Alterations in Calcium-Binding Properties of Sarcoplasmic Reticulum Membrane Proteins Following Cardiac Injury

Aims: The objective of present work is to investigate metabolic alterations associated with heart failure, particularly one of its manifestations, a sustained hypocalcemia that causes hemodynamic changes contributed to subsequent myocardial injury. Comparative study was carried out using experimental models of pancreatic necrosis (PN) and crush syndrome (CS) accompanied by cardiac damage down to myocardial infarction.

Study design: Wistar adult male rats randomly divided into groups (n=12/group). The controls are healthy intact animals. The pancreatic necrosis (PN) and crush syndrome (CS) groups were then randomly subdivided: PN group- into 3, 24 and 72 h groups concerning hemorrhage, early and late pancreatic necrosis respectively; CS group – into 2, 4, 24, and 48 h decompression stages. The rats were sacrificed to analyze spectra and calcium-binding properties of the membrane proteins isolated from the cardiomyocyte sarcoplasmic reticulum (SR). Development of pathological changes in the heart and pancreas were also monitored.

Place and Duration of Study: Department of Pathological Biochemistry and Radioisotope Methods, H. Buniatyan Institute of Biochemistry of Natl. Acad. Sci (NAS), Republic of Armenia (RA). Experiments conducted between May 2011 and October 2013.

Methodology: To study pathogenesis of hypocalcemia underlying myocardial damage a translocation of radioactive 45CaCI2 into cardiomyocytes and its intracellular distribution was examined. Binding of 45Ca2+ to the SR membrane proteins was measured after proteins separation by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and radioactivity from the gel plates was counted by a gas-flow meter Berthold–II. Isoelectric focusing of the protein isolated from the SR of cardiomyocyte was performed.

Results: Statistically significant changes in mean radio labeled calcium incorporation into a total protein fraction of the cardiomyocyte SR from control (13682±271) were determined by3h of PN (23055±168, P<.001), 24 h of PN (22876±240, P<.01), and by 72 h (3851±271,P<.01), P vs. control. Similarly, these parameters were detected following CS by 2h decompression (24179±225, P<.01), 4-24 hours decompression (21666±124, P<.001) and 48 h decompression (2941±189, P<.001), P vs. control. We demonstrate that drop in the binding calcium level observed was partially due to impaired affinity to calcium of the cardiomyocyte SR calcium-binding proteins during development of both PN and CS despite a simultaneous manifestation of affinity to calcium of the SR 32-kDa protein.

Conclusion: In the present study we have clearly shown that both experimental acute pancreatitis and long-term compression injury may cause similar changes,а loss the calcium-binding properties of the cardiomyocyte proteins, particularly those of SR serving as a main calcium depot under physiological circumstances and appear to be involved in common cellular and molecular mechanisms of myocardial injury contributing to hypocalcemia. Simultaneously, both PN and/or CS cause similar manifestations of the new calcium-binding properties of the cardiomyocyte SR 32-kDa membrane protein, and mirrored dynamic changes in its calcium affinity suggested by Scatchard plot analysis indicating a common mechanism that would be a transient attempt of certain heart cells to compensate hypocalcemia, and thus emerge from an otherwise pathological outcome. Thus, the above mentioned changes could be used to identify patients at high risk of cardiovascular disease in different pathologies.