Mechanisms of acetylcholine-mediated vasodilatation in young and aged human skin

Abstract

Thermoregulatory cutaneous vasodilatation (VD) is attenuated in aged skin. While acetylcholine (ACh) plays a role in thermally mediated VD, the precise mechanisms through which ACh-mediated VD acts and whether those downstream mechanisms change with ageing are unclear. We tested the hypotheses that both nitric oxide (NO)- and prostanoid-mediated pathways contribute to exogenous ACh-mediated VD, and that both are attenuated with advanced age. Twelve young (Y: 23 +/- 1 years) and 10 older (O: 69 +/- 1 years) subjects underwent infusions of 137.5 mum ACh at four intradermal microdialysis sites: control (C, Ringer solution), NO synthase inhibited (NOS-I, 10 mm l-NAME), cyclooxygenase inhibited (COX-I, 10 mm ketorolac) and NOS-I + COX-I. Red blood cell flux was monitored using laser-Doppler flowmetry, and cutaneous vascular conductance (CVC) was calculated (laser-Doppler flux/mean arterial pressure) and normalized to maximal CVC (%CVC(max)) (28 mm sodium nitroprusside + local heating to 43 degrees C). Baseline %CVC(max) was increased in the O at COX-I sites (COX-I 16 +/- 1, NOS-I + COX-I 16 +/- 2 versus C 10 +/- 1%CVC(max); P < 0.001) but not in the young, suggesting an age-related shift toward COX vasoconstrictors contributing to basal cutaneous vasomotor tone. There was no difference in peak %CVC(max) during ACh infusion between age groups, and the response was unchanged by NOS-I (O: NOS-I 35 +/- 5 versus C 38 +/- 5%CVC(max); P = 0.84) (Y: NOS-I 41 +/- 4 versus C 39 +/- 4%CVC(max); P = 0.67). COX-I and NOS-I + COX-I attenuated the peak CVC response to ACh in both groups (COX-I O: 29 +/- 3, Y: 22 +/- 2%CVC(max) versus C; P < 0.001 both groups; NOS-I + COX-I O: 32 +/- 3 versus Y: 29 +/- 2%CVC(max); versus C; P < 0.001 both groups). ACh mediates cutaneous VD through prostanoid and non-NO-, non-prostanoid-dependent pathways. Further, older subjects have a diminished prostanoid contribution to ACh-mediated VD.

Figures

Figure 1. Group mean responses in drug treatment sites to infusion of 137.5 μm acetylcholine (ACh)

A, young subjects (n = 12); B, older subjects (n = 10). • Control site, lactated Ringer infusion; ▿ nitric oxide synthase inhibited (NOS-I), 10 mmNG-nitro-l-arginine methyl ester (l-NAME); ▵ cylooxygenase inhibited (COX-I), 10 mm ketorolac; ⋄ NOS-I + COX-I, 10 mml-NAME+ 10 mm ketorolac. Standard error bars omitted for the NOS-I and NOS-I + COX-I sites for clarity. *Significant difference from the control site (P < 0.05).

Figure 2. Baseline %CVCmax in all drug treatment sites before infusion of 137.5 μm ACh

%CVCmax, percentage maximal cutaneous vascular conductance. Filled bars, young subjects (n = 12); open bars, older subjects (n = 10). Treatment with ketorolac alone (COX-I) and ketorolac +l-NAME (COX-I + NOS-I) significantly increased baseline in the older subjects. *P < 0.05, significant difference between groups; †P < 0.005, significant versus control site older subject group; ‡P < 0.005, significant versus control site young subject group.

Figure 3. Peak %CVCmax responses in all drug treatment sites to infusion of 137.5 μm ACh

Filled bars, young subjects (n = 12); open bars, older subjects (n = 10). Peak %CVCmax was attenuated in both subject groups in sites treated with ketorolac (COX-I), but not in sites treated with l-NAME (NOS-I) alone,*P < 0.05, significant difference between groups; †P < 0.005, significant versus control site older subject group; ‡P < 0.005, significant versus control site young subject group.