The first objective of the

The first objective of the present study was to investigate the impact of ageing on arginase activity in tissues which exhibit age-related impairments in their function. The second objective was to determine whether l-arginine supplementation modulated the effect of ageing on arginase activity. The third objective was to interrogate the clinical reliability of plasma l-arginine/l-ornithine ratio to estimate arginase activity in ageing. For this purpose, arginase activity in heart, vessel, brain, lung, kidney and liver as well as plasma l-arginine/l-ornithine ratio were measured in young rats and aged rats with or without l-arginine supplementation.
Material and methods
Results
Discussion The main results of the present study are that a) ageing decreased arginase activity in the liver and in the kidney, while it did not change arginase activity in other tissues including aorta, heart, lung and brain; b) l-arginine supplementation in aged rats did not increase arginase activity irrespective of the tissues studied and even decreased it in aorta; and c) ageing-induced decrease in plasma l-arginine levels and l-arginine/l-ornithine ratio was normalized by a chronic l-arginine supplementation but was independent on tissue arginase activity. The effect of ageing on arginase activity has been poorly investigated and controversial results were obtained. For instance, liver arginase activity was reported to be decreased by ageing in mice (Majaw and Sharma, 2015) whereas an increase (Shukla and Kanungo, 1969) or no change (Aldemir et al., 2003) were observed in aged rats. Opposite effect of ageing on arginase pathway were also reported in the Cyclosporin H receptor of aged rats (Gupta et al., 2012, Jing et al., 2013). All these studies only focused on a specific tissue. The present study is the first to analyze the effect of ageing on arginase activity in multiple tissues in the same animals, thus offering the opportunity to investigate a tissue-dependent effect of ageing on the arginase pathway. The new finding provided by our data is that ageing decreased arginase activity in tissues which express large amount of either Arg1 (liver) or Arg2 (kidney), while it failed to change arginase activity in tissues in which basal expression level was lower (brain, heart, aorta and lung). It is noteworthy that changes in liver and kidney arginase activity did not result from downregulation of the enzyme since Arg1 and Arg2 expression did not differ between aged and young rats in these tissues. By contrast, in tissues in which ageing did not change arginase activity, changes in Arg2 expression were observed. These data support the idea that reduction in Arg2 synthesis could be compensated, at least partially, by an increase in arginase activity, resulting in preservation of the global activity. However, the response of aorta to ageing appeared to be unique with respect to changes in the arginase pathway. Indeed in this tissue the lack of effect of ageing on arginase activity, as previously reported in certain studies (Delp et al., 2008) but not all (Berkowitz et al., 2003, Kim et al., 2009), coincided with Arg1 upregulation and Arg2 downregulation. Of note, as ex vivo measurement of arginase activity was performed on tissue homogenates exposed to an excess of l-arginine, the endogenous intracellular l-arginine concentration did not influence tissue arginase activity. Thus, arginase activity might be dependent on the presence of endogenous NOS inhibitors such as asymmetric dimethylarginine (ADMA) or enzymatic parameters which are modulated by post-transcriptional modifications. For instance, arginase 1 activity is stimulated by post-translational S-nitrosylation (Dunn et al., 2011) and arginase 2 activity is enhanced after its subcellular redistribution secondary to the activation of the RhoA/Rho kinase pathway (Ryoo et al., 2011, Santhanam et al., 2008). Taken together, these data exemplify the complexity of the effect of ageing on the arginase pathway with mismatches between expression and activity as well as tissue-dependent effects of ageing on arginase activity.