Cholinergic Ligand-dependent Modulation of Oxidative Phosphorylation Coupling in Digitonin-permeabilized BE(2)-C...

The objective of this study was to assess oxidative phosphorylation (OXPHOS) function in cultured cells using defined substrate-inhibitor combinations while retaining cellular structure and cytosolic context lost in isolated mitochondrial preparations. Because intact cells are poorly permeable to several Krebs cycle intermediates, direct assessment of substrate-supported respiration through specific electron transport chain (ETC) entry points is limited. To overcome this, we applied digitonin-mediated selective plasma membrane permeabilization and performed extracellular flux analyzer-based coupling and electron flow assays in BE(2)-C neuroblastoma cells. To determine cell-type dependence, digitonin was empirically titrated in HEK293 cells and primary rat dorsal root ganglion (DRG) neurons using succinate + rotenone to isolate Complex II-IV-driven respiration. Succinate-supported respiration with Complex I inhibition showed increased Complex II-IV-driven oxygen (O₂) consumption in permeabilized compared with non-permeabilized cells, consistent with improved access of a membrane-impermeant substrate to mitochondria. In contrast, respiration supported by substrates that enter via endogenous transport pathways (e.g., pyruvate/malate) showed smaller differences between conditions. Using this platform to test muscarinic ligands, we observed agonist- versus antagonist-associated differences in O₂ consumption in the coupling assay, whereas the electron flow assay revealed minimal ligand-associated effects under the tested conditions. These findings indicate that detectable ligand effects were more prominent at the level of coupling-defined respiratory states than maximal electron transfer capacity. Overall, selective permeabilization expands substrate accessibility in cultured-cell bioenergetic assays and enables analysis of pharmacologic modulation of mitochondrial respiration.