UPTAKE AND RETENTION OF HEXAKIS (2-METHOXYISOBUTYL ISONITRILE) TECHNETIUM(I) IN CULTURED CHICK MYOCARDIAL-CELLS - 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 (K0) 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 K0 buffer was also reduced; a resting plasma membrane potential of -87±6 mV was calculated from the Goldman flux equation using normal K0/high K0 Tc-MIBI influx ratios. Addition of the potassium ionophore valinomycin to cells incubated in 130 mM K0 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]i/ [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·nM0, 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 manner. By hyperpolarizing mitochondrial membrane potentials with the K+/H+ ionophore nigericin or the ATP synthase inhibitor oligomycin, net uptake and retention of Tc-MIBI were increased by 60±9% and 375±20%, respectively. Caffeine, as well as the respiratory chain electron transport inhibitor rotenone, did not significantly alter net cell uptake (p>0.2). These data indicate that the fundamental myocellular uptake mechanism of Tc-MIBI involves passive distribution across plasma and mitochondrial membranes and that at equilibrium Tc-MIBI is sequestered within mitochondria by the large negative transmembrane potentials.