Isotopic Estimation of Plasma Glucose Conversion to Plasma Lactate Using [6- 3 H] Glucose and [6- 14 C]Glucose

Abstract

Abstract

To determine whether [6-³H]glucose could be used to quantitatively estimate the rate of plasma glucose conversion to plasma lactate, we compared the relative transfer of [³H] and [¹⁴C]plasma glucose to plasma lactate in nine postabsorptive anesthetized rats infused to isotopic steady state with [6-³H]glucose and [6-¹⁴C]glucose. Glucose turnover (μmol/kg/min) measured with [6-³H]glucose (29.4 ± 1.7) and [6-¹⁴C] glucose (25.0 ± 1.5), Cori cycle activity (4.35 ± 0.79), and the percentage of plasma lactate derived from plasma glucose calculated from ¹⁴C-specific activities (59.8 ± 5.8) were all comparable to previously reported values for the overnight fasted rat. Although the percentage of plasma lactate derived from plasma glucose calculated from ³H-specific activities (30.0 ± 3.3) was only half that calculated from ¹⁴C-specific activities (P < 0.001), the ³H:¹⁴C ratios were constant over a wide range of percentage of lactate derived from glucose (r = 0.95, P < 0.001). We conclude that ³H-specific activity ratios of plasma lactate to plasma glucose cannot be directly used as a quantitative estimate of the percentage of plasma lactate derived from plasma glucose; however, correction for the apparent constant proportionality of detritiation of lactate relative to ¹⁴C loss could permit use of [6-³H]glucose for this purpose.