Rocco Barazzoni

Evidence for Acute Stimulation of Fibrinogen Production by Glucagon in Humans

Fibrinogen, an acute-phase protein, and glucagon, a stress hormone, are often elevated in many conditions of physical and metabolic stress, including uncontrolled diabetes. However, the possible mechanisms for this association are poorly known. We have studied the acute effects of selective hyperglucagonemia (raised from ∼200 to ∼350 pg/ml for 3 h) on fibrinogen fractional secretion rate (FSR) in eight normal subjects during infusion of somatostatin and replacement doses of insulin, glucagon, and growth hormone. Fibrinogen FSR was evaluated by precursor-product relationships using either Phe (n = 8) or Leu (n = 2) tracers. Hyperglucagonemia did not change either plasma Phe or Tyr specific activity. After hyperglucagonemia, fibrinogen FSR increased by ∼65% (from 12.9 ± 3.6 to 21.5 ± 6.1% per day, P < 0.025) using plasma Phe specific activity as the precursor pool. FSR increased by ∼80% (from 16.6 ± 4.8 to 29.4 ± 8.8% per day, P < 0.025) if plasma Phe specific activity was corrected for the ketoisocaproate/Leu enrichment (or specific activity) ratio to obtain an approximate estimate of intrahepatic Phe specific activity. FSR increased by ∼60% when using plasma Tyr specific activity as precursor pool (n = 8) (P < 0.05), as well as when using the Leu tracer precursorproduct relationship (n = 2). In conclusion, selective hyperglucagonemia for ∼3 h acutely stimulated fibrinogen FSR using a Phe tracer method. Thus, glucagon may be involved in the increase of fibrinogen concentration and FSR observed under stressed or pathologic conditions.

Protein metabolism in glucagonoma

Summary Although protein wasting and reduced amino acid concentrations are common findings in glucagonoma patients, the mechanisms underlying these alterations are unclear. Therefore, we studied basal postabsorptive leucine, phenylalanine and tyrosine turnover following L-[D3]-Leucine, L-[D5]-Phenylalanine and L-[D2]-Tyrosine i. v. infusions in one male and one female patient with glucagonoma, compared with healthy control volunteers. Plasma amino acid concentrations were reduced (–40 to 80 %, δ > 2 SD vs control subjects) in both patients. Plasma leucine, phenylalanine and tyrosine rates of appearance in patients with glucagonoma were similar to values in the control subjects, except leucine rate of appearence in the female patient with glucagonoma ( + &30 %, d > 2 SD). In contrast, the intracellular leucine rate of appearence, reflecting protein degradation, was considerably increased in both patients ( + 60–80 %, δ > 2 SD). Phenylalanine hydroxylation was moderately higher only in the male patient with glucagonoma ( + &30 %, d > 2 SD). Leucine, phenylalanine and tyrosine clearances ( + 100–300 %), as well as phenylalanine hydroxylative clearance ( + 75–100 %) were also increased in the patients. In conclusion, whole-body protein breakdown is enhanced in patients with glucagonoma compared with healthy control subjects. Phenylalanine hydroxylative clearance is also higher. Reduced plasma amino acid concentrations are probably due, at least in part, to their increased clearance. These alterations could contribute to the determination of the catabolic state of the glucagonoma syndrome. [Diabetologia (1999) 42: 326–329]

Differences in Estimates of Forearm Protein Synthesis Between Leucine and Phenylalanine Tracers Following Unbalanced Amino Acid Infusion

We compared the leucine (Leu) and phenylalanine (Phe) tracer-determined response of forearm protein synthesis (PS) before and after stimulation of protein anabolism by intravenous infusion of Leu-enriched, Phe-deficient amino acids and insulin (increased to approximately 100 microU/mL) with the euglycemic clamp. Six healthy subjects received primed-constant infusions of L-[ring-2H5]-Phe and L-[1-(14)C]-Leu, and steady-state forearm Phe and Leu kinetics were determined. Following the combined infusion, the arterial Leu concentration increased approximately 70% (P < .001), whereas Phe decreased about 15% (P < .01). Forearm PS and net balance (NB) increased (P < .05 or less v basal) using both amino acid tracers. However, the relative increments observed with the Leu tracer were more than 75% larger (P < .05 or less) than those observed with the Phe tracer, even when the data were corrected for the standard relative abundance of these two amino acids in forearm protein(s). Thus, the calculated changes of forearm PS and NB in response to an unbalanced amino acid infusion with hyperinsulinemia were affected by the plasma level of leucine and phenylalanine, whose tracers were used to estimate forearm protein turnover. Since these two essential amino acids share the same transport system, a competition at this level cannot be excluded.

Relationships between phenylalanine hydroxylation and plasma aromatic amino acid concentrations in humans

We investigated the relationships between phenylalanine hydroxylation (Phe Hy) and plasma concentrations of phenylalanine, tyrosine, and glucagon in healthy male volunteers (N = 13; age, 29 +/- 3 years). Phe Hy, as well as the Phe and Tyr rate of appearance (Ra), were measured during L-[2H5]-Phe and L-[2H2]-Tyr continuous intravenous (i.v.) infusions both under basal postabsorptive conditions (N = 13) and following divergent changes of plasma aromatic amino acids (AAA) concentrations. Namely, AAA were increased by administration of a balanced synthetic mixed meal (n = 6) or selectively decreased by i.v. infusion of insulin along with a Phe-deficient, Tyr and tryptophan-deprived amino acid mixture ([IAA] n = 7). Following the meal, plasma Phe (54 +/- 3 to 81 +/- 12 micromol/L), plasma Tyr (54 +/- 4 to 91 +/- 7), Phe Hy (0.09 +/- 0.01 to 0.15 +/- 0.02 micromol/kg x min), Phe Ra (0.65 +/- 0.04 to 0.96 +/- 0.07), and Tyr Ra (0.51 +/- 0.03 to 0.93 +/- 0.11) all significantly increased (P < or = .05 v basal). IAA infusion significantly decreased plasma Phe (to 47 +/- 3 micromol/L), plasma Tyr (to 25 +/- 4), Phe Hy (to 0.07 +/- 0.004 micromol/kg x min), and Tyr Ra (to 0.29 +/- 0.02; all P < or = .05 v sal), while Phe Ra did not change (0.64 +/- 0.04, NS). Plasma glucagon did not change in the three experimental periods (basal, 85 +/- 7; meal, 72 +/- 10; IAA, 92 +/- 14 pg/mL; NS). Using linear regression analysis, plasma Phe was positively related to both Phe Hy (R2 = .76, P < .001) and plasma Tyr (R2 = .80, P < .001); Phe Hy and plasma Tyr were also significantly correlated (R2 = .60, P < .001). No correlation was found between Phe Hy and basal plasma glucagon (R2 = .04, NS). Using multiple regression analysis with plasma Tyr as the dependent variable, plasma Phe was still correlation with plasma Tyr (t = 4.29, P = .0002), while the relationship between Phe Hy and plasma Tyr was no longer significant (t = 0.69, P = .49). These data indicate that plasma Phe is closely associated with its own hydroxylative disposal in humans, and confirm that Phe conversion to Tyr may play a physiological role in maintaining balanced plasma phenylalanine and tyrosine concentrations.

Decreased VLDL-Apo B 100 Fractional Synthesis Rate Despite Hypertriglyceridemia in Subjects With Type 2 Diabetes and Nephropathy

CONTEXT Subjects with type 2 diabetes mellitus (T2DM) and diabetic nephropathy (DN) often exhibit hypertriglyceridemia. The mechanism(s) of such an increase are poorly known. OBJECTIVE We investigated very low-density lipoprotein (VLDL)-Apo B 100 kinetics in T2DM subjects with and without DN, and in healthy controls. DESIGN Stable isotope (13)C-leucine infusion and modeling analysis of tracer-to-tracee ratio dynamics in the protein product pool in the 6-8-h period following tracer infusion were employed. SETTING Male subjects affected by T2DM, either with (n = 9) or without (n = 5) DN, and healthy male controls (n = 6), were studied under spontaneous glycemic levels in the post-absorptive state. RESULTS In the T2DM patients with DN, plasma triglyceride (TG) (mean ± SD; 2.2 ± 0.8 mmol/L) and VLDL-Apo B 100 (17.4 ± 10.4 mg/dL) concentrations, and VLDL-Apo B 100 pool (0.56 ± 0.29 g), were ∼60-80% greater (P < .05 or less) than those of the T2DM subjects without DN (TG, 1.4 ± 0.5 mmol/L; VLDL-Apo B 100, 9.9 ± 2.5 mg/dL; VLDL-Apo B 100 pool, 0.36 ± 0.09 g), and ∼80-110% greater (P < .04 or less) than those of nondiabetic controls (TG, 1.2 ± 0.4 mmol/L; VLDL-Apo B 100, 8.2 ± 1.7 mg/dL; VLDL-Apo B 100, 0.32 ± 0.09 g). In sharp contrast however, in the subjects with T2DM and DN, VLDL-Apo B 100 fractional synthesis rate was ≥50% lower (4.8 ± 2.2 pools/d) than that of either the T2DM subjects without DN (9.9 ± 4.3 pools/d; P < .025) or the control subjects (12.5 ± 9.1 pools/d; P < .04). CONCLUSIONS The hypertriglyceridemia of T2DM patients with DN is not due to hepatic VLDL-Apo B 100 overproduction, which is decreased, but it should be attributed to decreased apolipoprotein removal.