GABAergic Contribution to the Muscle Mechanoreflex‐Mediated Heart Rate Responses at the Onset of Exercise in Humans

Abstract

It is well stablished that the cardiovascular response to exercise is governed by both central and peripheral mechanisms. However the possible neurotransmitters involved in the autonomic responses during exercise are not fully understood. During exercise, Potts et al. (Neuroscience 2003;119:201–14) showed that electrically evoked muscular contraction inhibits arterial baroreceptors signaling within the nucleus of the tractus solitaries (NTS) via activation of GABAergic receptors in rats, suggesting that skeletal muscle afferents are a potential source synaptic drive to GABAergic neurons within the NTS. However, the extent to which these findings can be extrapolated to humans remains unclear. We reasoned that the GABAergic mechanisms involved in the heart rate (HR) responses at the onset of dynamic exercise is primarily mediated by muscle mechanoreceptors afferents. Therefore, in this study we sought to determine if central GABA mechanisms are involved in the muscle mechanoreflex‐mediated HR responses at the onset of exercise in humans. Twenty‐eight healthy subjects (14 men) aged between 18 to 35 years randomly performed three bouts of 5‐s of passive and active cycling under placebo and following oral administration of diazepam (10 mg), a benzodiazepine that produces an enhancement in GABAA activity. Beat‐to‐beat HR (electrocardiography) and arterial blood pressure (BP ‐ finger photopletysmography) were continuously measured. Electromyography of vastus lateralis was obtained to confirm no electrical activity during passive trials. HR increased from rest under placebo and further increased following administration of diazepam in both passive (Δ12 ± 1 beats min−1 vs. Δ17 ± 1 beats min−1, P < 0.01) and active (Δ14 ± 1 beats min−1 vs. Δ18 ± 1 beats min−1, P < 0.01) cycling. Arterial BP increased from rest similarly during all conditions. Importantly, no sex‐related differences were found in any variables during all experiments. These findings demonstrate for the first time that the central GABAergic mechanisms significantly contribute to the muscle mechanoreflex‐mediated cardiac vagal withdrawal at the onset of exercise in humans.Support or Funding InformationBrazilian National Council of Scientific and Technological Development (CNPq)Foundation for Research Support of Federal District (FAPDF)Brazilian Federal Agency for Support and Evaluation of Graduate Education (CAPES)This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.