Comparison of the noradrenergic sympathetic nerve contribution during local skin heating at forearm and leg sites in humans.

Purpose: We investigated the role of noradrenergic sympathetic nerves in the cutaneous circulation at rest and in response to local heating.

Methods: Dorsal forearm and lateral leg sites were each instrumented with 2 microdialysis fibers, 2 local skin heaters, and 2 laser-Doppler probes. All sites were heated from 33° to 42 °C. Each limb had 1 skin site treated with bretylium tosylate (BT) to block noradrenergic sympathetic neurotransmitter release and 1 site infused with lactated Ringer's (Control).

Results: During baseline (33 °C), cutaneous vascular conductance (CVC; laser-Doppler flux/blood pressure) at control (24 ± 2 %max) and BT-treated (29 ± 4 %max) sites in the leg was significantly higher than the forearm (control: 12 ± 1 %max; BT-treated: 17 ± 2 %max) (P = 0.032 and P = 0.042). At 42 °C local skin temperature, the initial peak CVC response with BT decreased compared to control at both forearm (62 ± 3 vs. 86 ± 6 %max, P < 0.01) and leg (67 ± 3 vs. 77 ± 2 %max, P = 0.035) sites. CVC at the forearm with BT was lower than that of the leg (P = 0.02). With control, plateau phase (~35 min at 42 °C) CVC was greater in the leg (98 ± 2 %max) than the forearm (89 ± 4 %max) (P = 0.027). BT reduced the peak CVC in the leg (90 ± 4 %max, P = 0.027) and in the forearm (69 ± 5 %max, P < 0.01). CVC at the BT-treated sites was reduced more in the forearm than in the legs (P < 0.01).

Conclusions: The contribution of noradrenergic sympathetic nerves during local heating differs between leg and forearm at rest and with skin heating.