Early effects of inorganic lead on immature rat brain mitochondrial respiration.

Extract: Inorganic lead, added to the diet of the suckling rat in high doses, produces an encephalopathy similar to that seen in the immature human. Pathologic changes of edema and hemorrhage are seen earliest and are most prominent in the cerebellum. In this study, we measured respiration in cerebral hemisphere and cerebellar mitochondria isolated from led-fed and age-matched normal rat pups. Lactating mothers were begun on ad libitum feedings containing 4% lead carbonate when their pups were 2 weeks old. Mitchondria were isolated by differential centrifugation. Oxygen consumption was measured polarographically. NAD-linked respiration was measured with oxidation of the substrate pair, glutamate and malate. Cytochrome oxidase (cytochrome c oxidase, EC. 1.9.3.1) activity was measured in the presence of tetramethyl-p-phenylenediamine dihydrochloride (TMPD) and ascorbate. Within 2 days of starting lead feedings, rat pups showed a significant loss in body weight (P < 0.02) and, after 1 week, a significant loss in cerebral hemisphere wet weight (P < 0.01) compared with controls. Overt encephalopathy appeared in pups from two of nine litters receiving lead feedings for 1 week and in half of the litters after 2 weeks of feedings. None of the lead-fed mothers developed encephalopathic signs. With oxidation of the NAD-linked substrate pair, there was a progressive decrease, relative to controls, in ADP/O ratios in both cerebellar and cerebral mitochrondria from lead-fed animals. After 2 weeks these differences were significant in mitochondria from both regions (cerebellum, P<0.02; cerebrum, P<0.005). Respiratory control ratios were significantly lower in cerebellar mitochondria from lead-fed rats within 2 days of beginning feedings (P<0.02) and in mitochondria from both regions after 2 weeks of lead feedings (cerebellum, P<0.01; cerebrum, P<0.05). The decrease in control ratios in cerebellar mitochondria from animals receiving lead feedings for 1 week or less was due to a small decrease in state 3 respiration and a large, but inconsistent, increase in state 4 respiration. The decrease in control ratios in both cerebellar and cerebral hemisphere mitochondria after 2 weeks of lead feedings was due to a marked inhibition of state 3 respiration, relative to controls (cerebellum, P<0.01; cerebral hemisphere, P<0.05).In cerebellar mitochondria from lead-fed animals, cytochrome oxidase activity showed similar changes compared with controls: a highly significant (P < 0.001) increase within 2 days of beginning feedings and a significant (P < 0.01) decrease after 2 weeks of feedings. Speculation: The effects of inorganic lead feedings were greater in mitochon- dria from cerebellum than from cerebral hemispheres. The changes in respiratory chain function, which appeared sooner after beginning lead feedings than any previously reported biochemical or morpho- logic effect on rat cerebellum, may constitute an early step in the pathogenesis of an encephalopathy which is predominant in this region of the suckling rat brain. In cerebellar mitochondria, inorganic lead may be acting as an uncoupling agent, such as dinitrophenol, affecting both NAD-linked respiration and cyto- chrome oxidase activity. The maturing brain may be most sensitive to the encephalopathic effects of lead at a critical period of rapid synthesis and increasing activity of respiratory enzymes. The greater effects on cerebellar mitochondria may be related to regional differences in mitochondria respiratory chain properties during this maturational period.