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Abstract— The effects of altered osmolarity on respiration and fine structure were compared in isolated rat cerebral versus liver mitochondria. Polarographic study of cerebral mitochondria in hypo‐osmolar media showed inhibition of State 3 (ADP‐dependent) respiration which was not reversed by dinitrophenol. In hyperosmolar media, State 3 respiration was transiently inhibited and State 4 (ADP‐independent) respiration increased with the NAD‐linked substrate pair, glutamate and malate. With succinate as substrate, respiration was not affected by moderate hyperosmolarity. In the most hyperosmolar medium, State 3 respiration was inhibited with both substrates. In contrast to the results with cerebral mitochondria, State 4 respiration was increased in hypo‐osmolar media and State 3 respiration was persistently inhibited in hyperosmolar media in liver mitochondria with both substrates. In both cerebral and liver mitochondria, cytochrome c oxidase (EC 1.9.3.1.) activity was mildly inhibited in hypo‐osmolar media and increased in hyperosmolar media. Electron microscopy showed that liver mitochondria were swollen in hypo‐osmolar media and condensed in hyperosmolar media. Cerebral mitochondria showed mild rarefaction in hypo‐osmolar media and, in hyperosmolar media, more than half the mitochondria showed either no or minimal changes in fine structure. Our results suggest that there are differences in metabolic control and structure between mitochondria from different cell types, which may be important in the cellular metabolic response to pathologic changes in water or osmolarity.

Microsomes of liver and many other tissues contain two hemoproteins, cytochromes b5 and P450, which appear to serve in a mixed-function oxidase system active in the metabolism of steroids, certain drugs, and environmental chemicals (Matsubara et al., 1976;Oshino, 1978). Cytochrome b5 has been found in small quantities in microsomes from mature rabbit, guinea pig, and bovine brains (Inouye and Shinagawa, 1965). These investigators could find no spectrophotometric evidence for the presence of cytochrome P450. Recently, Cohn et al. (1977) and Sasame et al. (1977) reported the presence of a small quantity of P450 in mature rat brain microsomes. In the course of studies on the effects of inorganic lead feedings on cytochromes in the rat brain, we have found spectrophotometric evidence for the presence of both cytochromes b5 and P450. We now are reporting quantitative estimates of these cytochromes in brain microsomes isolated from normal immature and adult rats.

The effects of altered osmolality on respiration and fine structure were studied in isolated cerebral mitochondria from mature rats (6C-100 days of age) and from rat pups in the first month of postnatal life (5, 10, 20 and 30 days). In the mature cerebral mitochondria, ADP-dependent respiration was inhibited in media of decreased osmolality. There was a transient inhibition of ADP-dependent respiration and a sustained increase in ADP-independent respiration in media of increased osmolality. In contrast, cerebral mitochondria from 5-day-old rats showed both inhibition of ADP-dependent respiration and increased ADP-independent respiration in hypo-osmolal media. In these mitochondria, inhibition of ADP-dependent respiration was stable and ADP-independent respiration was unchanged in media of increased osmolality. The transition to the mature respiratory response occurring with altered osmolality took place between 10 and 30 days of age. During this same age period, cerebral mitochondria showed an increasing resistance to matrix condensation in media of normal and increased osmolality.

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