Altered cardiac mitochondrial Ca ²⁺ regulation during rewarming following hypothermia

Abstract

The mechanisms underlying rewarming shock (cardiac failure during rewarming from profound hypothermia) are not known. Increased myocardial Ca2+ levels and altered mitochondrial ultrastructure have been reported. We hypothesized that mitochondrial buffering of cytosolic Ca2+ contributes to cardiac dysfunction. Enzymatically‐dissociated rat cardiac myocytes loaded with Ca2+ indicators fluo‐3 (cytosolic) and rhod‐2 (mitochondrial) were electrically stimulated (0.5 Hz) and visualized using real time confocal microscopy. Myocytes were cooled from 37 to 15 °C over 1h, maintained at 15 °C for 30min, and rewarmed over 1h back to 37 °C. Hypothermia increased baseline mitochondrial Ca2+ (134% of normothermic control), which returned to control levels after rewarming. In contrast, rewarming elevated cytosolic Ca2+ significantly compared to controls (126% vs. 94%). During hypothermia both cytosolic and mitochondrial peak Ca2+, time to peak, and time to 50% decay increased by 135%, 150% and 200%, respectively, but returned to control levels during rewarming. These data indicate that during hypothermia, mitochondria buffer cytosolic Ca2+, and during rewarming, mitochondrial Ca2+ is redistributed to the cytosol, leading to continued elevation in cytosolic Ca2+. The temporary Ca2+ overload in mitochondria may further affect subsequent energy production and myocyte function. Supported by Mayo Foundation Clinical Research grants, and NIH grants HL74309 and GM56686.