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 pharmacologic al studies have suggested that Tc-MIBI, a lipophilic cation, is seques tered in mitochondria in response to transmembrane potentials. To asse ss directly the subcellular distribution of the probe in heart tissue, cultured chick heart cells were analyzed by electron-probe X-ray micr oanalysis (EPXMA) following equilibration in micromolar concentrations of carrier-added Tc-99-MIBI, the ground-state radiopharmaceutical. Qu antitation of the physiological elements Na, Ca, Mg, K, S, P, and Cl w as 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) o f individual mitochondria was calculated as -117 mV using the Nernst e quation. Concentrations of Tc-99-MIBI >36 muM caused a significant eff lux of K and Mg from the cell, as well as an increase in Cl in the mit ochondria. Comparison of cell ultrastructure with conventional electro n microscopy at extracellular 99Tc-MIBI concentrations of 36-72 muM sh owed no changes compared with control. Tc-99-MIBI allows valuable in s itu investigation of cellular bioenergetics with EPXMA by quantitation of DELTApsi.