Cutaneous vascular and sudomotor responses to isometric exercise in humans

To identify whether isometric handgrip exercise (IHG) affects cutaneous vasoconstrictor and/or active vasodilator activities, seven subjects (6 men and 1 woman) performed 30% maximal voluntary contraction of a forearm under normothermic (1 bout) and hyperthermic (2 bouts) conditions. Skin blood flow was indexed by laser-Doppler flowmetry at a contralateral forearm site at which adrenergic vasoconstrictor function was blocked by iontophoresis of bretylium tosylate (BT) and therefore only has active vasodilation as a mechanism for reflex control. Skin blood flow was also monitored at an adjacent untreated site. Cutaneous vascular conductance (CVC) was calculated from the flow signal and noninvasive blood pressure. CVC was normalized to the value obtained from maximal vasodilation at that site. Sweat rate (SR) was measured at the same locations. During normothermia, IHG did not affect CVC at the control or BT-treated sites, nor did IHG affect SR (P > 0.05). The second bout of IHG in hyperthermia evoked significant reductions in CVC at the untreated (69.4 ± 3.4 to 58.9 ± 2.5% of maximum, P < 0.05) and BT- treated sites (75.4 ± 6.1 to 64.4 ± 6.2% of maximum, P < 0.05), whereas SR significantly increased (0.62 ± 0.16 to 0.70 ± 0.17 mg · cm^-2 · min^{- 1}, P < 0.05). These findings uniquely show that, in hyperthermia, IHG reduces active vasodilator activity while at the same time sudomotor activity is increasing. This suggests independent control of these effectors.