Na⁺/Ca²⁺ exchanger plays a key role in inducing apoptosis after hypoxia in cultured guinea pig ventricular myocytes

Altered Na⁺/Ca²⁺ exchanger (NCX) protein expression or activity is thought to contribute to various aspects of cardiac pathology. In guinea pig ventricular myocytes, NCX-mediated Ca²⁺ entry is almost entirely responsible for Ca²⁺ overload during hypoxia-reoxygenation. Because Ca²⁺ overload is a common initiator of apoptosis, the purpose of this study was to test the hypotheses that NCX activity is critically involved in initiating apoptosis after hypoxia-reoxygenation and that hypoxia-reoxygenation-induced apoptosis can be modulated by changes in NCX protein expression or activity. An NCX antisense oligonucleotide was used to reduce NCX protein expression in cultured adult guinea pig ventricular myocytes. Caspase-3 activation and cytochrome c release were used as markers of apoptosis. Hypoxia-reoxygenation-induced apoptosis was significantly decreased in antisense-treated myocytes compared with untreated control or nonsense-treated myocytes. Pretreatment of cultured myocytes for 24 h with either endothelin-1 or phenylephrine was found to increase both NCX protein expression and evoked NCX activity as well as enhance hypoxia-reoxygenation- induced apoptosis. Control experiments demonstrated that endothelin-1 and phenylephrine did not induce apoptosis on their own nor did they enhance the apoptotic response in a model of Ca²⁺-dependent, NCX-independent apoptosis. Additional control experiments demonstrated that the NCX antisense oligonucleotide did not alter the apoptotic response of myocytes to either H₂O₂ or isoproterenol. Taken together, these data suggest that the NCX has a critical and specific role in the initiation of apoptosis after hypoxia-reoxygenation in guinea pig myocytes and that hypoxia- reoxygenation-induced apoptosis is quite sensitive to changes in NCX activity.