Cerebral metabolism of plasma [14C]palmitate in awake, adult rat: subcellular localization.

Following intravenous injection of [U-14C]palmitate in awake adult rats, whole brain radioactivity reached a broad maximum between 15-60 min, then declined rapidly to reach a relatively stable level between 4 hr and 20 hr. At 44 hr total radioactivity was 57% of the 4 hr value (p less than 0.05). About 50% of palmitate which entered the brain from the blood was oxidized rapidly, producing 14C-labeled water-soluble components which later left the cytosol. Radioactivity in the cytosolic fraction peaked at 45 min and then declined, coincident with the decline in total brain radioactivity. Membrane fractions were rapidly labeled to levels which remained relatively stable from 1 to 44 hr. Increases in the relative distributions of radioactivity were seen between 1 and 4 hr for the microsomal and mitochondrial fractions, and beyond 4 hr for the synaptic and myelin membrane fractions (p less than 0.05). Radioactivity in membrane fractions was 80-90% lipid, 5-13% water-soluble components and 3-17% protein. The proportion of label in membrane-associated protein increased with time. Proportions of radioactivity in the combined membrane fractions increased from 65% to 76% to 80% at 4, 20 and 44 hr, respectively. The results show that plasma-derived palmitate enters oxidative and synthetic pathways to an equal extent, immediately after entry into the brain. At and after 4 hr, the radiolabel resides predominantly in stable membrane lipids and protein. Brain radioactivity at 4 hr can be used therefore, to examine incorporation of palmitate into lipids in vivo, in different experimental conditions.

Utilization of plasma fatty acid in rat brain: distribution of [14C]palmitate between oxidative and synthetic pathways.

Radioactivity within individual brain compartments was determined from 5 min to 44 h after intravenous injection of [14C]palmitate into awake Fischer-344 rats, aged 21 days or 3 months. Total radioactivity peaked broadly between 15 min and 1 h after injection, declined rapidly between 1 and 2 h, and then more slowly. In 3-month-old rats, the lipid and protein brain fractions were maximally labeled within 15 min after [14C]palmitate injection, then retained approximately constant label for up to 2 days. Radioactivity in the aqueous brain fraction comprised mainly radioactive glutamate and glutamine, and peaked at 45 min, when it comprised 48% of total brain radioactivity, then decreased to 27% of the total at 4 h, 15% at 20 h, and 10% at 44 h. Percent distribution of radioactivity within the different brain compartments, 4 h after intravenous injection of [14C]palmitate, was similar in 21-day-old and 3-month-old rats, despite higher net brain uptake in the younger animals. The results indicate that about 50% of plasma [14C]palmitate that enters the brain of adult rats is incorporated rapidly into stable protein and lipid compartments. The remaining [14C]palmitate enters the aqueous fraction after beta-oxidation, and is slowly lost. At 4 h after injection, 73% of brain radioactivity is within the stable brain compartments; this fraction increases to 86% by 20 h.

Gangliosides in axolemmal and synaptic membrane fractions from developing rats: effects of maternal ethanol consumption on offspring.

Previous work from this laboratory has shown that in utero exposure to ethanol significantly alters the synthesis of glycoproteins in synaptic, axolemmal, and myelin membranes from developing rats. In an attempt to determine whether in utero exposure to ethanol similarly alters the synthesis of other glycoconjugates involved with cell-cell interactions, the present study examined the influence of chronic maternal ethanol consumption prior to parturition on the content and synthesis of gangliosides in axolemmal and synaptic plasma membranes from developing rats. The results demonstrate that, in contrast to central nervous system glycoproteins, synaptic and axolemmal glycolipids are minimally affected by in utero exposure to ethanol. At all ages examined (17 to 34 days of age), the offspring of control and ethanol-treated rats had a comparable distribution of radiolabel among synaptic and axolemmal gangliosides, a normal concentration of ganglioside sialic acid in synaptic plasma membranes, and a near-normal distribution of sialic acid among synaptic gangliosides. The present study provides evidence which indicates that the radiolabeling patterns of axolemmal and synaptic membrane gangliosides are similar. Specifically, the most heavily labeled synaptic and axolemmal gangliosides were GT1b (20-37% of the total radioactivity) and GD1a (20-32%). A smaller proportion of radioactivity was associated with GD1b (approximately 11-16%), GM1 (5-10%), and GQ1b (4-11%), as well as with GD3 and the other monosialogangliosides (less than 5%). During the age period examined the proportion of radioactive GT1b increased in both membrane fractions.

Myelin gangliosides in developing rats: the influence of maternal ethanol consumption.

The present study examined myelin gangliosides in the developing offspring of rats that were pair-fed control or ethanol liquid diets prior to and during gestation. Between 17 and 31 days of age, we observed an increase in the proportion of GM1 in myelin (from 15% to 38% of ganglioside sialic acid) and a decrease in the proportion of GT1b (from 26% to 4%). GM4 was detected at all ages examined. Between 17 and 31 days of age, there was an increase in the proportion of N-acetylmannosamine-derived radioactivity associated with GM1 (from 16% to 22%) and GM4 (from 5% to 13%), and a decrease in that associated with GT1b (from 24% to 4%). Small, but significant (p less than 0.05), developmentally related differences were found in GD2 and GD3. Detection of GM4 in myelin of young rats in the present study appears to depend on the use of nonpartitioning methods of ganglioside extraction. Although the distribution of myelin gangliosides and radioactivity was near-normal in ethanol-treated pups, there was a consistent decrease in the proportion and radioactivity associated with the major myelin ganglioside, GM1.

Glycoproteins and proteins in an axolemma-enriched fraction and myelin from developing rats: effect of maternal ethanol consumption.

The present study examined proteins and glycoproteins from an axolemma-enriched fraction from the developing offspring of female rats that were pair-fed control or 6.6% (50 g/liter) ethanol liquid diets on a chronic basis prior to parturition. In addition, this study examined the synthesis of the major CNS myelin-associated glycoprotein (MAG) as an index of myelin maturation. The results of the latter study demonstrated normal MAG maturation in ethanol-treated rats. However, a significant decrease in the proportion of radioactivity associated with MAG was found in the developing offspring of ethanol-treated rats. The major axolemmal proteins from 32-day rats included those with molecular weights of 105 K, 81 K, 62 K, 55 K, 52 K, 36 K, and 33 K. Major peaks of radioactivity were associated with fucosylated axolemmal glycoproteins with apparent molecular weights of 150 K, 130 K, 85 K, 76 K, and 64 K. Several development-related changes in the protein composition of axolemma-enriched fractions were observed in control animals. Between 22 and 32 days of age control rats exhibited a significant (P less than .05) decrease in the proportion of axolemmal proteins that had apparent molecular weights of 150 K, 105 K, and 62 K. A development-related decrease in the 105 K axolemma-associated protein did not occur during the 22-32 day age period in the offspring of ethanol-treated animals. At 22 days the proportion of this 105 K protein in affected offspring was significantly (P less than .05) less than that in age-matched control rats and comparable to that in 32-day control rats. The relative distribution of radioactivity among fucosylated axolemmal glycoproteins also changed significantly between 22 and 32 days of age. These changes include a decrease in the proportion of radioactivity associated with the 110 K, 55 K, and 52 K fucosylated glycoproteins and an increase in the proportion of radioactivity associated with the 85 K and 67 K glycoproteins. Several small, but significant (P less than .05) alterations were found associated with glycoproteins in an axolemma-enriched fraction from 22- and 32-day ethanol-treated rats.