Effect of muscle metaboreflex activation on spontaneous cardiac baroreflex sensitivity during exercise in humans

We sought to determine whether the activation of metabolically sensitive skeletal muscle afferents (muscle metaboreflex) is a potential mechanism for the decrease in spontaneous cardiac baroreflex sensitivity (cBRS) during exercise in humans. In protocol 1, 15 male subjects (22 +/- 1 years) performed steady-state leg cycling at low (26 +/- 4W) and moderate workloads (105 +/- 7W), under free-flow conditions and with partial flow restriction (bilateral thigh cuff inflation at 100 mmHg) to evoke muscle metaboreflex activation during exercise. In protocol 2, rhythmic handgrip exercise at 35% maximum voluntary contraction was performed with progressive upper arm cuff inflation (0, 80, 100 and 120 mmHg) to elicit graded metaboreflex activation. Both protocols were followed by post-exercise ischaemia (PEI) to isolate the muscle metaboreflex. Leg cycling-induced increases in HR and mean BP were augmented by partial flow restriction (P < 0.05 vs. free flow), while HR and mean BP both remained elevated during PEI (P < 0.05 vs. rest). Leg cycling evoked an intensity-dependent decrease in cBRS (16 +/- 2, 7 +/- 1 and 2 +/- 0.2ms mmHg(-1) at rest, low and moderate workloads, respectively; P < 0.05), which was further reduced with partial flow restriction (by -2.6 +/- 0.8 and -0.4 +/- 0.1 ms mmHg(-1) at low and moderate workloads). cBRS remained suppressed during PEI following leg cycling with partial flow restriction (4 +/- 1 ms mmHg(-1); P < 0.05 vs. rest). cBRS was unchanged during handgrip under free-flow conditions, handgrip with partial flow restriction and PEI following handgrip (P > 0.05 vs. rest). These data indicate that the activation of metabolically sensitive skeletal muscle afferents (muscle metaboreflex) decreases cardiac baroreflex responsiveness during leg cycling exercise in humans.