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Partial blockade of skeletal muscle somatosensory afferents attenuates baroreflex resetting during exercise in humans

Scott A. Smith, Ross G. Querry, Paul J. Fadel, Kevin M. Gallagher, Morten Strømstad*, Kojiro Ide*, Peter B. Raven and Niels H. Secher*

During exercise, the carotid baroreflex is reset to operate around the higher arterial pressures evoked by physical exertion. The purpose of this investigation was to evaluate the contribution of somatosensory input from the exercise pressor reflex to this resetting during exercise. Nine subjects performed seven minutes of dynamic cycling at 30 % of maximal work load and three minutes of static one-legged contraction at 25 % maximal voluntary contraction before (control) and after partial blockade of skeletal muscle afferents with epidural anaesthesia. Carotid baroreflex function was assessed by applying rapid pulses of hyper- and hypotensive stimuli to the neck via a customised collar. Using a logistic model, heart rate (HR) and mean arterial pressure (MAP) responses to carotid sinus stimulation were used to develop reflex function stimulus-response curves. Compared with rest, control dynamic and static exercise reset carotid baroreflex-HR and carotid baroreflex-MAP curves vertically upward on the response arm and laterally rightward to higher operating pressures. Inhibition of exercise pressor reflex input by epidural anaesthesia attenuated the bi-directional resetting of the carotid baroreflex-MAP curve during both exercise protocols. In contrast, the effect of epidural anaesthesia on the resetting of the carotid baroreflex-HR curve was negligible during dynamic cycling whereas it relocated the curve in a laterally leftward direction during static contraction. The data suggest that afferent input from skeletal muscle is requisite for the complete resetting of the carotid baroreflex during exercise. However, this neural input appears to modify baroreflex control of blood pressure to a greater extent than heart rate.

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