The Roles of Tissue Nitrate Reductase Activity and Myoglobin in Securing Nitric Oxide Availability in Deeply Hypoxic Crucian Carp Marie N
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© 2016. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2016) 219, 3875-3883 doi:10.1242/jeb.149195 RESEARCH ARTICLE The roles of tissue nitrate reductase activity and myoglobin in securing nitric oxide availability in deeply hypoxic crucian carp Marie N. Hansen1, Jon O. Lundberg2, Mariacristina Filice3,4, Angela Fago4, Nanna M. G. Christensen1 and Frank B. Jensen1,* ABSTRACT NO, which provides an alternative pathway for NO generation under In mammals, treatment with low doses of nitrite has a cytoprotective hypoxia (Gladwin et al., 2005; Lundberg et al., 2009; van Faassen effect in ischemia/reperfusion events, as a result of nitric oxide et al., 2009). Accordingly, NO is generated by different mechanisms formation and S-nitrosation of proteins. Interestingly, anoxia-tolerant depending on oxygen levels and also tissue type, pH and redox lower vertebrates possess an intrinsic ability to increase intracellular status (Gladwin et al., 2005; Feelisch et al., 2008; Hill et al., 2010). nitrite concentration during anoxia in tissues with high myoglobin and The preservation of NO availability by nitrite reduction is indeed mitochondria content, such as the heart. Here, we tested the important in hypoxia, where it contributes to hypoxic vasodilation hypothesis that red and white skeletal muscles develop different and cytoprotection (Cosby et al., 2003; Shiva and Gladwin, 2009). nitrite levels in crucian carp exposed to deep hypoxia and assessed Several studies have documented that administration of nitrite can whether this correlates with myoglobin concentration. We also tested reduce cell death and infarct size after ischemia/reperfusion in heart whether liver, muscle and heart tissue possess nitrate reductase and liver tissues of mammals (Webb et al., 2004; Duranski et al., activity that supplies nitrite to the tissues during severe hypoxia. 2005; Hendgen-Cotta et al., 2008; Shiva and Gladwin, 2009). This relates to the binding of nitrite-derived NO to complex IV of the Crucian carp exposed to deep hypoxia (1<PO2<3 mmHg) for 1 day increased nitrite in red musculature to more than double the value in mitochondrial respiratory chain, which reduces respiration rate and S normoxic fish, while nitrite was unchanged in white musculature. extends the O2 gradient, as well as -nitrosation of complex I, which There was a highly significant positive correlation between tissue limits generation of reactive oxygen species (ROS) at the onset of concentrations of nitrite and nitros(yl)ated compounds. Myoglobin reoxygenation (Shiva et al., 2007; Murillo et al., 2011; Chouchani levels were 7 times higher in red than in white musculature, but there et al., 2013). Interestingly, anoxia-tolerant vertebrates (crucian carp was no clear correlation between nitrite and myoglobin levels. Finally, and freshwater slider turtles) seem to naturally exploit these we found a low but significant nitrate reductase activity in liver and mechanisms by elevating nitrite and nitros(yl)lated compounds in, white muscle, but not in cardiomyocytes. Nitrate reduction was for example, the heart during deep hypoxia and anoxia (Sandvik inhibited by allopurinol, showing that it was partly catalyzed by et al., 2012; Jensen et al., 2014; Hansen et al., 2016). xanthine oxidoreductase. We have previously shown that goldfish and crucian carp maintain tissue nitrite concentration during hypoxia and increase it KEY WORDS: Ethanol, Hypoxia, Nitrate reduction, Nitric oxide, in the heart during deep hypoxia and anoxia (Hansen and Jensen, Nitrite, Red muscle 2010; Sandvik et al., 2012; Hansen et al., 2016). This occurs in the face of a decrease in extracellular nitrite concentration. We INTRODUCTION suggested that nitrite is transferred from the extracellular to the Nitric oxide (NO) is a vital signaling molecule that exerts its intracellular space and that this could be facilitated by intracellular physiological effects by reversible binding/reacting with hemes, binding of nitrite to proteins, which would keep the cytosolic thiols or amines, forming iron-nitrosyl (FeNO), S-nitroso (SNO) and concentration of free nitrite low and permit inward diffusion N-nitroso (NNO) compounds (Hill et al., 2010). Furthermore, NO is (Hansen and Jensen, 2010). Because anoxia increases tissue nitrite short-lived and excess NO is rapidly oxidized to nitrite and nitrate. in the heart of crucian carp and red-eared slider turtles, as well as in Under normoxic conditions, NO is produced from the reaction of red pectoral muscle of turtles, but not white muscle of crucian carp, L-arginine with molecular oxygen, catalyzed by nitric oxide we hypothesized that myoglobin (Mb) or mitochondria, both synthase (NOS) enzymes, and this reaction is vulnerable to present at high levels in heart and red musculature, play a role hypoxia because of the requirement for O2 (Moncada, 1993; (Jensen et al., 2014). Specifically we suggested that the increased Bryan, 2006; Lundberg et al., 2008). However, it has been binding of negatively charged nitrite during anoxia could be established that both nitrite and nitrate can be reduced back to explained by a progressively more positively charged Mb due to H+ buffering caused by anoxia-induced acidosis (Jensen et al., 2014). One aim of the present study was to measure nitrite levels in red and 1Department of Biology, University of Southern Denmark, Odense M DK-5230, Denmark. 2Department of Physiology and Pharmacology, Karolinska Institute, white muscle of normoxic and deeply hypoxic crucian carp to Stockholm SE-17177, Sweden. 3Department of Biology, Ecology and Earth validate a difference in nitrite levels between the two muscle types Sciences, University of Calabria, 87036 Arcavacata di Rende (CS), Italy. within the same species. Further, we measured Mb concentrations in 4Zoophysiology, Department of Bioscience, Aarhus University, Aarhus C DK-8000, Denmark. red and white muscle to test for a possible correlation between tissue nitrite and Mb levels. *Author for correspondence ([email protected]) As mentioned above, extracellular nitrite is shifted into tissues of F.B.J., 0000-0001-6148-6663 hypoxic and anoxic fish. But the extracellular pool of nitrite is sparse, and nitrite needs to be supplemented from other sources to Received 2 September 2016; Accepted 27 September 2016 maintain or increase tissue nitrite concentration during long-term Journal of Experimental Biology 3875 RESEARCH ARTICLE Journal of Experimental Biology (2016) 219, 3875-3883 doi:10.1242/jeb.149195 hypoxia. We recently showed that crucian carp can utilize ambient separated. Tissues were rinsed in phosphate-buffered saline nitrite by taking it up across the gills and transporting it via the blood [50 mmol l−1 phosphate buffer pH 7.8; 85 mmol l−1 NaCl; to tissues, where it enters cells, notably in hypoxia (Hansen et al., 2.4 mmol l−1 KCl; 10 mmol l−1 N-ethylmaleimide (NEM); 2016). Another potential source of tissue nitrite is tissue nitrate. 0.1 mmol l−1 diethylenetriaminepentaacetic acid (DTPA)], dried Mammalian tissues have been shown to possess nitrate reductase on tissue paper and weighed. All samples were snap frozen in liquid activity (mediated by xanthine oxidoreductase and possibly other nitrogen and stored at −80°C until further analysis. proteins) that can reduce nitrate to nitrite, and this is most prominent The procedures and experiments were conducted in accordance during hypoxic/anoxic conditions (Jansson et al., 2008; Huang with Danish laws on animal experimentation. All chemicals were et al., 2010; Piknova et al., 2015). The existence of tissue nitrate purchased from Sigma-Aldrich (Steinheim, Germany). reductase activity in fish remains to be established, but it could be a valuable additional source of nitrite (and thus NO) in crucian carp, Nitrate reduction and determination of NO metabolites when they experience long-term hypoxia and anoxia in ice-covered The method for determination of tissue nitrate reductase activity was ponds during winter (Vornanen et al., 2009). For this reason, a mostly conducted as previously described (Jansson et al., 2008). In major aim of the present study was to investigate nitrate reductase short, subsamples of tissues weighing between 19 and 200 mg activity in selected tissues from normoxic and deeply hypoxic (depending on the tissue) were thawed and homogenized in 3 times crucian carp. The liver was examined because of its relatively high their weight of phosphate buffer (100 mmol l−1, pH 7.0) and nitrate reductase activity in mammals (Jansson et al., 2008; Piknova 1 vol % protease inhibitor (P2714, Sigma-Aldrich). Heart and liver et al., 2015). The heart was tested for nitrate reductase activity tissues were homogenized using zirconium oxide beads (ZROB05, because of the distinctive increase in nitrite concentration in this 5 mm, Next Advance, NY, USA) in a Bullet Blender Blue (Next tissue during anoxia and deep hypoxia (Sandvik et al., 2012; Advance) and muscle tissue was homogenized with a tissue grinder Hansen et al., 2016). Finally, white skeletal muscle was examined (Struers, Heidoloph, Denmark). Homogenates were centrifuged because it constitutes some 50% of the fish mass, and because it has (10 min, 500 g, 4°C), and supernatants were decanted and divided a unique role in anoxic crucian carp in converting lactate to ethanol into subsamples that were snap frozen, or used for determination of (for subsequent excretion across the gills), thereby limiting acidosis protein concentration, using the BioRad (München, Germany) during anaerobic metabolism (Shoubridge and Hochachka, 1980; protein assay based on Bradfords assay, with bovine serum albumin Johnston and Bernard, 1983; Vornanen et al., 2009). To ascertain as standard (Bradford, 1976). Final concentrations of protein ethanol production in deep hypoxia, we measured ethanol (7 mg ml−1), cofactors (1 mmol l−1 NADPH, 500 µmol l−1 NADH, concentration in muscle and plasma. 500 µmol l−1 NAD+, 500 µmol l−1 GSH) and nitrate (300 µmol l−1) were achieved by mixing appropriate amounts of buffer MATERIALS AND METHODS (250 mmol l−1 sucrose, 10 mmol l−1 Tris-HCl, pH 7.0), Animals, treatment and sampling homogenate, cofactor mix and nitrate in the given order.