G. Biolo

Hyperaminoacidaemia reduces insulin-mediated glucose disposal in healthy man

SummaryTo determine whether hyperaminoacidaemia may modify insulin-mediated glucose disposal, normal subjects were studied with the euglycaemic glucose-clamp technique, with or without an amino acid infusion, at a rate sufficient to duplicate the plasma concentration of most amino acids. Steady-state glucose infusion rates to maintain euglycaemia were 36% lower during hyperaminoacidaemia (7.3±1.0 versus 11.4±0.8mg· kg−1· min−1, p<0.01) at comparable insulin concentrations (92±6 versus 93±7 mU/l respectively). Thus, under conditions of hyperinsulinaemia, amino acids could compete with glucose as metabolic fuels.

Regulation of Postprandial Whole-Body Proteolysis in Insulin-Deprived IDDM

Suppression of tissue proteolysis is an important mechanism of postprandial protein anabolism, and it may be mediated by insulin, hyperaminoacidemia, or both. To evaluate whether insulin is essential in the regulation of this process, we have investigated the effect of mixed-meal ingestion on whole-body protein breakdown in insulin-deprived insulin-dependent diabetes mellitus (IDDM) patients and normal control subjects. Endogenous phe-nylalanine and leucine rate of appearance (Ra) from proteolysis were measured at steady-state conditions using a multiple stable isotope technique before and after the constant administration of a synthetic mixed meal. In the postabsorptive state, the IDDM patients exhibited accelerated intracellular leucine Ra (IDDM, 2.64 ± 0.19 μmol · min−1 · kg−1; control, 2.02 ± 0.08 μmol · min−1 · kg−1; P < 0.05) and plasma phenylalanine Ra (IDDM, 0.73 ± 0.03 μmol · min−1 · kg−1; control, 0.61 ± 0.04 μmol · min−1 · kg−1; P < 0.05). During meal ingestion, endogenous phenylalanine and leucine Ra values were suppressed in both the insulin-deficient IDDM (P < 0.05) and control subjects (P < 0.05). Although postmeal endogenous leucine and phenylalanine Ra values remained greater (P < 0.05) in IDDM, the Δ changes from the basal endogenous leucine Ra (IDDM, −0.56 ± 0.11 μmol · min−1 · kg−1; control, −0.56 ± 0.09 μmol · min−1 · kg−1) and phenylalanine Ra (IDDM, −0.13 ± 0.01 μmol · min−1 · kg−1; control, −0.14 ± 0.02 μmol · min−1 · kg−1) were similar in both groups. In the IDDM patients, the postmeal increases from the basal leucine concentration were onefold greater (P < 0.05) than in the control subjects. In conclusion, in IDDM patients, whole-body proteolysis was suppressed during meal ingestion despite insulin withdrawal, which was possibly mediated by excessive hyperaminoacidemia.

Splanchnic versus whole-body production of α-ketoisocaproate from leucine in the fed state

The extent to which dietary branched-chain amino acids are deaminated by the splanchnic tissues (ie, the liver and gut) in the fed state and released as ketoacids into the systemic circulation is not known. To determine this, we combined the oral (L-[1-13C]-leucine, [13C]-Leu) and intravenous (L-[5,5,5-2H3]leucine, [2H3]-Leu) leucine tracer infusion with the intravenous administration of an independent isotope of the leucine ketoanalog alpha-ketoisocaproic acid (KIC) ([4,5-3H]KIC). The study was conducted during constant administration of a complete mixed meal. We found that 26% +/- 5% of the orally administered leucine was taken up by the splanchnic organs at first pass, whereas 74% +/- 5% appeared in the systemic circulation. The rate of splanchnic KIC release from deamination of dietary leucine accounted for 3% +/- 0.2% of the oral leucine administration rate and 13% +/- 2% of leucine splanchnic uptake (fractional splanchnic deamination). The fraction of whole-body total leucine uptake that was deaminated to KIC was 41% +/- 5% (P < .05 v fractional splanchnic deamination of dietary leucine uptake). We conclude that (1) the release of KIC from leucine deamination within splanchnic tissues constitutes a minimal fraction of first-pass dietary leucine uptake, and (2) splanchnic tissues are relatively less efficient than the whole body in KIC production from leucine deamination.

Leucine and Phenylalanine Kinetics in Compensated Liver Cirrhosis: Effects of Insulin

BACKGROUNDnThe pathogenesis of the altered ratio of branched-chain amino acid to aromatic amino acid concentration in liver cirrhosis is poorly known. We explored the possible link between altered amino acid concentrations and kinetics in cirrhosis.nnnMETHODSnPost-absorptive leucine and phenylalanine rates of appearance (Ra) and their response to insulin were studied in patients with compensated, nondiabetic cirrhosis and in controls.nnnRESULTSnIn the cirrhotics, concentration of postabsorptive phenylalanine was greater and that of alpha-ketoisocaproate lower than in controls, whereas concentration of leucine was comparable. Leucine Ra was lower, phenylalanine Ra was greater, and the ratio of leucine Ra to phenylalanine Ra was markedly decreased (P < 0.001) in patients vs. controls (2.40 +/- 0.23 vs. 3.67 +/- 0.19, respectively). During an euglycemic-hyperinsulinemic clamp, glucose disposal was reduced and leucine Ra was suppressed more profoundly in cirrhotics than in controls, whereas suppression of phenylalanine Ra was comparable.nnnCONCLUSIONSnIn compensated liver cirrhosis, postabsorptive phenylalanine Ra is increased with respect to leucine Ra, suggesting the existence either of altered amino acid pools and/or transport or of abnormally sequenced proteins and/or peptides. Insulin resistance is restricted to glucose, but not to amino acid metabolism.

A fast high-performance liquid chromatographic method for the measurement of plasma concentration and specific activity of phenylalanine

A fast high-performance liquid chromatographic (HPLC) method for the measurement in plasma of phenylalanine concentration and specific activity is reported. One-to-two mL of acidified plasma are applied to an ion-exchange resin. The eluted amino acids are enzymatically converted into the corresponding alpha-ketoacids, i.e. phenylalanine is converted into phenylpyruvic acid. After a two-step extraction, phenylpyruvic acid is separated by reverse phase chromatography within 8-10 min. The use of an internal standard allows precise quantitation of plasma concentrations. The radioactivity eluted from the HPLC is divided by the amount injected to yield the specific activity. Concentration and rate of appearance of phenylalanine in man, calculated with the L-[2,6-3H]-phenylalanine tracer, are in the range of published data.

Relationship between plasma leucine concentration and clearance in normal and type 1 diabetic subjects.

In a series of studies in normal and type 1 diabetic subjects, we analysed the relationship between isotope-calculated leucine clearance and plasma leucine concentration. All studies were performed under euglycaemic conditions. Plasma leucine concentrations were either experimentally decreased by means of insulin infusion, or increased by means of exogenous amino acid infusion in the presence of hyperinsulinaemia. Leucine clearance rates were compared in normal and diabetic subjects at similar plasma insulin levels. The effect of hyperinsulinaemia was examined by measuring clearance rates in normal subjects at comparable leucine levels but different insulin concentrations. Our data show that leucine clearance is inversely related to leucine concentration, and that it is not independently stimulated by hyperinsulinaemia. Type 1 diabetes is not associated with decreased leucine clearance. A general equation relating leucine concentration and clearance is proposed. These data support the view that peripheral leucine utilization is not decreased in type 1 diabetes mellitus.