Fetal and newborn hearts have been shown to use lactate as the primary metabolic fuel, later switching to fatty acids; these hearts are less sensitive to hypoxia than adult hearts. Advancing our understanding of substrate selection prior to, and following hypoxia will assist in the development of improved techniques for metabolic/ nutritional support of critically ill infants. Therefore, 13C NMR isotopomer analysis was used to determine relative substrate utilization at baseline and following hypoxia.

Methods: Two groups of 4-day-old isolated, perfused rabbit hearts (n=5 each) were studied at 37°C with 10 mM glucose + insulin, 1 mM lactate and 0.25 mM acetate. Following 40 min. baseline, control hearts were switched to 3-13C lactate and 1,2-13C acetate, while glucose was unlabelled. After 30 min of labelled perfusion, hearts were freeze clamped, underwent PCA extraction, and 13C NMR analysis. Hypoxia/reoxygenation hearts were subjected to 60 min of hypoxia (20 torr) followed by reoxygenation for 60 min. and then perfused with the same 13C labels, extracted and analyzed by 13C NMR. Relative contributions of 13C acetyl-CoA was determined by analyzing the 13C NMR spectra of C4 glutamate, while comparing C4 to C3 glutamate was used to quantify labelled vs. non-labelled carbon sources. Stastical significance is defined as p ≤ 0.05, ANOVA.

Results: In control hearts, 74% of the acetyl-CoA was derived from lactate, 16% from acetate, and 10% from unlabelled sources. Following hypoxia/reoxygenation, lactate derived acetyl-CoA significantly decreased to 47%, acetate was unchanged at 18%, and unlabelled sources significantly increased to 35%.

Conclusions: Lactate was the predominant metabolic fuel under control conditions, unlabelled substrates (glucose, glycogen) increased in use following hypoxia/reoxygenation. This study demonstrates the feasibility of13 C isotopomer analysis for metabolic studies of the infant heart.