Muscle metaboreceptor modulation of cutaneous active vasodilation

Purpose: Isometric handgrip exercise in hyperthermia has been shown to reduce cutaneous vascular conductance (CVC) by inhibiting the cutaneous active vasodilator system. Methods: To identify whether this response was initiated by muscle metaboreceptors, in seven subjects two 3-min bouts of isometric handgrip exercise in hyperthermia were performed, followed by 2 min of postexercise ischemia (PEI). An index of forearm skin blood flow (laser-Doppler flowmetry) was measured on the contralateral arm at an unblocked site and at a site at which adrenergic vasoconstrictor function was blocked via bretylium iontophoresis to reveal active cutaneous vasodilator function unambiguously. Sweat rate was measured via capacitance hygrometry. CVC was indexed from the ratio of skin blood flow to mean arterial pressure and was expressed as a percentage of maximal CVC at that site. In normothermia, neither isometric exercise nor PEI affected CVC (P > 0.05). Results: The first bout of isometric handgrip exercise in hyperthermia reduced CVC at control sites and this reduction persisted through PEI (pre-exercise: 59.8 ± 5.4, exercise: 49.8 ± 4.9, PEI: 49.7 ± 5.3% of maximum; both P < 0.05), whereas there were no significant changes in CVC at the bretylium treated sites. The succeeding bout of isometric exercise in hyperthermia significantly reduced CVC at both untreated (pre-exercise: 59.0 ± 4.8, exercise: 47.3 ± 4.0, PEI: 50.1 ± 4.1% of maximum; both P < 0.05) and bretylium treated sites (pre-exercise: 61.4 ± 7.3, exercise: 50.6 ± 5.1, PEI: 53.9 ± 6.0% of maximum, both P < 0.05). At both sites, CVC during PEI was lower than during the pre-exercise period (P < 0.05). Sweat rate rose significantly during both bouts of isometric exercise and remained elevated during PEI. Conclusions: These data suggest that the reduction in CVC during isometric exercise in hyperthermia. including the inhibition of the active vasodilator system, is primarily mediated by muscle metaboreceptors, whereas central command or muscle mechanoreceptors have less influence.