Acute Heat Exposure Improves Microvascular Function in Skeletal Muscle of Aged Humans

Acute heat exposure improves microvascular function in the leg of aged adults as assessed using post-occlusive reactive hyperemia. However, reactive hyperemia measures whole-limb blood flow and cannot isolate perfusion among various tissues. Thus, it is unclear if the skeletal muscle circulation contributes to the improvement in microvascular function observed following acute heat exposure. We tested the hypothesis that acute hot water immersion would improve microvascular function in the vastus lateralis of aged adults. Seven aged adults (1 man, 71 ± 4 yrs) were immersed to the umbilicus for 60 min in thermoneutral (36 °C) or hot (40 °C) water. Body core temperature was measured via a telemetric pill. Two microdialysis probes were placed in the vastus lateralis ~30 min after immersion. Microdialysis was utilized to bypass the cutaneous circulation and directly assess endothelial-dependent and endothelial-independent microvascular function in skeletal muscle by measuring the local blood flow response to a graded infusion of acetylcholine (ACh, 27.5 and 55.0 mM) and sodium nitroprusside (SNP, 21 and 42 mM), respectively. Local blood flow was measured using the ethanol washout technique. Body core temperature increased by Δ1.1 ± 0.3 °C during hot water immersion but was relatively unchanged during thermoneutral immersion (Δ0.1 ± 0.3 °C). Baseline skeletal muscle blood flow did not differ between thermal conditions for the ACh probe (P = 0.9), nor the SNP probe (P = 0.7). The hyperemic response to 27.5 mM ACh did not differ between thermal conditions (thermoneutral immersion, Δ11.3 ± 11.5 ml/min/100g vs. hot water immersion, Δ18.6 ± 16.8 ml/min/100g; P = 0.7). However, the hyperemic response to 55.0 mM ACh was increased with prior hot water immersion (thermoneutral immersion, Δ30.7 ± 16.9 ml/min/100g vs. hot water immersion, Δ56.2 ± 19.7 ml/min/100g; P < 0.01). Similarly, the hyperemic response to 21 mM SNP did not differ between thermal conditions (thermoneutral immersion, Δ16.9 ± 16.8 ml/min/100g vs. hot water immersion, Δ18.2 ± 18.8 ml/min/100g; P = 0.9), but was increased with prior hot water immersion during the infusion of 42 mM SNP (thermoneutral immersion, Δ29.3 ± 14.4 ml/min/100g vs. hot water immersion, Δ58.5 ± 31.2 ml/min/100g; P = 0.02). These data suggest that acute heat exposure improves endothelial-dependent and endothelial-independent microvasculature function in skeletal muscle of aged humans. Furthermore, these data highlight the therapeutic potential of heat therapy to attenuate the hypoperfusion of skeletal muscle that occurs in aged adults during conditions that require an elevated blood supply such as exercise.