Calcium handling proteins in the failing human heart

There is accumulating evidence that disturbed calcium homeostasis may play a key role in the pathophysiology of human heart failure. Because disturbed calcium handling could result from altered protein expression, levels of calcium handling proteins were quantitated by Western Blot analysis in failing and nonfailing human myocardium from hearts with endstage failing dilated or ischemic cardiomyopathy. Protein levels of the sarcoplasmic reticulum calcium release channel (ryanodine receptor) and of calcium storage proteins (calsequestrin and calreticulin) were similar in failing and nonfailing human myocardium. However, proteins involved in calcium removal from the cytosol were significantly altered in the failing human heart: 1) SR-Ca²⁺-ATPase, relevant for removal of calcium from the cytosol into the lumen of the sarcoplasmic reticulum, was decreased; 2) phospholamban, which inhibits the SR-Ca²⁺-ATPase in the basal unphosphorylated state, was slightly decreased; 3) the ratio of SR-Ca²⁺-ATPase to phospholamban was decreased; 4) the sarcolemmal Na⁺-Ca²⁺-exchanger, relevant for transsarcolemmal calcium extrusion was increased in the failing hearts. In summary, altered levels of proteins involved in calcium removal from the cytosol suggest an increase in transsarcolemmal calcium elimination relative to sarcoplasmic reticulum calcium removal. These findings support the concept that reduced function of the sarcoplasmic reticulum to accumulate calcium may reflect a major defect in excitationcontraction coupling in human heart failure.