Induction of Apoptosis of Rat Neonatal Cardiomyocytes by Chemical Ischemia and Reoxygenation: The Role of Phosphatidylserine

Ischemia/reperfusion injury plays a crucial role in the induction of the cell death of myocytes. The precise mechanism of the cell death, however, has not been elucidated enough. This study examined the cell death of rat neonatal myocytes induced by chemical ischemia and reoxygenation with an in vitro model, in terms of apoptosis, and the role of phosphatidylserine, which is recognized with annexin V. Chemical ischemia and reoxygenation were conducted on the cultured myocytes obtained from 1- or 2-day-old Wistar rats. The cells were divided into 4 groups exposed to chemical ischemia for 9 h (Group A), 18 h (Group B) and 24 h (Group C) and one group not exposed to chemical ischemia (Control Group). DNA ladder formation on agarose gel electrophoresis was noted in Groups B and C followed by reoxygenation, but not in Group A, as well as all 4 groups without reoxygenation. There were cells positive to terminal deoxynucleotidyl transferase-mediated dUDP-biotin nick end labeling in all 3 groups except for the Control Group; after reoxygenation, the number of cells became larger in Groups B and C than in Group A. Flow cytometry revealed that annexin V-positive cells were 1.15 ± 0.82% in the Control Group, 4.07 ± 3.8% in Group A without reoxygenation and 15.5 ± 6.3% in Group A after 30-min reoxygenation, respectively; the value was significantly higher in the latter than the former two (P < 0.01). Although 18-h and 24-h ischemia increased the annexin V-positive cells, reoxygenation did not alter the number of cells in Groups B and C. These results indicate that i) chemical ischemia followed by reoxygenation variably induces apoptosis of rat myocytes, ii) long-term ischemia causes phosphatidylserine translocation on the cell surface membrane, regardless of reoxygenation and iii) mild ischemia necessitates reoxygenation to translocate phosphatidylserine, which might play a crucial role in the initiation of apoptosis of the myocytes.