Investigation of Metabolism of Exogenous Glucose at the Early Stage and Onset of Diabetes Mellitus in Otsuka Long-Evans Tokushima Fatty Rats Using [1, 2, 3-¹³C]Glucose Breath Tests

<div><p>This study aimed to evaluate changes in glucose metabolism at the early stage and onset of diabetes in Otsuka Long-Evans Tokushima Fatty (OLETF) rats. Specifically, after the oral administration of [1, 2, 3-¹³C]glucose, the levels of exhaled ¹³CO₂, which most likely originated from pyruvate decarboxylation and tricarboxylic acid, were measured. Eight OLETF rats and eight control rats (Long-Evans Tokushima Otsuka [LETO]) were administered ¹³C-glucose. Three types of ¹³C-glucose breath tests were performed thrice in each period at 2-week intervals. [3-¹³C]glucose results in a ¹³C isotope at position 1 in the pyruvate molecule, which provides ¹³CO₂. The ¹³C at carbons 1 and 2 of glucose is converted to ¹³C at carbons 2 and 1 of acetate, respectively, which produce ¹³CO₂. Based on metabolic differences of the labeled sites, glucose metabolism was evaluated using the results of three breath tests. The increase in ¹³CO₂ excretion in OLETF rats was delayed in all three breath tests compared to that in control rats, suggesting that OLETF rats had a lower glucose metabolism than control rats. In addition, overall glucose metabolism increased with age in both groups. The utilization of [2-¹³C]glucose was suppressed in OLETF rats at 6–12 weeks of age, but they showed higher [3-¹³C]glucose oxidation than control rats at 22–25 weeks of age. In the [1-¹³C]glucose breath test, no significant differences in the area under the curve until 180 minutes (AUC₁₈₀) were observed between OLETF and LETO rats of any age. Glucose metabolism kinetics were different between the age groups and two groups of rats; however, these differences were not significant based on the overall AUC₁₈₀ of [1-¹³C]glucose. We conclude that breath ¹³CO₂ excretion is reduced in OLETF rats at the primary stage of prediabetes, indicating differences in glucose oxidation kinetics between OLETF and LETO rats.</p></div

Results of [1-¹³C]glucose breath tests.

<p>(A) Changes in expired ¹³CO₂ levels after the oral administration of 100 mg/kg of [1-¹³C]glucose in OLETF rats at 5–11weeks, 14–17 weeks, and 20–23 weeks of age. (B) Changes in expired ¹³CO₂ levels after the oral administration of 100 mg/kg of [1-¹³C]glucose in LETO rats at 5–11 weeks, 14–17 weeks, and 20–23 weeks of age. Data are represented as mean ± SD.</p

[2-¹³C]glucose breath test between OLETF and LETO rats.

<p>Comparison of ¹³CO₂ excretion curves after the oral administration of 100 mg/kg of [2-¹³C]glucose between OLETF and LETO rats at 6–12 weeks (A), 15–18 weeks (B), and 21–24 weeks (C) of age. Data are represented as mean ± SD.</p

Results of [2-¹³C]glucose breath tests.

<p>(A) Changes in expired ¹³CO₂ levels after the oral administration of 100 mg/kg of [2-¹³C]glucose in OLETF rats at 6–12 weeks, 15–18 weeks, and 21–24 weeks of age. (B) Changes in expired ¹³CO₂ levels after the oral administration of 100 mg/kg of [2-¹³C]glucose in LETO rats at 6–12 weeks, 15–18 weeks, and 21–24 weeks of age. Data are represented as mean ± SD.</p

Fasting plasma glucose levels when breath tests were performed.

<p>Blood glucose level was significantly higher in OLETF rats than in LETO rats at every stage. (<i>P</i> < 0.01, Student’s <i>t</i>-test was performed on each diabetic stage.) Data are represented as mean ± SD.</p

[3-¹³C]glucose breath test between OLETF and LETO rats.

<p>Comparison of ¹³CO₂ excretion curves after the oral administration of 100 mg/kg of [3-¹³C]glucose between OLETF and LETO rats at 7–13 weeks (A), 16–19 weeks (B), and 22–25 weeks (C) of age. Data are represented as mean ± SD.</p

Changes in food intake per day.

<p>Dietary intake differed significantly between the two groups (two-way repeated-measures ANOVA: rat × time F(23,391) = 5.56, <i>P</i> < 0.01; time F(23,391) = 106.22, <i>P</i> < 0.01; rat F(1,17) = 276.72, <i>P</i> < 0.01). LETO, <i>n</i> = 8; OLETF, <i>n</i> = 8. Data are represented as mean ± SD.</p

Values of AUC₉₀ obtained from the three ¹³C-glucose breath tests for each age group.

<p>Values of AUC₉₀ obtained from the three ¹³C-glucose breath tests for each age group.</p

Tmax obtained from the three types of ¹³C-glucose breath tests for each age group.

<p>Tmax obtained from the three types of ¹³C-glucose breath tests for each age group.</p

Peak values obtained from the three types of ¹³C-glucose breath tests for each age group.

<p>Peak values obtained from the three types of ¹³C-glucose breath tests for each age group.</p