Peripheral artery disease (PAD) is an atherosclerotic disease that impairs blood flow and muscle function in the lower limbs. A skeletal muscle myopathy characterized by mitochondrial dysfunction and oxidative damage is present in PAD; however, the underlying mechanisms are not well established. We investigated the impact of chronic ischemia on skeletal muscle microcirculatory function and its association with leg skeletal muscle mitochondrial function and oxygen delivery and utilization capacity in PAD. Gastrocnemius samples and arterioles were harvested from patients with PAD (n = 10) and age-matched controls (Con, n = 11). Endothelium-dependent and independent vasodilation was assessed in response to flow (30 μL·min⁻¹), acetylcholine, and sodium nitroprusside (SNP). Skeletal muscle mitochondrial respiration was quantified by high-resolution respirometry, microvascular oxygen delivery, and utilization capacity (tissue oxygenation index, TOI) were assessed by near-infrared spectroscopy. Vasodilation was attenuated in PAD (P < 0.05) in response to acetylcholine (Con: 71.1 ± 11.1%, PAD: 45.7 ± 18.1%) and flow (Con: 46.6 ± 20.1%, PAD: 29.3 ± 10.5%) but not SNP (P = 0.30). Complex I + II state 3 respiration (P < 0.01) and TOI recovery rate were impaired in PAD (P < 0.05). Both flow and acetylcholine-mediated vasodilation were positively associated with complex I + II state 3 respiration (r = 0.5 and r = 0.5, respectively, P < 0.05). Flow-mediated vasodilation and complex I + II state 3 respiration were positively associated with TOI recovery rate (r = 0.8 and r = 0.7, respectively, P < 0.05). These findings suggest that chronic ischemia attenuates skeletal muscle arteriole endothelial function, which may be a key mediator for mitochondrial and microcirculatory dysfunction in the PAD leg skeletal muscle. Targeting microvascular dysfunction may be an effective strategy to prevent and/or reverse disease progression in PAD. NEW & NOTEWORTHY Ex vivo skeletal muscle arteriole endothelial function is impaired in claudicating patients with PAD, and this is associated with attenuated skeletal muscle mitochondrial respiration. In vivo skeletal muscle oxygen delivery and utilization capacity is compromised in PAD, and this may be due to microcirculatory and mitochondrial dysfunction. These results suggest that targeting skeletal muscle arteriole function may lead to improvements in skeletal muscle mitochondrial respiration and oxygen delivery and utilization capacity in claudicating patients with PAD.

Peripheral artery disease (PAD) is characterized by the development of atherosclerotic plaques in the lower-body conduit arteries. PAD is commonly accompanied by microvascular disease, which may result in poor wound healing, plantar ulcer development, and subsequent limb amputation. Understanding the mechanisms underlying the development of plantar ulcers is a critical step in the development of adequate treatment options for patients with PAD. Skin is classified into two major components: glabrous and non-glabrous. These skin types have unique microcirculation characteristics, making it important to differentiate between the two when investigating mechanisms for plantar ulcer development in PAD. There is evidence for a microcirculation compensatory mechanism in PAD. This is evident by the maintenance of basal microcirculation perfusion and capillary filling pressure despite a reduced pressure differential beyond an occlusion in non-critical limb ischemia PAD. The major mechanism for this compensatory system seems to be progressive vasodilation of the arterial network below an occlusion. Recently, heat therapies have emerged as novel treatment options for attenuating the progression of PAD. Heat therapies are capable of stimulating the cardiovascular system, which may lead to beneficial adaptations that may ultimately reduce fatigue during walking in PAD. Early work in this area has shown that full-body heating is capable of generating an acute cardiovascular response, similar to exercise, which has been suggested as the most efficient treatment modality and may generate adaptations with chronic exposure. Heat therapies may emerge as a conservative treatment option capable of attenuating the progression of PAD and ultimately impeding the development of plantar ulcers.

Skeletal muscle contraction evokes numerous biochemical alterations that underpin exercise benefits. This present study aimed to elucidate the mechanism for electrical pulse stimulation (EPS)-induced antioxidant adaptation in C2C12 myotubes. We found that EPS significantly upregulated Nrf2 and a broad array of downstream antioxidant enzymes involved in multiple antioxidant systems. These effects were completely abolished by pretreatment with a ROS scavenger, N-acetylcysteine. MitoSOX-Red, CM-H2DCFDA, and EPR spectroscopy revealed a significantly higher ROS level in mitochondria and cytosol in EPS cells compared to non-stimulated cells. Seahorse and Oroboros revealed that EPS significantly increased the maximal mitochondrial oxygen consumption rate, along with an upregulated protein expression of mitochondrial complexes I/V, mitofusin-1, and mitochondrial fission factor. A post-stimulation time-course experiment demonstrated that upregulated NQO1 and GSTA2 last at least 24 h following the cessation of EPS, whereas elevated ROS declines immediately. These findings suggest an antioxidant preconditioning effect in the EPS cells. A cell viability study suggested that the EPS cells displayed 11- and 36-fold higher survival rates compared to the control cells in response to 2 and 4 mM H₂O₂ treatment, respectively. In summary, we found that EPS upregulated a large group of antioxidant enzymes in C2C12 myotubes via a contraction-mitochondrial-ROS-Nrf2 pathway. This antioxidant adaptation protects cells against oxidative stress-associated cytotoxicity.

Prolonged sitting reduces lower leg skeletal muscle metabolic rate in healthy individuals. Artificial intelligence revealed that impaired post-occlusive reactive hyperemia after prolonged sitting is related to a reduced stimulus for vasodilation and may not be evidence of microvascular dysfunction. Current post-occlusive reactive hyperemia protocols may be insufficient to assess micro- and macrovascular function after prolonged sitting.

Pulse transit time (PTT) is the time it takes for pressure waves to propagate through the arterial system. Arterial stiffness assessed via PTT has been extensively examined in the conduit arteries; however, limited information is available about PTT to the skeletal muscle microcirculation. Therefore, the purpose of this study was to assess PTT to the skeletal muscle microcirculation (PTTm) with near-infrared spectroscopy (NIRS) and to determine whether PTTm provides unique information about vascular function that PTT assessed in the conduit arteries (PTTc) cannot provide. This pilot study was conducted with 10 (male = 5; female = 5) individuals of similar age (21.5 ± 1.2 yr). The feasibility of using the intersecting tangents method to derive PTTm with NIRS was assessed during reactive hyperemia with the cross-correlation of PTTm produced by the intersecting tangents method and a different algorithm that used signal spectral properties. To determine whether PTTm was distinct from PTTc, the cross-correlation of PTTm and PTTc during reactive hyperemia was assessed. Cross-correlation indicated agreement between PTTm derived from both algorithms (r² = 0.77, P < 0.01) and a lack of agreement between PTTm and PTTc during reactive hyperemia (r² = 0.07, P < 0.01). Therefore, we conclude that it is feasible to assess PTTm using NIRS, and PTTm provides unique information about vascular function, including skeletal muscle microvascular elasticity, which cannot be achieved with traditional PTTc. PTTm with NIRS may provide a comprehensive and noninvasive assessment of vascular function and health. NEW & NOTEWORTHY Pulse transit time to the skeletal muscle microcirculation can be assessed using near-infrared spectroscopy and the intersecting tangents method. Pulse transit analysis to the microcirculation provides a comprehensive assessment of the vascular response to postocclusive reactive hyperemia that pulse transit analysis in the conduit arteries cannot provide. Pulse transit time to the skeletal muscle microcirculation using near-infrared spectroscopy provides unique information about microvascular elasticity in the skeletal muscle. These findings indicate that the combination of near-infrared spectroscopy and pulse transit analysis may be a useful method for assessing the skeletal muscle microcirculation.