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.