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

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 μM 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°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.