Involvement of the sarcoplasmic reticulum calcium pump in myocardial contractile dysfunction: comparison between chronic pressure-overload and stunning.

Acute as well as chronic forms of heart failure involve mechanical dysfunction during systole and/or diastole. The rapid Ca2+ release from and Ca2+ reuptake into the tubuli of the sarcoplasmic reticulum are processes that critically determine normal systolic and diastolic myocardial function, which explains why in the last fifteen years so much attention has been paid to understand the performance of the sarcoplasmic reticulum Ca2+ pump during myocardial contractile dysfunction. In this communication we have reviewed the literature data on sarcoplasmic reticulum Ca2+ pump function in the chronically pressure-overloaded hypertrophied and stunned (post-ischemic reversibly injured) myocardium in the light of some new data from our laboratory. Results on the pressure-overloaded hypertrophied myocardium provide evidence that impaired relaxation is most likely due to a low capacity of the sarcoplasmic reticulum to pump Ca2+, a consequence of a lower density of Ca(2+)-pumping sites within the sarcotubular membranes. Contractile dysfunction in stunned myocardium is accompanied by an upregulation of the sarcoplasmic reticulum Ca2+ ATPase gene resulting in a slight increase of the Ca2+ pumping activity. The latter increase is likely an adaptive response of the reversibly injured myocardium which may contribute to the slow recovery of contractile function.