Substrate cycling between pyruvate and oxaloacetate in awake normal and 3,3'-5-triiodo-L-thyronine-treated rats

Abstract

Substrate cycling between pyruvate and oxaloacetate was assessed in awake 24-h fasted normal and triiodothyronine (T3)-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 T3-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 T3 treatment (normal 14.22 +/- 5.25 U/g liver vs. T3 treated 13.40 +/- 1.10 U/g liver), whereas both the enzyme activity ratio of PK (normal 0.47 +/- 0.15 vs. T3 treated 0.77 +/- 0.03, P < 0.05) and the activity of ME (normal 0.89 +/- 0.30 U/g liver vs. T3 treated 4.25 +/- 0.60 U/g liver, P < 0.05) increased with T3 treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of [3,4‐13C2]glucose to [6,6‐2H2]glucose as a tracer for glucose turnover by nuclear magnetic resonance

“…A basic assumption in measurement of glucose turnover is that a tracer molecule is not released from liver as the result of glycogenolysis or gluconeogenesis. A number of biochemical pathways could be followed by [3,[4][5][6][7][8][9][10][11][12][13] (Fig. 3).…”

“…Unlike [1,[6][7][8][9][10][11][12][13] C 2 ]MAG, [3,[4][5][6][7][8][9][10][11][12][13] C 2 ]MAG yields a characteristic pair of doublets in a 13 C NMR spectrum due to spin-spin coupling, which is easily observed. Most importantly, detection with NMR has the considerable advantage that the presence of 13 C does not interfere with measurement of 2 H enrichment by 2 H NMR at typical levels of enrichment.The goal of this study was to compare GP measured by a standard tracer, [6,6-2 H 2 ]glucose, to GP measured by NMR detection of [3,[4][5][6][7][8][9][10][11][12][13] C 2 ]glucose. Also, a convenient method to quantify 2 H enrichment in plasma glucose is introduced using an internal reference, deuterated dimethylformamide (DMF-d 7 ).…”

“…A simple derivative, monoacetone glucose (1,2-diisopropylidene glucofuranose, also termed MAG), offers well-resolved 2 H and 13 C NMR spectra while preserving the informative 2 H and 13 C distribution. Unlike [1,[6][7][8][9][10][11][12][13] C 2 ]MAG, [3,[4][5][6][7][8][9][10][11][12][13] C 2 ]MAG yields a characteristic pair of doublets in a 13 C NMR spectrum due to spin-spin coupling, which is easily observed. Most importantly, detection with NMR has the considerable advantage that the presence of 13 C does not interfere with measurement of 2 H enrichment by 2 H NMR at typical levels of enrichment.…”

“…The goal of this study was to compare GP measured by a standard tracer, [6,6-2 H 2 ]glucose, to GP measured by NMR detection of [3,[4][5][6][7][8][9][10][11][12][13] C 2 ]glucose. Also, a convenient method to quantify 2 H enrichment in plasma glucose is introduced using an internal reference, deuterated dimethylformamide (DMF-d 7 ).…”

Thyroid hormone action in metabolic regulation

“…However, RTH subjects have increased muscle mitochondrial uncoupling and fatty acid oxidation which may improve insulin sensitivity. This discrepancy probably is due to the net accumulation of IMCL and DAG caused by increased lipogenesis or enhanced delivery of free fatty acid to skeletal muscle which exceeds the oxidative capacity (Oppenheimer et al, 1991;Petersen et al, 1994;Petersen et al, 1995). Another possibility is that the RTH action might influence hepatic insulin sensitivity.…”