Nitric oxide, but not vasodilating prostaglandins, contributes to the improvement of exercise hyperemia via ascorbic acid in healthy older adults

Abstract

Acute ascorbic acid (AA) administration increases muscle blood flow during dynamic exercise in older adults, and this is associated with improved endothelium-dependent vasodilation. We directly tested the hypothesis that increase in muscle blood flow during AA administration is mediated via endothelium-derived vasodilators nitric oxide (NO) and prostaglandins (PGs). In 14 healthy older adults (64 ± 3 yr), we measured forearm blood flow (FBF; Doppler ultrasound) during rhythmic handgrip exercise at 10% maximum voluntary contraction. After 5-min steady-state exercise with saline, AA was infused via brachial artery catheter for 10 min during continued exercise, and this increased FBF ∼25% from 132 ± 16 to 165 ± 20 ml/min (P < 0.05). AA was infused for the remainder of the study. Next, subjects performed a 15-min exercise bout in which AA + saline was infused for 5 min, followed by 5 min of the nitric oxide synthase (NOS) inhibitor N(G)-monomethyl-l-arginine (l-NMMA) and then 5 min of the cyclooxygenase inhibitor ketorolac (group 1). The order of inhibition was reversed in eight subjects (group 2). In group 1, independent NOS inhibition reduced steady-state FBF by ∼20% (P < 0.05), and subsequent PG inhibition had no impact on FBF (Δ 3 ± 5%). Similarly, in group 2, independent PG inhibition had little effect on FBF (Δ -4 ± 4%), whereas subsequent NO inhibition significantly decreased FBF by ∼20% (P < 0.05). In a subgroup of five subjects, we inhibited NO and PG synthesis before AA administration. In these subjects, there was a minimal nonsignificant improvement in FBF with AA infusion (Δ 7 ± 3%; P = nonsignificant vs. zero). Together, our data indicate that the increase in muscle blood flow during dynamic exercise with acute AA administration in older adults is mediated primarily via an increase in the bioavailability of NO derived from the NOS pathway.

Fig. 1.

Experimental timeline. After placement of the brachial artery catheter and general setup, subjects performed 15 min of rhythmic handgrip exercise (RHG) at 10% maximal voluntary contraction (MVC). Noted as trial 1, exercise was performed for 5 min with saline infusion to achieve “steady-state” hemodynamics. Ascorbic acid was administered concurrently throughout the next 10 min of exercise. The dose of ascorbic acid was then reduced to 40% of the original dose and infused for the remainder of the experiment. After 20 min of rest, trial 2 was performed, which consisted of another 15-min bout of exercise. After 5 min, either N^G-monomethyl-l-arginine (l-NMMA) or ketorolac (Ket) was infused for 5 min. The other drug was infused for the last 5 min of exercise. Both l-NMMA and Ket were reduced to maintenance doses after trial 2. Subjects again rested for 20 min before trial 3, which consisted of 5 min of exercise. See text for further details.

Fig. 2.

Forearm blood flow (FBF) responses to ascorbic acid infusion and local inhibition of nitric oxide (NO) and prostaglandins (PGs). FBF is presented during rest and steady-state exercise (Ex). A: subjects in whom NO (l-NMMA) was inhibited before PG (ketorolac). B: subjects in whom PG inhibition occurred before NO inhibition. Respective drug infusions occurred as indicated. Ascorbic acid significantly increased FBF in both groups. Independent NO inhibition did cause a significant decrease in FBF, and subsequent PG inhibition had no further effect (A). Independent PG inhibition had no effect on FBF and combined PG-NO inhibition significantly attenuated FBF (A). *P < 0.05 vs. within-trial steady-state exercise condition. †P < 0.05 vs. previous trial resting condition. ‡P = 0.09 vs. previous trial resting condition.

Fig. 3.

Relative change in FBF after ascorbic acid infusion due to local inhibition of NO and/or PG. Independent NO inhibition decreased FBF ∼20% (A). In contrast, there was no significant change in exercise hyperemia with independent PG inhibition (B). Combined inhibition of NO and PG attenuated FBF ∼20% in both groups. *P < 0.05 vs. zero. †P < 0.05 vs. single-inhibition condition.

Fig. 4.

Improvement in exercise hyperemia due to ascorbic acid infusion. When ascorbic acid was infused with NO and PG synthesis pathways intact (+NO/PG), FBF improved ∼25%. When NO and PG synthesis were inhibited before ascorbic acid (protocol 2 subgroup; −NO/PG), the improvement due to ascorbic acid was abolished. *P < 0.05 vs. zero. †P < 0.05 vs. +NO/PG.