Passive heat stress shows promise as an effective strategy to reduce the risk of cardiovascular diseases (CVD) as an alternative or adjunct to exercise and/or medication. Improving function of the endothelium is one of the key targets underlying the reduction in CVD risk. Therefore, this thesis aimed to evaluate potential mechanisms of acute heat-related improvement of endothelial function and investigate the effectiveness of heat in selected clinical groups. Endothelial function was measured via the technique of flow-mediated dilation (FMD) and shear stress was estimated using duplex Doppler ultrasound imaging of large arteries. In study #1, we compared forearm vs. whole-body heating in in 12 young, healthy individuals. In addition to elevated shear stress, skin and core temperatures independently increased endothelial function. In study #2, we compared peripheral and cerebrovascular responses to whole-body heating with oral ingestion of an α-1 adrenergic antagonist (Prazosin) or placebo. Although α-1 adrenergic antagonism did not influence hyperthermia-induced reductions in cerebral blood flow or affect post-heating FMD, preliminary trends indicate a potential impact of sympathetic nervous system activity on FMD in the brachial artery. In order to test the effectiveness of passive heating in extremely sedentary individuals in study #3, we measured FMD in 15 participants with cervical spinal cord injuries (SCI) and 15 uninjured controls before and after acute leg heating. There was no change in either brachial or superficial femoral artery FMD in SCI or controls; however, systemically circulating microparticles reflective of endothelial activation were reduced by ~60% after heating in SCI but not controls These reductions in endothelial microparticles were associated with improved shear patterns. Study #4 demonstrated that FMD was not different between older adults who are habitually exposed to passive heat stress (via regular sauna use) compared to those who are not. Furthermore, an exploratory analysis suggested there was no difference in FMD in sauna users compared to non-users with coronary artery disease; these data help direct towards other mechanisms as factors underlying lower CVD risk with sauna use. Collectively, the findings within this thesis highlight thermal and hemodynamic responses to heating that acutely influence endothelial function in health and disease. These findings have implications on the design of future studies that will contribute to reducing CVD risk with aging and various disease states.Health and Social Development, Faculty of (Okanagan)Health and Exercise Sciences, School of (Okanagan)Graduat
