BAROREFLEX CONTROL OF THE CUTANEOUS ACTIVE VASODILATOR SYSTEM IN HUMANS

Cutaneous arterioles are controlled by vasoconstrictor and active vasodilator sympathetic nerves. To find out whether the active vasodilator system is under baroreceptor control, laser-Doppler velocimetry and the local iontophoresis of bretylium were combined to allow selective study of the active vasodilator system. Each of six subjects had two forearm sites (0.64 cm2) treated with bretylium to abolish adrenergic vasoconstrictor control. Laser-Doppler velocimetry was monitored at those sites and at two adjacent untreated sites. Subjects underwent 3 minutes of lower-body negative pressure (LBNP) and 3 minutes of cold stress to verify blockade of vasoconstrictor nerves. They were then subjected to whole-body heat stress (water-perfused suits), and the 3 minutes of LBNP was repeated. Finally, subjects were returned to normothermia, and LBNP and cold stress were repeated to verify the persistence of blockade. During the application of LBNP in normothermia, cutaneous vascular conductance (CVC) fell at untreated sites by 22.7 ± 4.7% (p < 0.01) but was unaffected at bretylium-treated sites (p > 0.20). During cold stress, CVC at untreated sites fell by 30.2 ± 1.7% (p < 0.01) and at treated sites rose by 0.7 ± 4.6% (p > 0.10). Both control and bretylium-treated sites reflexly vasodilated in response to hyperthermia. With LBNP during hyperthermia, CVC at untreated sites fell by 23.3 ± 7.1% (p < 0.05) and at treated sites by 17.9 ± 9.2% (p < 0.05) with no significant difference between sites (p > 0.10). After return to normothermia, neither LBNP application nor cold stress caused CVC to fall at treated sites (p > 0.10). Thus, the vasoconstrictor system was blocked by bretylium treatment throughout the study, whereas the active vasodilator response to heat stress was intact. Because LBNP in hyperthermia induced similar falls in CVC at both sites, we conclude that baroreceptor unloading elicits a withdrawal of active vasodilator tone and that the baroreflex has control of the active vasodilator system.