Muscle blood flow is independent of conduit artery diameter following prior vasodilation in males

Abstract

At the onset of exercise in humans, muscle blood flow (MBF) increases to a new steady-state that closely matches the metabolic demand of exercise. This increase has been attributed to "contraction-induced vasodilation," comprised of the skeletal muscle pump and rapid vasodilatory mechanisms. While most research in this area has focused on forearm blood flow (FBF) and vascular conductance, it is possible that separating FBF into diameter and blood velocity can provide more useful information on MBF regulation downstream of the conduit artery. Therefore, we attempted to dissociate the matching of oxygen delivery and oxygen demand by administering glyceryl tri-nitrate (GTN) prior to handgrip exercise. Eight healthy males (29 ± 9 years) performed two trials consisting of two bouts of rhythmic handgrip exercise (30 contractions·min^-1 at 5% of maximum) for 6 min, one for each control and GTN (0.4 mg sublingual) condition. Administration of GTN resulted in a 12% increase in resting brachial artery diameter that persisted throughout the duration of exercise (CON: 0.50 ± 0.01 cm; GTN: 0.56 ± 0.01 cm, p < 0.05). Resting FBF was greater following GTN administration compared to control (p < 0.05); however, differences in FBF disappeared following the onset of muscle contractions. Our results indicate that the matching of FBF to oxygen demand during exercise is not affected by prior vasodilation, so that any over-perfusion is corrected at the onset of exercise. Additionally, our findings provide further evidence that the regulation of vascular tone within the microvasculature is independent of the conduit artery diameter.

Keywords: exercise hyperemia; muscle pump; vascular tone; vasodilation.

FIGURE 1

Forearm blood flow. Forearm blood flow averaged at 10‐sec intervals for the first 2 min of exercise and every minute after until the end of the exercise protocol. Time = 0 seconds represents the start of contractions. Time point (−50) was assessed as the 30 s average of resting data for Control and prior to GTN administration. *indicates significance at p < 0.05. Two‐way ANOVA with repeated measures (time, condition) followed by Tukey's posttest

FIGURE 2

Changes in brachial artery diameter. Brachial artery diameter assessed at multiple time points throughout exercise. Time point (−50) was assessed as the 30 s average of resting data for Control and prior to GTN administration. *indicates significance at p < 0.05. Two‐way ANOVA with repeated measure (time, condition) followed by Tukey's posttest

FIGURE 3

Mean blood velocity. Mean blood velocity averaged at 10‐sec intervals for the first 2 min of exercise and every minute after until the end of the exercise protocol. Time point (−50) was assessed as the 30 s average of resting data for Control and prior to GTN administration. *indicates significance at p < 0.05. Two‐way ANOVA with repeated measures (time, condition) followed by Tukey's posttest

FIGURE 4

Antegrade and retrograde blood velocities. Antegrade velocity (a) was significantly higher prior to exercise in the GTN condition, but lower after 180 s of exercise. Retrograde blood velocity was significantly greater at all time points in the GTN condition. *indicates significance at p < 0.05. Two‐way ANOVA with repeated measures (time, condition) followed by Tukey’s posttest.

FIGURE 5

Vascular conductance. There were no differences between conditions in vascular conductance throughout the duration of exercise

FIGURE 6

Change in total hemoglobin concentration and de‐oxygenated hemoglobin as assessed by nearinfrared spectroscopy (NIRS). There were no differences in [THC] (a) between conditions. *represents difference between conditions for [HHb], p < 0.05 (b). Two‐way ANOVA with repeated measures (condition, time) followed by Tukey's posttest