Upregulation of arginase-II contributes to decreased age-related myocardial contractile reserve

Abstract

Arginase-II (Arg-II) reciprocally regulates nitric oxide synthase (NOS) and offsets basal myocardial contractility. Furthermore, decreased or absent myocardial NOS activity is associated with a depression in myocardial contractile reserve. We therefore hypothesized that upregulation of Arg-II might in part be responsible for depressed myocardial contractility associated with age. We studied arginase activity/expression, NOS expression, NO production in the presence and absence of the arginase inhibitor S-(2-boronoethyl)-L: -cysteine (BEC) in old (22months) and young (3months) rat hearts and myocytes. The spatial confinement of Arg-II and NOS was determined with immuno-electron-miocrographic (IEM) and immuno-histochemical studies. We tested the effect of BEC on the force frequency response (FFR) in myocytes, as well as NOS abundance and activity. Arginase activity and Arg-II expression was increased in old hearts (2.27±0.542 vs. 0.439±0.058nmol urea/mg protein, p=0.02). This was associated with a decrease in NO production, which was restored with BEC (4.54±0.582 vs. 12.88±0.432μmol/mg, p<0.01). IEM illustrates increased mitochondrial density in old myocytes (51.7±1.8 vs. 69±2.2×10(6)/cm(2), p<0.01), potentially contributing to increased Arg-II abundance and activity. Immunohistochemistry revealed an organized pattern of mitochondria and Arg-II that appears disrupted in old myocytes. The FFR was significantly depressed in old myocytes (61.42±16.04 vs. -5.15±5.65%), while inhibition of Arg-II restored the FFR (-5.15±5.65 vs. 70.98±6.10%). NOS-2 is upregulated sixfold in old hearts contributing to increased production of reactive oxygen species which is attenuated with NOS-2 inhibition by 1400W (4,735±427 vs. 4,014±314RFU/min/mg protein, p=0.005). Arg-II upregulation in aging rat hearts contributes to age-related decreased contractile function.