Modeling non-linear kinetics of hyperpolarized [1-(13)C] pyruvate in the crystalloid-perfused rat heart.

E Mariotti,M R Orton,O Eerbeek,J F Ashruf,R Southworth,C J Zuurbier,T R Eykyn

doi:10.1002/nbm.3464

E Mariotti, M R Orton + Show 5 more

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https://doi.org/10.1002/nbm.3464

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Abstract

Hyperpolarized 13C MR measurements have the potential to display non‐linear kinetics. We have developed an approach to describe possible non‐first‐order kinetics of hyperpolarized [1‐13C] pyruvate employing a system of differential equations that agrees with the principle of conservation of mass of the hyperpolarized signal. Simultaneous fitting to a second‐order model for conversion of [1‐13C] pyruvate to bicarbonate, lactate and alanine was well described in the isolated rat heart perfused with Krebs buffer containing glucose as sole energy substrate, or glucose supplemented with pyruvate. Second‐order modeling yielded significantly improved fits of pyruvate–bicarbonate kinetics compared with the more traditionally used first‐order model and suggested time‐dependent decreases in pyruvate–bicarbonate flux. Second‐order modeling gave time‐dependent changes in forward and reverse reaction kinetics of pyruvate–lactate exchange and pyruvate–alanine exchange in both groups of hearts during the infusion of pyruvate; however, the fits were not significantly improved with respect to a traditional first‐order model. The mechanism giving rise to second‐order pyruvate dehydrogenase (PDH) kinetics was explored experimentally using surface fluorescence measurements of nicotinamide adenine dinucleotide reduced form (NADH) performed under the same conditions, demonstrating a significant increase of NADH during pyruvate infusion. This suggests a simultaneous depletion of available mitochondrial NAD+ (the cofactor for PDH), consistent with the non‐linear nature of the kinetics. NADH levels returned to baseline following cessation of the pyruvate infusion, suggesting this to be a transient effect. © 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

Highlights

The heart is a highly metabolically active and energy demanding organ that is able to utilize a variety of energy substrates, dependent upon their relative abundance, prevailing hormonal conditions, workload and oxygen availability [1]
We studied two groups of hearts, one perfused with a standard Krebs–Henseleit buffer (KHB), where glucose was the only source of carbohydrate, and a second group perfused with KHB containing glucose supplemented with pyruvate
The 13C spectra from hearts perfused with glucose only exhibited two further peaks at about 175 parts per million and at about 172 ppm, which did not appear in spectra from hearts supplemented with pyruvate

Summary

Introduction

The heart is a highly metabolically active and energy demanding organ that is able to utilize a variety of energy substrates, dependent upon their relative abundance, prevailing hormonal conditions, workload and oxygen availability [1]. A number of experimental methods are available for investigating cardiac metabolism, including the use of radioactive tracer approaches [2,3] and steady-state incorporation of 13C labeled metabolites [4,5]. It has recently been shown to be possible to probe cardiac metabolism in real time Eerbeek Department of Anatomy, Embryology and Physiology, AMC, UvA, Amsterdam, The Netherlands preparations), by injecting hyperpolarized [1-13C] pyruvate and observing its conversion to bicarbonate, lactate and alanine by NMR or MRSI [6,7,8,9]. Fitting of the hyperpolarized 13C MRS time series of pyruvate and its metabolites to mathematical models [10,11] gives estimates of rate constants for their interconversion.

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