Muscle afferent activation causes ventilatory and cardiovascular responses during concurrent hypercapnia in humans

Abstract

Respiratory and cardiovascular responses to muscle mechanoreflex (passive calf stretch) and metaboreflex activation (local circulatory occlusion) were examined during inhalation of a hypercapnic gas mixture in four trials. These controlled for the effects of central command, metabolite sensitization of muscle afferents and hypercapnia-induced elevation of central respiratory drive. In an isokinetic dynamometer, with circulation through the right leg occluded by inflation of a thigh cuff, 13 participants either rested (control trial; CON) or plantarflexed their ankle at 50% maximal force for 1.5 min (voluntary exercise trial; EX). Thereafter, circulatory occlusion was maintained and the calf passively stretched before return to the resting position. Both trials were performed while breathing air, as well as while breathing a normoxic, hypercapnic (5% CO 2 ) gas mixture (CO 2 trial and CO 2 +EX trial). Hypercapnic gas inhalation increased baseline minute ventilation (V ), heart rate and mean arterial pressure (+27.67 ± 1.74 l min −1 , +7 ± 0.85 beats min −1 and +13 ± 3.41 mmHg, respectively; means ± SEM) above control values (9.78 ± 0.86 l min −1 , 62 ± 2.3 beats min −1 and 88 ± 2.6 mmHg, respectively). Voluntary exercise further increased these variables from baseline during both trials (P < 0.05). During the continued circulatory occlusion after voluntary exercise, mean arterial pressure remained significantly elevated (P < 0.05). Minute ventilation returned to baseline during circulatory occlusion following exercise in the EX trial, but in the CO 2 +EX trial theV remained elevated at end-exercise levels during this period (+7.12 ± 1.13 l min −1 ). Passive stretch caused further increases inV during CO 2 +EX and CO 2 trials but not in CON and EX. These results indicate that in the absence of central command, either muscle metaboreflex and/or mechanoreflex activation stimulates ventilation during concurrent hypercapnia.

Ventilatory responses to muscle metaboreflex activation in chronic obstructive pulmonary disease

“…Additionally, we have shown that in healthy participants activation of muscle afferents using PECO, combined with mild hypercapnia, generates a ventilatory response and there is evidence of a synergistic interaction between central chemoreflex activation and muscle afferent feedback (Lykidis et al . ; Bruce & White, , ).…”

“…(Lykidis et al . ; Bruce & White, , ). We hypothesise that chronically hypercapnic patients will generate a larger ventilatory response during PECO in comparison to normocapnic patients.…”

Reappraisal of systemic venous chemoreceptors: might they explain the matching of breathing to metabolic rate in humans?

“…; Lykidis et al . ; Olson, ; Bruce & White, , ). Only one found that breathing was sustained postexercise by arm occlusion (Piepoli et al .…”

Muscles bring breathing alive

“…Surprisingly, the role of these thin fibre muscle mechanoreceptors in respiratory control during exercise has been somewhat neglected, although it would be expected that cardiovascular and ventilatory changes are tightly integrated. The paper by Bruce & White (2011) in this issue of Experimental Physiology addresses this issue.…”

“…This is achieved by passive stretch of a muscle group by dorsiflexion of the foot (Gladwell & Coote, 2002). These techniques are cleverly exploited in the paper by Bruce & White (2011), in which the influence on ventilation, the heart and blood pressure of either metaboreceptors or mechanoreceptors and both together are tested. The ingenious part of the study was to increase the subliminal fringe of the central neuronal circuits by increasing the excitability of central respiratory and other autonomic neurones by raising the inspired carbon dioxide concentration.…”