Hellerstein M K, Neese R A, Letscher A, Linfoot P, Turner S
Department of Nutritional Sciences, University of California, Berkeley 94720-3104, USA.
Metabolism. 1997 Dec;46(12):1390-8. doi: 10.1016/s0026-0495(97)90137-2.
Measurement of hepatic glucose production (HGP) by standard isotope dilution reveals only the net release of glucose from the liver, not the flux across glucose-6-phosphatase ([G6Pase] or total hepatic glucose output), hepatic glucose cycling (HGC), irreversible glucose disposal into glycogen in the liver (hepatic Rd), or net hepatic glucose balance. We describe two independent isotopic techniques for measuring these parameters in vivo, both of which use secreted glucuronate (GlcUA). HGC can be quantified by measuring a correction factor for glucose label retained in hepatic glucose-6-phosphate (G6P), sampled as GlcUA. A complementary technique for measuring total hepatic glucose output is also described (reverse dilution), requiring administration of no labeled glucose but instead a labeled gluconeogenic precursor and unlabeled glucose. Hepatic Rd is calculated by multiplying the rate of appearance (Ra) of hepatic UDP-glucose ([UDP-glc] based on dilution of labeled galactose in GlcUA) times the direct entry of glucose into hepatic UDP-glc and the fraction of labeled UDP-glc retained in the liver. The sum of hepatic Rd plus HGC represents the total hepatic glucose phosphorylation rate. Rats received intravenous (i.v.) glucose infusions at a rate of 15 to 30 mg/kg/min after a 24-hour fast. Despite a suppression of net HGP more than 50%, total hepatic glucose output was not significantly decreased, because of increased HGC. Total hepatic glucose output calculated by reverse dilution yielded similar results during i.v. glucose infusions at 15 mg/kg/min, although values were higher than obtained by the correction-factor method at 30 mg/kg/min. The fraction of labeled UDP-glc released into blood glucose, representing a hepatic glycogen cycle, decreased from 35% (fasted) to nearly 0% (i.v. glucose 30 mg/kg/min). Hepatic Rd was 1.4, 4.6, and 7.5 mg/kg/min (fasted and i.v. glucose 15 and 30 mg/kg/min, respectively); total hepatic glucose phosphorylation increased substantially (from 4.2 to 8.5 to 12.7 mg/kg/min) and net hepatic glucose balance changed from negative to positive during i.v. glucose. In conclusion, hepatic G6Pase flux, glucose phosphorylation, HGC, disposal of glucose into glycogen, and net glucose balance can be measured noninvasively in vivo under various metabolic conditions by techniques involving the GlcUA probe.
通过标准同位素稀释法测量肝葡萄糖生成（HGP），只能揭示肝脏中葡萄糖的净释放量，而无法反映葡萄糖-6-磷酸酶（[G6Pase]，即总肝葡萄糖输出量）的通量、肝葡萄糖循环（HGC）、肝脏中葡萄糖不可逆地转化为糖原（肝Rd）或肝葡萄糖净平衡。我们描述了两种独立的同位素技术，用于在体内测量这些参数，这两种技术均使用分泌型葡萄糖醛酸（GlcUA）。通过测量作为GlcUA采样的肝葡萄糖-6-磷酸（G6P）中保留的葡萄糖标记的校正因子，可以对HGC进行定量。还描述了一种用于测量总肝葡萄糖输出量的补充技术（反向稀释法），该技术无需给予标记葡萄糖，而是给予标记的糖异生前体和未标记的葡萄糖。肝Rd通过将肝UDP-葡萄糖（[UDP-glc]，基于GlcUA中标记半乳糖的稀释）的出现率（Ra）乘以葡萄糖直接进入肝UDP-glc的速率以及肝中保留的标记UDP-glc的比例来计算。肝Rd与HGC之和代表总肝葡萄糖磷酸化速率。在禁食24小时后，大鼠以15至30mg/kg/min的速率接受静脉内（i.v.）葡萄糖输注。尽管净HGP受到超过50%的抑制，但由于HGC增加，总肝葡萄糖输出量并未显著降低。在以15mg/kg/min的速率进行静脉内葡萄糖输注期间，通过反向稀释法计算的总肝葡萄糖输出量产生了相似的结果，尽管在30mg/kg/min时的值高于通过校正因子法获得的值。释放到血糖中的标记UDP-glc的比例，代表肝糖原循环，从35%（禁食）降至近0%（静脉内葡萄糖30mg/kg/min）。肝Rd分别为1.4、4.6和7.5mg/kg/min（禁食以及静脉内葡萄糖15和30mg/kg/min时）；在静脉内输注葡萄糖期间，总肝葡萄糖磷酸化显著增加（从4.2增至8.5再增至12.7mg/kg/min），肝葡萄糖净平衡从负值变为正值。总之，通过涉及GlcUA探针的技术，可以在各种代谢条件下在体内非侵入性地测量肝G6Pase通量、葡萄糖磷酸化、HGC、葡萄糖转化为糖原以及葡萄糖净平衡。
