Passive Heat Treatment Increases Muscle Tissue Capillarization, But Not Protein Synthesis Rates In Humans

PURPOSE: This study assessed the impact of 8 weeks of passive heat treatment (PHT) in healthy, older adults on blood pressure, skeletal muscle tissue capillarization, perfusion kinetics, muscle protein synthesis rates, skeletal muscle hypertrophy, and strength. METHODS: Fourteen participants (9 males and 5 females; age: 73 ± 6 y) performed an identical experimental test day before and after 8 weeks of PHT (infrared sauna: 3x per week of 45 min at ~60 °C). During the experimental test days, postabsorptive and postprandial muscle protein synthesis rates were assessed by primed continuous L-[ring-13C6]-phenylalanine infusion with the collection of blood samples and muscle biopsies. Contrast-enhanced ultrasound was applied to assess muscle tissue perfusion kinetics before and 60 min after meal ingestion. Muscle tissue capillarization (by immunohistochemistry), blood pressure, vastus lateralis muscle cross-sectional area (CSA; ultrasound), and leg extension muscle strength (1RM) were measured before and after 8 weeks of PHT. RESULTS: Whereas diastolic blood pressure remained unchanged, systolic blood pressure decreased significantly (139 ± 10 vs 136 ± 13 mmHg; P = 0.019) in response to 8 weeks of PHT. Type I and type II muscle fiber capillarization increased significantly (capillary-to-fiber perimeter exchange index: +31 ± 18 and + 33 ± 30 %, respectively; P < 0.001) in response to PHT. However, no differences in basal (0.24 ± 0.27 vs 0.18 ± 0.11 AU; P = 0.266) and postprandial (0.20 ± 0.12 vs 0.18 ± 0.14 AU; P = 0.717) vastus lateralis muscle perfusion kinetics were observed between baseline and 8 weeks following PHT, respectively. Basal (0.048 ± 0.014 vs 0.051 ± 0.019 %/h; P = 0.630) and postprandial (0.041 ± 0.012 vs 0.051 ± 0.024 %/h; P = 0.199) muscle protein synthesis rates remained unchanged in response to 8 weeks of PHT. No significant changes were observed in vastus lateralis muscle CSA (15.3 ± 4.6 vs 15.2 ± 4.6 cm2; P = 0.768) or 1RM leg muscle strength (46 ± 12 vs 47 ± 12 kg; P = 0.087). CONCLUSIONS: Prolonged PHT lowers systolic blood pressure and increases muscle tissue capillarization in healthy older adults. The increase in tissue capillarization does not improve basal or postprandial muscle tissue perfusion or increase muscle protein synthesis rates, and does not augment muscle mass or strength.