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Existing methods of measuring glucose kinetics are subject to errors. There is considerable interest in improved methods of measuring glucose kinetics to allow the development of new regimes for the treatment of diabetes mellitus. 3-O-Methyl-D-glucose is transported but not metabolized and therefore allows independent estimation of transport parameters. We describe a method by which 3-O-methyl-D-glucose in plasma samples can be measured in protocols during which glucose flux is assessed with simultaneous use of two isotopically labeled glucoses to quantitate and validate measurements of the rate of glucose appearance and disappearance. Quantitative gas chromatographic/mass spectrometric (GC/MS) analysis of 3-O-methyl-D-glucose, D-glucose, D-[U-13C] glucose and D-[6,6-2H2] glucose in human plasma using methoxime-trimethylsilyl ether derivatives is described. Measurements of all four derivatives were performed together in a small sample volume (50 microliters) with high precision. The intra-assay (inter-assay) coefficients of variation at an isotope content of 0.25 atom% excess for D-[6,6-2H2] glucose, D-[U-13C] glucose and 3-O-methyl-D-glucose were 0.8% (1.0%), 0.5% (4.0%) and 0.1% (3.7%), respectively. This method provides the basis for quantitative estimation of parameters of glucose kinetics in man and the rates of glucose flux across the cell membrane.

The effect of infusions of recombinant insulin-like growth factor-I (IGF-I) (34, 103 or 688 pmol kg-1 min-1), insulin (3.4, 10.3 or 68.8 pmol kg-1 min-1) or combined infusions (34 pmol IGF-I + 3.4 pmol kg-1 min-1 insulin or 103 pmol IGF-I + 3.4 pmol kg-1 min-1 insulin) on glucose metabolism was investigated in dogs using a [3-3H]-glucose infusion and euglycaemic clamp. All insulin doses decreased glucose production rate (Ra) in a dose-dependent manner (P < 0.05). All IGF-I doses decreased glucose Ra (P < 0.05) but this decrease was not dose dependent. The decrease in glucose Ra with the combined infusion of 34 pmol kg-1 min-1 IGF-I + 3.4 pmol kg-1 min-1 insulin was greater than 34 pmol kg-1 min-1 IGF-I (P < 0.05) but not different from 3-4 pmol kg-1 min-1 insulin. All insulin and IGF-I doses increased glucose utilization rate (Rd) in a dose-dependent manner (P < 0.01). The increase in glucose utilization was greater following both combined infusions than with either component infused alone (P < 0.05). Although at the doses selected, insulin and IGF-I had similar effects on glucose utilization with additive effects when the two peptides were combined, IGF-I was less effective than insulin in suppressing glucose production.

The relationship between changes in ketone concentrations and leucine metabolism (seven obese subjects), glucose and alanine metabolism (seven obese subjects) was investigated using radioisotopic techniques after 12 h, 60 h and 2 weeks starvation. Leucine metabolism was also measured in five lean subjects after 12 h and 60 h starvation. In the obese subjects leucine concentration increased after 60 h starvation and leucine metabolic clearance rate, glucose and alanine concentration decreased (P < 0.05). Glucose and alanine production rate (Ra) decreased after 2 weeks (P < 0.05) but there was no change in leucine Ra after 60 h or 2 weeks. In the lean subjects leucine concentration, production rate and oxidation rate were increased after 60 h (P < 0.005, P < 0.05, P < 0.05). Ketone concentration was inversely related to alanine Ra (r = -0.51, P < 0.02) but was not related to measurements of protein metabolism in the obese subjects. This study demonstrates that the effect of short-term starvation on protein metabolism differs in lean and obese subjects. The decrease in glucose Ra during long-term starvation may be in part due to a decreased supply of alanine for gluconeogenesis.

The metabolic and cardiovascular effects of recombinant human IGF-I were compared to insulin in six normal subjects. Subjects were studied twice and intravenously received an infusion of [6,6-2H2]glucose (0-480 min) and in random order either IGF-I 20 micrograms kg-1 h-1 (43.7 pmol kg-1 min-1 or insulin 0.5 mU kg-1 min-1 (3.4 pmol kg-1 min-1) with an euglycaemic clamp. One subject was withdrawn following a serious adverse event. During the IGF-I infusion glucose appearance rate (Ra) decreased from 1.79 +/- 0.13 at baseline (150-180 min) to 0.35 +/- 0.26 mg kg-1 min-1 (P < 0.01) at 360 min, and glucose utilization rate (Rd) increased from 1.79 +/- 0.28 to 4.17 +/- 0.84 mg kg-1 min-1 (P < 0.01). There was no change in free fatty acids (FFA) and an increase (percentage change from pre-infusion mean) in cardiac output +l37.3% +/- 9% (P < 0.01), heart rate +13% +/- 2% (P < 0.01) and stroke volume +21% +/- 7% (P < 0.05). During the insulin infusion glucose Ra decreased from 1.89 +/- 0.13 to 0.34 +/- 0.33 mg kg-1 min-1 (P < 0.01) and FFA from 0.546 mmol l-1 to 0.198 mmol l-1 (P < 0.01), glucose Rd increased from 1.89 +/- 0.18 to 5.41 +/- 1.47 mg kg-1 min-1 (P < 0.01) and there were no significant changes in the cardiovascular variables.

The effect of infusions of recombinant insulin-like growth factor-I (IGF-I) (34, 103 or 688 pmol min-1 kg-1), insulin (3.4, 10.3 or 68.8 pmol min-1 kg-1) or combined infusions (34 pmol IGF-I + 3.4 pmol min-1 kg-1 insulin or 103 pmol IGF-I + 3.4 pmol min-1 kg-1 insulin) on protein metabolism, using an infusion of [1-14C]leucine was investigated in anaesthetized fasted dogs. Leucine concentration, production rate (measure of protein degradation), oxidation rate and non-oxidative disappearance rate (measure of protein synthesis) were decreased in a similar dose dependent manner by the IGF-I and insulin infusions (P < 0.01). The decrease in these measurements of leucine metabolism were greater following 34 pmol IGF-I + 3.4 pmol insulin than with either component infused alone (P < 0.05). Free fatty acid concentrations were decreased by all insulin doses (P < 0.01) but only by 103 and 688 pmol min-1 kg-1 insulin-like growth factor (P < 0.05, P < 0.01). These data demonstrate that IGF-I, like insulin, has a dose dependent effect on protein metabolism and that combined insulin and IGF-I infusions have additive effects on protein metabolism.

Turnover rates of glucose, free fatty acids (FFA) and leucine have been measured in newly diagnosed, uncontrolled insulin-dependent diabetic (IDD) patients. The results have been compared with data collected from the same patients while on conventional insulin therapy as well as after overnight intravenous infusion of insulin with sustained normoglycaemia. The data have been analysed by compartmental and non-compartmental methods and the results have been compared with simultaneously collected data on respiratory exchange. Oxidation rates of 14C-labelled substrates have also been measured. Tracer studies were done on established diabetics after insulin withdrawal and subsequent intravenous infusion of insulin at different rates. The results confirm the in vivo importance of the glucose-fatty acid cycle, indicating that when glucose, FFA and ketone bodies are available in excess it is FFA and ketones that are metabolized in preference to glucose. The data emphasize the importance of increased production rates rather than decreased utilization rates in producing high concentrations of substrates in the plasma of insulin-deficient patients.