Ascorbic acid improves brachial artery vasodilation during progressive handgrip exercise in the elderly through a nitric oxide-mediated mechanism

Abstract

The proposed mechanistic link between the age-related attenuation in vascular function and free radicals is an attractive hypothesis; however, direct evidence of free radical attenuation and a concomitant improvement in vascular function in the elderly is lacking. Therefore, this study sought to test the hypothesis that ascorbic acid (AA), administered intra-arterially during progressive handgrip exercise, improves brachial artery (BA) vasodilation in a nitric oxide (NO)-dependent manner, by mitigating free radical production. BA vasodilation (Doppler ultrasound) and free radical outflow [electron paramagnetic resonance (EPR) spectroscopy] were measured in seven healthy older adults (69 ± 2 yr) during handgrip exercise at 3, 6, 9, and 12 kg (∼13-52% of maximal voluntary contraction) during the control condition and nitric oxide synthase (NOS) inhibition via N(G)-monomethyl-L-arginine (L-NMMA), AA, and coinfusion of l-NMMA + AA. Baseline BA diameter was not altered by any of the treatments, while L-NMMA and L-NMMA + AA diminished baseline BA blood flow and shear rate. AA improved BA dilation compared with control at 9 kg (control: 6.5 ± 2.2%, AA: 10.9 ± 2.5%, P = 0.01) and 12 kg (control: 9.5 ± 2.7%, AA: 15.9 ± 3.7%, P < 0.01). NOS inhibition blunted BA vasodilation compared with control and when combined with AA eliminated the AA-induced improvement in BA vasodilation. Free radical outflow increased with exercise intensity but, interestingly, was not attenuated by AA. Collectively, these results indicate that AA improves BA vasodilation in the elderly during handgrip exercise through an NO-dependent mechanism; however, this improvement appears not to be the direct consequence of attenuated free radical outflow from the forearm.

Keywords: aging; endothelium; l-NMMA; vascular function.

Fig. 1.

Experimental timeline. After placement of the arterial and venous catheters and general setup, subjects performed progressive handgrip exercise at 3, 6, 9, and 12 kg under 4 experimental conditions: control, N^G-monomethyl-l-arginine (l-NMMA), ascorbic acid (AA), and l-NMMA + AA. Venous and arterial blood samples were collected after resting baseline (both with and without the infusions) and during the final portion of 6- and 12-kg handgrip exercise [i.e., after Doppler assessment of brachial artery (BA) diameter and blood velocity]. After the control and AA trials subjects rested for 30 min. A 2-h washout period was performed after infusion of l-NMMA. AA and l-NMMA were infused at loading doses and then reduced to a maintenance dose 1 min before baseline measures. See text for further details.

Fig. 2.

BA vasodilation (A), shear rate (B), and the contribution of nitric oxide (NO) to BA vasodilation (C) during progressive handgrip exercise. The contribution of NO to BA vasodilation was determined by the following equations: 1) NO contribution in control = ΔBAdiameter_Control − ΔBAdiameter_L-NMMA and 2) NO contribution in AA = ΔBAdiameter_AA − ΔBAdiameter_L-NMMA+AA. Absolute changes from baseline (BL) are presented for BA vasodilation and shear rate, and % change in BA diameter is expressed on right y-axis in A. Resting BL without (BL) and with infusion (BL infusion) is documented for each condition. All variables exhibited significant intensity-dependent increases during handgrip exercise (symbols denoting significant increases across workloads are not included for clarity). Values are means ± SE. *Significant difference between control and l-NMMA, ^#significant difference between control and AA, †significant difference between AA and l-NMMA + AA (P < 0.05).

Fig. 3.

Individual BA vasodilatory responses to handgrip exercise at 12 kg in control and AA conditions: BA diameter (A), absolute change in BA diameter (B), and % change in BA diameter (C). On average, BA diameter was significantly increased in the AA condition. *Significant difference between conditions (P < 0.05); n = 7.

Fig. 4.

BA blood flow and blood pressure during progressive handgrip exercise. A: blood flow. B: mean arterial pressure (MAP). Resting baseline with and without the infusions is documented for each condition. All variables exhibited significant intensity-dependent increases during handgrip exercise (symbols denoting significant increases across workloads are not included for clarity). Values are means ± SE. *Significant difference between control and l-NMMA, ^#significant difference between control and AA, †significant difference between AA and l-NMMA + AA (P < 0.05).

Fig. 5.

Individual BA blood flow response to handgrip exercise at 12 kg in control and AA conditions. On average, BA blood flow was significantly increased in the AA condition. *Significant difference between conditions (P < 0.05); n = 7.

Fig. 6.

Forearm free radical outflow as measured by α-phenyl-tert-butylnitrone (PBN) adduct during progressive handgrip exercise. Resting baseline with and without the infusions is documented for each condition. Free radical outflow increased during progressive handgrip exercise in all conditions (symbols denoting significant increases across workloads are not included for clarity). AU, arbitrary units. Values are means ± SE. *Significant difference between control and l-NMMA + AA (P < 0.05).

Fig. 7.

Relationship between shear rate and the change in BA diameter during progressive handgrip exercise. In addition to the present data from older subjects during both the control and AA infusion conditions, previously published control data from young subjects (57) are presented for reference. The slope of the relationship between shear rate and BA vasodilation was significantly attenuated in the old compared with the young (P < 0.05). AA increased the slope of the relationship between shear rate and BA vasodilation in the old such that the slope of this relationship was no longer different from that in the young (P = 0.98).