Nicotinic receptor activation augments muscarinic receptor-mediated eccrine sweating but not cutaneous vasodilatation in young males

Acetylcholine released from cholinergic nerves can activate both muscarinic and nicotinic receptors; each is known independently to induce cutaneous vasodilatation and eccrine sweating in humans. However, it is not known whether the two receptors interact in order to mediate cutaneous vasodilatation and eccrine sweating. In 10 young men (27 +/- 6 years old), cutaneous vascular conductance and sweat rate were evaluated at intradermal microdialysis sites that were continuously perfused with either lactated Ringer's solution (control) or three different concentrations of nicotine (0.1, 1 and 10 mm), a nicotinic receptor agonist. Co- administration of methacholine, a muscarinic receptor agonist, was performed at all skin sites in a dose-proportional fashion (0.0125, 0.25, 5, 100 and 2000 mm, each for 25 min). Administration of nicotine alone caused dose-dependent transient increases in cutaneous vascular conductance and sweat rate (all P <= 0.05), which thereafter returned to pre-nicotine levels, except that a portion of transient responses remained with continuous administration of 10 mm nicotine (both P <= 0.05). Cutaneous vascular conductance was increased by administration of >= 0.25 mm methacholine at the control site, and this response was likewise observed in the presence of co-administration of all doses of nicotine used (all P <= 0.05). Sweat rate at the control site was increased by administration of >= 0.25 mm methacholine, but the lowest dose of methacholine (0.0125 mm) was able to increase sweat rate in the presence of 10 mm nicotine (P <= 0.05). We conclude that nicotinic receptor activation lowers the muscarinic receptor agonist threshold for eccrine sweating ( augmentation of muscarinic sweating) but does not affect muscarinic cutaneous vasodilatation in young men in normothermic resting conditions.