Measurement of Myocardial MembranePotential by Primed Constant Infusion

Introduction: We recently reported a bolus method with dynamic imaging ¹⁸F-TPP⁺ (¹⁸F-labeled tetraphenylphosphonium) for measuring the tissue membrane potential (ΔΨ_T), a proxy for the mitochondrial membrane potential (ΔΨ_m) [1]. The total volume of distribution (V_T) of ¹⁸F-TPP⁺ is sensitive to two independent variables; the tissue fraction of the extra-cellular space (f_ECS) and tissue membrane potential (ΔΨ_T). The original methodology [1] requires imaging with blood sampling for 120 minutes. This study reports a new methodology, using bolus plus infusion, referred as primed constant infusion, of ¹⁸F-TPP⁺ to image a steady state which is more suitable for human investigation. Furthermore, we tested the hypothesis that intra-coronary infusion of BAM15, a mitochondrial proton gradient uncoupler [2,3], during imaging depolarizes myocardium membrane potential resulting in lower ¹⁸F-TPP⁺ concentration in relevant heart segments.

Methods: Four mini-pigs (three normal and one pig with myocardial infarction) were administered ¹⁸F-TPP⁺ by intravenous bolus injection followed by a constant infusion for up to 180 min. A PET/CT (GE Discovery MI) scanner was used to measure dynamic ¹⁸F-TPP⁺ during the experimental period. And whole blood and plasma activity concentration history was also obtained. Pre-contrast and post-contrast CT scans [4] were used to produce a map of f_ECS. V_T and ΔΨ_T were measured at secular equilibrium (90-105 min). After 105 min, BAM15 was infused into the left anterior descending (LAD) coronary artery for 25 min. Two pig studies were used to establish BAM15 dosing and two pigs were studied with the complete protocol. Tissue-to-plasma concentration at equilibrium, V_T, was determined using 17 segment polar mapping.

Results: The average f_ECS map value was 24.2±2.5%. The quality of f_ECS maps is adequate for calculation of ΔΨ_T. During the “control” secular equilibrium state (i.e. 90-105 min, no BAM15), average normal and infarcted segments values of V_T were 27.1±5.0 and 16.4±2.5, which yielded ΔΨ_T values of -128.4±5.1 meV and -111.2±2.7 meV in agreement with literature values and with our previous bolus-only measurements [1]. ¹⁸F-TPP⁺ concentration decreased after BAM15 infusion in apical-septal and apical regions, corresponding to the LAD territory, while other segments showed stable tissue concentration during the scan (Figure).

Conclusions: Primed constant infusion of ¹⁸F-TPP⁺ can be used to measure V_T and ΔΨ_T with the advantage that subjects need to be scanned during secular equilibrium. The effect of the mitochondrial proton uncoupler, BAM15, demonstrates the sensitivity of the technique to depolarization of the inner mitochondrial membrane. Research Support: P41EB022544 R01HL137230 T32EB013180