Zigzagging: Theoretical insights on climbing strategies

Human and animal trails on steep hillsides often exhibit dramatic switchbacks and shortcuts. Helbing et al. have recently examined the emergence of human trail systems on flat terrains while Minetti and Margaria established the effect of gradients on human metabolic efficiency. In this paper we use these ideas to develop a semi-quantitative theoretical model of the behaviour of humans moving on a terrain with relief. The model determines the direction of movement by minimising metabolic cost per unit of distance in a desired direction. The structure of the theory resembles the Landau Theory of Phase Transitions, much used in theoretical physics. We find that both hairpin bends (switchbacks) and shortcuts appear as efficient strategies for downhill walkers, while uphill walkers retain switchbacks. For weakly inclined slopes, the best strategy involves walking directly uphill or downhill. For sufficiently steep slopes, however, we find that the best strategy should undergo a transition to a broken symmetry solution corresponding to the switchback trail patterns typical of rugged environments. The critical slope at which this transition takes place should be less steep for uphill and downhill walkers. The theory should be amenable to empirical investigation. Amongst other applications, this model will enable us to generalize the work of previous authors to real landscapes, eventually permitting the reconstruction of ancient patterns of movement in archaeological landscapes.

Anticoagulants and other preanalytical factors interfere in plasma nitrate/nitrite quantification by the Griess method

Nitric oxide (NO) is a signal molecule with functions such as neurotransmission, local vascular relaxation, and anti-inflammation in many physiological and pathological processes. Various factors regulate its intracellular lifetime. Due to its high reactivity in biological systems, it is transformed in the bloodstream into nitrates (NO(-)(3)) by oxyhemoglobin. The Griess reaction is a technically simple method (spectrophotometric, 540 nm) for the analysis of nitrites (NO(-)(2)) in aqueous solutions. We studied the interference of common anticoagulants in the quantification of nitrate and nitrite in plasma samples by the Griess method. We obtained rat plasma using heparin or sodium EDTA as anticoagulants, then added, or otherwise, known NO(-)(3) amounts in order to calculate their recovery. We also studied the effect of ultra-filtration performed before Griess reaction on plasma and aqueous solutions of various anticoagulants (heparin, EDTA, and also sodium citrate) to compare the recoveries of added NO(-)(3) or NO(-)(2). We used standards of NO(-)(3) or NO(-)(2) for quantification. We conclude that: (i) The bacterial nitrate reductase used to reduce NO(-)(3) to NO(-)(2) is unstable in certain storage conditions and interferes with different volumes of plasma used. (ii) The ultrafiltration (which is sometimes performed before the Griess reaction) of plasma obtained with EDTA or citrate is not recommended because it leads to overestimation of NO(minus sign)(3). In contrast, ultrafiltration is necessary when heparin is used. (iii) The absorbance at 540 nm attributed to plasma itself (basal value or background) interferes in final quantification, especially when ultrafiltration is not performed. For the quantification of plasma NO(-)(3) we recommend: sodium EDTA as anticoagulant, no ultrafiltration of plasma, and measurement of the absorbance background of each sample.

Secretion of hepatic lipase by perfused liver and isolated hepatocytes

Hepatic lipase is found in liver and in adrenal glands and ovaries. Because in adult rats, neither adrenals nor ovaries synthesize this enzyme, it is assumed that the liver is the origin of their hepatic lipase. Our aim was to study the secretion of hepatic lipase by the liver. We observed that plasma of both fed and fasted rats contained hepatic lipase activity. This activity was significantly correlated with that in the liver. Isolated livers, perfused with heparin-free medium, secreted fully active hepatic lipase to the perfusate. The addition of heparin resulted in a rapid and larger release of hepatic lipase to the perfusate. In isolated hepatocytes, heparin did not affect the secretion of hepatic lipase mass, although it increased the stability of the enzyme activity. To study the degradation of hepatic lipase by hepatocytes, protein synthesis was blocked with cycloheximide, and both secreted and intracellular hepatic lipases were analyzed by Western blotting. We observed that the amount of hepatic lipase secreted equaled the decrease of intracellular mass. The total mass of the enzyme (inside and outside the cells) remained constant, at least for 90 min. In the next experiment, 0.7 nM 125I-hepatic lipase was added to hepatocyte suspensions, and the appearance of trichloracetic acid-soluble products was analyzed. Only 12% of the radioactivity added was associated with the cells after 90 min of incubation, and less than 2% of the hepatic lipase added was degraded. Although the association was decreased in the presence of heparin, the amount of 125I-hepatic lipase degraded was not affected. Taking all these results into account, we propose a model for the continuous secretion of hepatic lipase by the liver.

Effect of fasting on hepatic lipase activity in the liver of developing rats

The effect of fasting on hepatic lipase was studied during postnatal development in the rat. It was found that fasting produced a significant decrease in hepatic lipase only in neonatal (1-day-old) and adult (60-day-old) rats. We studied the effect of fasting on the distribution of hepatic lipase between extracellular (heparin-releasable) and intracellular (liver-retained or residual) compartments in perfused livers, and on the secretion of hepatic lipase by isolated hepatocytes. Fasting had similar effects in neonates and adults: it decreased both the heparin-releasing and the residual activities in perfused livers, and also decreased the rate of hepatic lipase secretion by isolated hepatocytes. Finally, the effect of fasting on hepatic lipase mRNA relative abundance in developing rat livers was determined. No difference was observed among the groups studied. It is concluded that the mechanisms involved in the effect of fasting on hepatic lipase appear to be similar in neonates and adult animals and may involve the post-translational processing of the enzyme.

Effects of growth and differentiation factors on the epithelial-mesenchymal transition in cultured neonatal rat hepatocytes

BACKGROUND/AIMS

Loss of specific differentiation markers, adoption of a migrating morphology and progressive replacement of the cytokeratin network by vimentin intermediate filaments characterize the epithelial-mesenchymal transition of cultured neonatal rat hepatocytes. In a previous study (Hepatology 1997; 25: 598-606), we reported that this process can be differentially regulated by EGF and DMSO, two agents that affect hepatocyte growth and differentiation. The aim of the present study was to determine if growth activation or differential gene expression could explain the differences in EMT observed between these two factors.

METHODS

We compared the effects of EGF, HGF, TGF-beta1 and DMSO on growth, proto-oncogene expression, epithelial-mesenchymal transition markers and expression of liver transcription factors in cultured neonatal rat hepatocytes using thymidine incorporation, Northern blotting and Western blotting analysis.

RESULTS

When TGF-beta1 or DMSO was added to the cultures supplemented with EGF and HGF, the mitogenic activity induced by these factors was inhibited. DMSO down-regulated c-myc and c-fos expression. mRNA levels of some liver-specific genes such as albumin, or liver-enriched transcription factors such as C/EBPdelta, HNF-4 and HNF-1beta were slightly different in cultures supplemented with DMSO or TGF-beta1. However, no differences were found when DMSO or TGF-beta1 was added to the cultures supplemented with EGF. Western blotting analysis showed that TGF-beta1 decreased cytokeratin and increased vimentin levels, while DMSO decreased both cytokeratin and vimentin. When DMSO or TGF-beta1 was added in combination with EGF or HGF, both factors maintained the increase in albumin and cytokeratin induced by the growth factors although DMSO, but not TGF-beta1, inhibited vimentin expression.

CONCLUSIONS

Activation of vimentin expression produced in cultures supplemented with the mitogenic factors (EGF and HGF) is independent of the activation of cell growth, because DMSO but not TGF-beta1 can abolish vimentin synthesis, although both inhibited growth. Moreover, the vimentin expression in these cultures seems to be independent of the mRNA levels of transcription factors associated with the differentiated liver phenotype.

Acquired lipoprotein lipase deficiency associated with chronic urticaria. A new etiology for type I hyperlipoproteinemia

Type I hyperlipoproteinemia (type I HLP) is a rare disorder of lipid metabolism characterized by fasting chylomicronemia and reduced postheparin plasma lipoprotein lipase (LPL) activity. Most cases of type I HLP are due to genetic defects in the LPL gene or in its activator, the apolipoprotein CII gene. Several cases of acquired type I HLP have also been described in the course of autoimmune diseases due to the presence of circulating inhibitors of LPL. Here we report a case of type I HLP due to a transient defect of LPL activity during puberty associated with chronic idiopathic urticaria (CIU). The absence of any circulating LPL inhibitor in plasma during the disease was demonstrated. The LPL genotype showed that the patient was heterozygous for the D9N variant. This mutation, previously described, can explain only minor defects in the LPL activity. The presence of HLP just after the onset of CIU, and the elevation of the LPL activity with remission of the HLP when the patient recovered from CIU, indicate that type I HLP was caused by CIU. In summary, we report a new etiology for type I HLP - a transient decrease in LPL activity associated with CIU and with absence of circulating inhibitors. This is the first description of this association, which suggests a new mechanism for type I HLP.

Partial hepatectomy and/or surgical stress provoke changes in the expression of lipoprotein lipase and actin in liver and extrahepatic tissues

The expression of lipoprotein lipase (LPL) and actin genes was examined in heart, muscle and white adipose tissue (WAT) and the expression of albumin and actin genes was examined in regenerating liver after 2/3 hepatectomy. Both surgical stress and partial hepatectomy (PH) affected LPL and actin mRNA levels in muscle and WAT, but not in heart. The changes in LPL mRNA suggest transcriptional regulation of the enzyme during hepatic regeneration. Our results show for the first time that the LPL gene expression in the different tissues studied is altered not only by the surgical stress, but also by PH per se. Actin expression is also affected in some tissues. In liver, PH and surgical stress altered the expression of albumin and total mRNA. The total mRNA of the other tissues studied did not change. The changes observed in LPL in different tissues, especially in WAT and muscle, may be responsible for some of the changes in lipidic metabolism, thus allowing for some plasma lipoproteins to be used as substrates by the LPL activity that arises in the liver during hepatic regeneration. The fatty acids derived from these lipoproteins would constitute not only an energy source but also the building material needed in the process of restoration of the lost hepatic mass. It is suggested that hormonal changes taking place after surgery are responsible for the variation in the levels of the different mRNAs studied.

Circadian rhythms of lipoprotein lipase and hepatic lipase activities in intermediate metabolism of adult rat

Although intermediate metabolism is known to follow circadian rhythms, little information is available on the variations in lipoprotein lipase (LPL) and hepatic lipase (HL) activities during the 24-h period, and there is also a lack of adequate statistical analysis. Here, adult male rats were fed ad libitum and kept at 21 degrees C under 12:12-h light-dark cycles. They were killed in batches every 3 h over a 24-h period. Lipase activities were determined in plasma and fresh homogenates of epididymal white adipose tissue (EWAT), interscapular brown adipose tissue (IBAT), heart, skeletal muscle, and liver. Plasma insulin, corticosterone, glucose, triacylglycerol (TAG), cholesterol, glycerol, beta-hydroxybutyrate, and liver and muscle glycogen were determined. Cosinor analysis was used to evaluate the presence (significance of fit of cosine curve to data and variance explained by rhythm) and characteristics of possible circadian rhythms [acrophase (phi), mesor, and amplitude]. Statistically significant circadian rhythms were detected for 1) all metabolites studied, except TAG, cholesterol, and liver HL activity; 2) LPL and HL activity in plasma (both phi in light phase); and 3) LPL activity in all tissues studied (phi: heart in light phase; skeletal muscle, IBAT, and EWAT in dark phase). Liver also showed a circadian rhythm of LPL activity, with phi near that in plasma. These findings demonstrate for the first time that, in physiological conditions, LPL activities in plasma and various tissues, including liver, and HL activity in plasma follow circadian rhythms. Their metabolic significance is discussed.

Growth and differentiation factors inhibit the migratory phenotype of cultured neonatal rat hepatocytes induced by HGF/SF

Previous studies have demonstrated that neonatal rat hepatocytes cultured in a serum-free medium adopt a fibroblast-like morphology and form a well-developed vimentin network. By immunoblotting, phase-contrast microscopy, and immunolocalization studies we report here that hepatocyte growth factor (HGF) induces a pronounced migratory/scatter phenotype in neonatal hepatocytes in primary culture. These fibroblast-like cells are highly elongated and adopt a pronounced spindle shape in the presence of this growth factor. Most of them coexpress vimentin and cytokeratin intermediate filaments. Both EGF and TGF-beta1 also induced vimentin expression in cytokeratin-positive cells but this effect was not correlated with a change of epithelial phenotype. Only in DMSO-supplemented cultures was vimentin expression of hepatocytes inhibited and no coexpression was observed in the presence of this factor. Remarkably, the effect induced by HGF was totally inhibited when DMSO, TGF-beta1, or EGF was added to HGF-supplemented cultures. Epithelial sheets of well-defined hepatocytes were observed with these combinations although the complete epithelial morphology was achieved by combining DMSO with a growth factor. Taken together these results suggest that growth and differentiation factors modulate the migratory effect of HGF/SF in hepatocytes.

Epithelial-mesenchymal transition of cultured rat neonatal hepatocytes is differentially regulated in response to epidermal growth factor and dimethyl sulfoxide

Neonatal rat hepatocytes cultured in the absence of added growth factors dedifferentiate by epithelial-mesenchymal transition (EMT). This involves the loss of their typical differentiation markers, the acquisition of a migrating morphology, and a change in the expression of the intermediate filament (IF) proteins. We attempted to determine whether the EMT of cultured neonatal rat hepatocytes could be modulated by factors that maintain and promote the differentiation state of adult and fetal hepatocytes such as epidermal growth factor (EGF) and dimethyl sulfoxide (DMSO). By (3H)-thymidine incorporation, Western blotting analysis, flow cytometry, and double-immunofluorescence studies, we found that both factors have marked but opposite effects on the EMT and on proliferation of neonatal liver cells. In DMSO treatment, albumin levels were higher than in the nontreated cells at all days studied. Moreover, DMSO reduced cytokeratin levels and inhibited cell proliferation, acquisition of the fibroblast-like morphology, and vimentin expression typical of the EMT. The increase in vimentin-positive cells in serum-free medium was not observed in DMSO cultures. EGF also increased albumin levels at all days studied. In contrast, EGF treatment induced hepatocyte proliferation and enhanced vimentin and cytokeratin expression. However, the increase in vimentin levels did not correlate with a significant increase in the number of vimentin-positive cells. Moreover, vimentin-positive cells in EGF treatment were also cytokeratin-positive and albumin-positive, and they maintained epithelioid morphology in spite of the vimentin network. These results indicate that EMT of cultured rat neonatal hepatocytes is differentially regulated in response to EGF and DMSO.

Hepatic regeneration induces changes in lipoprotein lipase activity in several tissues and its re-expression in the liver

We examined the expression of lipoprotein lipase (LPL) gene and LPL activity following a two-thirds hepatectomy and during liver regeneration. In most of the tissues studied, LPL activity increased a few hours after partial hepatectomy, but soon returned to normal levels. The greatest increase was found in the adrenal glands, plasma and liver. This increase in LPL activity in the liver could be partially due to an increase in the influx of the enzyme from extrahepatic tissues. There is, however, also a re-expression of LPL mRNA in the liver after partial hepatectomy (during the first hours). It is well known that LPL is expressed in the liver of neonatal animals, but progressively decreases during post-natal development, to reach adult levels around the time of weaning. Our results show by the first time that the remaining liver re-expresses LPL gene during the regeneration process and that the hepatocytes de-differentiate and acquire some of the neonatal characteristics. The increase in LPL mRNA will contribute to the rise in LPL activity after hepatectomy. This presence of LPL could enable the liver to take up fatty acids from the circulating triacylglycerols, which are needed as energetic and plastic substrates during the process of hepatic regeneration.

Decrease in the expression of hepatic lipase activity following partial hepatectomy

Variations in hepatic lipase (HL) activity were studied for the first time in liver, plasma and adrenal glands of partially hepatectomized (70%), sham-operated and intact rats. Activity profiles performed during 7 days in liver, plasma and adrenal glands of sham-operated rats were similar to those obtained in intact animals. However, HL activity in intact animals appeared to be slightly higher during the first 24 hours. Following surgery, hepatectomized animals showed a reduction of about 300 U in liver HL activity which persisted for 7 days. Plasma HL activity of hepatectomized rats was undetectable at 6 hours post-surgery but in increased afterwards. A high correlation between liver and adrenal gland HL activity was found in hepatectomized but not in sham-operated animals. HL mRNA levels in hepatectomized rats showed a 40% decrease during the first 24 hours after surgery, but they returned to the normal range later. On the other hand, HL mRNA values increased in sham-operated rats but no increase in HL activity was detected in these animals. To conclude, our results show that HL activity decreases dramatically during hepatic regeneration due to a concomitant decrement in the expression of the gene that encodes the enzyme and to other undetermined factors.

Vimentin filaments follow the preexisting cytokeratin network during epithelial-mesenchymal transition of cultured neonatal rat hepatocytes

Changes in cell cytoskeleton are known to play an important role in differentiation and embryogenesis and also in carcinogenesis. Previous studies indicated that neonatal hepatocytes undergo an epithelial-mesenchymal transition when cultured in a serum-free medium for several days. Here we show by Western blotting of neonatal rat liver cells cultured for 3 days that vimentin and cytokeratin were expressed by these cells. Epidermal growth factor treatment induced high coexpression of vimentin and cytokeratin filaments in hepatocytes from neonatal livers, as detected by double immunofluorescence microscopy. Confocal scanning laser microscopy was used to determine the spatial and cell distribution of cytokeratin and vimentin intermediate filament networks. Vimentin-expressing hepatocytes were mainly located on the periphery of epithelial clusters and presented a migratory morphology, suggesting that vimentin expression was related to the loss of cell-cell contact. Short vimentin filaments were mainly located at the cytoplasmic sites behind the extending lamella. Horizontal and vertical dual imaging of double immunofluorescence with anti-vimentin and anti-cytokeratin antibodies indicated that both filaments colocalize strongly. Three-dimensional reconstruction of serial optical sections revealed that newly synthesized vimentin distributed following the preexisting cytokeratin network and, when present, both filament scaffolds codistributed inside cultured hepatocytes. Immunoelectron microscopy performed in whole-mount-extracted cultured cells revealed that both filaments are closely interrelated but independent. However, a high degree of immunogold colocalization was found in the knots of the filament network. Further experiments with colcemide and cytochalasin treatment indicated that vimentin filament distribution, but not cytokeratin, was dependent on an intact microtubule network. These results are consistent with a mechanism of vimentin assembly, whereby growth of vimentin intermediate filaments is dependent on microtubules in topographically restricted cytoplasmic sites, in close relation to the cytokeratin cytoskeleton and to changes in cell-cell contact and cell shape.

Hormonal regulation of lipoprotein lipase activity from 5-day-old rat hepatocytes

Lipoprotein lipase (LPL) activity is known to be synthesized, active and functional in the 1-day-old rat liver: it peaks just at birth triggered by parturition. During suckling LPL mRNA, LPL synthesis and LPL activity are still high at 5 days and then fade reaching adult values at weaning. How LPL expression is gradually extinguished is not known. Therefore we studied the effect of different doses of several hormones on LPL activity released by incubated hepatocytes from 5-day-old rats. In the presence of heparin the release of LPL activity in the medium was linear until 3 h and was always significantly increased vs. without heparin. At 3 h in the presence of heparin the main hormonal effects were: dose-dependent increase (30-60%) with dexamethasone; dose-dependent increase (20-60%) with glucagon; dose-independent decrease (50-60%) with ethinylestradiol, testosterone, progesterone and prolactin; no effect with insulin; 20-40% increase with adrenaline < 1 mM but 40-50% decrease with noradrenaline < 10 microM. Increase of LPL release by glucagon and adrenaline agrees with the increased LPL expression we previously found in an undifferentiated hepatoma cell line when the adenylate cyclase/protein kinase A pathway was activated. The effect of glucagon is concordant with our previous observations that fasting increases liver LPL activity in neonatal rats. The high estradiol levels known to be present in male and female 9-19-day-old rats might contribute to liver LPL extinction during suckling.

Epithelial-mesenchymal transition in cultured neonatal hepatocytes

When hepatocyte-enriched fractions from neonatal rat livers were cultured for different times in the absence of added growth factors, a population of highly proliferating and migrating fibroblastlike cells appeared. Double immunofluorescence with antibodies to cytokeratin and to vimentin showed a progressive reduction in the number of cytokeratin-positive cells parallel to an increase in the vimentin-positive cells. Some cells with transitional epithelial or migrating morphology coexpressed both intermediate filament proteins. Immunofluorescence with antibodies against hepatocyte differentiation markers showed that shortly after seeding most of the cells were positive to anti-albumin antibodies, but after 1 week in culture, only 10% were positive. Cells presenting albumin and cytokeratin appeared morphologically epithelial. Fibroblastlike cells were not positive for albumin, but some cells with transitional epithelial morphology presented some labels for albumin and for vimentin. Immunofluorescence with antibodies to glutathione-S-transferase subunit Pi and vimentin showed that many fibroblastlike cells were positive for both markers, some of them binucleate. Cultures performed in the presence of dexamethasone, absence of arginine, or on collagen type I matrix had no effect on the behavior of neonatal hepatocytes. The appearance of fibroblastlike cells was ontogenically regulated because the highest increase in the percentage of vimentin-positive cells was observed in cell cultures from livers of 7- and 15-day-old animals. These data provide evidence that neonatal hepatocytes in culture have the potential to dedifferentiate by epithelial-mesenchymal transition and contribute to an understanding of hepatic growth development.

Lipoprotein lipase activity in developing rat brain areas

Lipoprotein lipase (LPL) is a key extracellular enzyme that enables tissue to import fatty acids from triacylglyceride-rich lipoproteins. LPL is present in most tissues of the body, but in the brain its functional significance remains unclear. Lipids constitute the main components of myelin and undergo significant changes during maturation. However, nothing is known of the postnatal evolution of LPL activity in the brain areas during postnatal development. Here we found that LPL activity is relatively high in the newborn brain and peaks between the 5th and the 10th days after birth, reaching activities 5 times higher than in the adult brain. In all the areas studied (olfactory bulbs, cortex, thalamus, cerebellum, hippocampus, striatum, brain-stem and spinal cord) LPL also increases sharply during postnatal development. Hippocampus shows the highest LPL activity levels, which are between 5 and 11 times higher than in the other regions. The significance of these high LPL activity levels is discussed.

Lipoprotein lipase and hepatic lipase in Wistar and Sprague-Dawley rat tissues. Differences in the effects of gender and fasting

To evaluate the effects of strain, gender and fasting in the regulation of lipoprotein lipase (LPL) and hepatic lipase (HL) activities were measured in tissues of male and female Wistar and Sprague-Dawley rats after feeding or a 24-h starvation period. It is noteworthy that an effect of gender on LPL activity was observed in Wistar, but not in Sprague-Dawley rats, not only in the basal (fed) activity in several tissues, such as white and brown adipose tissues, heart, and brain, but also in response to fasting which affected LPL activity in brown adipose tissue, heat and lung of female but not of male Wistar rats. By contrast, HL activity in liver, plasma and adrenals of Sprague-Dawley rats was higher in females than in males. No effect of gender on HL activity was observed in Wistar rats. Our results indicate that differences exist between Wistar and Sprague-Dawley rats in the regulation of both LPL and HL. Some of the contradictory results found in the literature may be explained by the differences between rat strains and gender, as well as differences in the nutritional status of the animals.

Coexisting subdural and intercostal haemangiomata

Haemangioma in the chest wall is a very rare entity, seldom described in the literature. We report the case of a 23 year old woman presenting with independent, cervical subdural and intercostal haemangiomata. She suffered several episodes of recurrent, self-limiting subdural haemorrhage before diagnosis was made by magnetic nuclear resonance, aortography and selective intercostal angiography. Surgical excision was performed in both locations with excellent results.

Coexisting subdural and intercostal haemangiomata

Haemangioma in the chest wall is a very rare entity, seldom described in the literature. We report the case of a 23 year old woman presenting with independent, cervical subdural and intercostal haemangiomata. She suffered several episodes of recurrent, self-limiting subdural haemorrhage before diagnosis was made by magnetic nuclear resonance, aortography and selective intercostal angiography. Surgical excision was performed in both locations with excellent results.

Epidermal growth factor interferes with the effect of adrenaline on glucose production and on hepatic lipase secretion in rat hepatocytes

We studied the interaction of epidermal growth factor (EGF) and adrenaline in the control of several metabolic functions in isolated hepatocytes from fed rats. EGF did not modulate glucose release, urea production or hepatic lipase secretion, but interfered with the stimulatory effect of adrenaline on both glucose and urea production and also with the inhibitory effect of this hormone on hepatic lipase secretion. EGF also interfered with the effect of both angiotensin II and vasopressin on glucose release and on hepatic lipase secretion. While the effect of EGF interfering with the action of adrenaline on glucose release was potentiated in the absence of extracellular calcium, the effect on the inhibition of hepatic lipase secretion was abolished. These results suggest that EGF interfered with catecholamine actions in the liver at a site distal from the generation of the calcium signal.

Lipoprotein lipase in developing rat tissues: differences between Wistar and Sprague-Dawley rats

Lipoprotein lipase in animal tissues is known to be affected by fasting, but contradictory results have been published concerning this effect in particular tissues. For example, we reported that lipoprotein lipase activity expressed in the liver of neonatal rats was either increased or not affected by fasting. To evaluate the influence of the rat strain used as experimental animal model, we studied differences between Wistar and Sprague-Dawley rats in the development and in the effect of fasting on lipoprotein lipase and hepatic lipase activities in tissues of neonatal rats. Beside some minor differences in the development of lipoprotein lipase in some tissues like brown adipose tissue and lungs, we found quite remarkable differences between both strains in the development of lipoprotein lipase and hepatic lipase activities in the liver. In 1-day-old neonates, differences between both strains were also observed in the effect of fasting on lipoprotein lipase activity both in liver and lungs. In the liver of Wistar pups lipoprotein lipase activity was increased by fasting by 350%, but only by 50% in the liver of Sprague-Dawley pups. In contrast, in the lungs of Wistar pups lipoprotein lipase activity was increased by fasting by 280%, but by 580% in lungs of Sprague-Dawley rats. Therefore, our results indicate that quantitative differences exist between Wistar and Sprague-Dawley rats in the regulation of lipoprotein lipase.

Neonatal extinction of liver lipoprotein lipase expression

In contrast to the complete absence of lipoprotein lipase (LPL) mRNA in adult rat liver, fetal and neonatal rat liver contain substantial amounts of LPL mRNA, which is translated in active LPL protein as can be deduced from the presence of LPL activity in this organ. At this neonatal stage, both the relative abundance of LPL mRNA and LPL activity increased with starvation. During the suckling period, LPL mRNA and LPL activity gradually decreased until both parameters were undetectable. While the administration of L-thyroxine or hydrocortisone enhanced the disappearance of LPL mRNA, induced hypothyroidism delayed its disappearance. In adult animals induced hypothyroidism could not reactivate LPL mRNA production in the liver. The data presented suggest that liver LPL production responds to changes in the nutritional state and becomes extinguished during development, in a fashion reminiscent to the extinction of alpha-fetoprotein. This extinction of LPL gene expression is influenced by hormonal factors.

Lipoprotein lipase and hepatic lipase activities are differentially regulated in isolated hepatocytes from neonatal rats

Lipoprotein lipase and hepatic lipase are members of the lipase gene family sharing a high degree of homology in their amino acid sequences and genomic organization. We have recently shown that isolated hepatocytes from neonatal rats express both enzyme activities. We show here that both enzymes are, however, differentially regulated. Our main findings are: (i) fasting induced an increase of the lipoprotein lipase activity but a decrease of the hepatic lipase activity in whole liver, being in both cases the vascular (heparin-releasable) compartment responsible for these variations. (ii) In isolated hepatocytes, secretion of lipoprotein lipase activity was increased by adrenaline, dexamethasone and glucagon but was not affected by epidermal growth factor, insulin or triiodothyronine. On the contrary, secretion of hepatic lipase activity was decreased by adrenaline but was not affected by other hormones. (iii) The effect of adrenaline on lipoprotein lipase activity appeared to involve beta-adrenergic receptors, but stimulation of both beta- and alpha 1-receptors seemed to be required for the effect of this hormone on hepatic lipase activity. And (iv), increased secretion of lipoprotein lipase activity was only observed after 3 h of incubation with adrenaline and was blocked by cycloheximide. On the contrary, decreased secretion of hepatic lipase activity was already significant after 90 min of incubation and was not blocked by cycloheximide. We suggest that not only synthesis of both enzymes, but also the posttranslational processing, are under separate control in the neonatal rat liver.

Lipoprotein lipase in lungs, spleen, and liver: synthesis and distribution

Lipoprotein lipase (LPL, E C 3.1.1.34) is the enzyme responsible for hydrolysis of triacylglycerols in plasma lipoproteins, making the fatty acids available for use by subjacent tissues. LPL is functional at the surface of endothelial cells, but it is not clear which cells synthesize the enzyme and what its distribution within tissues and vessels is. In previous studies we reported that in the major LPL-producing tissues (muscles, adipose tissue, and mammary gland) the enzyme is made by the major cell types. In the present work we have studied in adult guinea pigs some tissues that present LPL activity but in lower amounts (lung, spleen, and liver). On cryosections of these tissues we have searched for specific cell expression of the LPL gene (by in situ hybridization using a RNA probe) and for the corresponding protein distribution (by immunocytochemistry). Based on morphological criteria we can suggest that, contrary to the main LPL-producing tissues, in these tissues the enzyme is made by scattered cells, such as macrophages in the lung and spleen and Kupffer cells in the liver; endothelial cells present but do not synthesize the enzyme, indicating that the endothelial LPL originates in other cells. In the liver strong immunoreaction was detected in the sinusoid in contrast to the low level of mRNA expression, suggesting that liver takes up circulating LPL from blood.

Involvement of catecholamines in the effect of fasting on hepatic endothelial lipase activity in the rat

The effect of fasting on hepatic endothelial lipase activity in the liver of adult rats was investigated. We found that, both in male and female rats, fasting produced a progressive decrease of the hepatic endothelial lipase activity. Upon refeeding, the activity returned to control values in 48 h. In isolated livers from fed male rats, a sharp peak of hepatic endothelial lipase activity appeared in the perfusate upon heparin addition. It accounted for 75% of the total activity (heparin-released + residual) of the tissue. Fasting (24 h) decreased the heparin-releasable activity, and this effect was responsible for most of the decrease found in whole tissue. We suggest that the effect might be due to a decreased synthesis and/or secretion of the enzyme by hepatocytes, since isolated hepatocytes from fasted rats, incubated at 37 C, released 65% less activity to the incubation medium than hepatocytes from fed rats. Adrenaline, but not insulin, glucagon, dexamethasone, epidermal growth factor, or T3, decreased the amount of hepatic endothelial lipase activity released by hepatocytes isolated from fed rats. The effect of adrenaline appears to be mediated by alpha 1-receptors since phenylephrine but not isoprenaline reproduced, and prazosin but not propranolol blocked, the effect of the catecholamine. In the presence of cycloheximide, adrenaline also decreased the amount of activity released. We suggest that, in our incubation conditions (up to 3 h), the hormone affects the posttranslational processing of the enzyme. In vivo administration of prazosin blocked the effect of both noradrenaline and fasting on hepatic endothelial lipase activity in whole liver. Those results suggest that catecholamines are involved in the decreased hepatic endothelial lipase activity found in the liver of fasted rats, and points out the role of these hormones in the acute modulation of an enzyme involved in reverse cholesterol transport.

Lipoprotein lipase enables triacylglycerol hydrolysis by perfused newborn rat liver

Fasted 1-day-old rat liver has high heparin-releasable (endothelial) lipoprotein lipase (LPL) activity, and its hepatocytes synthesize LPL protein. To test the physiological role of this LPL, we perfused the isolated organ with a 0.8 mM triacylglycerol (TAG) (Intralipid + glycerol tri[3H]oleate) 6.3% serum medium. Samples of the recirculated perfusate were taken at different times to determine 3H in TAG, free fatty acid (FFA), and water-soluble (WS) fractions. In the medium [3H]TAG disappeared and [3H]FFA and [3H]WS fractions appeared linearly with time. This TAG hydrolysis was 1) absent when medium was recirculated without liver, 2) not affected by chloroquine addition, 3) inhibited by anti-LPL immunoglobulins, 4) absent when serum was omitted from the medium, and 5) restituted when apolipoprotein CII was added to the medium without serum. Therefore, lysosomal lipase is not involved in this TAG hydrolysis, the features of which are characteristic of LPL, not of the so-called "hepatic endothelial lipase." Thus LPL activity enables the neonatal rat liver to hydrolyze and take up circulating TAG, i.e., has the same function as extrahepatic LPL.

Lipoprotein lipase: cellular origin and functional distribution

Lipoprotein lipase (LPL, E.C. 3.3.1.34) is the enzyme responsible for hydrolysis of triacylglycerols in plasma lipoproteins, making the fatty acids available for use by subjacent tissues. LPL is functional at the surface of endothelial cells, but it is not clear which cells synthesize the enzyme and what its distribution is within tissues and vessels. We have searched for specific cell expression of the LPL gene by in situ hybridization using a RNA probe and for the corresponding protein distribution by immunocytochemistry on cryosections of some LPL-producing tissues of guinea pigs. In white and brown adipose tissues, heart and skeletal muscle, and lactating mammary gland, there was positive hybridization for LPL mRNA over all members of the major cell types, indicating that mature and immature adipocytes, muscle cells, and mammary epithelial cells are main sources of LPL. In large vessels, LPL expression was detected in some smooth muscle cells in the media layer. There was no positive hybridization for LPL mRNA over endothelial cells in any of the tissues studied, but there was immunoreaction for LPL protein at endothelial surfaces of all blood vessels. In the kidney, there was strong immunofluorescence at the vascular endothelium, particularly in the glomeruli, but little or no LPL mRNA was detected in the surrounding cells. These observations suggest that in some tissues LPL is synthesized by parenchymal cells and spreads along the vascular mesh. Transfer to the vascular endothelium is, however, not the only route taken by LPL. In the mammary gland most of the enzyme protein appeared to be secreted, partly in association with milk fat droplets.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of starvation on lipoprotein lipase activity in the liver of developing rats

Liver lipoprotein lipase activity in neonatal (1- and 5-day-old) rats was 2-3-times than in the liver of adult rats. In mid-suckling (15-day-old) or weaned (30-day-old) animals, it was not significantly different from the low activity detected in adult rats. Starvation resulted in a 3-fold increase of lipoprotein lipase activity in the neonatal liver, but did not affect the activity in the liver of mid-suckling, weaned or adult rats. When isolated livers from both 1- and 5-day-old pups were perfused with heparin, a sharp peak of lipoprotein lipase activity appeared in the perfusate. In fed neonates, the peak area accounted for about 70% of the total (released + non-releasable) activity. In starved neonates, the proportion of heparin-releasable activity increased up to about 90%. These results indicate that neonatal rat liver lipoprotein lipase activity is markedly affected by changes in the nutritional status of the animal, and the effect is restricted to the vascular pool of the enzyme, as was reported in extrahepatic tissues from adult rats.

Localization of lipoprotein lipase to discrete areas of the guinea pig brain

Lipoprotein lipase is a key enzyme in lipoprotein metabolism present primarily in extrahepatic tissues with high turnover of fatty acids. Using immunocytochemistry we have explored where lipoprotein lipase is localized in guinea pig brain. The enzyme was found to be associated with neuronal cells and vascular endothelial surfaces. The distribution was strikingly uneven with intense reaction in some areas, and virtually no reaction in adjacent areas. The highest reactivity was in neocortex, in hippocampus, in Purkinje cells of the cerebellum and in some motor nuclei of the brainstem. The results suggest marked differences between individual brain areas in utilization of plasma lipoproteins.

Lipoprotein lipase activity in the liver of starved pregnant rats

We have previously described the appearance of lipoprotein lipase (LPL)-like activity in the liver of 24-hour-starved 21-day pregnant rat, but it is not known up to what point the appearance of this activity depends on the gestation stage and/or the length of the starvation period. We found that 24 h of starvation resulted in the appearance of LPL-like activity in the liver of 21-day-pregnant but not in 17-day- or nonpregnant rats. This appearance was found only after 24 h but not after 48 or 72 h of starvation. We demonstrate that this activity actually corresponds to LPL, since it is inhibited by either 1.5 M NaCl or 1.5 mg/ml protamine sulfate, is serum-dependent, and could be separated from hepatic lipase activity by using heparin-Sepharose affinity chromatography. The possible relationship between the appearance of LPL activity in the liver and the enhanced metabolic response to starvation in pregnant rats at term is discussed. It is suggested that the presence of this enzyme in the liver would allow the direct uptake by the liver of circulating triacylglycerols.

Permanent abnormal response to a glucose load after prenatal ethanol exposure in rats

Postnatal development of the glucose and insulin balance in offspring of ethanol-treated and control rats has been studied. Newborn rats were separated from their mothers and placed with normal lactating, nonethanol-treated dams. Prenatal exposure to ethanol led to hypoglycemia on the first day of extrauterine life and a general tendency to hyperinsulinemia during the entire postnatal period studied. The glucose-tolerance test in weaned rats (30 days old) gave a greater and faster increase than controls in levels of both glucose and plasma insulin. At adult age (90 days) the response of blood glucose to an oral glucose load in offspring from ethanol-treated mothers was not different from that in offspring from controls, but the insulin response was higher. This abnormal insulin response, such a long time after the end of ethanol exposure, suggests either a permanent alteration in the pancreatic response, or a peripheral insulin resistance and/or differences in the rate of insulin degradation in these animals.

Hepatic endothelial lipase activity in neonatal rat liver

Hepatic endothelial lipase (HEL) activity is as high in the neonatal (1-day old) rat liver as in adults. Most of the HEL activity is located at the capillaries since 75% of the total activity is released by heparin or collagenase perfusion. The residual activity (non-releasable) is located in hepatocytes and not in hemopoietic cells, which are the major cell type in neonatal liver. Per mg of protein, the HEL activity is 50% higher in neonatal than in adult hepatocytes. We suggest that neonatal hepatocytes have an increased capacity to synthesize and secrete HEL activity, so maintaining a high activity in the whole organ. It might contribute to the hepatic uptake of cholesterol from circulating lipoproteins, in a period in which endogenous cholesterol synthesis is known to be inhibited in the liver.

Lipoprotein lipase activity in neonatal-rat liver cell types

The lipoprotein lipase activity in the liver of neonatal (1 day old) rats was about 3 times that in the liver of adult rats. Perfusion of the neonatal liver with collagenase decreased the tissue-associated activity by 77%. When neonatal-rat liver cells were dispersed, hepatocyte-enriched (fraction I) and haemopoietic-cell-enriched (fraction II) populations were obtained. The lipoprotein lipase activity in fraction I was 7 times that in fraction II. On the basis of those activities and the proportion of both cell types in either fraction, it was estimated that hepatocytes contained most, if not all, the lipoprotein lipase activity detected in collagenase-perfused neonatal-rat livers. From those calculations it was also concluded that haemopoietic cells did not contain lipoprotein lipase activity. When the hepatocyte-enriched cell population was incubated at 25 degrees C for up to 3 h, a slow but progressive release of enzyme activity to the incubation medium was found. However, the total activity (cells + medium) did not significantly change through the incubation period. Cycloheximide produced a time-dependent decrease in the cell-associated activity. Heparin increased the amount of lipoprotein lipase activity released to the medium. Because the cell-associated activity was unchanged, heparin also produced a time-dependent increase in the total activity. In those cells incubated with heparin, cycloheximide did not affect the initial release of lipoprotein lipase activity to the medium, but blocked further release. The cell-associated activity was also decreased by the presence of cycloheximide in those cells. It is concluded that neonatal-rat hepatocytes synthesize active lipoprotein lipase.

Low intestinal lactase activity in offspring from ethanol-treated mothers

Some aspects of small intestine maturation have been studied in the newborns from chronic ethanol-treated pregnant rats (25% ethanol in drinking fluid) immediately after birth (before suckling) and after 30 days of life. Litters delivered by mothers fed ad libitum with a standard diet diluted 50% with cellulose were used as a nutritional control. At birth, pups from ethanol-treated mothers showed significant decreases in total intestinal length and thickness, low total lactase activity and low somatostatin intestinal content. The intestinal alterations of these neonatal parameters are not present in newborns from mothers on fiber-diluted diet. From delivery, pups from different experimental groups were nursed by normal lactating dams. At 30 days of age neither of those parameters differed among the groups. We propose that the low levels of total lactase activity in newborns from alcoholic mothers, that are a consequence of a lower intestinal mucosa content, are a direct effect of ethanol in utero on the fetal gastrointestinal system.

Acid lipase activity in neonatal rat liver cell types. Effect of starvation

The acid lipase activity in the liver of neonatal (1-day-old) rats was studied. It was found that (i) in whole liver, the activity was 50% lower than in adult rats; (ii) in neonatal livers, the activity was 7.7-fold higher in hepatocytes than in hemopoietic cells; (iii) neonatal hepatocytes contained about 25% of the activity detected in adult hepatocytes; (iv) all the differences disappeared when expressed per mg of protein; and (v) starvation did not affect the activity either in adult or in neonatal rat liver.

Metabolic response to starvation at late gestation in chronically ethanol-treated and pair-fed undernourished rats

To study the role of undernourishment in the negative effects of ethanol during pregnancy and to determine whether maternal ethanol intake modifies metabolic response to starvation at late gestation, female rats receiving ethanol in their drinking water before and during pregnancy (ethanol group) were compared with animals that received the same amount of solid diet as the ethanol group rats (pair-fed group) and with normal rats fed ad libitum (control group). All animals were killed on the 21st day of gestation, either in the fed state or after 24-hours fasting. The body weight of ethanol rats was lower than that of controls but higher than that of pair-fed rats. When compared with controls, ethanol and pair-fed rats had reduced fetal body weights, whereas fetal body length was reduced only in the former. In the fed state, blood glucose concentration was lower in the ethanol and pair-fed rats and fetuses than in controls. Twenty-four-hour starvation caused a reduction in this parameter only in control and ethanol mothers. In the fed state, maternal liver glycogen concentration was lower in ethanol and higher in pair-fed mothers than in controls. Blood beta-hydroxybutyrate levels were higher in ethanol-treated mothers than in the others, and 24-hour starvation increased this parameter in ethanol and control rats to a greater extent than in the pair-fed ones. Liver triacylglyceride concentration was higher in ethanol-treated mothers than in the other two groups, and starvation caused this concentration to increase in ethanol and control groups but not in the pair-fed group.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake and metabolism of Intralipid by rat liver: an electron-microscopic study

The metabolism of Intralipid (intravenously injected) was studied in rats fasted for 48 h. At all doses used, the Intralipid triacylglycerols disappeared rapidly from circulation and concomitantly the hepatic content of triacylglycerols and the level of circulating ketone bodies increased, indicating an active metabolism of Intralipid by the liver. To study this possibility further we used an ultrastructural approach. In rats given Intralipid we detected numerous lipid particles in the spaces of Disse, retained in the interdigitations of the hepatocyte. There were also lipid particles attached to the luminal surface of the endothelial cells. Small lipid particles were seen in close contact with endocytic vesicles internalized into hepatocytes but were present mainly in endothelial cells. Inside the endothelial cells, the endocytic vesicles were detected in contact with lysosomes. Inside hepatocytes, a process of sterification seemed to occur in the endoplasmic reticulum as deduced from the presence of small lipid droplets with ill-defined outlines. Large lipid droplets were seen in close contact with mitochondria, indicating a mitochondrial uptake and metabolism of fatty acids to synthesize and release ketone bodies. The possible role of lipoprotein lipase in the liver for the hepatic uptake of Intralipid particles is discussed.

High liver lipoprotein lipase activity in hyperlipemic developing rats from undernourished pregnant mothers

To study the potential relationship between circulating triacylglycerol (TAG) levels and lipoprotein lipase (LPL) activity in the newborn rat liver, pups from undernourished or normal control mothers were nursed by normal dams, and studied at 0, 1, 15 or 30 days of age. Plasma TAG levels and liver TAG concentration increased more in pups from undernourished mothers than they did in controls. At birth, liver LPL activity was similarly high in both groups but, whereas in controls it decreased progressively after birth, in pups from undernourished mothers it remained stable until 15 days of age. Results suggest that the hypertriglyceridemia present in pups from undernourished mothers may be responsible for the sustained high LPL activity in their liver which may enhance the hepatic uptake of circulating TAG.

Lipoprotein lipase uptake by the liver: localization, turnover, and metabolic role

We have studied the binding and metabolism of 125I-labeled bovine lipoprotein lipase (LPL) by use of isolated, perfused rat livers. Our data suggest the presence of two types of binding sites, i.e., heparin-sensitive sites that bind primarily the catalytically active form of the lipase and are present at the endothelium in all blood vessels and heparin-insensitive sites that bind both active and inactive forms and are present only within the sinusoids. Forty minutes after uptake by the liver, approximately 50% of the LPL had lost its catalytic activity or been degraded. Three processes were evident: 1) colchicine-sensitive degradation to acid-soluble products, 2) partial proteolysis to fragments similar to those formed by limited digestion with trypsin or plasmin, and 3) a conformational change leading to loss of catalytic activity. Exogenous LPL bound in the liver caused a dramatic increase in the utilization of a perfused triacylglycerol emulsion (Intralipid), with rapid formation of free fatty acids and water-soluble metabolites. When the liver was flushed with heparin, it lost its ability to utilize the fat emulsion. Measurement of the hepatic extraction showed that rat livers take up 100-200 mU endogenous LPL per hour.

Comparative metabolic effects of chronic ethanol intake and undernutrition in pregnant rats and their fetuses

Female rats receiving ethanol in the drinking water before and during gestation (ET) were compared to pair-fed animals (PF) and normal controls (C) fed ad libitum. On the 21st day of gestation the maternal body and liver weight, blood glucose, and plasma protein concentrations were lower in ET and PF animals as compared to C. In contrast to C or PF mothers, ET-fed mothers had higher circulating beta-hydroxybutyrate and triacyglyceride levels and beta-hydroxy-butyrate/acetoacetate ratio. Liver triacylglycerides were increased whereas liver glycogen concentration was reduced in ET-fed animals. Only fetal body and liver weights and blood glucose were lower in both ET and PF than in C. Blood beta-hydroxybutyrate was increased and liver glycogen was decreased only in ET fetuses. There were no differences among the groups in fetal circulating beta-hydroxy-butyrate/acetoacetate ratio, plasma proteins, and triacylglycerides or liver triacyglyceride content. Results indicate that certain changes in ET mothers are specifically produced by the ethanol intake rather than undernutrition. Further, metabolic changes occurring in the fetus are influenced by the ethanol effects in the mother and these actions may be added to those directly produced by the ethanol crossing the placenta. However, the collaterals were three times more likely to report more drinking days than the patients; 40.4% (86/213) of the cohabiting contacts reported more drinking days compared to 12.7% (27/213) of the patients reporting more drinking days (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoprotein lipase in liver. Release by heparin and immunocytochemical localization

We have previously demonstrated that infusion of Intralipid to rats causes a pronounced increase of the lipoprotein lipase activity in the liver. In this paper we study where in the liver this lipoprotein lipase is located. When isolated livers from Intralipid-treated rats were perfused with heparin, substantial amounts of lipoprotein lipase were released into the perfusate. The identity of the lipase activity was demonstrated by specific inhibition with antisera to lipoprotein lipase, and to hepatic lipase, respectively, and by separation of the two lipase activities by chromatography on heparin-Sepharose. We have also studied the localization of both enzymes by an immunostaining procedure based on post-embedding incubation of ultrathin tissue sections with specific antibodies which were then visualized using protein A-colloidal gold complexes. There was no marked difference in localization for the two enzymes which were both seen at the luminal side of endothelial cells, at the interdigitations of the space of Disse and inside both hepatocytes and endothelial cells. Thus, lipoprotein lipase is present in the liver in positions similar to where the functional pool of hepatic lipase is located and analogous to where lipoprotein lipase is found in extrahepatic tissues. These results raise the possibility that the enzyme has a functional role in the liver.

Morphological recovery of hippocampal pyramidal neurons in the adult rat exposed in utero to ethanol

Reduced numbers of dendritic spines on the secondary apical dendritic branches and basilar dendrites of CA1 and CA3 pyramidal neurons were observed in ethanol-treated rats during embryonic life aged 15 days when compared with age-matched controls. However, differences were no longer present at the age of 90 days. These results suggest that recovery of some morphological parameters of pyramidal hippocampal neurons may occur in rats exposed in utero to ethanol.

Synthesis of lipoprotein lipase in the liver of newborn rats and localization of the enzyme by immunofluorescence

In newborn rats, lipoprotein lipase (LPL) activity was higher in the liver than in several other tissues, such as heart, diaphragm or lungs, and accounted for about 3% of total LPL activity in the body. There was no significant correlation between LPL activity in liver and in plasma. Thus transport of the enzyme from extrahepatic tissues was probably not the major source of LPL in liver. To study LPL biosynthesis directly, newborn rats were injected intraperitoneally with [35S]methionine, and LPL was isolated by immunoprecipitation and separation by SDS/polyacrylamide-gel electrophoresis. Radioactivity in LPL increased with a similar time course in all tissues studied, including the liver. Substantial synthesis of LPL was also demonstrated in isolated perfused livers from newborn rats, whereas synthesis was low in livers from adult rats. There was strong LPL immunofluorescence in livers from newborn rats, mainly within sinusoids and along the walls of larger vessels. This labelling disappeared after perfusion with heparin, which indicates that much of the enzyme is in contact with blood and can take part in lipoprotein metabolism.

Characterization of lipoprotein lipase activity in the newborn rat liver

Acetone ether powders of livers from starved newborn rats were applied to heparin-Sepharose affinity columns. The so-called hepatic triacylglycerol lipase was eluted with 0.9 M NaCl and a second lipolytic activity peak was eluted with 1.5 M NaCl. The behaviour of this 1.5 M NaCl-eluted fraction against increasing concentrations of serum, NaCl, protamine sulfate and heparin in the assay mixture was almost identical to that shown by partially purified lipoprotein lipase from adult rat adipose tissue, and clearly different from that shown by partially purified hepatic triacylglycerol lipase from the adult rat liver. We conclude that the newborn rat liver contains a lipoprotein lipase activity with similar properties to those found in adult adipose tissue lipoprotein lipase activity. It is suggested that this enzyme enables the neonatal liver to take up circulating triacylglycerols directly.