Substrate cycling between pyruvate and oxaloacetate in awake normal and 3,3'-5-triiodo-L-thyronine-treated rats
Petersen, K.F.; Cline, G.W.; Blair, J.B.; Shulman, G.I.
American Journal of Physiology 267(2 Pt 1): E273-E277
1994
ISSN/ISBN: 0002-9513 PMID: 8074207 Document Number: 428252
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 (3-13C)alanine (99% enriched, 12 mg/min) the 13C enrichments of liver glucose and alanine carbons were analyzed by 13C and 1H 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 13C 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 lt 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 lt 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 lt 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.