Date Published: January 16, 2018
Publisher: Public Library of Science
Author(s): Nathaniel M. Alpert, Nicolas Guehl, Leon Ptaszek, Matthieu Pelletier-Galarneau, Jeremy Ruskin, Moussa C. Mansour, Dustin Wooten, Chao Ma, Kazue Takahashi, Yun Zhou, Timothy M. Shoup, Marc D. Normandin, Georges El Fakhri, Cecilia Zazueta.
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.
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 (fECS) was measured in 3 pigs. We derived equations expressing ΔΨT as a function of VT and the volume-fractions of mitochondria and fECS. Seventeen segment polar maps and parametric images of ΔΨT were calculated in millivolts (mV).
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.
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.
Mitochondria produce approximately 90% of cellular adenosine triphosphate (ATP) through oxidative phosphorylation . The electron transport chain (ETC) of the mitochondrion is ultimately responsible for converting the foods we eat into electrical and chemical energy gradients by pumping protons across the inner membrane in the mitochondrial intermembrane space. The energy stored in the electric field, referred to as mitochondrial membrane potential (ΔΨm), is then used to power the conversion of ADP to ATP. In a typical cell, the ΔΨm remains constant with time and is about -140 mV . Table 1 lists ΔΨm for mitochondria of different cell types.
In this paper, we introduce the concept of quantitative mapping of ΔΨT for monitoring mitochondrial status. Our development emphasizes measurement of the total membrane potential, ΔΨT, while making clear that ΔΨT is a proxy for and tightly correlated to ΔΨm. Direct measurements of ΔΨm require a separate measurement of the cellular membrane potential that is currently not possible in vivo. Just as in the early studies conducted with 3H-TPP+, our method relies on measuring the total concentration of a lipophilic cationic tracer which is then analyzed by using a compartment model of the tissue and the steady state formulation of the Nernst equation, the same equation that is fundamental to electrochemistry  and cardiac electrophysiology [39, 40].
This study is the first to demonstrate the feasibility of quantitative in vivo mapping of total membrane potential, ΔΨT, a proxy of ΔΨm. In vivo measurements of ΔΨT obtained with our new method yielded values remarkably constant within and across the hearts of domestic swine that are comparable to results from in vitro bench top experiments. We have derived a theory explaining, for the first time, the major factors affecting the transport and residence time of lipophilic cations in tissue, including ΔΨT and fECS. The fact that we can measure ΔΨT in mV suggests that it may be possible to compare individual’s studies with normative data. Given the critical role of mitochondrial function in numerous pathologies, the potential applications of this new imaging method are immense. This novel technique could eventually be proved useful in numerous clinical and research scenarios.