Impact of heat therapy on exercise performance, collateral artery growth and skeletal muscle capillarization in a peripheral artery disease model

Abstract

IntroductionRepeated exposure to heat therapy (HT) has been shown to improve exercise tolerance in patients with symptomatic peripheral artery disease (PAD), but the mechanisms underlying this effect remain unknown. We investigated the effects of repeated exposure to HT on endurance running capacity, collateral growth, and skeletal muscle capillarization in a preclinical model of PAD.MethodsMale C57Bl/6 mice (32‐week‐old) were fed a high‐fat diet for 10 weeks. At 42 weeks of age, the animals underwent bilateral ligation of the femoral artery to induce hindlimb ischemia. After two weeks of recovery, the animals were randomly allocated to receive HT (n=12) or a control intervention (n=13). Heat therapy was applied by placing the animal in a flat bottom restrainer and immersing the lower half of the body in a glass container filled with water at 39°C for 30 min daily over 3 consecutive weeks. Animals assigned to the control group were also restrained, but were placed in an empty container. At baseline and one day after the last treatment session, the animals completed a treadmill exercise test (10 m/min for 5 min at 10% grade followed by an increase in speed by 1 m/min every minute until exhaustion) for the assessment of endurance exercise performance. One day after the final treadmill test, the animals were anesthetized, the infrarenal aorta was cannulated with polyethylene‐50 tubing, and a vascular casting compound (Microfil) was injected into the cannula after perfusion‐fixation. Microcomputed tomography (micro‐CT) was used for the measurement of the average diameter of the anterior and posterior gracilis arteries. Skeletal muscle capillarization was assessed in the soleus and extensor digitorum longus (EDL) muscles in a subset of animals (Control: n=8 vs. HT: n=7).ResultsThe change from baseline to 3 weeks in maximum running speed (Control: 1.0±1.1 m/min vs. HT: 3.5±0.7 m/min, p=0.072), running distance (Control: 25.7±23.5 m vs. HT: 82.2±15.8 m, p=0.065) and total running time (Control: 62.7±63.5 s vs. HT: 217.3±39.0 s, p=0.056) tended to be greater in the animals exposed to HT as compared to the control group. However, there were no differences between groups in the average diameter of the upper and lower gracilis arteries (Control: 0.14±0.00 mm vs. HT: 0.13±0.00 mm, p=0.119) or the number of capillaries around a fiber in both the EDL (Control: 4.61±0.24 vs. HT: 4.64±0.16, p=0.923) and the soleus (Control: 5.17±0.33 vs. HT: 5.28±0.39, p=0.834) muscles.ConclusionThese findings indicate that the improvement in exercise tolerance following repeated exposure to HT does not appear to stem from collateral artery enlargment or enhanced muscle capillarization in a mouse model of PAD.Support or Funding InformationFunding: Ralph W. and Grace M. Showalter Research Trust Award