Nicotinic receptors modulate skin perfusion during normothermia, and have a limited role in skin vasodilatation and sweating during hyperthermia

Abstract

New findings: What is the central question of this study? What is the role of nicotinic receptors in the regulation of normothermic cutaneous blood flow and cutaneous vasodilatation and sweating during whole-body heating induced following resting in a non-heat-stress condition? What is the main finding and its importance? Nicotinic receptors modulated cutaneous vascular tone during rest in a non-heat-stress condition and in the early stage of heating, but they had a limited role in mediating cutaneous vasodilatation when core temperature increased >0.4°C. Further, the contribution of nicotinic receptors to sweating was negligible during whole-body heating. Our findings provide new insights into the role of nicotinic receptors in end-organ function of skin vasculature and sweat glands in humans.

Abstract: Nicotinic receptors are present in human skin including cutaneous vessels and eccrine sweat glands as well as peripheral nerves. We tested the hypothesis that nicotinic receptors do not contribute to the control of cutaneous vascular tone in the normothermic state, but are involved in mediating cutaneous vasodilatation and sweating during a whole-body passive heat stress in humans. We first performed a nicotinic receptor blocker verification protocol in six young adults (one female) wherein increases in cutaneous vascular conductance and sweating elicited by 10 mm nicotine were blocked by administration of 500 µm hexamethonium to confirm effective blockade. Thereafter, 12 young males participated in a passive heating protocol. After an instrumentation period in a non-heat-stress condition, participants rested for a 10 min baseline period. Thereafter, oesophageal temperature was increased by 1.0°C using water-perfusion suits. Cutaneous vascular conductance, sweat rate, active sweat gland density and sweat output per individual gland were assessed with and without 500 µm hexamethonium administered via intradermal microdialysis. Hexamethonium reduced cutaneous vascular conductance by 22-34% during normothermia and the early stage of heating. However, this effect was diminished as oesophageal temperature increased >0.4°C. Active sweat gland density was reduced by hexamethonium when oesophageal temperature was elevated by 0.4-0.6°C above baseline resting. However, this was paralleled by a marginal increase in sweat gland output. Consequently, sweat rate remained unchanged. We showed that nicotinic receptors modulate cutaneous perfusion during normothermia and the early stage of heating, but not when core temperature increases >0.4°C. Additionally, they play a limited role in mediating sweating during heating.

Keywords: acetylcholine; cholinergic nerves; heat stroke; microcirculation; perspiration; thermoregulation.