Progressive muscle metaboreflex activation gradually decreases spontaneous heart rate baroreflex sensitivity during dynamic exercise

Abstract

Ischemia of active skeletal muscle elicits a pressor response termed the muscle metaboreflex. We tested the hypothesis that in normal dogs during dynamic exercise, graded muscle metaboreflex activation (MMA) would progressively attenuate spontaneous heart rate baroreflex sensitivity (SBRS). The animals were chronically instrumented to measure heart rate (HR), cardiac output (CO), mean and systolic arterial pressure (MAP and SAP), and left ventricular systolic pressures (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 or SAP during spontaneous sequences of > or =3 consecutive beats when HR changed inversely vs. pressure] decreased during mild exercise from the resting values (-5.56 +/- 0.86 vs. -2.67 +/- 0.50 beats.min(-1).mmHg(-1), P <0.05), and in addition, these LRs were shifted upward. Progressive MMA gradually and linearly increased MAP, CO, and HR; linearly decreased SBRS; and shifted LRs upward and rightward to higher HR and pressures denoting baroreflex resetting. Moderate exercise caused a substantial reduction in SBRS (-1.57 +/- 0.38 beats.min(-1).mmHg(-1), P <0.05) and both an upward and rightward resetting. Gradual MMA at this higher workload also caused significant progressive increases in MAP, CO, and HR and progressive decreases in SBRS, and the LRs were 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.

Fig. 1.

Example of the nonlinear pattern of 1 animal's hemodynamic (mean arterial pressure) response to graded reductions in hindlimb blood flow (HLBF). Free-flow ex, free-flow exercise.

Fig. 2.

A and C: prevailing heart rate (HR) and left ventricular systolic pressure (LVSP) with corresponding mean slopes at rest, during free-flow exercise, and during exercise + HLBF step reductions in 1 animal during mild (A) and moderate exercise (C). bpm, Beats/min. B and D: in the same animal, the heart rate spontaneous baroreflex sensitivity level (HR-SBRS) during free-flow exercise and during exercise + HLBF step reductions in mild (B) and moderate (D) exercise.

Fig. 3.

Average HR, stroke volume (SV), cardiac output (CO), mean arterial pressure (MAP), LVSP, and HR-SBRS at rest and during mild and moderate exercise without any imposed reductions in HLBF (free flow) at threshold and approximately maximum activation of the muscle metaboreflex (maximal possible HLBF reduction). *P < 0.05, significant increase from rest to mild or moderate exercise. †P < 0.05, significant increase from mild or moderate exercise threshold to approximately maximum activation of the muscle metaboreflex.

Fig. 4.

HLBF at metaboreflex threshold for HR-SBRS, HR, CO, LVSP, and MAP for mild (top) and moderate (bottom) exercise. Note that no statistical difference was found.

Fig. 5.

Relationship between HR-SBRS and LVSP beyond metaboreflex threshold for each experiment at mild (top) and moderate (bottom) workloads. The thick solid line represents the average of the slopes and intercepts of the individual lines.