Acetylcholine chloride as a potential source of variability in the study of cutaneous vascular function in man

IntroductionLaser-Doppler flowmetry (LDF) coupled with acetylcholine chloride (ACh) iontophoresis is increasingly recognized as a reliable non-invasive method to study the endothelial function. However, ACh-vasodilation measurement appears highly variable possibly due to the ACh pharmacological properties itself. These problems may be partially overcome by using methacholine chloride (MCh), a more stable synthetic agonist of muscarinic receptors, instead of ACh. Therefore, we first studied the correlation between the two drugs and then the effects of (1) spatial variability (inter-site measurements), (2) temporal variability (inter-day measurements), (3) intra-day variability (morning versus evening), and (4) age on the variability of both ACh-vasodilation and MCh-vasodilation.MethodsThe endothelium-dependent vasodilation response to simultaneous iontophoretic applications (4 doses of 10 s at 0.1 mA with 2 min of current-free interval) of ACh (11 mM) or MCh (10 mM) was studied on the forearm of 40 healthy subjects (36 males, median 28 yr, range 21–59 yr). The percent change in perfusion (CVCpeak) from baseline and the area under the curve (CVCAUC) during iontophoresis were assessed. Inter-site, inter-day and intra-day coefficients of variation (CV) were studied for each drug as well as correlations between drugs and age.ResultsA linear relationship was found between ACh- and MCh-CVCpeak (r² = 0.75, p = 0.01) and between ACh- and MCh-CVCAUC (r² = 0.55, p = 0.02). MCh inter-site CV for both CVCpeak (12.2%) and CVCAUC (13.8%) was significantly lower than ACh inter-site CV for CVCpeak (15.5%) and CVCAUC (15.3%), respectively. MCh inter-day CV for CVCpeak (17.2%) and CVCAUC (14.6%) was significantly lower than ACh inter-day CV for CVCpeak (19.7%) and ACh CVCAUC (21.2%). For ACh and MCh, the CVCpeak and CVCAUC were higher at 16:00 pm than at 11:00 am (p < 0.05 for all). Finally, both ACh- and MCh-CVCpeak exhibited a logarithmic decrease with age (r² = 0.61, p < 0.01 and r² = 0.58, p < 0.01).ConclusionAlthough both drugs exhibited circadian and age variability, MCh exhibited less inter-site and interday variabilities than did ACh for the evaluation of cutaneous endothelium-dependent vasodilation. These findings should be taken into account in studies of cutaneous vascular function by iontophoresis coupled with laser Doppler flowmetry.

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► We examine changes in ACH and MCh induced vasodilation and their variability.
► ACh- and MCh-vascular effects are correlated but variability is lesser using MCh.
► We recommend the use of MCh for cutaneous vascular reactivity assessment.
► Vascular reactivity depends on age and time of the test, other factors of variability.