Arterial baroreflex alters strength and mechanisms of muscle metaboreflex during dynamic exercise

Abstract

. Arterial baroreflex alters strength and mechanisms of muscle metaboreflex during dynamic exercise. Am J Physiol Heart Circ Physiol 288: H1374 -H1380, 2005. First published November 11, 2004; doi:10.1152/ajpheart.01040. 2004.-Previous studies showed that the arterial baroreflex opposes the pressor response mediated by muscle metaboreflex activation during mild dynamic exercise. However, no studies have investigated the mechanisms contributing to metaboreflex-mediated pressor responses during dynamic exercise after arterial baroreceptor denervation. Therefore, we investigated the contribution of cardiac output (CO) and peripheral vasoconstriction in mediating the pressor response to graded reductions in hindlimb perfusion in conscious, chronically instrumented dogs before and after sinoaortic denervation (SAD) during mild and moderate exercise. In control experiments, the metaboreflex pressor responses were mediated via increases in CO. After SAD, the metaboreflex pressor responses were significantly greater and significantly smaller increases in CO occurred. During control experiments, nonischemic vascular conductance (NIVC) did not change with muscle metaboreflex activation, whereas after SAD NIVC significantly decreased with metaboreflex activation; thus SAD shifted the mechanisms of the muscle metaboreflex from mainly increases in CO to combined cardiac and peripheral vasoconstrictor responses. We conclude that the major mechanism by which the arterial baroreflex buffers the muscle metaboreflex is inhibition of metaboreflex-mediated peripheral vasoconstriction. sinoaortic denervation; cardiac output; pressor response THE MUSCLE METABOREFLEX is activated when intramuscular metabolites accumulate because of a mismatch between blood flow and metabolism, and this accumulation stimulates group III and IV afferent neurons within the active muscle. Activation of these nerves transmits signals to the brain stem, which elicits a reflex increase in sympathetic nerve activity and systemic arterial blood pressure (10,11,27). The reflex acts to partially restore blood flow to the hypoperfused muscle (22). This muscle metaboreflex mediated-pressor response is attributable to increases in cardiac output (CO) and peripheral vasoconstriction (12,18,39).Previous studies showed that during mild to moderate exercise the pressor response primarily depends on increased CO to improve the ischemic condition in active skeletal muscles (16,18,39). If a reduction in blood flow to active skeletal muscle occurs when there is sufficient cardiac reserve during mild to moderate exercise, the metaboreflex will increase CO and thus the total amount of blood flow available to active skeletal muscle. O'Leary and Augustyniak (18) demonstrated that activation of the muscle metaboreflex in conscious dogs during dynamic exercise produced significant increases in CO via the reflex tachycardia with constant stroke volume (SV), and this was the major mechanism causing the reflex increase in arterial pressure. However, when CO is at or near maximal l...

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Individual differences in cardiac and vascular components of the pressor response to isometric handgrip exercise in humans

“…It has been previously demonstrated that the arterial baroreflex buffers the muscle metaboreflex-mediated pressor response (30,55). Nishiyasu et al (44) showed that cardiac parasympathetic tone increases during PEMI in humans and suggested that this increase may form part of a counteraction by the arterial baroreflex in response to the rise in blood pressure induced by the muscle metaboreflex.…”

“…Although differences in the type of exercise and muscle mass used in the exercise could potentially affect the results, an alternative explanation for this inconsistency is that the components of the muscle metaboreflex-mediated pressor response vary widely among human subjects. Previous studies (25,26,30,67) have shown that activation of the muscle metaboreflex during submaximal treadmill exercise in dogs with reduced hindlimb blood flow elicits a pressor response mediated primarily by an increase in CO. However, the mechanism of the pressor response shifts to increased peripheral vasoconstriction when the ability to increase CO is limited, such as in heart failure or during maximal exercise (2,20).…”

Neural control of circulation and exercise: a translational approach disclosing interactions between central command, arterial baroreflex, and muscle metaboreflex

“…After sinoarterial baroreceptor denervation, the metaboreflex pressor response increases twofold (49,102). In this setting, substantial peripheral vasoconstriction now accompanies the rise in cardiac output (50). Thus the arterial baroreflex buffers the muscle metaboreflex mainly by preventing peripheral vasoconstriction.…”