No abstract

We show that calcium overload is a characteristic feature of the beating heart during deep hypothermia, which aggravates by increasing duration of exposure. The relatively low decline in [Ca(2+) ](i) during the rewarming period indicates difficulties in recovering calcium homoeostasis, which in turn may explain cardiac contractile dysfunction observed after rewarming.

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.

Rewarming shock (acute cardiac failure) during resuscitation from profound hypothermia is partially ameliorated by low dose epinephrine (EPI). However, hypothermia‐induced cardiac Ca2+ overload persists with rewarming. The reactive oxygen species (ROS) scavenger superoxide dismutase (SOD) protects isolated hearts from hypothermia‐induced Ca2+ overload. In electrically stimulated rat cardiac myocytes loaded with Ca2+ indicators fluo‐3 (cytosolic) and rhod‐2 (mitochondria) and imaged using confocal microscopy, we investigated the effects of EPI and SOD on cytosolic vs. mitochondrial Ca2+ during rewarming. Myocytes were cooled from 37 to 15 °C over 1h, maintained at 15 °C for 30min, and rewarmed over 1h to 37 °C. EPI 0.125 μg/ml was administered from 27 °C upwards, and SOD 0.1 mg/ml from 15 °C upwards during rewarming. Control cells did not receive drugs. Other cells received EPI alone. Hypothermia resulted in elevated cytosolic and mitochondrial Ca2+. Compared to normothermia, mitochondrial Ca2+ uptake and release was slowed by hypothermia, and not immediately reversed with rewarming. EPI prolonged mitochondrial Ca2+ overload during rewarming. With SOD, mitochondrial as well as cytosolic Ca2+ were comparable to controls during hypothermia and rewarming. These data suggest that during rewarming, mitochondrial Ca2+ buffering remains impaired, and that ROS may play a role in Ca2+ overload. Administration of EPI may exacerbate mitochondrial dysfunction, potentially worsening rewarming shock. Supported by Mayo Foundation Clinical Research grants, and NIH grants HL74309 and GM56686.