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Sarcoplasmic reticular Ca2+ pump ATPase activity in congestive heart failure due to myocardial infarction
1Institute of Cardiovascular Sciences, St Boniface General Hospital Research Centre, Winnipeg, Manitoba.
Insights
Congestive heart failure depresses sarcoplasmic reticulum (SR) Ca2+ pump activity. Altered SR phospholipid composition and enzyme phosphorylation contribute to this reduced Ca2+ pump ATPase activity in failing hearts.
Area of Science:
- Cardiovascular Physiology
- Molecular Cardiology
- Biochemistry
Background:
- Congestive heart failure (CHF) following myocardial infarction (MI) is associated with impaired sarcoplasmic reticulum (SR) Ca2+ handling.
- Previous studies indicate reduced SR Ca2+ uptake and Ca2+ pump ATPase expression in failing hearts.
Purpose of the Study:
- To elucidate the mechanisms underlying the depressed SR Ca2+ pump activity in a rat model of post-MI heart failure.
- To investigate alterations in SR Ca2+ pump ATPase characteristics, phosphorylation, and membrane composition.
Main Methods:
- Heart failure was induced in rats via left coronary artery occlusion (16 weeks); sham-operated rats served as controls.
- Isolated SR membranes were analyzed for Ca2+ pump ATPase activity, response to varying ion concentrations, cAMP-dependent protein kinase and Ca(2+)-calmodulin induced phosphorylation, phospholipid composition, and sulfhydryl group content.
Main Results:
- Ca2+-stimulated SR Ca2+ pump ATPase activity was significantly depressed in failing hearts, independent of enzyme affinity for Ca2+ or ATP.
- Phosphorylation of the SR Ca2+ pump by both cAMP-dependent protein kinase and Ca(2+)-calmodulin was reduced in failing hearts.
- Significant alterations in SR membrane phospholipid composition were observed in the failing hearts, while SH-group content remained unchanged.
Conclusions:
- Depressed SR Ca2+ pump ATPase activity in heart failure is partly attributable to abnormalities in SR membrane phospholipid composition.
- Altered phosphorylation of the SR Ca2+ pump enzyme also contributes to the impaired function observed in the failing heart.
Objective:
Earlier studies have shown a depression in the sarcoplasmic reticular (SR) Ca2+ uptake and gene expression in Ca2+ pump ATPase protein in congestive heart failure subsequent to myocardial infarction. It is the objective of this study to understand further the mechanisms of depressed SR Ca2+ pump activity in the failing heart.
Methods:
Heart failure in rats was induced by occluding the left coronary artery for 16 weeks and the viable left ventricle was processed for the isolation of SR membranes. Sham-operated animals were used as control. The characteristics of SR Ca2+ pump ATPase in the presence of different concentrations of K+, Ca2+ and ATP were examined and the purity of these membranes was monitored by determining the marker enzyme activities. In addition to measuring changes in cyclic adenosine monophosphate (cAMP) protein kinase and Ca(2+)-calmodulin induced phosphorylation, alterations in SR phospholipid composition as well as sulfhydryl (SH) group content were investigated.
Results:
Ca(2+)-stimulated ATPase activity, unlike Mg(2+)-ATPase activity, was depressed in the left ventricular SR from failing hearts as compared to control. The decrease in Ca(2+)-stimulated ATPase activity was seen at different concentrations of Ca2+, K+ and ATP but no changes in the affinities of the enzyme for Ca2+ and ATP were evident. The SR Ca(2+)-stimulated ATPase activities in the presence of both cAMP-dependent protein kinase and Ca(2+)-calmodulin were markedly decreased in the failing hearts when compared to control preparations. Furthermore, the 32P incorporation in the presence of cAMP-dependent protein kinase or Ca(2+)-calmodulin was also reduced in the experimental heart SR membranes. The phospholipid composition of the SR membranes from the failing heart was markedly altered. No changes in SH-group or the degree of cross contamination with other membranes were apparent in the failing heart SR.
Conclusions:
These results suggest that abnormalities in membrane phospholipid composition and phosphorylation of the enzyme may partly explain the observed depression in SR Ca2+ pump ATPase activity in heart failure following myocardial infarction.