Skeletal Muscle Bioenergetics in Critical Limb Ischemia and Diabetes.

INTRODUCTION: Mitochondrial dysfunction is implicated in the metabolic myopathy accompanying peripheral artery disease (PAD) and critical limb ischemia (CLI). Type-2 diabetes mellitus (T2DM) is a major risk factor for PAD development and progression to CLI and may also independently be related to mitochondrial dysfunction. We set out to determine the effect of T2DM in the relationship between CLI and muscle mitochondrial respiratory capacity and coupling control. METHODS: We studied CLI patients undergoing revascularization procedures or amputation, and non-CLI patients with or without T2DM of similar age. Mitochondrial respiratory capacity and function were determined in lower limb permeabilized myofibers by high-resolution respirometry. RESULTS: Fourteen CLI patients (65 ± 10y) were stratified into CLI patients with (n = 8) or without (n = 6) T2DM and were compared to non-CLI patients with (n = 18; 69 ± 5y) or without (n = 19; 71 ± 6y) T2DM. Presence of CLI but not T2DM had a marked impact on all mitochondrial respiratory states in skeletal muscle, adjusted for the effects of sex. Leak respiration (State 2, P < 0.025 and State 4o, P < 0.01), phosphorylating respiration (P < 0.001), and maximal respiration in the uncoupled state (P < 0.001), were all suppressed in CLI patients, independent of T2DM. T2DM had no significant effect on mitochondrial respiratory capacity and function in adults without CLI. CONCLUSIONS: Skeletal muscle mitochondrial respiratory capacity was blunted by ∼35% in patients with CLI. T2DM was not associated with muscle oxidative capacity and did not moderate the relationship between muscle mitochondrial respiratory capacity and CLI.

Muscle carnitine availability plays a central role in regulating fuel metabolism in the rodent.

KEY POINTS: Meldonium inhibits endogenous carnitine synthesis and tissue uptake, and accelerates urinary carnitine excretion, although the impact of meldonium-mediated muscle carnitine depletion on whole-body fuel selection, and muscle fuel metabolism and its molecular regulation is under-investigated. Ten days of oral meldonium administration did not impact on food or fluid intake, physical activity levels or body weight gain in the rat, whereas it depleted muscle carnitine content (all moieties), increased whole-body carbohydrate oxidation and muscle and liver glycogen utilization, and reduced whole-body fat oxidation. Meldonium reduced carnitine transporter protein expression across muscles of different contractile and metabolic phenotypes. A TaqMan PCR low-density array card approach revealed the abundance of 189 mRNAs regulating fuel selection was altered in soleus muscle by meldonium, highlighting the modulation of discrete cellular functions and metabolic pathways. These novel findings strongly support the premise that muscle carnitine availability is a primary regulator of fuel selection in vivo. ABSTRACT: The body carnitine pool is primarily confined to skeletal muscle, where it regulates carbohydrate (CHO) and fat usage. Meldonium (3-(2,2,2-trimethylhydrazinium)-propionate) inhibits carnitine synthesis and tissue uptake, although the impact of carnitine depletion on whole-body fuel selection, muscle fuel metabolism and its molecular regulation is under-investigated. Male lean Zucker rats received water (control, n = 8) or meldonium-supplemented water (meldonium, n = 8) for 10 days [1.6 g kg-1 body mass (BM) day-1 days 1-2, 0.8 g kg-1  BM day-1 thereafter]. From days 7-10, animals were housed in indirect calorimetry chambers after which soleus muscle and liver were harvested. Food and fluid intake, weight gain and physical activity levels were similar between groups from days 7 to 10. Compared to control, meldonium depleted muscle total carnitine (P < 0.001) and all carnitine esters. Furthermore, whole-body fat oxidation was less (P < 0.001) and CHO oxidation was greater (P < 0.05) compared to the control, whereas soleus and liver glycogen contents were less (P < 0.01 and P < 0.01, respectively). In a second study, male Wistar rats received water (n = 8) or meldonium-supplemented water (n = 8) as above, and kidney, heart and extensor digitorum longus muscle (EDL) and soleus muscles were collected. Compared to control, meldonium depleted total carnitine content (all P < 0.001), reduced carnitine transporter protein and glycogen content, and increased pyruvate dehydrogenase kinase 4 mRNA abundance in the heart, EDL and soleus. In total, 189 mRNAs regulating fuel selection were differentially expressed in soleus in meldonium vs. control, and a number of cellular functions and pathways strongly associated with carnitine depletion were identified. Collectively, these data firmly support the premise that muscle carnitine availability is a primary regulator of fuel selection in vivo.

Mitochondrial Bioenergetics in the Metabolic Myopathy Accompanying Peripheral Artery Disease.

Peripheral artery disease (PAD) is a serious but relatively underdiagnosed and undertreated clinical condition associated with a marked reduction in functional capacity and a heightened risk of morbidity and mortality. The pathophysiology of lower extremity PAD is complex, and extends beyond the atherosclerotic arterial occlusion and subsequent mismatch between oxygen demand and delivery to skeletal muscle mitochondria. In this review, we evaluate and summarize the available evidence implicating mitochondria in the metabolic myopathy that accompanies PAD. Following a short discussion of the available in vivo and in vitro methodologies to quantitate indices of muscle mitochondrial function, we review the current evidence implicating skeletal muscle mitochondrial dysfunction in the pathophysiology of PAD myopathy, while attempting to highlight questions that remain unanswered. Given the rising prevalence of PAD, the detriment in quality of life for patients, and the associated significant healthcare resource utilization, new alternate therapies that ameliorate lower limb symptoms and the functional impairment associated with PAD are needed. A clear understanding of the role of mitochondria in the pathophysiology of PAD may contribute to the development of novel therapeutic interventions.