Mitochondrial and Plasma Membrane Potential Dependence

The fundamental myocellular uptake and retention mechanisms of hexakis (2-methoxyisobutyl isonitrile) technetium(I) (Tc-MIBI), a technetium-99m-based myocardial perfusion imaging agent, are unresolved. Because of the lipophilic cationic nature of Tc-MIBI, it may be distributed across biological membranes in response to transmembrane potential. To test this hypothesis, net uptake and retention of Tc-MIBI in cultured chick embryo ventricular myocytes were determined under conditions known to alter mitochondrial and plasma membrane potentials. Isovolumic depolarization of plasma membrane potentials in 130 mM extracellular K (KO) 20 mM extracellular Cl buffer reduced net accumulation of Tc-MIBI from 171+16 (control) to 29±3.3 fmol intracellular Tc-MIBI/mg protein nM extracellular Tc-MIBI. Unidirectional influx of Tc-MIBI in cells depolarized in 30 mM K. buffer was also reduced; a resting plasma membrane potential of -87±+6 mV was calculated from the Goldman flux equation using normal KO/high KO Tc-MIBI influx ratios. Addition of the potassium ionophore valinomycin to cells incubated in 130 mM KO buffer to additionally depolarize mitochondrial membrane potentials further reduced net uptake of Tc-MIBI to levels comparable to that found in nonviable freeze-thawed preparations ([Tc-MIBI]j/ [Tc-MIBI]0=1). By depolarizing mitochondrial (and in part plasma membrane) potentials with the protonophores 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone (CCCP) Tc-MIBI was rapidly depleted from 181±+16 (control) to 16±2.6 and 31±4.2 fmol/mg protein nMo, respectively, with kinetics that did not correlate with loss of cellular ATP content. CCCP alone inhibited 90±3% of net accumulation or 66±3% of unidirectional influx of Tc-MIBI in a concentration-dependent