The role of lipoproteins in the degradation of platelet-activating factor

Measuring both platelet-stimulating activity and liberation of acetate, the capacity of serum and individual lipoproteins to degrade the platelet-activating factor (PAF) was studied. The highest degrading effect relative to the protein content was found in very low density lipoproteins (VLDL) and in low density lipoproteins (LDL). The effect is about 10- and 100-fold higher than that of high density lipoproteins (HDL) and serum, respectively. In lipoprotein deficient serum (LPDS) less than 5% of serum activity is detectable. Considerable individual variations are observed measuring the degradation of PAF under standard conditions in plasma from 37 healthy volunteers. Moreover, this activity is shown to correlate strongly with the plasma concentration of LDL. On the other hand, a significant negative correlation was found between the PAF-degrading capacity and the plasma concentration of HDL. In contrast, the PAF-degradation is unrelated to the concentration of plasma triglycerides. The results point to a possible role of plasma lipoproteins in regulating the degradation of PAF released into the circulation.

Endogenous lipoproteins modify the thromboxane formation capacity of platelets

A significantly negative correlation was demonstrated between HDL-cholesterol levels of serum and the thromboxane B2 (TXB2) formation in clotted whole blood, whereas a significantly positive correlation was estimated between the LDL cholesterol or apolipoprotein B (apo B) levels and the TXB2 formation in clotted whole blood. Similar relationships were observed between the HDL and apo B serum levels and the thrombin-induced malondialdehyde formation in platelet-rich plasma. The levels of HDL2 cholesterol, total cholesterol, apo A-I and of triglycerides were not significantly correlated with the TXB2 or malondialdehyde formation in both systems. The results in this study support the hypothesis that the TXB2 formation may be modulated by endogenous lipoproteins: high level of LDL stimulates and high level of HDL inhibits the TXB2 formation.

Glucocorticoid receptors in sheep brain tissues during development

In this study we describe changes in the number of glucocorticoid binding sites that occur in cytosols of pituitary, hypothalamic, and hippocampal tissue obtained from fetal, newborn, and adult sheep. We observed specific, saturable binding of [3H]triamcinolone acetonide by cytosols of all three tissues. There were both age-related and tissue-related differences in the number of binding sites observed. By midgestation (70 days) significant quantities of binding sites were present in pituitary and brain tissues, and the number remained relatively constant throughout gestation and in the neonatal period. The number of binding sites declined markedly in cytosols from adult tissues. At all ages there were more binding sites present in pituitary cytosols than in cytosols of hypothalamus and hippocampus. The sucrose density gradient profile of the radioactive ligand-binding site complex is characteristic of a glucocorticoid receptor. Thus, by midgestation at least a portion of the biochemical machinery mediating glucocorticoid effects is present in the fetal pituitary and in specific brain tissues considered important in the modulation of ACTH secretion by corticosteroids.

Synthesis of arachidonic acid in the human placenta in vitro

The syntheses of arachidonic acid and total fatty acids were measured in human placentas at the end of the first trimester and at term using 14C-acetate in in vitro incorporation experiments. The rates of arachidonic acid synthesis were determined to be 0.70 (first trimester) and 0.64 (term) mumol/h/100 g of placental tissue. The calculated rates of total fatty acid synthesis amount to 0.5 (first trimester) and 0.6 (term) mumol palmitate-equivalents/h/100 g. The results suggest that the human placenta seems to be not dependent on maternal arachidonic acid. Near term the placenta even could be source of arachidonic acid.

Concentration and composition of the lipoprotein classes in human umbilical cord serum

The concentration and composition of the lipoprotein density classes d less than 1.006 (VLDL), d = 1.006 - 1.064 (LDL), and d = 1.064 - 1.21 (HDL) of human umbilical cord serum were investigated by means of preparative ultracentrifugation. The concentrations of all the density classes, in particular that of the VLDL, are lower than in adults; the values correspond to 8.1 (VLDL), 59.2 (LDL), and 77.4 (HDL) mg/100 ml. About 15% of the total cholesterol and roughly 40% of the total lipid phosphorus did not float at the highest density of 1.21. The composition of the LDL and the HDL was principally in agreement with the pattern obtained in adults. The VLDL of the cord serum, however, showed a lower amount of triglycerides (45%) and a higher content of proteins (22%) than the VLDL from serum of adults. In cord serum only 25% of the total triglycerides are associated with the VLDL fraction, while more than 50% of the triglycerides circulate as a constituent of the LDL.

In vivo metabolism of 14C-labeled palmitic acid in serum and in brown and white adipose tissues of well-fed and starved newborn rabbits. I. Tracer kinetic analysis of free fatty acid metabolism in serum

After i.v. injection of 20.10⁶ cpm/100 g body weight ¹⁴C-1-palmitic acid at a thermoneutral environment (Ta 35 °C) the primary FFA pool was in 7 days old well fed rabbits (Group I) 9,57 µmole and in rabbits of the same age but previously starved for 48 h at 20 °C (group II) 3.17 and in those starved 72 h at 35 °C (group II) 1.89. There was no significant difference in disappearance rate (λ) half time (t_0.5) and turnover time (t_t) indicating that the rate at which the label disappeared from the forerunner FFA pool was more dependent on the Ta maintained during the tracer kinetic experiment, than on the duration and the thermal conditions of the previous starvation. The absolute turnover rate of serum FFA (ms) was 9.38 µmole FFA/min in group I and 3.44 and 2.67 in group II and group III, respectively; pointing out that the transfer of FFA from the forerunner pool into the fatty acid compartments of various organs and tissues was significantly reduced by starvation.

In vivo metabolism of 14C-labeled palmitic acid in serum and in brown and white adipose tissue of well-fed and starved newborn rabbits. II. Tracer kinetic analysis of the incorporation rate of serum free fatty acids into the main lipid fractions of brown and white adipose tissues

Starvation at an ambient temperature (T_a) of 35 °C for 72 h or at T_a 20 °C for 48 h affected differently the free fatty acid (FFA), phospholipid and glyceride pools as well as the total flux of fatty acid (msf) between the intracellular and extracellular lipid compartments of brown (BAT) and white adipose tissues (WAT). Tracer kinetic studies with 20.10⁶ cpm/100 g body weight ¹⁴C-1-palmitic acid injected at T_a 35 °C showed that BAT takes up a consistently higher proportion of the initial serum FFA pool (ms) than WAT in both fed (group I) and fasting newborn rabbits (group II at T_a 20 °C; group III at T_a 35 °C. BAT of animals starved in the cold prior to the tracer’s injection (group II) shared a considerably larger fraction of ms than that of fed littermates (group I). Fasting in a thermoneutral environment (T_a 35 °C) consistently resulted in a very low absolute (msf) and relative (percentual msf) flow rates of FFA into BAT. In starving rabbits msF and percentual msF in WAT were consistently lower than in well-fed animals and were even more markedly reduced in animals previously starved in the cold (Ta 20 °C) than in a thermoneutral environment (T_a 35 °C).