Sprint interval and endurance training are equally effective in increasing muscle microvascular density and eNOS content in sedentary males

Abstract

Sprint interval training (SIT) has been proposed as a time efficient alternative to endurance training (ET) for increasing skeletal muscle oxidative capacity and improving certain cardiovascular functions. In this study we sought to make the first comparisons of the structural and endothelial enzymatic changes in skeletal muscle microvessels in response to ET and SIT. Sixteen young sedentary males (age 21 ± SEM 0.7 years, BMI 23.8 ± SEM 0.7 kg m(-2)) were randomly assigned to 6 weeks of ET (40-60 min cycling at ∼65% , 5 times per week) or SIT (4-6 Wingate tests, 3 times per week). Muscle biopsies were taken from the m. vastus lateralis before and following 60 min cycling at 65% to measure muscle microvascular endothelial eNOS content, eNOS serine(1177) phosphorylation, NOX2 content and capillarisation using quantitative immunofluorescence microscopy. Whole body insulin sensitivity, arterial stiffness and blood pressure were also assessed. ET and SIT increased skeletal muscle microvascular eNOS content (ET 14%; P < 0.05, SIT 36%; P < 0.05), with a significantly greater increase observed following SIT (P < 0.05). Sixty minutes of moderate intensity exercise increased eNOS ser(1177) phosphorylation in all instances (P < 0.05), but basal and post-exercise eNOS ser(1177) phosphorylation was lower following both training modes. All microscopy measures of skeletal muscle capillarisation (P < 0.05) were increased with SIT or ET, while neither endothelial nor sarcolemmal NOX2 was changed. Both training modes reduced aortic stiffness and increased whole body insulin sensitivity (P < 0.05). In conclusion, in sedentary males SIT and ET are effective in improving muscle microvascular density and eNOS protein content.

Figures

Figure 1. Effects of endurance training (ET) and sprint interval training (SIT) on eNOS content

A, widefield microscopy images of skeletal muscle pre- (left) and post- (right) endurance training (top) and sprint interval training (bottom). Skeletal muscle eNOS expression was revealed using Alexa-Fluror 594 conjugated secondary antibody (red). Bar = 50 μm. B, mean fluorescence intensity of eNOS is summarised. The mean level of eNOS pre-training was assigned a value of 1, and the relative intensity of eNOS post-training was calculated (ET n= 8, SIT n= 8). *P < 0.05, different from pre-training. †P < 0.05, different from ET post-training

Figure 2. Effects of acute exercise and endurance training (ET) and sprint interval training (SIT) on eNOS serine1177 phosphorylation

A, widefield microscopy images of skeletal muscle pre-training pre-exercise (Pre, pre), post-training pre-exercise (Pre, post), pre-training post-exercise (Pre, post) and post-training post-exercise (Post, post) in endurance training (top) and sprint interval training (bottom). Skeletal muscle eNOS serine1177 (ser1177) phosphorylation was revealed using Alexa-Fluror 594 conjugated secondary antibody (red). Bar = 5 μm. B, mean fluorescence intensity of eNOS ser1177 is summarised (ET n= 7, SIT n= 8). The mean level of eNOS ser1177 pre-training pre-exercise was assigned a value of 1, and the relative intensity of eNOS ser1177 post-training or post-exercise was calculated. *P < 0.05, main effect of training. †P < 0.05, main effect of time.

Figure 3. Effects of endurance training (ET) and sprint interval training (SIT) on NOX2 content

A, widefield microscopy images of skeletal muscle pre- (left) and post- (right) endurance training (top) and sprint interval training (bottom). Skeletal muscle NOX2 content was revealed using Alexa-Fluror 594 conjugated secondary antibody (red). Bar = 50 μm. B, mean fluorescence intensity of NOX2 within the endothelium is summarised (ET n= 7, SIT n= 8). C, mean fluorescence intensity of NOX2 within the muscle membrane is summarised (ET n= 7, SIT n= 8). The mean level of NOX2 pre-training was assigned a value of 1, and the relative intensity of NOX2 post-training was calculated.

Figure 4. Effects of endurance training (ET) and sprint interval training (SIT) on skeletal muscle capillarisation

Composite widefield microscopy images of skeletal muscle pre- (left) and post- (right) endurance training (top) and sprint interval training (bottom). Skeletal muscle microvessels were visualised using Ulex europaeus–FITC conjugated lectin (green) and the skeletal muscle membrane was revealed using wheat germ agglutinin-350 (blue). Bar = 50 μm.

Figure 5. Effect of endurance training (ET) and sprint interval training (SIT) on systemic wave reflections and central and peripheral artery stiffness

A, systemic wave reflections measured using augmentation index normalised to 75 bpm (AIx@75bpm) following ET and SIT. B, central artery (aortic) stiffness measured using pulse wave velocity (PWV) following ET and SIT. C, peripheral artery (brachial artery) stiffness measured using pulse wave velocity following ET and SIT. *P < 0.05, main effect of training.