Enhanced gene expression of Na⁺/Ca²⁺exchanger attenuates ischemic and hypoxic contractile dysfunction

Enhanced gene expression of the Na(+)/Ca(2+) exchanger in failing hearts may be a compensatory mechanism to promote influx and efflux of Ca(2+), despite impairment of the sarcoplasmic reticulum (SR). To explore this, we monitored intracellular calcium (Ca(i)(2+)) and cardiac function in mouse hearts engineered to overexpress the Na(+)/Ca(2+) exchanger and subjected to ischemia and hypoxia, conditions known to impair SR Ca(i)(2+) transport and contractility. Although baseline Ca(i)(2+) and function were similar between transgenic and wild-type hearts, significant differences were observed during ischemia and hypoxia. During early ischemia, Ca(i)(2+) was preserved in transgenic hearts but significantly altered in wild-type hearts. Transgenic hearts maintained 40% of pressure-generating capacity during early ischemia, whereas wild-type hearts maintained only 25% (P < 0.01). During hypoxia, neither peak nor diastolic Ca(i)(2+) decreased in transgenic hearts. In contrast, both peak and diastolic Ca(i)(2+) decreased significantly in wild-type hearts. The decline of Ca(i)(2+) was abbreviated in hypoxic transgenic hearts but prolonged in wild-type hearts. Peak systolic pressure decreased by nearly 10% in hypoxic transgenic hearts and >25% in wild-type hearts (P < 0.001). These data demonstrate that enhanced gene expression of the Na(+)/Ca(2+) exchanger preserves Ca(i)(2+) homeostasis during ischemia and hypoxia, thereby preserving cardiac function in the acutely failing heart.