Methylmalonate increases glucose uptake in vitro by brain of young rats

Abstract

Methylmalonic acidaemia is an inherited metabolic disorder generally associated with severe clinical features. Those who survive the first months present a variable degree of mental retardation (Rosemberg, 1983). Methylmalonate (MMA) is the major metabolite which accumulates in tissues of methylmalonic acidaemic patients. To our knowledge, only two studies have been done on the effects of MMA on brain metabolism (Frenkel et a/., 1973; Patel et a/., 1976). Since glucose is the major substrate that throughout life supports the energy requirements of the brain, we studied the effects of MMA on glucose uptake in rat brain in vitro.,Wistar rats bred in our laboratory were used in this study. Free water and a 20% (w/w) protein commercial chow were provided. The mother and eight pups were kept per cage. Half of the animals were left fasting 24 h before the experiments. The rats were killed by decapitation without anaesthesia at 15 days of age, their brains were rapidly removed. The olfactory bulbs, cerebellum and pons/medulla were discarded. The rest of the brain (cerebrum) was cut in two perpendicular directions with a Mcllwain chopper into prisms of 40 p m width. Brain prisms from fed and fasting animals were incubated under 0 , / C 0 2 (19:l) mixture at 37°C for 30 min in Krebs-Ringer bicarbonate buffer, pH 7.0 (100 mg of tissue/ml of buffer) containing: glucose 5.0 mM and one of the three concentrations of MMA, 1.0, 2.5 and 5.0 mM. Control experiments did not contain MMA in the incubation medium. Glucose was measured by the glucose oxidase method (Trinder, 1969) and the uptake determined by subtracting the amount found after the incubation from the total amount placed in the medium.It can be seen in Table 1 that MMA had no effect on glucose incorporation by brain prisms of fed rats, but, surprisingly, it increased glucose uptake by brain prisms of fast-Abbreviation used: MMA, methylmalonate.ing rats at levels as low as 1.0 mM. Further assays revealed that MMA doses of 0.5 mM also produced such an effect. On the other hand, propionate in doses up to 10.0 mM did not affect glucose incorporation (results not shown), ruling out a possible non-specific effect due to an acidic component added to the incubation medium. Both MMA and propionate in the doses used did not change the buffer pH. It can also be seen in the Table that the brains of fasting rats incorporated less glucose in the control experiments (incubation medium free from MMA).In another experiment, we applied three subcutaneous injections of buffered MMA, pH 7.2, with an interval of 90 min between them, to 15-day-old fed and 24 h fasting rats in such doses (10.5 mg of MMA/100 g body weight) that the animals achieved MMA serum levels of around 2.5 mM (concentration found in methylmalonic acidaemic patients at crises). Control animals received saline in the same volumes. Ninety minutes after the last injection animals were decapitated and the brain processed as before, excepting for the presence of MMA in the incubation medium. Again we observed that only brain...

Effect of methylmalonate onin vitro lactate release and carbon dioxide production by brain of suckling rats

“…Therefore, it appears that more pyruvate is diverted to lactate instead of being oxidized in the Krebs cycle. This is in agreement with our previous findings showing that MMA inhibits brain succinate dehydrogenase activity in a dose-response way (Dutra et al, 1989).…”

“…We therefore assume that brain lactate release in our assay represents lactate production. Total brain glucose uptake also increased with MMA concentrations in the incubation medium (F(3,28) = 2.215, p < 0.05) (Table 1) in a dose-dependent pattern (regression coefficient 0.774, p < 0.05), thus confirming our previous findings (Dutra et al, 1989(Dutra et al, , 1991. The table also shows that lactate production was proportional to glucose utilization, leading one to speculate that the increment in brain glucose uptake caused by MMA was mainly diverted to lactate.…”

“…Very little has been done to identify the mechanisms underlying the neurological dysfunction of these patients. We have previously found that MMA induces a higher glucose utilization by brain of fasting young rats (Dutra et al, 1989(Dutra et al, , 1991. Since brain of young mammals releases considerable amounts of lactate into blood (Hawkins et aI., 1971;Cremer and Health, 1974;Dahlquist and Persson, 1976), in this study we investigated the effect of MMA on lactate and CO2 production by rat brain in an attempt to better understand the fate of the augmented brain glucose uptake provoked by this acidic metabolite.…”

Inhibition of succinate dehydrogenase and β‐hydroxybutyrate dehydrogenase activities by methylmalonate in brain and liver of developing rats

“…We recently found that MMA inhibits succinate dehydrogenase (SDH) activity in brain of young rats (Dutra et al 1989). In addition, we also demonstrated that MMA diminishes fl-hydroxybutyrate (BHB) but not acetoacetate incorporation into brain prisms of suckling rats, owing to inhibition of fl-hydroxybutyrate dehydrogenase (HBDH) (EC.I.I.I.30) activity (Dutra et al 1991).…”

“…Those who survive present with a variable degree of physical and mental retardation (Rosenberg and Fenton 1989).Various metabolic effects have been attributed to MMA and methylmalonyl-CoA. We have previously found important effects of MMA on brain metabolism (Wajner et al 1982(Wajner et al , 1988Dutra et al 1989Dutra et al , 1991.…”