Investigating Mitochondrial Bioenergetics in Intact Heart and Brain Tissue Slices.

Mitochondria are essential for maintaining the high energetic demands of the heart and brain, generating ATP required for contractile function, neuronal signaling, and ionic homeostasis. In both tissues, metabolic flexibility is critical for maintaining bioenergetic efficiency, redox balance, and cellular viability. Despite their importance, existing experimental approaches to assess mitochondrial bioenergetic function present notable limitations. Isolated mitochondria and permeabilized cell assays provide precise control over substrates and respiratory states but disrupt organelle integrity and remove native cellular and extracellular context. Conversely, measurements in isolated or cultured cells preserve intact mitochondria but introduce phenotypic and metabolic artifacts. These constraints highlight the need for an intermediate platform that preserves native tissue architecture and cellular diversity while enabling quantitative assessment of mitochondrial bioenergetics. Building on prior demonstrations of respiration measurements in intact cardiac and neural tissue, we describe a tissue punch-based approach that enables region-specific analysis of mitochondrial function in small ex vivo tissue slices of intact heart and brain. This method preserves cytoarchitecture, and intercellular interactions while remaining compatible with high-resolution Seahorse respirometry analysis. Tissue punches allow multiple technical replicates from individual organs, reduce variability associated with isolation procedures, and enable assessment of regional metabolic heterogeneity, such as atrial versus ventricular myocardium or discrete brain regions. Here we present detailed and reproducible workflow protocols for brain and cardiac tissue punch preparation and extracellular flux analysis, including guidance on sample acquisition, punch sizing, normalization strategies, and data interpretation, with considerations for adapting the protocols across multiple pre-clinical models. © 2026 Wiley Periodicals LLC. Support Protocol: Preparation, Reagent Setup, and Instrumentation Basic Protocol 1: Cardiac tissue preparation Basic Protocol 2: Brain tissue preparation: rodents Basic Protocol 3: Brain tissue preparation: nonhuman primates Basic Protocol 4: Placement of tissue punches on Seahorse organoid plate Basic Protocol 5: Mitochondrial respiration measurements Basic Protocol 6: Tissue disruption for total protein quantification.