Muscle co-activity tuning in Parkinsonian hand movement: disease-specific changes at behavioral and cerebral level.

We investigated simple directional hand movements based on different degrees of muscle co-activity, at behavioral and cerebral level in healthy subjects and Parkinson's disease (PD) patients. We compared "singular" movements, dominated by the activity of one agonist muscle, to "composite" movements, requiring conjoint activity of multiple muscles, in a center-out (right hand) step-tracking task. Behavioral parameters were obtained by EMG and kinematic recordings. fMRI was used to investigate differences in underlying brain activations between PD patients (N = 12) and healthy (age-matched) subjects (N = 18). In healthy subjects, composite movements recruited the striatum and cortical areas comprising bilaterally the supplementary motor area and premotor cortex, contralateral medial prefrontal cortex, primary motor cortex, primary visual cortex, and ipsilateral superior parietal cortex. Contrarily, the ipsilateral cerebellum was more involved in singular movements. This striking dichotomy between striatal and cortical recruitment vs. cerebellar involvement was considered to reflect the complementary roles of these areas in motor control, in which the basal ganglia are involved in movement selection and the cerebellum in movement optimization. Compared to healthy subjects, PD patients showed decreased activation of the striatum and cortical areas in composite movement, while performing worse at behavioral level. This implies that PD patients are especially impaired on tasks requiring highly tuned muscle co-activity. Singular movement, on the other hand, was characterized by a combination of increased activation of the ipsilateral parietal cortex and left cerebellum. As singular movement performance was only slightly compromised, we interpret this as a reflection of increased visuospatial processing, possibly as a compensational mechanism.

Role of thiol homeostasis and adenine nucleotide metabolism in the protective effects of fructose in quinone-induced cytotoxicity in rat hepatocytes.

Freshly-isolated rat hepatocytes were exposed in glucose (15 mM) or fructose (5 mM) medium to menadione (2-methyl-1,4-naphthoquinone) (85 microM) or 1,4-naphthoquinone (NQ) (50 microM). Menadione and NQ are closely related quinones and have an approximately equal potential to induce redox cycling. However, NQ has a higher potential to arylate and is more toxic than menadione. During 2 hr of incubation, cell viability, thiol status, adenine nucleotide level and lactate production were determined. LDH-leakage was used as a measure of cell viability. In glucose medium, exposure of hepatocytes to menadione or NQ resulted in a faster excretion rate of oxidized glutathione as compared to those cells in fructose medium. As a result, quinone-exposed hepatocytes in fructose medium retained higher amounts of oxidized glutathione. Menadione-exposed hepatocytes in fructose medium exhibited a diminished rate of transthiolation of protein thiols with oxidized glutathione as compared to those cells in glucose medium. The adenine nucleotide level of hepatocytes in glucose medium was markedly higher than in fructose medium. This was caused by an ATP decrease in hepatocytes in fructose medium resulting in a low energy charge (E.C.) (0.6) as compared to hepatocytes in glucose medium (0.9). Only menadione caused a decrease in the E.C. in glucose medium while NQ caused a decrease of all three adenine nucleotides. In fructose medium, quinone-exposed hepatocytes showed no change in their adenine nucleotides as compared to control cells. Despite the higher oxidized glutathione content and the lower ATP level of NQ-exposed hepatocytes in fructose medium, they had a better viability than those cells in glucose medium. From our results we conclude that a high ATP content is not always beneficial for cell survival.

Kinetic modeling of slow dissociation of bromosulphophthalein from albumin in perfused rat liver: toxicological implications.

Due to strong binding between organic anions and albumin, the kinetics of the binding process must be carefully considered in biologically-based models used for predictive toxicology applications. Specifically, the slow dissociation rate of an organic anion from the protein may lead to reduced availability of free anion in its flow through the capillaries of an organ. In this work, the effect of the dissociation rate of the anion bromosulphophthalein (BSP) from albumin was studied in isolated, perfused rat livers in the presence of albumin concentrations of 0.25, 1, and 4% (w/v) and an initial BSP concentration of 20 microM. The uptake of BSP from the perfusion medium was modeled using a biologically-based kinetic model of the sinusoidal and intracellular liver compartments. The best fit of the model to data resulted in the prediction of a slow dissociation rate constant for the BSP-albumin of between 0.097 and 0.133 s(-1). Assuming BSP and albumin to be in binding equilibrium in the sinusoidal space, with rapid binding-rate constants, as is often done, produced an unacceptable fit. These results indicate that the strong binding interaction between BSP and albumin, beyond keeping the concentration of free chemical low due to a small equilibrium dissociation constant, can also reduce uptake by an organ due to the slow release of BSP from the protein during passage through the capillaries. The implication of this dissociation-limited condition, when extrapolating to other doses and in-vivo situations, is discussed.

Effect of alkylating and redox cycling quinones on insulin receptor autophosphorylation.

Autophosphorylation is thought to be an essential step in the insulin receptor signal transduction cascade. Previous studies have shown that thiol alkylation of the receptor can block this receptor autophosphorylation, whereas an oxidative environment can increase this process. Since the toxicity of quinones can be related to two mechanisms-redox cycling resulting in oxidative stress, and arylation of cellular nucleophilic groups-the effects of 1,4-naphthoquinone and menadione on insulin receptor autophosphorylation were investigated. The results show that these two quinones have a dual effect: lower concentrations leading to oxidative stress increase insulin receptor autophosphorylation, whereas higher concentrations cause a thiol depletion and inhibit the normal insulin receptor autophosphorylation.

Kinetics of trichloroacetic acid and dichloroacetic acid in the isolated perfused rat liver.

Trichloroacetic acid (TCA) and dichloroacetic acid (DCA) are environmental contaminants that are suspected human carcinogens. To obtain more detail on the role of the liver in the kinetics of TCA and DCA, experimental studies in the isolated perfused rat liver (IPRL) system were conducted. The IPRL system was dosed with either 5 or 50 micromol of either TCA or DCA (25 or 250 microM initial concentration, respectively). TCA and DCA concentrations were followed in perfusion medium and bile for 2 h. The chemical concentration in liver was determined at the end of exposure. Liver viability was monitored by measuring leakage of lactate dehydrogenase (LDH) into perfusion medium and the rate of bile production. Studies performed with TCA showed that the total TCA concentration in perfusion medium decreased slightly during the first 30 min of exposure and remained constant thereafter. Most TCA, greater than 90% of total, was bound to albumin in the perfusion medium. A low, linear excretion rate of TCA in bile was obtained. The calculated free TCA concentration in the liver intracellular water space was higher than the unbound TCA concentration in the perfusion medium. Parallel studies with DCA showed that the DCA concentration in perfusion medium decreased rapidly. Of the total DCA in the perfusion medium, 60% was bound to albumin. The concentration of DCA in bile decreased over time. There was no DCA detectable in the liver after 2 h of exposure at both DCA concentrations. Enzyme leakage and bile production did not change in the presence of TCA or DCA, indicating that these concentrations were not acutely cytotoxic to the liver.

Cytotoxicity of menadione and related quinones in freshly isolated rat hepatocytes: effects on thiol homeostasis and energy charge.

The cytotoxic events in freshly isolated rat hepatocytes following exposure over 2 h to menadione (2-methyl-1,4-naphthoquinone) and two closely related quinones, 2,3-dimethyl-1,4-naphthoquinone (DMNQ) and 1,4-naphthoquinone (NQ), were examined. These quinones differ in their arylation capacity (NQ > menadione >> DMNQ) and in their potential to induce redox cycling (NQ approximately menadione >> DMNQ) The glutathione status (reduced and oxidized glutathione) of the hepatocytes was determined using HPLC after derivatization with monobromobimane. Protein thiols were measured spectrophotometrically and the energy charge of the cells was determined with HPLC using ion pair chromatography. The leakage of lactate dehydrogenase was used as a marker for cell viability. All three quinones caused alterations of the glutathione status of the exposed cells but the effects were markedly different. Exposure to DMNQ resulted in a slow decrease of reduced glutathione and an increase of mixed disulfides. The other two quinones caused an almost complete depletion of reduced glutathione within 5 min. Hepatocytes exposed to NQ accumulated oxidized glutathione whereas menadione-exposed hepatocytes showed increased levels of mixed disulfides. We did not find any effects of DMNQ (200 microM) on protein thiols, energy charge or cell viability. There was a clear difference in the effects of menadione and NQ on protein thiols, energy charge and cell viability; exposure to NQ resulted in a more extensive decrease of protein thiols and energy charge and an earlier onset of lactate dehydrogenase leakage.(ABSTRACT TRUNCATED AT 250 WORDS)