HEPATIC GLUCOSE-6-PHOSPHATASE FLUX AND GLUCOSE PHOSPHORYLATION, CYCLING, IRREVERSIBLE DISPOSAL, AND NET BALANCE IN-VIVO IN RATS - MEASUREMENT USING THE SECRETED GLUCURONATE TECHNIQUE

Measurement of hepatic glucose production (HGP) by standard isotope di lution 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 balanc e. We describe two independent isotopic techniques for measuring these parameters in vivo, both of which use secreted glucuronate (GlcUA). H GC can be quantified by measuring a correction factor for glucose labe l retained in hepatic glucose-6-phosphate (G6P), sampled as GlcUA. A c omplementary technique for measuring total hepatic glucose output is a lso described (reverse dilution), requiring administration of no label ed glucose but instead a labeled gluconeogenic precursor and unlabeled glucose. Hepatic Rd is calculated by multiplying the rate of appearan ce (Ra) of hepatic UDP-glucose ([UDP-glc] based on dilution of labeled galactose in GlcUA) times the direct entry of glucose into hepatic UD P-glc and the fraction of labeled UDP-glc retained in the liver. The s um of hepatic Rd plus HGC represents the total hepatic glucose phospho rylation rate. Rats received intravenous (IV) glucose infusions at a r ate 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 signifi cantly decreased, because of increased HGC. Total hepatic glucose outp ut calculated by reverse dilution yielded similar results during IV gl ucose infusions at 15 mg/kg/min, although values were higher than obta ined by the correction-factor method at 30 mg/kg/min. The fraction of labeled UDP-glc released into blood glucose, representing a hepatic gl ycogen cycle, decreased from 35% (fasted) to nearly 0% (IV glucose 30 mg/kg/min). Hepatic Rd was 1.4, 4.6, and 7.5 mg/kg/min (fasted and IV glucose 15 and 30 mg/kg/min, respectively); total hepatic glucose phos phorylation increased substantially (from 4.2 to 8.5 to 12.7 mg/kg/min ) and net hepatic glucose balance changed from negative to positive du ring IV glucose. In conclusion, hepatic GGPase flux, glucose phosphory lation, HGC, disposal of glucose into glycogen, and net glucose balanc e can be measured noninvasively in vivo under various metabolic condit ions by techniques involving the GlcUA probe. Copyright (C) 1997 by W. B. Saunders Company.