Competition between cutaneous active vasoconstriction and active vasodilation during exercise in humans

Cutaneous vasoconstriction occurs in response to the initiation of dynamic exercise in hyperthermia. To find whether this response was due to increased vasoconstrictor activity or to withdrawal of active vasodilator activity, blood flow monitoring with laser-Doppler flowmetry (LDF) was combined with the local iontophoresis of bretylium. Each of six male subjects had two forearm sites treated with bretylium for selective local blockade of noradrenergic vasoconstrictor nerves in skin. LDF was monitored at those sites and at two adjacent untreated sites. Mean arterial pressure (MAP) was measured, and cutaneous vascular conductance (CVC) was calculated as LDF/MAP. After iontophoresis, subjects underwent 3 min of cold stress (water-perfused suits) to verify vasoconstrictor blockade. CVC at untreated sites fell by 35.9 ± 3.1% (P < 0.01) and at bretylium-treated sites was not significantly changed (P > 0.10). During strenuous exercise in normothermia, CVC at untreated sites fell by 16.1 ± 4.1% (P < 0.05) and was unchanged at bretylium-treated sites (+12.7 ± 6.6%, P > 0.05). Whole body heat stress was then applied. When exercise was repeated in hyperthermia, CVC at untreated sites fell by 11.6 ± 3.8% (P < 0.05) but was not significantly changed at bretylium-treated sites (+3.6 ± 3.0%, P > 0.30). Following return to normothermia, cold stress verified the persistence of the blockade. We conclude that exercise initiation causes a cutaneous vasoconstriction largely or entirely due to enhanced active vasoconstrictor tone in both normothermia and hyperthermia. Little or no role in this response can be ascribed to reduced active vasodilator activity.