Abstract

The outcome of myocardial ischemia-reperfusion has been partially attributed to the degree of apoptosis in cardiomyocytes. Aggregating platelets by release of transforming growth factor-β₁ (TGF-β₁) protect the isolated heart against ischemia-reperfusion injury and preserve myocardial TGF-β₁ content. To gain more insight into the modulation of hypoxia-reoxygenation-induced injury (apoptosis and necrosis) to myocytes by TGF-β₁ and aggregating platelets, cultured adult rat myocytes were exposed for 48 or 72 h to hypoxia alone, or to hypoxia followed by 3 h of reoxygenation. Apoptosis in the cells was determined by in situ terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling staining and DNA fragmentation on gel electrophoresis. Hypoxia alone caused a time-dependent increase in myocyte apoptosis (number of apoptotic cells: 19 ± 3% at 48 h and 39 ± 5% at 72 h compared with 5 ± 1% in control cells, based on a 500-cell count). Three hours of reoxygenation after 48 h of hypoxia further increased the number of apoptotic cells (34 ± 8 versus 19 ± 3% in hypoxia for 48 h), but reoxygenation after 72 h of hypoxia did not additionally increase the number of apoptotic cells, perhaps because of extensive cell necrosis on prolonged hypoxia. Forty-eight hours of hypoxia followed by 3 h of reoxygenation also resulted in a decrease in Bcl-2 and an increase in Fas protein level. Incubation of myocytes with either recombinant TGF-β₁ (0.5–5 ng/ml) or aggregated platelet supernatant (from 2–3 × 10⁷ platelets/ml, containing ∼0.5 ng/ml of TGF-β₁) markedly (P < .01) decreased the number of apoptotic cells after hypoxia-reoxygenation. Incubation with TGF-β₁ also reduced myocyte necrosis as evident from lactate dehydrogenase release and trypan blue dye exclusion. These data demonstrate that hypoxia-reoxygenation results in apoptosis and necrosis in cultured adult rat myocytes; this can be attenuated by TGF-β₁. Similarity of data with TGF-β₁ and aggregated platelet supernatant suggests that platelet-mediated cardioprotection during hypoxia-reoxygenation may relate in part to the release of TGF-β₁.