Objectives The abnormality of Sarcoplasmic reticulum (SR) Ca2+ handling proteins was shown to be essential in myocardial dysfunction after acute reperfusion injury, which might be attenuated by NO therapy. Cardiac arrest (CA)/CPR process is an acute global I/R process, so we hypothesised that nitrite therapy would improve postresuscitation myocardial dysfunction by increasing plasma and myocardial NO generation, and the mechanism involved the modulation of SR Ca2+ handling proteins.
Methods Cardiac arrest in rats was induced by intravenous bolus of KCL (40 μg/g). Nitrite (1.2nmol/g) or saline were administered respectively when CPR was initiated. Hemodynamic parameters including blood pressure, dP/dtmax,-dP/dtmax and electrocardiogram were monitored for 1 hour after ROSC. Cardiac function was evaluated as ejection fraction and fractional shortening by echocardiogram. Myocardial sample was harvested 5 minutes and 1 hour after ROSC. The expression of sarcoplasmic reticulum Ca2+ ATPase (SERCA2a), phospholamban (PLB) and rynodine receptors (RyRs) were analysed by Western blot analysis. Myocardial NO five minutes after ROSC were measured using Nitric Oxide Assay Kit.
Results Myocardial function was significantly compromised in nitrite and placebo groups after resuscitation, and cardiac function with nitrite was significantly better than with placebo. Nitrite administration increased the myocardial level of nitric oxide after resuscitation when compared with placebo group. No significant differences were found in the expression of myocardial SERCA2a and RyR proteins. The level of phosphorylated PLB in nitrite group was significantly higher than in placebo group after resuscitation, indicating nitrite during CPR increased the phosphorylation of PLB.
Conclusions Recent studies have demonstrated that NO was an important regulator of SR function by modulating the phosphorylation of PLB via cGMP and PKG pathway during acute I/R process, and the elevation of NO generation by enhancing nitric oxide synthase (NOS) expression or plasma nitrite would prevent acute myocardial dysfunction by preserving phosphorylated PLB.Improved NO formation would also improve cardiac dysfunction after CPR. Thus, the protection effects of nitrite might be associated with the increase of NO and the preservation of PLB after ROSC.
On the other hand, in the first minutes of reperfusion, impaired SR Ca2+ handling leads to Ca2+ oscillation and sequential mitochondrial injury by a Ca2+ related manner, inducing abnormal energy metabolism and myocardial dysfunction. Recent studies indicated that the impairment of Ca2+ handling in the first few minutes played an important role in the myocardial stunning after acute I/R. In our study, nitrite significantly increased the myocardial NO level and the PLB phosphorylation five minutes after ROSC, suggesting that the protection effect also involves the preservation of phosphorylated PLB and SR Ca2+ uptake in the opportunity window at early stage of reperfusion.
Above all, our findings suggest that the impairment of PLB is involved in the mechanism of postresuscitation myocardial dysfunction. Exogenous nitrite, acting as a NO donor, significantly increases myocardial NO, and therefore preserves phosphorylated PLB and postresuscitation myocardial function in CA/CPR model. The clinical effects of NO therapies are needed to be approved by further studies.