The brain at high altitude: hypometabolism as a defense against chronic hypoxia?

Decreased rat brain cytochrome oxidase activity after prolonged hypoxia

Three weeks of hypoxic exposure results in a spectrum of systemic physiological and local brain tissue adaptations. Cytochrome oxidase histochemistry was used as an indicator of changes in energy demand in response to hypoxia. We found overall cytochrome oxidase activity decreased in hypoxic adapted rats as compared to normoxic control rats. Some regions, notably layer 3 of the frontal cerebral cortex, layer 4 of the parietal sensory barrel fields, lacunosum moleculare hippocampi, and specific nuclei of the rostral ventral medulla, though exhibiting cytochrome oxidase activity decreases of 16-27%, still retain their relatively higher levels of activity. We conclude that there is a hypometabolic component of the rat brain adaptation to continued hypoxia.

Estimation of the degree of acclimatization to high altitude by a rapid and simple physiological examination

The recent expansion in the geographical areas open to human activity has made it desirable to have an objective method to evaluate the degree of high-altitude acclimatization. In this study, we measured the arterial oxygen saturation value at rest and just after exercise in healthy high-altitude trekkers using a transportable pulse oximeter. During a 100-day stay at high altitude (around 4000 m), the degree of arterial hemoglobin saturation measured at rest was relatively stable. However, shortly after arrival at high altitude, even light exercise induced an acute reduction in the degree of arterial hemoglobin saturation; this reduction was ameliorated as the trekkers became acclimatized to the high altitude. Preliminary short trekking to high altitudes does not appear sufficient to induce this response. It is suggested that this rapid and simple physiological examination, the measurement of arterial oxygen saturation value after light exercise, could be a convenient means of estimating the level of high-altitude acclimatization among healthy subjects.

Reduced mitochondrial respiration in mouse cerebral cortex during chronic hypoxia

Respiratory activity and NADH CoQ reductase (complex I) and cytochrome c oxidase (complex IV) activities were measured in free (non-synaptosomal) mitochondria isolated from cerebral cortex of male Balb/c mice exposed to intermittent hypobaric hypoxia (450 Torr; 4300 m) for 21 days and compared to normoxic (sea level) controls. In the hypoxic we found a 47% reduction of oxygen uptake during state 3 (ADP and substrate present), 12% reduction during state 4 (no ADP present) and 20% reduction in the uncoupled respiration rate with pyruvate plus malate as substrates. Respiratory control ratio (RCR) decreased by 24%. No change in the ADP/O ratio was seen. NADH CoQ reductase activity decreased by 30% and cytochrome c oxidase by 17%, suggesting that under conditions of chronic hypoxia, the reductions of mitochondrial respiratory activities are caused, at least in part, by enzymatic alterations of the electron transport chain (complex I and complex IV). The decreased activity of these enzymes could contribute to alterations in neuronal activity by reducing brain energy metabolism during development under conditions of chronic hypoxia.

Evidence of mitochondrial DNA diversity in South American aboriginals

The absence in South American aboriginals of an Asian-specific marker, a 9-bp deletion between the genes for the second subunit of cytochrome oxidase II and lysine transfer RNA in region V, has been interpreted as a bottlenecking effect at the Isthmus of Panama during the peopling of the Americas. We screened mitochondrial DNA (mtDNA) for this 9-bp tandem repeat and for polymorphisms in specific regions of the mtDNA in 2 ancient and 31 contemporary samples from South American aboriginals. We found additional (mtDNA) diversity in South American aboriginals in three ways. First, an Asian-specific marker not previously reported in South American aboriginals was identified by a sequencing analysis in both the contemporary Andean and Amazonian aboriginal peoples. Second, two new haplotypes so far unique to South American aboriginals were found. Additionally, we show that South American aboriginals fall into discrete populations. These results suggest that the prehistoric colonization of South America is the outcome of multiple migrations; the data do not support a bottlenecking effect at the Isthmus of Panama.