br Materials and methods br Results br Discussion Heart hype

Materials and methods
Results
Discussion Heart hypertrophy is initially physiological to compensate for the loss of heart functions. However, sustained stresses lead to a pathological status, in which myocardium becomes stiffened by interstitial fibrosis and thereby diastolic dysfunction induces global remodeling of the heart, dilated cardiomyopathy and heart failure [23], [24]. Heart failure is one of the most devastating diseases in which cardiac hypertrophy is a determinant of the clinical course [1], [25]. The lifetime risk of heart failure is 1 in 5 among both men and women. Despite the advances in the development of new therapies in past decades, the survival rate after the onset of heart failure remains very low [26], [27]. Therefore, a novel anti-hypertrophic drug or a potential therapeutic target for the treatment of cardiac hypertrophy and heart failure is extremely needed. The present study demonstrates for the first time that glycine may antagonize pressure overload induced cardiac hypertrophy and fibrosis in rodents. Glycine has been regarded as a ‘‘nutritionally nonessential amino acid’’ for humans due to the presence of its endogenous synthesis in the body [28], [29]. Plasma level of glycine is normally about 300μM in adult humans [30], [31], which is similar to the mouse level measured by the present study. However, it has been reported that the amount of glycine synthesized in vivo would be insufficient to meet metabolic demands in humans and animals [32]. Moreover, accumulative lines of evidence have revealed that glycine is associated with the cytoprotection, anti-inflammatory responses, and animal development [33], [34], [35]. Implication with high doses of glycine exhibits the beneficial effects on gabexatemesilate induced lung endothelial STA-21 injury [36], I/R injury in cultured cells, perfused organs, and in vivo models including heart, liver, kidney, skeletal muscle, and small intestine [7], [37], [38]. In the present study, the plasma concentrations of glycine were higher than 0.5mM, the effective level for protection against I/R injury in rat small intestine [31], for more than 8h/day after intraperitoneal injection of 700mg/kg glycine. Meanwhile, ammonia, one of the catabolic products of glycine, was sustained at the normal level for more than 22h/day. The administered glycine may be excreted in the urine only in a minor amount. The majority is taken up by cells and metabolized via a variety of other routes. Therefore, the observed improvements in the cardiac structures and functions would be attributed to the implanted glycine. The doses of 800mg/kg body weight would be acceptable for intraperitoneal injection of glycine [39]. A medical record for five years on oral administration of 50–66g/day glycine has been tracked for the treatment of refractory obsessive-compulsive disorder and body dysmorphic disorder. No obvious side-effect has been found [40]. However, high doses of glycine can cause serious adverse effects. Intravenous infusion of 1.5% glycine causes the cardiotoxic effects in rabbits [41]. Rat myocardial cells are supposed to die in more than 1.0% (≈133mM) glycine in vitro [42]. In humans, the threshold concentration of plasma glycine would be 5mM at which overt adverse effects may start to develop. A significant finding of this study is that the protection of glycine might be mediated by glyR α2. It is interesting that only glyR α2 among the four family members is expressed in the cardiomyocyte. GlyR α2 is highly expressed at the embryonic and neonatal stages but postnatally is largely replaced by glyR α1 in central nervous system [43]. The definite role of glyR α2 in the heart is not fully known yet. GlyR α2 has been reported to be less sensitive to ethanol, tropisetron and Zinc than glyR α1 [19]. It might be that the presence of glyR α2 endows the myocardium with special functions in the development and metabolism, although a glyR α2 deficiency mouse model seems to be phenotypically normal [43], [44]. It has been reported that extrasynaptic activation of glyRs containing the α2 subunit in cortical interneurons by endogenous glycine activates voltage-gated calcium channels and promotes calcium influx, which further modulates actomyosin contractility to fine-tune nuclear translocation during migration [45]. Nobles et al. reported that selective glyR α2 subunit control of crossover inhibition between the on and off retinal pathways and is involved in regulation of interneuron differentiation during spinal cord development [46]. Moreover, our results indicate that glyR α2 in the myocardium may be a novel target to antagonize heart failure.