Cutaneous blood flow and sweat rate responses to exogenous administration of acetylcholine and methacholine

Abstract

The aim of this study was to evaluate cutaneous vasodilation and sweating responses to exogenous administration of acetylcholine (ACh) and methacholine (MCh), which have different sensitivities to endogenous cholinesterase. Four intradermal microdialysis probes were placed in dorsal forearm skin: two sites were perfused with ACh (1 x 10(-7)-1 M) and the other two with the same molar concentrations of MCh. Sweat rate (SR) and cutaneous blood flow were simultaneously assessed directly over each microdialysis membrane. Dose-response curves were constructed, and the effective concentration of the drug resulting in 50% of the maximal response (EC(50)) was identified. For SR and cutaneous vascular conductance (CVC), there were no significant differences in EC(50) between sites receiving the same drug: -1.52 +/- 0.18 and -1.19 +/- 0.09 log-molar concentration of ACh at distal and proximal sites, respectively, and -2.35 +/- 0.24 and -2.42 +/- 0.23 log-molar concentration of MCh at distal and proximal sites, respectively, for SR (P > 0.05) and -3.87 +/- 0.32 and -3.97 +/- 0.27 log-molar concentration of ACh at distal and proximal sites, respectively, and -4.78 +/- 0.17 and -4.46 +/- 0.16 log-molar concentration of MCh at distal and proximal sites, respectively, for CVC (P > 0.05). However, the EC(50) for CVC and SR was significantly lower at the MCh than at the ACh sites. A second procedure was performed to confirm that differences in responses between ACh and MCh could be attributed to different cholinesterase sensitivities. Similarly, four microdialysis membranes were placed in dorsal forearm skin: two sites were perfused with ACh and other two with MCh. However, one of each of the ACh and MCh sites was also perfused with 10 microM neostigmine (an acetylcholinesterase inhibitor). Neostigmine at the ACh site induced a leftward shift (i.e., lower EC(50)) of the SR and CVC dose-response curves compared with the site treated with ACh alone, resulting in no difference in the EC(50) for SR and CVC between the ACh + neostigmine and the MCh site. These results suggest that elevations in SR and CVC occur earlier with MCh than with ACh treatment because of differences in cholinesterase susceptibility between these drugs.

Fig. 1

Sweat rate (SR) dose-response curves to exogenous administration of 10⁻⁷−1 M ACh (A) and methacholine (MCh, B) at proximal and distal sites. EC₅₀ values were not significantly different between proximal and distal sites treated with the same drug. *Significant difference between sites at indicated dose (P < 0.05).

Fig. 2

Cutaneous vascular conductance (CVC) dose-response curves to exogenous administration of 10⁻⁷−1 M ACh (A) and MCh (B) at proximal and distal sites. EC₅₀ values were not significantly different between proximal and distal sites treated with the same drug.

Fig. 3

SR dose-response curves to exogenous administration of 10⁻⁷−1 M ACh and MCh. Before analysis, data from both sites treated with the same drug (for each subject) were averaged, resulting in a single value for each dose. EC₅₀ was significantly greater for the ACh than for the MCh dose-response curve. *Significant difference between sites at the indicated dose (P < 0.05). †Significant difference between EC₅₀ values (P < 0.05).

Fig. 4

CVC dose-response curves to exogenous administration of 10⁻⁷−1 M ACh and MCh. Before analysis, data from both sites treated with the same drug (for each subject) were averaged, resulting in a single value for each dose. EC₅₀ was significantly greater for ACh than for MCh dose-response curve. *Significant difference between sites at the indicated dose (P < 0.05). †Significant difference between EC₅₀ values (P < 0.05).

Fig. 5

Effect of inhibition of acetylcholinesterase (AChE) on SR dose-response curves to exogenous administration of ACh and MCh. Inhibition of AChE shifted ACh dose-response curve to the left compared with control site, such that EC₅₀ was significantly greater for ACh than for ACh + neostigmine (Neo), MCh, or MCh + neostigmine. †Significant difference between EC₅₀ values (P < 0.05).

Fig. 6

Effect of inhibition of AChE on CVC dose-response curves to exogenous administration of ACh and MCh. Inhibition of AChE shifted ACh dose-response curve to the left compared with the control site, such that EC₅₀ was significantly greater for ACh than for ACh + neostigmine, MCh, or MCh + neostigmine. †Significant difference between EC₅₀ values (P < 0.05).