Ferric derisomaltose augments intrinsic skeletal muscle electron transport chain activity in heart failure: A FERRIC-HF II molecular substudy

Abstract

Aims: Skeletal muscle energetic augmentation might be a mechanism via which intravenous iron improves symptoms in heart failure, but no direct measurement of intrinsic mitochondrial function has been performed to support this notion. This molecular substudy of the FERRIC-HF II trial tested the hypothesis that ferric derisomaltose (FDI) would improve electron transport chain activity, given its high dependence on iron-sulfur clusters which facilitate electron transfer during oxidative phosphorylation.

Methods and results: Vastus lateralis skeletal muscle biopsies were taken before and 2 weeks after randomization. Mitochondrial complex I, II, and I&II respiration were quantified with respirometry of permeabilized fresh skeletal muscle biopsies. Net respiratory capacities, reflecting respiration that is truly available for adenosine triphosphate generation, were calculated by subtracting non-phosphorylating LEAK respiration. Complex I-V and myoglobin protein levels, and skeletal muscle fibre type composition were assayed. Patients randomised to FDI (n = 21) or placebo (n = 19) were similar (age 66 ± 13 years, 73% men, left ventricular ejection fraction 37 ± 8%, 48% New York Heart Association class III, 50% diabetic). After 2 weeks, total complex I-linked respiration (0.33 [interquartile range 0.24-0.37] vs. 0.19 [0.06-0.27] nmol/min/mg, p = 0.03) and net complex I-linked respiration (0.21 [0.16-0.24] vs. 0.11 [0.04-0.16] nmol/min/mg, p = 0.01) were higher in patients allocated to FDI. There was no intergroup difference in other respiratory states, in mitochondrial abundance as reflected by complex I-V protein levels, and in skeletal muscle myoglobin and oxidative fibre type content.

Conclusions: Iron repletion induces an early, selective, and potentially direct enhancement of mitochondrial complex I-dependent respiration in the skeletal muscle of heart failure patients. This could be harnessed to optimize repletion protocols to maximize patient benefits.

Keywords: Energetics; Heart failure; Iron; Mitochondria; Muscle; Respirometry.

Figure 1

Skeletal muscle respiration. Fresh muscle bundles obtained from vastus lateralis biopsies (A) were subjected to respirometry on a Hansatech instrument (B) with the activity of the electron transport chain (C) compared between the ferric derisomaltose (FDI) and placebo groups (D). Substrate oxidation releases energy‐rich electrons to the reducing equivalents, nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2). NADH receives electrons for complex I (CI)‐linked respiration while FADH2 receives electrons for complex II (CII)‐linked respiration. Iron–sulfur (Fe‐s) cluster and haem prosthetic groups in ETC proteins CI, CII, coenzyme Q (CoQ), complex III (CIII), and cytochrome C (Cyt C) facilitate electron transfer which releases redox energy that fuels the generation of the proton gradient that drives ATP synthesis. Box plots show the median (line), 25th and 75th percentiles (box), and the range (whiskers). ADP, adenosine diphosphate; CI&II, complex Iⅈ ETS, electron transport system capacity; FDI, ferric derisomaltose; JO₂, oxygen consumption; RCR, respiratory control ratio; P/E, phosphorylation system control ratio.

Figure 2

Levels of mitochondrial complex proteins and myoglobin. Complex I to V protein levels normalized to β‐actin are shown at baseline and at week 2 for the ferric derisomaltose (FDI) and placebo groups (A), with representative western blots (B). Similarly, myoglobin protein levels normalized to β‐actin are also shown at baseline and at week 2 for the randomized treatment groups (C) with representative western blots (D). *p = 0.008 against baseline. Box plots show the median (line), 25th and 75th percentiles (box), and the range (whiskers).

Figure 3

Skeletal muscle fibre type composition. The percentage of skeletal muscle fibres expressing slow myosin heavy chain (MHC) isoforms are shown at baseline and at 2 weeks for the ferric derisomaltose (FDI) and placebo groups. Representative images of immunostained skeletal muscles are shown above with fibres positive (+) and negative (−) for slow MHC isoforms (= oxidative fibres) marked in the merged image. Scale bar: 100 μm. Box plots show the median (line), 25th and 75th percentiles (box), and the range (whiskers).