Quantitative in vivo mapping of myocardial mitochondrial membrane potential

Abstract Background: Mitochondrial membrane potential (ΔΨm) arises from normal function of the electron transport chain. Maintenance of ΔΨm within a narrow range is essential for mitochondrial function. Methods for in vivo measurement of ΔΨm do not exist. We use 18F-labeled tetraphenylphosphonium (18F-TPP+) to measure and map the total membrane potential, ΔΨT, as the sum of ΔΨm and cellular (ΔΨc) electrical potentials. Methods: Eight pigs, five controls and three with a scar-like injury, were studied. Pigs were studied with a dynamic PET scanning protocol to measure 18F-TPP+ volume of distribution, VT. Fractional extracellular space (f_ECS) was measured in 3 pigs. We derived equations expressing ΔΨT as a function of VT and the volume-fractions of mitochondria and f_ECS. Seventeen segment polar maps and parametric images of ΔΨT were calculated in millivolts (mV). Results: In controls, mean segmental ΔΨT = -129.4±1.4 mV (SEM). In pigs with segmental tissue injury, ΔΨT was clearly separated from control segments but variable, in the range -100 to 0 mV. The quality of ΔΨT maps was excellent, with low noise and good resolution. Measurements of ΔΨT in the left ventricle of pigs agree with previous in in-vitro measurements. Conclusions: We have analyzed the factors affecting the uptake of voltage sensing tracers and developed a minimally invasive method for mapping ΔΨT in left ventricular myocardium of pigs. ΔΨT is computed in absolute units, allowing for visual and statistical comparison of individual values with normative data. These studies demonstrate the first in vivo application of quantitative mapping of total tissue membrane potential, ΔΨT.研究背景：线粒体膜电位（mitochondrial membrane potential, ΔΨm）源于电子传递链（electron transport chain）的正常生理功能。将ΔΨm维持在狭窄区间内，是线粒体功能正常发挥的必要条件。目前尚无针对ΔΨm的体内检测方法。本研究采用氟-18标记四苯基鏻（18F-labeled tetraphenylphosphonium, ¹⁸F-TPP+），将线粒体膜电位ΔΨm与细胞电位（ΔΨc, cellular electrical potentials）之和定义为总膜电位ΔΨT，并对其进行测量与分布成像。 研究方法：本研究共纳入8头实验猪，其中5头为对照组，3头构建瘢痕样组织损伤模型。采用动态正电子发射断层扫描（positron emission tomography, PET）扫描方案，检测¹⁸F-TPP+的分布容积（volume of distribution, VT）。对其中3头猪测量细胞外间隙分数（fractional extracellular space, f_ECS）。本研究推导了以VT、线粒体体积分数及f_ECS为自变量的ΔΨT计算公式。最终生成17个心肌节段极坐标图及ΔΨT参数图像，单位为毫伏（mV）。 研究结果：对照组猪的心肌节段平均ΔΨT为-129.4±1.4 mV（均值标准误，standard error of the mean, SEM）。存在节段性组织损伤的猪，其ΔΨT与对照组节段区分显著，但数值波动范围为-100~0 mV。ΔΨT成像质量优异，噪声水平低且分辨率良好。猪左心室心肌（left ventricular myocardium）的ΔΨT测量结果与既往体外实验结果一致。 研究结论：本研究分析了影响电压敏感显像剂摄取的相关因素，建立了一种微创的猪左心室心肌总膜电位ΔΨT定量成像方法。ΔΨT以绝对单位计算，可实现个体测量值与参考标准的可视化及统计学对比。本研究首次实现了组织总膜电位ΔΨT的体内定量绘图。