Plasma Biomarkers of Muscle Metabolism During Exercise to the Assessment of Insulin Resistance in CKD Dialysis Patients

Chronic renal failure (or CKD) is a chronic disease requiring end-stage replacement of renal function by extra-renal purification via hemodialysis (HD). CKD patients on dialysis suffer from numerous severe co-morbidities, limiting the possibility of renal transplantation.

Metabolic impairments are numerous and complex: dyslipidemia, phospho-calcium disorders, hormonal and nutritional disturbances.

In these patients, the onset of insulin resistance is virtually systematic, but its assessment remains a challenge. In fact, the classic screening method known as HOMA-IR, using simultaneous measurement of insulin and plasma glucose levels, is greatly affected by dialysis and the timing of its implementation. The reference method known as the hyperinsulinemic-euglycemic clamp is difficult to perform in HD patients.

While insulin resistance is a condition that can be reversed by physical activity in chronic pathologies, the absence of a validated assessment method for CKD in HD represents a missed opportunity for prevention in CKD, at a time when muscle rehabilitation programs are developing in CKD.

Moreover, insulin resistance is associated with skeletal striated muscle dysfunction in CKD patients, and constitutes another major comorbidity in these patients, as skeletal striated muscle is the main tissue utilizing glucose in response to insulin. The research team has shown that muscular dysfunctions associating reduced muscle strength and/or muscular atrophy are a morbi-mortality factor in dialysis patients. Finally, molecular abnormalities associate a switch of muscle fibers towards glycolytic fibers and mitochondrial abnormalities strongly suggesting an energy dysfunction.

In a preliminary study, the research team were able to demonstrate in dialysis patients an alteration in energy metabolism, comparable to a loss of "metabolic flexibility", mimicking that observed in insulin-resistant patients without renal dysfunction. A better understanding of the components of energy anomalies and insulin resistance could guide muscle rehabilitation protocols and prevent complications associated with insulin resistance.

Currently, the only valid, non-invasive method used in clinical practice to demonstrate impairment of muscular oxidative metabolism is the measurement of carbohydrate/lipid utilization rate during an exercise test. This "metabolic stress test" makes it possible to determine the point of maximum lipid utilization (LIPOXMAX), the current reference marker of muscle metabolism during exercise.

Nevertheless, LIPOXMAX alone cannot provide an understanding of underlying metabolic abnormalities in energy pathways, and the assay of different muscle metabolites released into plasma during exercise (and their combination) has recently emerged as a specific and early assessment of insulin resistance.

In particular, parameters such as lactate or pyruvate measured in plasma at rest have been positively correlated with the HOMA-IR index and associated with an increased risk of developing type 2 diabetes. During exercise (metabolic stress test), lactate levels in insulin-resistant patients differ from those in healthy controls.

In view of the abnormalities of oxidative muscle metabolism in CKD and insulin-resistant patients, the measurement of lactate at rest and during exercise, adjusted for V'O2, could reveal a profile enabling discrimination of CKD and insulin-resistant patients from healthy control subjects.

With the addition of plasma metabolic intermediates associated with energy disorders linked to insulin resistance, The investigators hypothesize that their measurement during exercise will improve the sensitivity of the assessment of muscle oxidative metabolism abnormalities, as reported in exercise-intolerant subjects.

The metabolomics approach during exercise would provide a biological and functional "signature" of muscle-induced insulin resistance in haemodialysis patients. A better understanding of metabolic abnormalities could guide muscle rehabilitation.

This prospective, cross-sectional, repeated-measures comparative study comparing functional and biochemical response profiles to exercise between 2 groups of chronically ill patients and a group of healthy subjects.

This comparison of the metabolisms of the 3 groups is analyzed using repeated plasma sampling at different effort intensities.

• Plasma lactate adjusted to V'O2 (ratio) at rest and during exercise.
• V'O2-adjusted metabolic profile at rest and during exercise through a combination of exercise energy metabolism intermediates based on candidate metabolites with a significant correlation coefficient >0.7 compared to LIPOXmax and/or HOMA-IR, among the following:
• Krebs cycle cofactors/substrates (lactate, pyruvate, malate, citrate, succinate, fumarate and alpha-cetoglutarate)
• ß-oxidation cofactors/substrates (free fatty acids, acyl-carnitine profile, beta-hydroxybutyrate, acetoacetate)
• Amino acid profile
• Acyl carnitine profile