CKD and Muscle Mitochondrial Energetics

Chronic kidney disease (CKD) leads to the retention of uremic solutes that disrupt skeletal muscle function¹ leading to reduced physical performance and mobility limitation². Disruption of muscle mitochondrial energetics in CKD may precede the onset of detectable functional limitations. In particular, reduced coupling of ATP production to oxygen consumption (P/O ratio) within mitochondria of skeletal muscle indicates oxidative stress, impaired metabolism, and reduced exercise efficiency^3–6. Comprehensive analysis of skeletal muscle mitochondrial energetics in CKD patients has been limited by lack of precise, real-time, non-invasive techniques.

We used in vivo magnetic resonance spectroscopy and optical spectroscopy (³¹P MRS/OS) procedures to measure resting mitochondrial energetics for the first time among non-diabetic CKD patients (GFR<60ml/min/1.73m²). Mitochondrial function parameters were measured according to published protocols⁷ (Supplemental methods). We compared these measures with results from a reference population of healthy community dwelling adults free of mobility limitation, known kidney and cardiovascular comorbidity (Supplemental Table 1). Briefly, MRS and separate OS measurements were performed during an initial resting state followed by a period of reversible ischemia to separate oxygen uptake from ATPflux. The first dorsal interosseus muscle (FDI) of the hand was selected because it contains well coupled mitochondria⁸, coupling of ATP generation per unit of oxygen consumption is sensitive to the impact of age and disease, and it is relatively insensitive to differences in locomotory physical activity. Oxygen uptake (O₂uptake) by muscle was measured from the deoxygenation of hemoglobin and myoglobin during ischemia using optical spectroscopy. Combining measures of O₂uptake with ATPflux yields a direct measurement of energy coupling (P/O) in vivo, a measure of mitochondrial efficiency. We also measured physical performance among the 12 participants with CKD.

CKD participants had a mean age of 53.1±12.8years (Table 1; Supplemental table 1–2) with mean eGFR of 39.3±13.2ml/min/1.73m². In comparison, the mean age of controls was 48.9±21.1 years (p=0.53). Among the CKD group, handgrip strength and timed up and go test were not significantly different from normative values (p=0.8 and 0.2, respectively).

Table 1.

Characteristics of participants with chronic kidney disease and healthy controls.

	CKD participants (n=12)	Controls (n=26)
Demographics
Age (years)	53.1 ±12.8	48.9 ±21.1
Male, No. (%)	9 (75)	16 (62)
Body mass index (kg/m2)	29.4 ±5.1
Black, No. (%)	3 (25)
Laboratory data
eGFR CKD-EPI (ml/min/1.73m2)	39.3 ±13.2
Urine albumin-creatinine ratio (mg/g Cr), median (IQR)	61.7 (7.4, 493.4)
Serum Bicarbonate (mmol/L)	25.5 ±2.9
Hemoglobin (g/dL)	13 ±1.6
Serum Albumin (g/dL)	3.7 ±0.3
Physical Performance data
Mean grip strength (kg)	42.7 ±10.9
Percent of predicted grip strength	100 ±24
Mean Timed get up and go (seconds)	8.4 ±2.4
Percent of predicted timed up and go	112.7 ±24.4
Medications
Using statins, No. (%)	5 (42)

Abbreviations: eGFR CKD-EPI: estimated glomerular filtration rate by Creatinine based CKD EPI equation.

Mean mitochondrial coupling ratio (P/O) was 34% lower in the CKD group versus controls (p=0.002) (Figure 1a). Mean P/O values were 1.5±0.7 in the CKD group versus 2.2±0.6 in controls. This difference in P/O was unaltered by age adjustment (adjusted difference 0.7; p=0.002). Muscle oxygen uptake was greater among CKD participants versus controls (p=0.03) (Supplemental figure 1a–1c).

Figure 1.

Left panel (A): Box plot ATP/O₂ ratio (P/O ratio) values among participants with CKD versus healthy controls. P-value represents the age-adjusted P-value for the association. Right panel (B): Box plots of P/O among those with CKD comparing those with of low eGFR (≤40ml/min/1.73m²) vs high eGFR (41–60ml/min/1.73m²). Open circles represent individual data points. Each box represents interquartile range of data with the center horizontal lines representing the median value in each group. P-value represents the age-adjusted P-value for the continuous association of eGFR with P/O.

In the CKD group, lower eGFR was associated with lower P/O after adjustment for age (Figure 1b). CKD participants who were using statins had a mean P/O ratio of 1.1±0.6 compared to 1.7±0.7 in CKD participants not using statins (p=0.12). There was no observed relationship of muscle oxygen uptake with ATPase rate in the CKD group (ρ=−0.35; p=0.3), whereas greater oxygen uptake was associated with progressively greater ATPase rate in control subjects (ρ=0.59; p=0.001) (Supplemental figure 2). Among the CKD group, age and albuminuria were modestly correlated with muscle O₂uptake but were not statistically significant (Supplemental table 3).

Our primary finding demonstrates elevated resting skeletal muscle oxygen consumption among CKD patients who are non-diabetic CKD and have preserved physical performance. These findings in the hand muscle indicate that altered mitochondrial metabolism may be related to CKD rather than differences in physical activity. Greater resting muscle mitochondrial metabolism in CKD suggests uncoupling of oxidative phosphorylation (lower P/O) as an early pathophysiological manifestation of kidney disease.

Elevated muscle O₂uptake with stable ATPflux is an indication of disrupted muscle mitochondrial metabolism and uncoupling of oxidative phosphorylation in CKD. Heightened oxidative stress characteristic of CKD may underlie the altered mitochondrial metabolism in patients with moderate CKD. This energy uncoupling may occur via oxidative stress induced activation of mitochondrial channels that raise O₂uptake without changing ATP generation^3,9. Thus the presence of elevated muscle mitochondrial metabolism and uncoupling in CKD points to elevated oxidative stress in the hand muscles of these patients. To our knowledge no studies have directly measured oxidative stress in muscles of CKD patients.

This preliminary study is limited by the small sample size restricting inferences to the general CKD population. Furthermore we cannot distinguish whether the observed differences in mitochondrial coupling are directly related to kidney disease or to other conditions that may differ between CKD patients and controls. Future studies are needed to replicate our findings and assess the role of physical activity and oxidative stress in mitochondrial energetics.

In summary we demonstrate muscle mitochondrial metabolic changes in patients with CKD who have preserved physical performance. These findings of uncoupling of muscle mitochondria in early CKD may indicate processes leading to impaired physical performance characteristic of more severe disease. Further investigation is needed to clarify if inefficient mitochondrial coupling is associated with declines in exercise efficiency among those with CKD similar to other populations¹⁰ and to precisely identify mechanisms and clinical consequences in CKD. Such investigation may help identify therapeutic targets for improving muscle function and physical performance.