Heart mitochondria in rats submitted to chronic hypoxia

The effect of prolonged exposure to normobaric hypoxia on the mitochondria of myocard of rats exposed for several weeks to 8 and 7% O-2 has been morph ometrically evaluated. Twelve male Wistar rats housed in Nalgene cages (2 p er cage) with a batch of six cages placed in plexiglass chambers were maint ained in air/N-2 mixtures containing different concentrations of O-2. Six a nimals kept in similar cages under normoxia served as controls. When at day 60 the FIO2 was reduced to 8%, the weight increase stagnated and after the 81st test day, on which the hypoxic animals were subdivided into 8% and 7% groups the weight curve showed a decrease in the mean body weight for both groups. The arrest and the following loss of weight beyond the 85th day ma y be interpreted as the expression of a limit reached in the compensation c apacity. In the 8%-group the shape of the mitochondria varied more markedly often with budding and furrowing of the surface. In the 7%-group bizarre s hapes and wide variations in size with a decided shift towards larger mitoc hondria were noteworthy. While rats kept under 8% oxygen exhibited a numeri cal increase in myocardial mitochondria compared to controls, the mitochond ria of the 7%-group were numerically reduced. The results suggest that hypo xia of 8% oxygen is compensatable, if only to some extent, by an increasing surface of mitochondrial membranes, and that further reduction of oxygen c auses compensation mechanisms to fail as seen by the severe alterations of the mitochondrial population of the cardiomyocyte in the 7%-group.