Resumption of intracellular Ca2+ cycling as a novel therapeutic strategy for heart failure

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Altered cellular Ca^<2+> handling plays a key role in the pathophysiology of heart failure. A typical aspect of failing heart cells is a decrease in the ability to load Ca^<2+> in the sarcoplasmic reticulum (SR), which results in a decreased amplitude and a slowed decay rate of Ca^<2+> transients, and an increased diastolic [Ca^<2+>]_i. This unloaded SR Ca^<2+> could be ascribed to a decrease in Ca^<2+> re-uptake by SR Ca^<2+> ATPase (SERCA), an increase in Ca^<2+> extrusion by the over-expression of Na^+/Ca^<2+> exchange (NCX), and an increase in the SR Ca^<2+> leak by the dissociation of FK506-binding proteins from the SR Ca^<2+> release channel. Many inotropic agents including digitalis and β-receptor agonists have failed to improve long-term prognosis of heart failure in clinical studies. For the SR Ca2+ uptake, drugs that enhance SR Ca^<2+> uptake in a cAMP-independent manner, including protein phosphatase inhibitors and MCC-135, are candidates. In 2005, we initially investigated … More the effect of a direct SERCA activator, MCC-135, on the profiles of Ca^<2+> transients and cell contraction in isolated rat cardiomyocytes. Unfortunately, we could not obtain significant inotropic effects of MCC-135. In 2006, we studied the effect of I-1, a specific protein phosphatase-1 inhibitor on cellular Ca^<2+> handling in permeabilized rat ventricular myocytes. As a result, I-1 increased SR Ca^<2+> content without altering SR Ca^<2+> release manner. For NCX, we clarified the inhibition modality of novel specific inhibitors of NCX, SEA0400 and SN-6 using patch clamp technique in guinea pig ventricular myocytes. SN-6 inhibited Ca^<2+> influx mode of NCX selectively, and SEA0400 equally blocked both Ca^<2+> efflux and influx mode. SEA0400 also protected rat perfused hearts against ischemia/reperfusion injury by preserving high energy metabolism. Despite limitations for clinical use, the combination of these agents may restore cellular Ca^<2+> cycling and improve cardiac function with less toxicity. Less