Metabolism and distribution of pharmacological homoarginine in plasma and main organs of the anesthetized rat.

L-Homoarginine (hArg) and guanidinoacetate (GAA) are produced from L-arginine (Arg) by the catalytic action of arginine:glycine amidinotransferase. Guanidinoacetate methyltransferase methylates GAA on its non-guanidine N atom to produce creatine. Arg and hArg are converted by nitric oxide synthase (NOS) to nitric oxide (NO). NO is oxidized to nitrite and nitrate which circulate in the blood and are excreted in the urine. Asymmetric dimethylarginine (ADMA), an NOS inhibitor, is widely accepted to be exclusively produced after asymmetric N G-methylation of Arg residues in proteins and their regular proteolysis. Low circulating and urinary hArg concentrations and high circulating concentrations of ADMA emerged as risk markers in the human renal and cardiovascular systems. While ADMA's distribution and metabolism are thoroughly investigated, such studies on hArg are sparse. The aim of the present pilot study was to investigate the distribution of exogenous hArg in plasma, liver, kidney, lung, and heart in a rat model of takotsubo cardiomyopathy (TTC). hArg hydrochloride solutions in physiological saline were injected intra-peritoneally at potentially pharmacological, non-toxic doses of 20, 220, or 440 mg/kg body weight. Vehicle (saline) served as control. As hArg has been reported to be a pro-oxidant, plasma and tissue malondialdehyde (MDA) was measured as a biomarker of lipid peroxidation. hArg administration resulted in dose-dependent maximum plasma hArg concentrations and distribution in all investigated organs. hArg disappeared from plasma with an elimination half-life ranging between 20 and 40 min. hArg administration resulted in relatively small changes in the plasma and tissue content of Arg, GAA, ADMA, creatinine, and of the NO metabolites nitrite and nitrate. Remarkable changes were observed for tissue GAA, notably in the kidney. Plasma and tissue MDA concentration did not change upon hArg administration, suggesting that even high-dosed hArg is not an oxidant. The lowest hArg dose of 20 mg/kg bodyweight increased 25-fold the mean hArg maximum plasma concentration. This hArg dose seems to be useful as the upper limit in forthcoming studies on the putative cardioprotective effects of hArg in our rat model of TTC.

Homoarginine supplementation improves blood glucose in diet-induced obese mice.

L-Homoarginine (hArg) is an endogenous amino acid which has emerged as a novel biomarker for stroke and cardiovascular disease. Low circulating hArg levels are associated with increased mortality and vascular events, whereas recent data have revealed positive correlations between circulating hArg and metabolic vascular risk factors like obesity or blood glucose levels. However, it is unclear whether hArg levels are causally linked to metabolic parameters. Therefore, the aim of our study was to investigate whether hArg directly influences body weight, blood glucose, glucose tolerance or insulin sensitivity. Here, we show that hArg supplementation (14 and 28 mg/mL orally per drinking water) ameliorates blood glucose levels in mice on high-fat diet (HFD) by a reduction of 7.3 ± 3.7 or 13.4 ± 3.8 %, respectively. Fasting insulin concentrations were slightly, yet significantly affected (63.8 ± 11.3 or 162.1 ± 39.5 % of control animals, respectively), whereas body weight and glucose tolerance were unaltered. The substantial augmentation of hArg plasma concentrations in supplemented animals (327.5 ± 40.4 or 627.5 ± 60.3 % of control animals, respectively) diminished profoundly after the animals became obese (129.9 ± 16.6 % in control animals after HFD vs. 140.1 ± 8.5 or 206.3 ± 13.6 %, respectively). This hArg-lowering effect may contribute to the discrepancy between the inverse correlation of plasma hArg levels with stroke and cardiovascular outcome, on the one hand, and the direct correlation with cardiovascular risk factors like obesity and blood glucose, on the other hand, that has been observed in human studies. Our results suggest that the glucose-lowering effects of hArg may reflect a compensatory mechanism of blood glucose reduction by hArg upregulation in obese individuals, without directly influencing body weight or glucose tolerance.

Compartmentation of amino acids in the rat kidney.

Amino acid concentrations ([AA]) were determined in cortical, outer and inner medullary (OM and IM), and papillary tissue of rat kidney (Cti, mmol/kg wet wt), in plasma (Cpl), and in urine. In all regions, Cti values were highest for Tau, Gly, and Glu-, making up 54-65% of the total [AA]:27, 21, and 11 mmol/kg wet wt in cortex, OM, and IM and papilla, respectively. Cortical cell water [AA] values (CcH2O, mmol/kgH2O) were between 12.4 (Tau) and 0.09 (Orn+), representing cell water-to-plasma water ratios (CcH2O/CpH2O) between 134 (Asp-) and 0.9 (Thr and Cit). Short-term water diuresis did not change the total tissue [AA] throughout the kidney. Treatment of the tissue with Triton X-100 instead of sulfosalicylic acid (SSA) resulted in much higher [AA], except for Glu-, Glu-NH2, Tau, and exogenous L-homoarginine+ (hoArg+). When hoArg+ was infused (leading to a Cpl = 5.9 mmol/l), Cti of hoArg+ was similar throughout the kidney (13-22 mmol/kg wet wt). In the presence of hoArg+, CcH2O/CpH2O of Arg+ rose 13-fold. We conclude that 1) AA contribute 20% to cytosolic osmolality in renal cortex, 2) total [AA] decreases from cortex to papilla, 3) cellular uptake of Tau and anionic AA must be rheogenic, whereas cationic AA (except for Arg+ in cortex) are passively distributed, and 4) AA do not seem to contribute quantitatively to short-term medullary osmotic adaptation during diuresis.