MICROPROBE ANALYSIS OF TC-MIBI IN HEART-CELLS - CALCULATION OF MITOCHONDRIAL-MEMBRANE POTENTIAL

Hexakis (2-methoxyisobutylisonitrile technetium-99m (Tc-99m-MIBI) is a gamma-emitting radiopharmaceutical probe currently in clinical use to evaluate myocardial perfusion. Biochemical and cellular pharmacological studies have suggested that Tc-MIBI, a lipophilic cation, is sequestered in mitochondria in response to transmembrane potentials. To assess directly the subcellular distribution of the probe in heart tissue, cultured chick heart cells were analyzed by electron-probe X-ray microanalysis (EPXMA) following equilibration in micromolar concentrations of carrier-added Tc-99-MIBI, the ground-state radiopharmaceutical. Quantitation of the physiological elements Na, Ca, Mg, K, S, P, and Cl was correlated with exposure to increasing concentrations of Tc-99-MIBI. EPXMA signals indicated that Tc-99-MIBI was concentrated up to 1,000 times into mitochondria in a dose-dependent fashion based on measured Tc content in the mitochondria. Inner membrane potential (DELTApsi) of individual mitochondria was calculated as -117 mV using the Nernst equation. Concentrations of Tc-99-MIBI >36 muM caused a significant efflux of K and Mg from the cell, as well as an increase in Cl in the mitochondria. Comparison of cell ultrastructure with conventional electron microscopy at extracellular 99Tc-MIBI concentrations of 36-72 muM showed no changes compared with control. Tc-99-MIBI allows valuable in situ investigation of cellular bioenergetics with EPXMA by quantitation of DELTApsi.