated. Regional anaesthesia to attenuate skeletal muscle afferent feedback abolishes the exercise-induced increase in middle cerebral artery mean blood velocity (MCA Vmean). However, such exercise-related increases in cerebral perfusion are not preserved during post exercise muscle ischaemia (PEMI) where the activation of metabolically sensitive muscle afferents is isolated. We tested the hypothesis that a hyperventilation-mediated decrease in the arterial partial pressure of CO2, hence cerebral vasoconstriction, masks the influence of muscle metaboreceptor stimulation on MCA Vmean during PEMI. Ten healthy men (20 ± 1 years old) performed two trials of fatiguing isometric hand-grip exercise followed by PEMI, in control conditions and with end-tidal CO2 (P ET ,CO2) clamped at ∼1 mmHg above the resting partial pressure. In the control trial, P ET ,CO2 decreased from rest during hand-grip exercise and PEMI, while MCA Vmean was unchanged from rest. By design, P ET ,CO2 remained unchanged from rest throughout the clamp trial, while MCA Vmean increased during hand-grip (+10.6 ±1.8 cm s-1) and PEMI (+9.2 ± 1.6 cm s-1; P < 0.05 versus rest and control trial). Increases in minute ventilation and mean arterial pressure during hand-grip and PEMI were not different in the control and P ET ,CO2 clamp trials (P > 0.05). These findings indicate that metabolically sensitive skeletal muscle afferents play an important role in the regional increase in cerebral perfusion observed in exercise, but that influence can be masked by a decrease in P ET ,CO2 when they are activated in isolation during PEMI.