Metabolism of chylomicrons by the isolated rat liver

Isolated livers perfused with washed corn oil chylomicrons labeled in vivo with palmitic acid-1-(14)C removed a large proportion of the chylomicrons. Slices from these livers oxidized chylomicron fatty acid esters to both carbon dioxide and acetoacetate. The liver slices also generated free fatty acids from chylomicron lipids and converted chylomicron triglycerides to phospholipids. Similar activities were observed in rat liver slices prepared shortly after the intravenous administration of chylomicrons to intact rats. The observed chylomicron uptake and lipid conversions were similar in livers from both fed and fasted rats. Fasting increased the oxidation of chylomicron fatty acid esters by livers labeled in vivo and by perfusion. In livers removed from intact rats given labeled chylomicrons, the triglyceride-(14)C to phospholipid-(14)C ratio was high, a finding unexpected if the liver had acquired this (14)C by removal of circulating fatty acids formed by extrahepatic lipolysis. These results demonstrate the ability of the liver to remove and utilize chylomicrons directly and suggest that direct removal accounts for a significant portion of the chylomicron fatty acids utilized by the liver of intact rats.     

Microvascular albumin permeability in isolated perfused lung: effects of EDTA

We examined the effects of decreases in perfusate concentrations of calcium and magnesium on the pulmonary vascular permeability in the isolated perfused rabbit lung. The albumin permeability-surface area product (PS) and the albumin reflection coefficient (sigma) were determined in the same lung using 125I- and 131I-labeled albumin tracers. Decreases in vascular Ca2+ and Mg2+ concentrations were induced by adding ethylenediaminetetraacetic acid (EDTA) to the perfusate. Decreases in the concentration of these cations resulted in an increase in the PS from a control value of 1.18 +/- 0.13 X 10(-3) to 7.69 +/- 0.75 X 10(-3) cm3 X min-1 X g wet lung wt-1 and a decrease in the sigma from 0.96 +/- 0.01 to 0.74 +/- 0.02. The decrease in sigma suggests an increase in the calculated equivalent pore radius from 44 to 63 A. The results indicate that Ca2+ and Mg2+ play a role in the maintenance of normal pulmonary vascular permeability to proteins.     

Structural changes of nucleosomal particles and isolated core-histone octamers induced by chemical modification of lysine residues

Treatment of nucleosomal particles and isolated core-histone octamers with dimethylmaleic anhydride, but not with acetic anhydride, is accompanied by a biphasic release of the two H2A.H2B dimers, the first dimer being more easily released than the second. With both kinds of particles, 50% of histones H2A and H2B are released for modification of approximately 35% of the histone amino groups. The similar behavior of nucleosomal particles and isolated core-histone octamers is consistent with the same structure of the histone octamer in the nucleosomal particle and in the free octamer in 2 M NaCl. The described release of H2A.H2B dimers allows the preparation of nucleosomal particles deficient in one H2A.H2B dimer and of the histone hexamers H2A.H2B.(H3.H4)2. For more extensive modifications, both reagents, acetic and dimethylmaleic anhydrides, cause the dissociation of nucleosomal particles with liberation of double-stranded DNA, which suggests that lysine amino groups are involved in the binding of histones to DNA. The modified nucleosomal particles are more sensitive to ionic strength than those untreated, and the presence of salt (NaCl) increases the extent of DNA release. The histones corresponding to the liberated DNA, except H2A and H2B released with dimethylmaleic anhydride, are apparently bound to the DNA-containing particles as extra histones.     

Comparison of the metabolic effects of chylomicrons and their remnants on isolated hepatocytes

A comparison was made between the effects of chylomicrons and chylomicron remnants on metabolic processes of isolated hepatocytes. Since isolated triacylglycerol-rich lipoproteins are contaminated with nonesterified fatty acids, control incubations were conducted with an amount of fatty acid equivalent to the contaminating fatty acids present in the chylomicrons and the remnant preparations, respectively. Chylomicron remnants, produced in vitro by incubation of chylomicrons in postheparin rat plasma, caused marked inhibition of glycolysis, fatty acid synthesis, and cholesterol synthesis, along with marked stimulation of ketogenesis. These effects were traced to the release of nonesterified fatty acids from these remnant particles as a consequence of contamination with lipoprotein lipase, picked up by the particles during the incubation with rat plasma. Fatty acids inhibit glycolysis, cholesterol, and fatty acid synthesis, but enhance ketone body formation by isolated hepatocytes. Chylomicrons and remnants prepared in vivo by the injection of chylomicrons into functionally hepatectomized rats were not contaminated with lipoprotein lipase and did not inhibit glycolysis and cholesterol synthesis nor increase ketone body formation. These lipoprotein particles did, however, cause significant inhibition of fatty acid synthesis, with the chylomicrons being more effective on a protein basis than the remnants produced in vivo. The mechanism responsible for the inhibition of fatty acid synthesis by chylomicrons and remnants prepared in vivo remains to be resolved.     

Physical and chemical cues affect oviposition by Neoleucinodes elegantalis

Recognition and acceptance of a suitable host plant by phytophagous insects requires the integration of visual, physical and chemical cues. The present study investigates the host cues that a specialist insect integrates to optimize oviposition decisions and whether these cues are weighted in a specific way. The study also determines whether the tomato fruit borer Neoleucinodes elegantalis (Guenée) (Lepidoptera: Crambidae), an important pest on Solanaceae in Brazil, shows a preference for oviposition sites that differ in physical and/or chemical cues. When styrofoam balls are provided as artificial fruits, N. elegantalis deposit significantly more eggs on rough artificial fruits than on smooth ones. Hexane fruit extracts applied to the artificial fruits stimulate female oviposition strongly. Physical and chemical cues also affect the oviposition of females when offered together. Furthermore, certain parts of the artificial fruits are prefered, irrespective of the presence of chemical cues. Both physical and chemical cues affect oviposition decisions; hence, the fruit borer relies on cues of different sensory modalities.     

Physical, chemical and functional changes following platelet activation in normal and "giant" platelets

Unactivated discocytes in healthy human donors have mean volumes of approximately 6.0 microns3 (range 3.8-7.5 microns3), while mean values for similarly-shaped discocytes obtained from donors with the hereditary "giant" platelet syndromes were either normal (one Bernard-Soulier syndrome (BSS) and all five members of a family with the Montreal platelet syndrome (MPS) or, on average, up to twice normal (range 6.4-13.8 microns3). This apparent heterogeneity is complicated by the much more consistent and significant observation that both BSS and MPS platelets undergo a defective hypervolumetric shape change following activation which is prolonged indefinitely, in contrast to a transient hypervolumetric change measureable in 1-5 s following ADP addition to normal platelets. It is suggested that the hypervolumetric shape change in both normal and "giant" platelets is accompanied by an increase in externalized plasma membrane surface area, with the most probable source being surface-connected canalicular system. Membrane glycoprotein I abnormalities were not detectable in platelets for 2/3 sibling MPS donors. The precise relation of these membrane changes to altered platelet functions is compared for normal and "giant" platelets, but largely remains to be experimentally determined. Early shape change appears tightly associated with early microscopically-measured aggregation (PA), with both PA and turbidimetrically-measured macroaggregation generally appearing normal to elevated for "giant" platelets.     

Energy state of hepatocytes of fed rats isolated by the means of EDTA and vibration

The energy state of mitochondria in fed rat hepatocytes isolated by the use of non-enzymatic method including liver perfusion with an EDTA-containing solution with a further mild mechanical effect of tissue fragments by vibration has been studied. The isolation procedure used permits to obtain significant amounts of hepatocytes whose viability is not less than 80%. The endogenous respiration rate (10-15 nm O2/min.mln cells) is slightly stimulated by succinate. In the course of incubation in a balanced salt medium, the cells accumulate ATP and the lipophilic cation, tetraphenylphosphonium. Data from the inhibitory analysis testify to the fact that tetraphenylphosphonium accumulation reflects the membrane potential of intact cell mitochondria, which are in a metabolic state similar to state 3.     

Physical and chemical scavenging of singlet molecular oxygen by tocopherols

Singlet molecular oxygen (1O2) arising from the thermal decomposition of the endoperoxide of 3,3'-(1,4-naphthylidene) dipropionate was used to assess the effectiveness of alpha-, beta-, gamma-, and delta-tocopherol in the physical quenching as well as the chemical reaction of 1O2. The relative physical quenching efficiencies of the tocopherol homologs were found to decrease in the order of alpha greater than or equal to beta greater than gamma greater than delta-tocopherol. The ability of physical quenching depends on a free hydroxyl group in position 6 of the chromane ring. Chemical reactivity of the tocopherol homologs with 1O2 was low, accounting for 0.1-1.5% of physical quenching with beta-tocopherol showing particularly low reactivity, resulting in the sequence alpha greater than gamma greater than delta greater than beta-tocopherol. Tocopheryl quinones were products of all tocopherol homologs, and in addition a quinone epoxide was a major product from gamma-tocopherol. This quinone epoxide was not cleaved by rat liver microsomal epoxide hydrolase; however, it reacted further with 1O2. It is concluded that methylation in position 5 of the chromane ring enhances physical quenching of 1O2, whereas chemical reactivity is favored by a methylated position 7. In view of the fact that beta-tocopherol is as effective as alpha-tocopherol in physical quenching of 1O2 but shows very low chemical reactivity, this tocopherol homolog might be particularly suitable for biological conditions in which an accumulation of oxidation products might weaken the antioxidant defense.     

Calcium and pH-induced structural changes in skinned muscle fibers: prevention by N-ethylmaleimide.

Optical diffractometry was used to investigate structural changes in contractile proteins from chemically skinned muscle fibers. The intensity of the first order diffraction line decreased in response to calcium and alkaline pH. Fibers pre-treated with sulfhydryl blocking agents did not exhibit the calcium- or pH- induced intensity decreases. We suggest that the decreases in intensity result from structural changes in the thick filament. Optical diffractometry may be a useful technique for the investigation of conformational changes in myosin.     

Competition between chylomicrons and their remnants for plasma removal: a study with artificial emulsion models of chylomicrons

In previous studies, protein-free emulsions of defined lipid composition were shown capable of simulating either the metabolism of chylomicrons (chylomicron-like emulsion) or their remnants (remnant-like emulsion), depending on the content of free, unesterified cholesterol. To validate further the assumption that remnant-like and chylomicron-like emulsion have metabolic pathways in common with their natural counterparts, studies of competition for plasma removal were undertaken: the remnant-like emulsion labeled with [3H]triolein was injected sequentially twice in the carotid arteries of rats to compare the clearance of remnant-like emulsion of the second injection with the first (control). Prior to the second injection, a large bolus of the chylomicron-like emulsion or rat lymph chylomicron was injected, to check the hypothesis that remnant generated from chylomicron-like emulsion or natural chylomicrons could compete with and displace remnant-like emulsion particles from their tissue receptor sites. Experiments were also performed in rats treated with Triton WR-1339, to block the generation of remnants. Results showed that remnants derived from either natural chylomicrons or chylomicron-like emulsion both strongly competed with the remnant-like emulsion. In contrast, when transformation of remnants was prevented by Triton, the undegraded particles of chylomicron-like emulsion or natural chylomicron were unable to compete with or displace remnant-like emulsion from its sites of removal from the plasma. In agreement with plasma clearance data, the hepatic uptake of the remnant-like emulsion was inhibited by the surplus dose of natural chylomicrons. In contrast, the spleen uptake was unaffected by it.     

Electrical changes in isolated rat jejunum induced by hypertonicity

Mucosal hypertonicity produces a drop of transepithelial potential difference in isolated rat jejunum with a half time of about 15 s. The same effect is obtained when the osmorality of both bathing solutions is raised simultaneously. Serosal hypertonicity produces an increase of transepithelial potential difference an order of magnitude lower than the drop produced by mucosal hypertonicity. The change in the short circuit current parallels the one in potential difference.When the transepithelial potential difference is varied by adding different concentrations of glucose to the bathing media, the potential drop induced by mucosal hypertonicity is linearly related to the magnitude of the transepithelial potential difference just before the increase in osmolarity.The drop of potential can be explained by a decrease of the electrical resistance of the extracellular shunt pathway due to opening of the tight junctions. The results can be accounted for in terms of an equivalent electrical circuit proposed for small intestine. Using this equivalent circuit model it is possible to obtain estimates of the values of the diffusion potential and the salt gradient across the tight junction.