Contribution of proteolysis and de novo synthesis to alanine production in diabetic rat skeletal muscle: a 15 N/ 1 H nuclear magnetic resonance study

Abstract

Although glutamine has recently emerged as a major source of new carbon for gluconeogenesis, alanine is still considered to be an important glucose precursor in humans [1][2][3][4]. The mechanisms of altered alanine turnover rates during diabetes, however, are not completely understood. Alanine release is increased from rat muscle in experimental diabetes [5][6][7] and the interorgan flux of this amino acid is slightly elevated in spite of decreased plasma alanine in diabetic humans [8,9]. From these data, skeletal muscle and splanchnic bed have been determined as the major sites of alanine release and uptake, respectively. However, to our knowledge, it has not been clearly demonstrated whether alanine release from muscle in diabetes originates from the increased protein catabolism -a known consequence of insulin deficiency [10] -or from de novo alanine synthesis.At the whole body level, leucine was found to be a major nitrogen donor for de novo alanine synthesis in dogs [11] and postabsorptive humans [12,13], but this role remains poorly documented during diabetes mellitus. Leucine is known to be transaminated and (at least in part) oxidized in skeletal muscle and the whole body. Leucine oxidation is enhanced both in insulin-dependent diabetic humans [1,9] and in rat experimental diabetes [14,15]. Moreover, a recent study suggests that, in whole body and leg tissues in type 1 diabetic patients, a substantial acceleration in Diabetologia (1997Diabetologia ( ) 40: 1159Diabetologia ( -1165 Contribution of proteolysis and de novo synthesis to alanine production in diabetic rat skeletal muscle: a 15 N/ 1 H nuclear magnetic resonance study Summary To assess the role of leucine as a precursor of alanine a -amino nitrogen in skeletal muscle during diabetes, extensor digitorum longus muscles from control (n = 7 experiments) and streptozotocin-diabetic rats (n = 8 experiments) were isolated and superfused with [ 15 N]leucine (3 mmol/l) in the presence of glucose (10 mmol/l) for 2 h. Muscle perchloric acid extraction was performed at the end of superfusion in order to quantify newly synthesized alanine by 15 N/ 1 H nuclear magnetic resonance. Release of [ 15 N]alanine in the superfusion medium was also measured. The pool of newly synthesized [ 15 N]alanine was significantly increased ( ∼ 40 %) in extensor digitorum longus muscles from streptozotocin-diabetic rats. Whereas a significant enhancement of total alanine release from muscle was induced by diabetes (20 %), only a slight increase in [ 15 N]alanine release was detectable under our experimental conditions. Consequently, we conclude that streptozotocin-diabetes in growing rats induces in skeletal muscle: 1) an increase in nitrogen exchange between leucine and alanine leading to newly synthesized [ 15 N]alanine; and 2) an increase of total alanine release from muscle originating from both proteolysis and de novo synthesis. [Diabetologia (1997[Diabetologia ( ) 40: 1159[Diabetologia ( -1165

Study on free amino acid concentrations in umbilical cord blood plasma of newborns from normoglycaemic and gestational diabetes-complicated pregnancies

“…No statistically important differences between normoglycaemic women and those with GDM were reported for ILE and LEU, only VAL results were significant (p=0.01). Meynial-Denis et al [37] showed an increased level of LEU transformation into ALA in skeletal muscle in diabetes. They also observed an increase in ALA concentration, as well as an increased rate of metabolism of these amino acids.…”

“…Freyse et al [36] and Meynial-Denis et al [37] report higher levels of ALA in animals with diabetes. This level is not confirmed by the current research.…”

Simultaneous in vivo assessment of cardiac and hepatic metabolism in the diabetic rat using hyperpolarized MRS

“…Also in the liver, we saw a decreased hepatic conversion of pyruvate to alanine, potentially indicating an increased supply of alanine from outside the liver due to insulin resistance. [21][22][23] The data could be representative of a change in the relative flux through the exchange reaction mediated by ALT, decreasing the incorporation of the 13 C label from pyruvate into the hepatic pool of alanine.…”

Challenges in Small Animal Noninvasive Imaging

“…More extensive studies in MRS have been conducted with a number of tracer molecules permitting the analysis of the intracellular milieu, metabolite kinetics, drug distribution and metabolism, ion fluxes, and metabolite dynamics. These nuclides used as tracers in MRS include l33 Cs (Schornack et al 1997), 2 H (Eng et al 1990), 15 N (Kanamori andRoss 1997;Meynial-Denis et al 1997), 17 O (Arai et al 1990Fiat et al 1992;Ronen et al 1998;Sibson et al 1998), 13 C (Bhujwalla et al 1994;Pascual et al 1998;Peled-Kamar et al 1998;Sibson et al 1998), 87 Rb (Cross et al 1995;Deslauriers and Kupriyanov 1998;Syme et al 1990), and 19 F (Hees and Sotak 1993;Mason et al 1993;McSheehy et al 1997).…”