Allometry of mitochondrial efficiency is set by metabolic intensity

Area of Science:

• Physiology
• Biochemistry
• Metabolic Scaling

Background:

• Metabolic activity influences resource uptake and allocation, with body size being a key factor.
• Previous studies used oxygen consumption as a proxy for energy output, neglecting the efficiency of ATP production.
• Understanding mitochondrial coupling efficiency (ATP/O) is crucial for accurate metabolic rate assessments.

Purpose of the Study:

• To investigate the allometry of mitochondrial coupling efficiency (ATP/O) in skeletal muscle across 12 mammal species.
• To determine how body mass and metabolic intensity affect mitochondrial efficiency in mammals.

Main Methods:

• Isolated skeletal muscle mitochondria from 12 mammal species (6 g to 550 kg).
• Measured mitochondrial coupling efficiency (ATP/O) at various phosphorylation states.
• Analyzed the relationship between body mass, metabolic intensity, and efficiency.

Main Results:

• Mitochondrial efficiency increased significantly with higher metabolic rates.
• Body mass dependence of efficiency varied with metabolic intensity.
• Efficiency positively correlated with body mass near basal metabolic rate but was independent at maximal rates.
• Large mammals demonstrated a more rapid ATP/O increase than small mammals during the transition from basal to maximal activity.

Conclusions:

• Mitochondrial coupling efficiency in mammalian skeletal muscle is dependent on both body mass and metabolic intensity.
• The invariant maximal coupling efficiency across species may explain the scaling exponent near 1 at maximal metabolic rates.