Heart mitochondrial nitric oxide metabolism during adaptation to chronic ischemic hypoxia: effect of sildenafil and L-NAME

ZAOBORNYJ T; VALDEZ LB; BOVERIS A

Mitochondria and Cell Signaling in Health and Disease

Taylor and Francis

Año: 2011; p. 171 - 189

Heart and lung physiological function provides through blood hemoglobin the convective transport of O2 to tissues. Within cells, O2 diffuses to mitochondria where it is reduced to H2O coupled to ATP synthesis in the energy-yielding process of oxidative phosphorylation. Mitochondria are by far the largest cell consumers of O2 and the determinants of cytosolic PO2 and of the O2 gradient between alveoli and cells. Mitochondria control cell survival by providing the ATP required for endergonic processes and the molecular signals that command genetic expression and metabolic regulation (Darley-Usmar 2004). Superoxide radical (O2-) is generated through the monovalent reduction of O2 in the mitochondrial electron transport chain, mainly by autooxidation of the semiquinones of ubiquinone (UQH·) and of flavin mononucleotide (FMNH·), in a process which rate is determined by the metabolic state and the redox state of the mentioned respiratory chain components (Boveris, Oshino, and Chance 1972).  The enhanced mitochondrial generation of O2- and of hydrogen peroxide (H2O2), the product of O2- dismutation, appears implicated in the onset of both deleterious and cardioprotective mechanisms within the heart (Bell, Emerling, and Chandel 2005). Hydrogen peroxide and nitric oxide (NO) are uncharged molecules normally produced in mitochondria that are highly diffusible through biological membranes and suitable for subcelullar and cellular signaling. It has been observed that myocardium resistance to O2 deprivation is increased by systemic hypoxia, pharmacological intervention or ischemic preconditioning. These experimental therapeutic strategies enhance cardiac tolerance to all major deleterious consequences of acute O2 deprivation, reducing infarct size and alleviating post-ischemic contractile dysfunction and ventricular arrhythmias (Murry, Jennings and Reimet, 1986). Interestingly, mitochondria that are tightly modulated by NO steady state levels (Poderoso et al. 1998; Antunes, Boveris, and Cadenas 2004; Valdez et al. 2006, Boveris, Carreras, and Poderoso 2010), participate in NO-dependent cardioprotection (Jones and Bolli 2006). Nitric oxide exerts several distinct effects in mitochondria: it competitively inhibits cytochrome oxidase (Brown and Cooper 1994; Cleeter et al. 1994; Poderoso et al. 1996); it regulates the rate of O2- and H2O2 production by autooxidation of UQH· at complex III (Poderoso et al. 1996; Boveris, Carreras, and Poderoso 2010); it activates mitoKATP channels (Sasaki et al. 2000); and it prevents the formation of the permeability transition pore (Brookes et al. 2000). Nitric oxide is indeed the first molecule that fulfills the requirement for a physiological modulator of respiration on the basis of its O2-competitive inhibition of cytochrome oxidase (Antunes, Boveris, and Cadenas 2004); it is produced in the tissues at a rate high enough to exhibit an effective inhibitory effect on cytochrome oxidase that, as a consequence of the reduced O2 uptake, extends the O2 diffusion distance into the tissues (Poderoso et al. 1996; Thomas et al. 2001). At physiological O2 levels (about 25 mM), a mitochondrial NO steady state concentration of about 50-100 nM produces an inhibition of about 50 % in state 3 O2 consumption (Antunes, Boveris, and Cadenas, 2004). Furthermore, NO was found to trigger mitochondrial biogenesis in several cell types and tissues including heart (Nisoli et al. 2003). This wide spectrum of actions supports the idea that mitochondrial NO is implicated in the mechanisms involved in cardioprotection. Cyclic guanosine monophosphate (cGMP) is synthesized by soluble guanylyl cyclases (sGC) as a second messenger of NO, which activates guanylyl cyclase (Fig. 1). cGMP activates a specific group of protein kinases, in particular protein kinase G (PKG), which serve as a key effector of cGMP by reducing intracellular calcium and therefore promoting muscle relaxation (Ignarro et al. 1997). Phosphodiesterases (PDE) are a family of at least 11 enzymes which are ubiquitous throughout the body and that present a diversity of functions. PDE5 catalyzes the breakdown of cGMP. It is the most important enzyme in the corpus cavernosum, and plays a role in penile function. PDE5 inhibitors are vasoactive drugs that have been developed for treatment of erectile dysfunction (Boolell et al. 1996) and have been used in trials as cardioprotective agents. The mechanism of action involves active competitive inhibition of the PDE5 enzyme resulting in an increase in cGMP steady state levels and smooth muscle relaxation. Sildenafil citrate (Viagra) was the first PDE5 inhibitor approved for the treatment of erectile dysfunction. The chemical structure of sildenafil is similar to that of cGMP (Fig. 2) and the inhibition of PDE5 by this drug is achieved by the binding to the cGMP-catalytic sites (Corbin and Francis 2002) and allows the accumulation of cGMP in the tissue. Sildenafil is highly specific for PDE5 inhibition with relatively minor cross reactivity with PDE6. The functional effects of PDE5 inhibitors are given by their effects on cGMP steady levels. Soon after the approval of this drug for the treatment of erectile dysfunction in 1998, potential cardiovascular effects of sildenafil attracted attention again.  In the pulmonary circulation, vasodilatory effects were found to be beneficial in the treatment of primary arterial pulmonary hypertension, which finally led to a new indication for oral treatment with sildenafil (Ghofrani et al. 2004). Subsequently, research interest shifted to the field of cardioprotection. Different studies performed in experimental animals showed infarct size reducing effects for sildenafil in myocardial ischemia and reperfusion, in part similar to those of ischemic preconditioning. In addition, potential beneficial effects were confirmed in experimental models of congestive heart failure and left ventricular hypertrophy (Das et al. 2002).