Passive Heat Therapy as a Novel Approach for Inducing Angiogenesis in Humans: Roles of Nitric Oxide

Chronic passive heat therapy is a novel health intervention which may have widespread cardiovascular benefits. Studies in animals suggest chronic heat exposure promotes angiogenesis via NO, independently of VEGF. We sought to determine whether the same occurs in humans using an endothelial tubule formation assay to quantify serum angiogenic balance. METHODS Serum was collected from ten young (23±2 years), healthy human subjects before and after 8 weeks of passive heat therapy (36 sessions of hot water immersion sufficient to maintain rectal temperature ≥38.5°C for 60min per session; post‐heat therapy serum was collected at least 36h after the last session). All analyses were performed in duplicate using human sera collected pre and post heat therapy. To assess serum angiogenic balance, human umbilical vein endothelial cells (HUVECs) were plated onto serum‐free Matrigel, exposed to human sera, and incubated for 10h, after which total tubule length per frame was determined by two blinded investigators. To investigate whether potential improvements in serum angiogenic balance following heat therapy were NOS‐dependent, angiogenesis experiments were repeated in which HUVECs were additionally treated with 1mM L‐NNA, a competitive inhibitor of NOS. In separate experiments, HUVECs were exposed to human sera for 24h and eNOS protein was determined using Western blot, normalized to vinculin loading control. Lastly, serum VEGF concentration was determined by ELISA. RESULTS Serum angiogenic balance was improved following 8 weeks of heat therapy (total tubule length: 71.8±1.0 to 74.3±1.1mm, p=0.03) and this improvement was prevented by L‐NNA (73.1±1.7 to 73.9±1.1, p=0.56). eNOS protein expression increased by 18.6±3.4% (p=0.05) in endothelial cells exposed to human sera following heat therapy relative to pre‐heat therapy. Heat therapy decreased serum VEGF concentration (370±77 to 331±69 pg/ml, p=0.05). CONCLUSIONS Chronic passive heat therapy in humans upregulates circulating factor(s) which promote angiogenesis in a NOS‐dependent and VEGF‐independent manner, possibly through increasing eNOS expression. These results are consistent with those reported in animal studies and suggest chronic passive heat therapy could be a viable treatment option for improving vascularization in at‐risk populations. Support or Funding Information Supported by AHA Grant #14PRE20380300 and the Eugene and Clarissa Evonuk Memorial Foundation