Response of leucine metabolism to hyperinsulinemia under amino acid replacement in experimental hyperthyroidism

Alternative equations for whole-body protein synthesis and for fractional synthetic rates of proteins

In a constant infusion study of a mass isotope of leucine, two alternative equations are commonly available to calculate amino acid oxidation rate and, thence, whole-body protein synthesis. One, developed by Matthews et al (Am J Physiol Endocrinol Metab. 1980;238:E473-E479), is shown here to require assuming a tracee steady state (TSS), namely, that tracee (unlabeled) amino acid concentrations and fluxes (rates of oxidation and incorporation into protein) are unaltered compared with the preinfusion state. The other, developed by Garlick and coworkers (Melville et al, Metabolism 1989;38:248-255), stems from a protein steady state (PSS) assumption, namely, that protein synthesis is unaffected by the tracer infusion. We derive here a simple expression for the relative difference in whole-body protein synthesis computed from the two assumptions, and a simple test of the validity of TSS in the form of an equality that must be satisfied by plasma measurements at all times. We also propose two experiments to discriminate between the two assumptions. Theoretical reasons and experimental evidence from the literature are offered to support PSS. The two assumptions result in different expressions for fractional synthetic rates (FSRs) of individual or organ proteins-TSS requires the use of tracer-to-tracee ratios and PSS the use of enrichments. An expression is derived here for the relative difference in FSR with TSS vs PSS. For both whole-body synthesis and for FSR, the TSS assumption consistently results in an underestimate, the relative bias roughly equal to the precursor amino acid enrichment.

Effect of T(3)-induced hyperthyroidism on mitochondrial and cytoplasmic protein synthesis rates in oxidative and glycolytic tissues in rats

Hyperthyroidism increases metabolic rate, mitochondrial ATP production, and protein synthesis, but it remains to be determined whether all tissues and synthesis of specific protein pools are equally affected by hyperthyroidism. Previous studies showed that mitochondrial function was less responsive to elevated triiodothyronine (T(3)) levels in the low-oxidative plantaris muscle compared with other tissues in rats. We tested the hypothesis that in T(3)-treated animals mitochondrial protein synthesis would increase in oxidative but not glycolytic tissues. Male rats received either T(3) (200 mug/day, n = 10) or saline (controls, n = 9) by subcutaneous pump for 14 days, and then in vivo protein synthesis rates were measured using [(15)N]phenylalanine in liver, heart, plantaris, and red gastrocnemius (Red Gast). Mitochondrial protein synthesis rate in T(3)-treated rats was higher than in controls by 62% in Red Gast and plantaris and 89 and 115% in liver and heart, respectively (P < 0.01). Cytoplasmic protein synthesis rates in the T(3) group were 107-176% higher than control values (P < 0.01). There was also indirect evidence that protein breakdown was increased in all tissues of the T(3)-treated rats. Phosphorylation of selected regulators of protein synthesis in plantaris and Red Gast (mTOR, p70 S6 kinase, 4E-BP1), however, were not significantly affected by T(3). We conclude that T(3) infusion stimulates a general increase in mitochondrial and cytoplasmic protein synthesis rate among tissues and that this does not appear to explain the tissue-specific responses in mitochondrial oxidative capacity.

Whole body leucine flux in HIV-infected patients treated with or without protease inhibitors

The present study was carried out to assess the effects of protease inhibitor (PI) therapy on basal whole body protein metabolism and its response to acute amino acid-glucose infusion in 14 human immunodeficiency virus (HIV)-infected patients. Patients treated with PIs (PI+, 7 patients) or without PIs (PI-, 7 patients) were studied after an overnight fast during a 180-min basal period followed by a 140-min period of amino acid-glucose infusion. Protein metabolism was investigated by a primed constant infusion of l-[1-(13)C]leucine. Dual-energy X-ray absorptiometry for determination of fat-free mass (FFM) and body fat mass measured body composition. In the postabsorptive state, whole body leucine balance was 2.5 times (P < 0.05) less negative in the PI+ than in the PI- group. In HIV-infected patients treated with PIs, the oxidative leucine disposal during an acute amino acid-glucose infusion was lower (0.58 +/- 0.09 vs. 0.81 +/- 0.07 micromol x kg FFM(-1) x min(-1) using plasma [(13)C]leucine enrichment, P = 0.06; or 0.70 +/- 0.10 vs. 0.99 +/- 0.08 micromol x kg FFM(-1) x min(-1) using plasma [(13)C]ketoisocaproic acid enrichment, P = 0.04 in PI+ and PI- groups, respectively) than in patients treated without PIs. Consequently, whole body nonoxidative leucine disposal (an index of protein synthesis) and leucine balance (0.50 +/- 0.10 vs. 0.18 +/- 0.06 micromol x kg FFM x (-1) x min(-1) in PI+ and PI- groups respectively, P < 0.05) were significantly improved during amino acid-glucose infusion in patients treated with PIs. However, whereas the response of whole body protein anabolism to an amino acid-glucose infusion was increased in HIV-infected patients treated with PIs, any improvement in lean body mass was detected.

Whole body and forearm substrate metabolism in hyperthyroidism: evidence of increased basal muscle protein breakdown

Thyroid hormones have significant metabolic effects, and muscle wasting and weakness are prominent clinical features of chronic hyperthyroidism. To assess the underlying mechanisms, we examined seven hyperthyroid women with Graves' disease before (Ht) and after (Eut) medical treatment and seven control subjects (Ctr). All subjects underwent a 3-h study in the postabsorptive state. After regional catheterization, protein dynamics of the whole body and of the forearm muscles were measured by amino acid tracer dilution technique using [15N]phenylalanine and [2H4]tyrosine. Before treatment, triiodothyronine was elevated (6.6 nmol/l) and whole body protein breakdown was increased 40%. The net forearm release of phenylalanine was increased in hyperthyroidism (microg.100 ml(-1).min(-1)): -7.0 +/- 1.2 Ht vs. -3.8 +/- 0.8 Eut (P = 0.04), -4.2 +/- 0.3 Ctr (P = 0.048). Muscle protein breakdown, assessed by phenylalanine rate of appearance, was increased (microg.100 ml(-1).min(-1)): 15.5 +/- 2.0 Ht vs. 9.6 +/- 1.4 Eut (P = 0.03), 9.9 +/- 0.6 Ctr (P = 0.02). Muscle protein synthesis rate did not differ significantly. Muscle mass and muscle function were decreased 10-20% before treatment. All abnormalities were normalized after therapy. In conclusion, our results show that hyperthyroidism is associated with increased muscle amino acid release resulting from increased muscle protein breakdown. These abnormalities can explain the clinical manifestations of sarcopenia and myopathy.