Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C19
Muscle afferent feedback contributions to ventilatory and cardiovascular control in humans
R. M. Bruce1, M. J. White1
1. School of Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom.
The role of muscle metaboreceptive and mechanoreceptive afferents in human cardiorespiratory control can be examined using post exercise circulatory occlusion (PECO) and passive muscle stretch respectively avoiding the involvement of central command (Drew et al 2008). In a series of trials, the respiratory and cardiovascular responses to combinations of passive calf muscle stretch and PECO during inhalation of a hypercapnic gas mixture were examined. These controlled for the effects of 1) the sensitising effects on muscle afferents of metabolites produced in exercise and 2) hypercapnia induced elevated central respiratory drive, in the absence of central command. With local Ethical Committee approval, 12 healthy males performed 4 trials whilst positioned in a Biodex isokinetic dynamometer. Their right foot was attached to a footplate with the centre of rotation of their ankle aligned with that of the machine. During the test phases of each trial, circulation through the right leg was occluded by inflation of a thigh cuff to 200mmHg. Subsequently subjects either rested for 1.5 min, Control Trial (Con) or plantarflexed their ankle at 50% maximum force for 1.5min, Exercise Trial (Ex). Thereafter in each trial a further 7.5 min of occlusion followed which comprised a 3.5min rest period, a 3min sustained calf stretch period (performed automatically by the Biodex) and a further 1min rest period post stretch. These two trials were performed breathing air and were repeated breathing a normoxic, hypercapnic (5% CO2) gas mixture (CO2 trial and CO2+Ex trial). Minute ventilation (VE), mean arterial pressure (MAP) and heart rate (HR) were continuously recorded. Inhalation of the hypercapnic gas increased baseline VE, HR and MAP (+26.5+/-2.3 L.min-1, +6 +/-2.5 beats.min-1, +9.7+/-3 mmHg; Mean+/-S.E.M.) above control values (9.0+/-0.7 L.min-1, 63+/-2.5 beats.min-1, 87.6+/-2.6 mmHg) respectively. Exercise caused a further significant increase in these variables from baseline during both trials (P<0.05) ANOVA. MAP remained significantly elevated above baseline during PECO (P<0.05). VE returned to baseline levels during PECO in the Ex trial but remained at end exercise levels (+7.6+/-1.9 L.min-1) during PECO in the CO2+Ex trial. Passive stretch caused a 2.2+/-0.6 L.min-1 further increase in VE during the CO2+Ex trial and a 3.3+/-1.3 L.min-1 increase in the CO2 trial. Stretch caused no increase in VE when participants inhaled air. These results indicate that feedback from muscle metabo and mechanoreceptive afferents stimulate ventilation during concurrent hypercapnia. It is as yet unclear whether the hypercapnic background contributes to the observed alterations in ventilation through a central or peripheral mechanism
Where applicable, experiments conform with Society ethical requirements