The nitric oxide synthase (NOS)/nitric oxide (NO) system integrates cellular biochemical machinery and energetics. In heart microenvironment, dynamic NO behaviour depends upon the presence of superoxide anions, haemoglobin (Hb), and myoglobin (Mb), being hemoproteins are major players disarming NO bioactivity. The Antarctic icefish, which lack Hb and, in some species, also cardiac Mb, represent a unique model for exploring Hb and Mb impact on NOS/NO function. We report in the (Hb
−/Mb
−) icefish Chaenocephalus aceratus the presence of cardiac NOSs activity (NADPH-diaphorase) and endothelial NOS (eNOS)/inducible NOS (iNOS) zonal immuno-localization in the myocardium. eNOS is localized on endocardium and, to a lesser extent, in myocardiocytes, while iNOS is localized exclusively in myocardiocytes. Confronting eNOS and iNOS expression in Trematomus bernacchii (Hb
+/Mb
+), C. hamatus (Hb
−/Mb
+) and C. aceratus (Hb
−/Mb
−) is evident a lower expression in the Mb-less icefish.
NO signaling was analyzed using isolated working heart preparations. In T. bernacchii, l-arginine and exogenous (SIN-1) NO donor dose-dependently decreased stroke volume, indicating decreased inotropism. l-arginine-induced inotropism was NOSs-dependent, being abolished by NOSs-inhibitor NG-monomethyl-l-arginine (l-NMMA). A SIN-1-induced negative inotropism was found in presence of SOD. NOS inhibition by l-N5-N-iminoethyl-l-ornithine (l-NIO) and l-NMMA confirmed the NO-mediated negative inotropic influence on cardiac performance. In contrast, in C. aceratus, l-arginine elicited a positive inotropism. SIN-1 induced a negative inotropism, which disappeared in presence of SOD, indicating peroxynitrite involvement. Cardiac performance was unaffected by l-NIO and l-NIL. NO signaling acted via a cGMP-independent mechanism. This high conservation degree of NOS localization pattern and signaling highlights its importance for cardiac biology.
