176 Current Enzyme Inhibition, 2009, 5, 176-183 1573-4080/09 $55.00+.00 © 2009 Bentham Science Publishers Ltd. Cardiac and Renal Nitric Oxide in the Adaptation to Hypovolemic Shock Andrea L. Fellet* ,1 , Alberto Boveris 2 , Cristina T. Arranz 1 and Ana M. Balaszczuk 1 1 Department of Physiology; 2 Laboratory of Free Radical Biology, School of Pharmacy and Biochemistry, University of Buenos Aires, IQUIMEFA-CONICET, Buenos Aires, Argentina Abstract: Evidence is mounting that NO plays a role in the cardiovascular and renal adaptation to the hemorrhagic shock secondary to major blood loss. Recently, we reported cardiac and renal NO-dependent protective mechanisms activated by the hypovolemic state that include stimulation of NO synthase (NOS) expression and activity. In the heart, an increase in en- dothelial NOS is an early response to regulate cardiac function after blood loss. Meanwhile, inducible NOS increases heart dysfunction in later stages of sustained hemorrhagic shock. Additionally, the hypovolemic state induced changes in water homeostasis associated with alterations in inner medullary aquaporin water channel type 2 (AQP2) protein expres- sions and subcellular localization. NO production is one of the causes of the age-associated response that modulates AQP2 expression/trafficking in the inner collecting duct principal cells in response to hemorrhage. The complex regula- tion of NOS in cardiovascular and renal physiology depends on the magnitude of hemorrhage and age. The adaptive en- hancement of NO synthesis and availability activates or increases expression of other protective factors, including heat shock proteins, antioxidants and prostaglandins, making the protection more robust and sustained. The work with labora- tory animals and experimental hemorrhagic shock identified mitochondria as a sensitive subcellular compartment in- volved in the NO-mediated response to hypovolemic state. In a separate line of research, mtNOS was probed as a likely pacemaker of mammalian aging. Finally, further research based on the participation of mtNOS and the relationships be- tween NO and other modulators of heart and kidney function, as well as the impact of aging in tissue adaptation to hypo- volemia, are new research lines and hypothetical therapeutic strategies for the prevention and early treatment of tissue dysfunction. Keywords: Nitric oxide synthase, mitochondrial nitric oxide synthase, hypovolemia, heart, kidney. 1. NITRIC OXIDE AND NITRIC OXIDE SYNTHASES Since its discovery as the endothelium-derived relaxing factor, nitric oxide (NO) was regarded as mediator of nu- merous physiological processes that range from the regula- tion of cardiovascular and renal homeostasis to the participa- tion in memory formation and in immunological processes [1-4]. The current concept concerning NO and heart function is that NO is a physiological regulator of vascular tone [5], vascular remodelling [6], and baroreflex function [7]. In the kidney, NO regulates salt and fluid reabsorption [8], kidney hemodynamics [9], renin secretion [10], and tubuloglomeru- lar feedback [11]. There is a family of three NO synthases (NOS) in mam- malian aerobic cells encoded by genes that have been identi- fied in humans [12, 13]. Neuronal NOS (nNOS, NOS-1) coded in chromosome 12 and endothelial NOS (eNOS, NOS-3), coded in human chromosome 7, are membrane bound constitutive and Ca-regulated. These two enzymes, nNOs and eNOS, are expressed not only in nervous tissue and endothelium, respectively, but also in other types of cells and tissues, such as heart, kidney, muscle and thymus [14]. The third isoform is inducible NOS (iNOS, NOS2), coded in human chromosome 17 and expressed in the cytosol of macrophages and neutrophils, cells that are active in the process of phagocytosis and in other cells of the immu- nologic system such as lymphocytes and thymocytes [4]. Another type of classification of NOS, more meaningful in *Address correspondence to this author at the Department of Physiology, School of Pharmacy and Biochemistry, Junin 956, C1113AAD Buenos Aires, Argentina; Tel/Fax: (54-11) 4964-8280; E-mail: afellet@ffyb.uba.ar terms of cell physiology, is made according to their subcellu- lar localization, with mitochondrial (mtNOS) and endoplas- mic-sarcoplasmic reticulum isoforms [15-18]. The mito- chondrial isoform, mtNOS, has been reported in heart, kid- ney, liver, brain, smooth muscle and thymus with homoge- nous activities (about 1mol NO/ min x mg protein) and with a controversial antibody reactivity [19]. The cytosolic types constitute another family by themselves [15, 17, 20]. The mitochondrial and endoplasmic-sarcoplasmic types of NOS share the common property of being associated to membranes, with covalently attached fatty acids that anchor the enzymes to the membrane phospholipid bilayer [21]. It is apparent that all aerobic mammalian cells express two types of functional NOS, one isoform, mtNOS is located at the N side (the side facing the matrix) of the inner membrane in the mitochondrial compartment, and the other isoform (eNOS) located in the cyosolic compartment of heart, muscle, liver, kidney and other tissues [22]. The physiological function of this bipolar distribution of the NO-producing enzymes in the cell and the mechanisms that regulate NOS activities in mi- tochondria and cytosol are at moment hypotheses that are starting to be studied. All NOS isoforms are transcriptionally and post-transcriptionally regulated [14]. The production of NO by the NOS reaction requires L-arginine, O 2 , NADPH, and tetrahydrobiopterin (BH 4 ), substrates that are available in both mitochondrial and cytosolic compartments. In situa- tions where there is L-arginine and/or BH 4 deficiency, the NADPH-reduced NOS isoforms generate superoxide radical (O 2 - ) by autooxidation of the reduced flavins [23]. 1.1. Nitric Oxide Effects The multiple effects of NO in biological systems have generated an intense interest in the molecular mechanisms of