Progressive Muscle Metaboreflex Activation Gradually Decreases Spontaneous Heart Rate Baroreflex Sensitivity During Dynamic Exercise

Abstract

We tested whether graded muscle metaboreflex activation (MMA) during dynamic exercise would progressively attenuate spontaneous heart rate baroreflex sensitivity (SBRS) in dogs. We measured heart rate (HR), cardiac output (CO), mean arterial and left ventricular systolic pressures (MAP, LVSP) at rest and during mild or moderate treadmill exercise before and during progressive MMA [via graded reductions of hindlimb blood flow (HLBF)]. SBRS (slopes of the linear relationships (LRs) between HR and LVSP during spontaneous sequences of ≥ 3 consecutive beats when HR changed inversely vs. pressure) decreased during mild exercise from the values at rest (−6.0 ± 1.2 vs. −3.0 ± 0.7 bpm/mmHg, P < 0.05) and these LRs were shifted upwards. Progressive MMA gradually and linearly increased MAP, CO, and HR and linearly decreased SBRS and shifted LRs upward and rightward denoting baroreflex resetting. Moderate exercise caused a substantial reduction in SBRS (−1.8 ± 0.5 bpm/mmHg, P < 0.05) and both an upward and rightward resetting. Gradual MMA again caused significant increases in MAP, CO and HR and progressive decreases in SBRS and LRs shifted to higher MAP and HR. Our results demonstrate that gradual MMA during mild and moderate dynamic exercise progressively decreases SBRS. In addition, baroreflex control of HR is progressively reset to higher blood pressure and HR in proportion to the extent of MMA. NIH HL 55473