During heat stress, blunted increases in skin sympathetic nervous system activity (SSNA) and reductions in end-organ vascular responsiveness contribute to the age-related reduction in reflex cutaneous vasodilation. In older adults, folic acid supplementation improves the cutaneous vascular conductance (CVC) response to passive heating; however, the influence of folic acid supplementation on SSNA:CVC transduction is unknown. Fourteen older adults (66±1yrs, 8M/6F) ingested folic acid (5mg·day-1) or placebo for 6 weeks in a randomized, double-blind, crossover design. In protocol 1, esophageal temperature (Tes) was increased by 1.0ºC (water-perfused suit) while SSNA (peroneal microneurography) and red cell flux in the innervated dermatome (laser Doppler flowmetry; dorsum of the foot) were continuously measured. In protocol 2, two intradermal microdialysis fibers were placed in the skin of the lateral calf for graded infusions of acetylcholine (ACh; 10-10 to 10-1M) with and without nitric oxide synthase (NOS) blockade (20mM L-NAME). Folic acid improved reflex vasodilation (46±4% vs. 31±3 %CVCmax for placebo; P<0.001) without affecting the increase in SSNA (Δ506±104% vs. Δ415±73% for placebo; NS). Folic acid increased the slope of the SSNA:CVC relation (0.08±0.02 vs. 0.05±0.01 for placebo; P<0.05) and extended the response range. Folic acid augmented ACh-induced vasodilation (83±3% vs. 66±4 %CVCmax for placebo; P=0.002); however there was no difference between treatments at the NOS-inhibited site (53±4% vs. 52±4% CVCmax for placebo; NS). These data demonstrate that folic acid supplementation enhances reflex vasodilation by increasing the sensitivity of skin arterioles to central sympathetic nerve outflow during hyperthermia in aged human subjects.
- sympathetic nervous system
- folic acid
- skin blood flow
- Copyright © 2017, American Journal of Physiology-Regulatory, Integrative and Comparative Physiology