S. Inchiostro

Kidney, splanchnic, and leg protein turnover in humans. Insight from leucine and phenylalanine kinetics.

The rate of kidney protein turnover in humans is not known. To this aim, we have measured kidney protein synthesis and degradation in postabsorptive humans using the arterio-venous catheterization technique combined with 14C-leucine, 15N-leucine, and 3H-phenylalanine tracer infusions. These measurements were compared with those obtained across the splanchnic bed, the legs (approximately muscle) and in the whole body. In the kidneys, protein balance was negative, as the rate of leucine release from protein degradation (16.8 +/- 5.1 mumol/min.1.73 m2) was greater (P < 0.02) than its uptake into protein synthesis (11.6 +/- 5.1 mumol/min. 1.73 m2). Splanchnic net protein balance was approximately 0 since leucine from protein degradation (32.1 +/- 9.9 mumol/min. 1.73 m2) and leucine into protein synthesis (30.8 +/- 11.5 mumol/min. 1.73 m2) were not different. In the legs, degradation exceeded synthesis (27.4 +/- 6.6 vs. 20.3 +/- 6.5 mumol/min. 1.73 m2, P < 0.02). The kidneys extracted alpha-ketoisocaproic acid, accounting for approximately 70% of net splanchnic alpha-ketoisocaproic acid release. The contributions by the kidneys to whole-body leucine rate of appearance, utilization for protein synthesis, and oxidation were approximately 11%, approximately 10%, and approximately 26%, respectively; those by the splanchnic area approximately 22%, approximately 27%, and approximately 18%; those from estimated total skeletal muscle approximately 37%, approximately 34%, and approximately 48%. Estimated fractional protein synthetic rates were approximately 42%/d in the kidneys, approximately 12% in the splanchnic area, and approximately 1.5% in muscle. This study reports the first estimates of kidney protein synthesis and degradation in humans, also in comparison with those measured in the splanchnic area, the legs, and the whole-body.

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.

A stepwise approach to assess the impact of clustering cardiometabolic risk factors on carotid intima-media thickness: the metabolic syndrome no-more-than-additive

Background Cardiovascular risk factors cluster in the metabolic syndrome (MS), but it is not known whether the risk associated with the syndrome is higher than the sum of its parts. In this study, we explored the relationship between clustering cardiometabolic risk factors and carotid intima-media thickness (c-IMT). Methods and results Cardiovascular parameters and c-IMT were determined in 240 middle-aged healthy participants, divided into groups according to their number of MS components. Higher number of MS components were associated with higher mean c-IMT. Analysis of synergy revealed that c-IMT increase at component clustering fitted an additive model. Redefinition of cutpoints for MS traits, optimized to detect high c-IMT, did not improve the interaction between components. When metabolic factors were rendered independent, a synergistic interaction between factors in increasing the likelihood of having a high c-IMT was detected. Synergic as well was the interaction between metabolic factors with other risk factors that are not consequence of insulin resistance, such as low-density lipoprotein-cholesterol level and smoking habit. Conclusion A stepwise approach reveals that the lack of synergy in the interactions between MS components is attributable to their mutual interdependence, possibly owing to the common pathophysiological background. Indeed, if MS is a unique clinical entity, it should be no more than the sum of its parts.

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.

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

BACKGROUND The 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. METHODS Post-absorptive leucine and phenylalanine rates of appearance (Ra) and their response to insulin were studied in patients with compensated, nondiabetic cirrhosis and in controls. RESULTS In 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. CONCLUSIONS In 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.

Hyperglucagonemia stimulates phenylalanine oxidation in humans

Glucagon stimulates in vitro liver phenylalanine (Phe) degradation, thus inducing net protein catabolism. Whether these effects occur also in vivo in humans is not known. Therefore, we studied the effects of physiological hyperglucagonemia on Phe rate of appearance (Rα), hydroxylation, and oxidation in seven normal volunteers during infusions of somatostatin with replacement doses of insulin and growth hormone. Steady-state Phe kinetics were evaluated using the l-[1-14C]Phe tracer both at the end of a 3-h basal glucagon replacement period (glucagon concentration: 212 ± 115 ng/l) and after a 3-h hormone infusion at the rate of ∼ 3 ng · kg−1 · min−1 (→654 ± 280 ng/l). Hyperglucagonemia did not change plasma Phe concentration and Ra but increased Phe oxidation by ∼ 30% (P < 0.01). Oxidation was also increased by ∼ 24% (P < 0.01) using plasma [14C]tyrosine (Tyr) specific activity as a precursor pool. Phe hydroxylation to Tyr estimated by assuming a fixed ratio of Tyr to Phe Rα (0.73) did not change. Nonhydroxylated Phe disposal decreased by ∼ 6% (P = 0.08). These data show that in humans in the postabsorptive state, hyperglucagonemia, with near maintenance of basal insulin and growth hormone concentrations, stimulates Phe oxidation but not Phe hydroxylation, suggesting a different regulation of these two Phe catabolic steps. Glucagon may also reduce Phe availability for protein synthesis.

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.