title: 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 the V remained elevated at end-exercise levels during this period (+7.12 ± 1.13 l min(-1)). Passive stretch caused further increases in V 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.
authors: School of Sport and Exercise Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
authors: Richard M
authors: RM
authors: Bruce
authors: Michael J
authors: MJ
authors: White
mesh_terms: D000328:Adult; D000842:Ankle / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D002319:Cardiovascular System / Q000503:physiopathology*; D015444:Exercise / Q000502:physiology; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology*; D007866:Leg / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D012123:Pulmonary Ventilation / Q000502:physiology*; D012018:Reflex / Q000502:physiology*; D012119:Respiration; D012146:Rest / Q000502:physiology; D019415:Torque; D055815:Young Adult