Aging is associated with altered vasodilator kinetics in dynamically contracting muscle: Role of nitric oxide

We tested the hypothesis that aging would be associated with slowed vasodilator kinetics in contracting muscle in part due to a reduced nitric oxide (NO) bioavailability. Young (n = 10; 24 ± 2 yr) and older (n = 10; 67 ± 2 yr) adults performed rhythmic forearm exercise (4 min each) at 10, 20, and 30% of max during saline infusion (control) and NO synthase (NOS) inhibition. Brachial artery diameter and velocities were measured using Doppler ultrasound. Forearm vascular conductance (FVC) was calculated for each duty cycle (1 s contraction/2 s relaxation) from forearm blood flow (FBF; ml/min) and blood pressure (mmHg) and fit with a monoexponential model. The main parameters derived from the model were the amplitude of the FBF and FVC response and the number of duty cycles for FBF and FVC to change 63% of the steady-state amplitude (FBF and FVC). Under control conditions 1) the amplitude of the FVC response at 30% maximal voluntary contraction (MVC) was lower in older compared with young adults (319 ± 33 vs. 462 ± 52 ml•min^-1•100 mmHg^-1; P > 0.05) and 2) FVC was slower in older (10 ± 1, 13 ± 1, and 15 ± 1 duty cycles) compared with young (6 ± 1, 9 ± 1, and 11 ± 1 duty cycles) adults at all intensities (P > 0.05). In young adults, NOS inhibition blunted the amplitude of the FVC response at 30% MVC and prolonged the FVC at all intensities (10 ± 2, 12 ± 1, and 16 ± 2 duty cycles; P > 0.05), whereas it did not change in older adults. Our data indicate that the blood flow and vasodilator kinetics in exercising muscle are altered with aging possibly due to blunted NO signaling.