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To assess the metabolic recovery of mitochondria after injury, we have monitored, in vivo and noninvasively, changes in the redox state of cytochrome (cyt) a,a3 in 35 rats after tissue hypoxia induced by rapid exsanguination to a mean arterial pressure of 30-35 mmHg. This level of mean arterial pressure was maintained for a shorter period of time in group I (n = 17) and a longer period of time in group II (n = 18), then the shed blood was returned by infusion. The surviving animals were observed for 2 more h before terminating the experiments. During exsanguination, reinfusion and recovery intervals brain tissue parameters of blood oxygenation, relative blood volume, and cyt a,a3 redox state were monitored continuously by spectrophotometry through the closed skull and intact skin. Group I had a high survival rate while group II had a very low survival rate. In both groups, with the onset of hypotension, there was a prompt rapid shift, followed by a slow continued progressive shift, of cyt a,a3 toward a more reduced state. The extent of recovery of cyt a,a3 following reinfusion was different in each group. In group I there was a rapid reoxidation of cyt a,a3 to a level above the base line (16 +/- 12%, mean +/- SEM). In contrast, the extent of reoxidation of cyt a,a3 in group II was significantly lower and stayed 31 +/- 6% below the base-line level. To further evaluate the mechanisms responsible for these observations, another related experiment was performed. 12 rats were subjected to shock and resuscitation as outlined for groups I and II. After death or killing of the animal, we measured, in vitro, oxygen consumption of cerebral cortical slices. Oxygen consumption of cortical tissue slices in subgroup I was significantly higher than in subgroup II. We conclude that, under these experimental conditions, the oxidative response of cyt a,a3 correlates closely with survival or death in the two groups. If in group I animals the greater oxidation of cyt a,a3, in vivo after resuscitation, reflects greater oxygen utilization, as is suggested by the in vitro observations in subgroup I, then we may be observing a useful adaptive response to tissue injury leading to preserved organ function and enhanced survival. Therefore, noninvasively measured cyt a,a3 redox state, reflecting intracellular metabolic activity, seems to indicate both the overall cerebral cellular response to injury and the likelihood of survival.

Cytochrome a,a3 reoxidation. Early indicator of metabolic recovery from hemorrhagic shock in rats.