A rapid and strong apoptotic process is triggered by hyperosmotic stress in cultured rat cardiac myocytes

In all cell types, the maintenance of normal cell volume is an essential homeostatic function. Relatively little is known about the induction of apoptosis by hyperosmotic stress and its molecular mechanism in terminally differentiated cardiac myocytes. We compared the apoptotic response of cultured neonatal rat cardiomyoctes to hyperosmotic stress by sorbitol (SOR) with those induced by doxorubicin (Doxo) or angiotensin II (Ang II). We also examined the apoptotic-signaling pathway stimulated by the hyperosmotic stress. Apoptosis was assessed by the observation of: (1) cell viability, (2) DNA fragmentation detected by the TUNEL method and by agarose gel electrophoresis, and (3) poly(ADP-ribose)polymerase (PARP) degradation, and Bcl-X_S and Bcl-X_L levels by Western blot analysis. Exposure of cardiomyocytes to 0.3 M SOR for 24 h resulted in decreased cell viability and increased generation of oligosomal DNA fragments (2.5-fold of controls). At this time, 83±5% of SOR-treated myocytes were TUNEL-positive (vs 23.7±6.8% in controls; P<0.01). PARP levels also decreased by approximately 42% when cardiac myocytes were exposed to SOR. Hyperosmotic stress induced a more rapid and stronger apoptotic response in cardiomyocytes than Doxo or Ang II. In addition, SOR increased 3.2-fold Bcl-X_S proapoptotic protein without changes in Bcl-_XL antiapoptotic protein levels and in the p53-transactivating activity. Taken together, these results strongly suggest that hyperosmotic stress triggers cardiac myocyte apoptosis in a p53-independent manner, being earlier and stronger than apoptosis induced by Doxo and Ang II.