We measured acute vascular responses to heat stress to examine the hypothesis that macrovascular endothelial-dependent dilation is improved in a shear-dependent manner, which is further modified by skin temperature. Twelve healthy males performed whole body heating (+1.3°C esophageal temperature), bilateral forearm heating (∼38°C skin temperature), and a time-matched (∼60 min) control condition on separate days in a counterbalanced order. Bilateral assessments of blood flow and brachial artery flow-mediated dilation (FMD) were performed before and 10 min after each condition with duplex Doppler ultrasound. To isolate the influence of shear stress, a pneumatic cuff was inflated (∼90 mmHg) around the right forearm during each condition to attenuate heat-induced rises in blood flow and shear stress. After forearm heating, FMD increased [cuffed: 4.7 (2.9)% to 6.8 (1.5)% and noncuffed: 5.1 (2.8)% to 6.4 (2.6)%] in both arms (time P < 0.01). Whole body heating also increased FMD in the noncuffed arm from 3.6 (2.2)% to 9.2 (3.2)% and in the cuffed arm from to 5.6 (3.0)% to 8.6 (4.9)% (time P < 0.01). After the time control, FMD decreased [cuffed: 6.3 (2.4)% to 4.7 (2.2)% and noncuffed: 6.1 (3.0)% to 4.5 (2.6)%] in both arms (time P = 0.03). Multiple linear regression (adjusted R² = 0.421 P = 0.003) revealed that changes in esophageal temperature, skin temperatures, and heart rate explained the majority of the variance in this model (34%, 31%, and 21%, respectively). Our findings indicate that, in addition to shear stress, skin and core temperatures are likely important contributors to passive heating-induced vascular adaptations.NEW & NOTEWORTHY The primary determinant of vascular adaptations to lifestyle interventions, such as exercise and heat therapy, is repeated elevations in vascular shear stress. Whether skin or core temperatures also modulate the vascular adaptation to acute heat exposure is unknown, likely due to difficulty in dissociating the thermal and hemodynamic responses to heat. We found that skin and core temperatures modify the acute vascular responses to passive heating irrespective of the magnitude of increase in shear stress.

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