Early effects of inorganic lead on immature rat brain mitochondrial respiration

Abstract

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, the authors 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 wk 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 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 wk, a significant loss in cerebral hemisphere wet weight (P<0.01) compared with controls. Overt encephalopathy appeared in pups from 2 of 9 litters receiving lead feedings for 1 wk and in half of the litters after 2 wk 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 wk 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 wk 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 wk or less was due to a small decrease in state 3 respiraton and a large, but inconsistent, increase in state 4 respiration. The decrease in control ratios in both cerebellar and cerebral hemisphere mitochondria after 2 wk 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 wk of feedings.