The Influence of Baroreceptor Loading Status on Muscle Metaboreceptor Modulation of Heat Loss Responses

Abstract

Muscle metaboreceptors have been shown to have a profound effect on the control of the local heat loss responses of sweating (LSR) and skin blood flow (SkBF). Under experimental conditions which employ the ischemic isometric hand-grip exercise (IHG) model, muscle metaboreceptors tend to reduce active vasodilation and attenuate sweating. However, it remains unclear if non-pharmacologically-induced changes in baroreceptor status may mitigate this response. PURPOSE: To evaluate the influence of baroreceptor loading status on LSR and SkBF during postexercise ischemia. METHODS: Seven physically active males (19-24 years) completed two experimental trials under a normothermic (NS) and hyperthermic (whole-body warming, HS) state. Subjects performed 1-min of IHG exercise at 60% of their maximal voluntary contraction followed by 2-min of ischemia under simultaneous lower body positive pressure (LBPP, +40 mm Hg), negative pressure (LBNP, -20 mm Hg) and no pressure (CON). The sequence of these conditions was performed in random order with a 20-min recovery period between conditions to minimize muscular fatigue. RESULTS: No significant differences were observed during baseline rest or at the end of the IHG exercise. Under both NS and HS, mean arterial pressure (MAP) was increased above baseline resting during ischemia. However, the magnitude of increase in MAP was greater during LBPP for both NS (p<0.001) and HS (p<0.002), but lower during LBNP (p<0.028) under HS relative to CON. Despite differences in baroreceptor loading status under NS, no significant difference in LSR or SkBF were observed. In HS, no differences in SkBF were measured between CON and LBPP. However, SkBF response during ischemia was significantly reduced during LBNP as compared to CON (p=0.015) and LBPP (p=0.023). No differences in LSR were observed between CON and LBNP during ischemia. However, a marked reduction in LSR was measured in LBPP relative to CON (p=0.024) but not LBNP (p=0.092). CONCLUSIONS: These results suggest that baroreceptor loading status can mitigate muscle metaboreceptor-induced changes in local heat loss responses. However, this only appears to occur in a hyperthermic state. Funding support from the Natural Sciences and Engineering Research Council of Canada (held by Dr. G. Kenny).