Interaction of the muscle mechanoreflex and graded metaboreflex activation in reducing carotid‐cardiac baroreflex gain in man.

Abstract

We have shown that muscle mechanoreflex stimulation during concurrent metaboreflex activation reduces maximal gain (Gmax) of the carotid‐cardiac baroreflex in man. The effect of graded metabolite accumulation and metaboreflex activation on mechanoreflex modulation of carotid‐cardiac Gmax is unknown. Mechanoreflex stimulation by a standard passive calf stretch was performed during local circulatory occlusion in the resting limb, and after isometric calf contraction for 90s at 10, 30, 50 and 70% of maximum voluntary contraction (MVC). Heart rate and blood pressure were measured continuously. Throughout the protocol, pulsatile neck pressure trains (+40 to ‐80 Torr) were applied to derive carotid cardiac and vasomotor baroreflex function curves. As expected, there was a graded increase in blood pressure with graded metaboreflex activation. Gmax of the carotid‐cardiac baroreflex function curve was similarly reduced from rest during stretch in the 30, 50 and 70 % trials, by ‐50±7, ‐44±2 and ‐51±16 % respectively (P<0.05). No significant reduction from rest was detected during the 0 and 10% trials. Gmax of the carotid‐vasomotor baroreflex function curves was unaffected by stretch. The results suggest that afferent feedback due to passive stretch in combination with metabolites accumulated during exercise at 30% MVC and above is sufficient to alter carotid‐cardiac baroreflex gain.