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 (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).

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.

Fig 1. Volume of distribution model for¹⁸F-TPP⁺ in a PET image voxel.

The outer black line represents the voxel boundary. C_p, C_inter, C_cyto, and C_mito represent the concentrations of the plasma, interstitial space, cytosol, and mitochondria respectively. The arrows represent ¹⁸F-TPP⁺ transport between the different compartments. f_ECS represents the voxel volume fraction occupied by ECS and f_mito represents the cellular volume fraction occupied by mitochondria.

Fig 2. Dependence of V_T on mitochondrial membrane potential and fractional ECS computed with Eq 3.

ΔΨ_T is more negative inside membranes. Black lines indicate variation (systematic error) in ΔΨ_T range at constant volume of distribution due to neglecting fractional ECS volume.

Fig 3. Variation of plasma and tissue concentration following intravenous bolus injection of ¹⁸F-TPP⁺.

Plasma concentration decreases monotonically over the first two hours; whereas, myocardial concentration is nearly constant.

Fig 4. Whole blood-to-plasma concentration ratio measured as a function of time after bolus injection of ¹⁸F-TPP⁺.

After about 15 minutes, whole blood and plasma concentrations equilibrate with equal concentration.

Fig 5. Typical ¹⁸F-TPP⁺ SUV image, integrated from 60–120 minutes after IV bolus injection.

SUV is highest in liver, followed by LV myocardium, with lower activity in the visible in bone marrow.

Fig 6. Parametric images of V_T and ΔΨ_T.

Each panel of three x two images shows short axis, vertical and horizontal slices. Images of a representative control pig are shown in the left panel. Images of a representative scar pig are shown in the right panel. The top row of each panel depicts the TPP⁺ volume of distribution and bottom row the membrane potential.

Fig 7. Comparison of ΔΨ_T in 17 "bull’s eye" segments Results shown for five control (blue squares) and three pigs with injury to the LAD territory (red circles).