Substrate cycling between pyruvate and oxaloacetate was assessed in awake 24-h fasted normal and triiodothyronine (T-3)-treated rats. After a 20- or 60-min infusion of [3-C-13]alanine (99% enriched, 12 mg/min) the C-13 enrichments of liver glucose and alanine carbons were analyzed by C-13 and H-1 nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry. Substrate cycling from phosphoenolpyruvate to pyruvate [via pyruvate kinase (PK)] and from oxaloacetate to pyruvate [via malic enzyme (ME)] relative to the pyruvate carboxylase (PC) flux [i.e., (PK + ME)/PC] was assessed by the ratio of the C-13 enrichment of C-2 alanine relative to that in C-5 glucose. In the normal rats (PK + ME)/PC was 0.26 +/- 0.07 (n = 7, t = 20 min) and 0.37 +/- 0.08 (n = 4, t = 60 min). In the T-3-treated rats the (PK + ME)/PC increased four- to fivefold to 1.03 +/- 0.19 (n = 8, t = 20 min) and to 1.83 +/- 0.19 (n = 3, t = 60 min) (P < 0.05 vs. normal rats). The liver enzyme activity of PK did not change with T-3 treatment (normal 14.22 +/- 5.25 U/g liver vs. T-3 treated 13.40 +/- 1.10 U/g liver), whereas both the enzyme activity ratio of PK (normal 0.47 +/- 0.15 vs. T-3 treated 0.77 +/- 0.03, P < 0.05) and the activity of ME (normal 0.89 +/- 0.30 U/g liver vs. T-3 treated 4.25 +/- 0.60 U/g liver, P < 0.05) increased with T-3 treatment. In conclusion substrate cycling between pyruvate and oxaloacetate is substantial in the awake 24-h fasted rat, accounting for about one-third of the gluconeogenic flux, and is increased four- to fivefold with T-3 treatment. This increased substrate cycling contributes to the hypermetabolic state of hyperthyroidism.
