Evidence for a role of insulin in the regulation of lipogenesis in lactating rat mammary gland. Measurements of lipogenesis in vivo and plasma hormone concentrations in response to starvation and refeeding

Maternal adaptations of the pancreas and glucose homeostasis in lactation and after lactation

During lactation, the maternal physiology adapts to bear the nutritional requirements of the offspring. The exocrine and endocrine pancreas are central to nutrient handling, promoting digestion and metabolism. In concert with prolactin, insulin is a determinant factor for milk synthesis. The investigation of the pancreas during lactation has been scattered over several periods. The investigations that laid the foundation of lactating pancreatic physiology and glucose homeostasis were conducted in the decades of 1970-1980. With the development of molecular biology, newer studies have revealed the molecular mechanisms involved in the endocrine pancreas during breastfeeding. There has been a surge of information recently about unexpected changes in the pancreas at the end of the lactation period and after weaning. In this review, we aim to gather information on the changes in the pancreas and glucose homeostasis during and after lactation and discuss the outcomes derived from the current discoveries.

The adaptation of maternal energy metabolism to lactation and its underlying mechanisms

Maternal energy metabolism undergoes a singular adaptation during lactation that allows for the caloric enrichment of milk. Changes in the mammary gland, changes in the white adipose tissue, brown adipose tissue, liver, skeletal muscles and endocrine pancreas are pivotal for this adaptation. The present review details the landmark studies describing the enzymatic modulation and the endocrine signals behind these metabolic changes. We will also update this perspective with data from recent studies showing transcriptional and post-transcriptional mechanisms that mediate the adaptation of the maternal metabolism to lactation. The present text will also bring experimental and observational data that describe the long-term consequences that short periods of lactation impose to maternal metabolism.

Morphological, hormonal, and molecular changes in different maternal tissues during lactation and post-lactation

Milk supply and quality during lactation are critical for progeny survival. Maternal tissues and metabolism, influenced by hormonal changes, undergo modification during lactation to sustain breastfeeding. Two organs that suffer essential adjustment are the mammary glands and the bone; however, renal calcium conservation and calcium absorption from the intestine are also modified. Lactation leads to a transient loss of bone minerals to provide adequate amounts of minerals, including calcium for milk production. Physiological, metabolic, and molecular changes in different tissues participate in providing nutrients for milk production. After weaning, the histological, metabolic, and hormonal modifications that take place in lactation are reverted, and bone remineralization is a central function at this time. This study focuses on the hormonal, metabolic, molecular, and tissue modifications that occur in mammary glands, bone, intestine, and kidneys in the mother during lactation and post-weaning periods.

The Role of Plasticity and Adaptation in the Incipient Speciation of a Fire Salamander Population

Phenotypic plasticity and local adaptation via genetic change are two major mechanisms of response to dynamic environmental conditions. These mechanisms are not mutually exclusive, since genetic change can establish similar phenotypes to plasticity. This connection between both mechanisms raises the question of how much of the variation observed between species or populations is plastic and how much of it is genetic. In this study, we used a structured population of fire salamanders (Salamandra salamandra), in which two subpopulations differ in terms of physiology, genetics, mate-, and habitat preferences. Our goal was to identify candidate genes for differential habitat adaptation in this system, and to explore the degree of plasticity compared to local adaptation. We therefore performed a reciprocal transfer experiment of stream- and pond-originated salamander larvae and analyzed changes in morphology and transcriptomic profile (using species-specific microarrays). We observed that stream- and pond-originated individuals diverge in morphology and gene expression. For instance, pond-originated larvae have larger gills, likely to cope with oxygen-poor ponds. When transferred to streams, pond-originated larvae showed a high degree of plasticity, resembling the morphology and gene expression of stream-originated larvae (reversion); however the same was not found for stream-originated larvae when transferred to ponds, where the expression of genes related to reduction-oxidation processes was increased, possibly to cope with environmental stress. The lack of symmetrical responses between transplanted animals highlights the fact that the adaptations are not fully plastic and that some level of local adaptation has already occurred in this population. This study illuminates the process by which phenotypic plasticity allows local adaptation to new environments and its potential role in the pathway of incipient speciation.

The demands of lactation promote differential regulation of lipid stores in fasting elephant seals

Fasting animals must ration stored reserves appropriately for metabolic demands. Animals that experience fasting concomitant with other metabolically demanding activities are presented with conflicting demands of energy conservation and expenditure. Our objective was to understand how fasting northern elephant seals regulate the mobilization of lipid reserves and subsequently milk lipid content during lactation. We sampled 36 females early and 39 at the end of lactation. To determine the separate influences of lactation from fasting, we also sampled fasting but non-lactating females early and late (8 and 6 seals, respectively) in their molting fasting period. Mass and adiposity were measured, as well as circulating non-esterified fatty acid (NEFA), triacylglycerol (TAG), cortisol, insulin and growth hormone levels. Milk was collected from lactating females. Milk lipid content increased from 31% in early to 51% in late lactation. In lactating females plasma NEFA was positively related to cortisol and negatively related to insulin, but in molting seals, only variation in cortisol was related to NEFA. Milk lipid content varied with mass, adiposity, NEFA, TAG, cortisol and insulin. Surprisingly, growth hormone concentration was not related to lipid metabolites or milk lipid. Suppression of insulin release appears to be the differential regulator of lipolysis in lactating versus molting seals, facilitating mobilization of stored lipids and maintenance of high NEFA concentrations for milk synthesis. Milk lipid was strongly impacted by the supply of substrate to the mammary gland, indicating regulation at the level of mobilization of lipid reserves.

Olanzapine promotes fat accumulation in male rats by decreasing physical activity, repartitioning energy and increasing adipose tissue lipogenesis while impairing lipolysis

Olanzapine and other atypical antipsychotics cause metabolic side effects leading to obesity and diabetes; although these continue to be an important public health concern, their underlying mechanisms remain elusive. Therefore, an animal model of these side effects was developed in male Sprague-Dawley rats. Chronic administration of olanzapine elevated fasting glucose, impaired glucose and insulin tolerance, increased fat mass but, in contrast to female rats, did not increase body weight or food intake. Acute studies were conducted to delineate the mechanisms responsible for these effects. Olanzapine markedly decreased physical activity without a compensatory decline in food intake. It also acutely elevated fasting glucose and worsened oral glucose and insulin tolerance, suggesting that these effects are adiposity independent. Hyperinsulinemic-euglycemic clamp studies measuring (14)C-2-deoxyglucose uptake revealed tissue-specific insulin resistance. Insulin sensitivity was impaired in skeletal muscle, but either unchanged or increased in adipose tissue depots. Consistent with the olanzapine-induced hyperglycemia, there was a tendency for increased (14)C-2-deoxyglucose uptake into fat depots of fed rats and, surprisingly, free fatty acid (FFA) uptake into fat depots was elevated approximately twofold. The increased glucose and FFA uptake into adipose tissue was coupled with increased adipose tissue lipogenesis. Finally, olanzapine lowered fasting plasma FFA, and as it had no effect on isoproterenol-stimulated rises in plasma glucose, it blunted isoproterenol-stimulated in vivo lipolysis in fed rats. Collectively, these results suggest that olanzapine exerts several metabolic effects that together favor increased accumulation of fuel into adipose tissue, thereby increasing adiposity.

Exercise training in rats impairs the replenishment of white adipose tissue after partial lipectomy

The aim of this study was to evaluate the effect of exercise training on the metabolism of rats following the partial removal of fat pads. Three-month-old male Wistar rats were subjected to the partial removal (L) of retroperitoneal white adipose tissue (RET) and epididymal white adipose tissue (EPI), or a sham operation (Sh). Seven days after surgery, both sets of rats were subdivided into exercised (LE or ShE) (swimming 90 min/day, 5 days/week, 6 weeks) and sedentary (LS or ShS) groups. Partial removal of the fat pads increased the lipogenesis rates in both the RET and EPI and decreased the weight and lypolysis rate of the EPI, while the RET weight was not significantly affected by lipectomy. In both lipectomized and sham-operated groups, exercise training caused a reduction in carcass lipid content, food intake, RET and EPI weights, and RET lipogenesis rate. On the other hand, the exercise training increased the percentage of diet-derived lipid accumulation in both tissues, either in sham and lipectomized rats. These results confirmed that regrowth is not uniform and depends on the particular fat pad that is excised. They also demonstrated that exercise training following the partial removal of fat pads modified adipose tissue metabolism, impaired the replenishment of adipose tissue, and decrease body adiposity.

Fatty acid oxidation and fatty acid synthesis in energy restricted rats(1)

The importance of fat oxidation and fatty acid synthesis were examined in rats fed approximately one half their ad libitum food intake for a period of 13 days followed by 7 days of ad libitum feeding (refed rats). This study was undertaken because previous reports demonstrated that refed rats rapidly accumulated body fat. Our results confirmed this observation: refed rats accrued body fat and body weight at rates that were approximately 3 times higher than controls. Evidence for a period of increased metabolic efficiency was demonstrated by measuring the net energy requirement for maintenance over the refeeding period: refed rats had a reduced metabolic rate during the period of energy restriction (approximately 30% lower than control) and this persisted up to 2 days after the reintroduction of ad libitum feeding. The major factor responsible for the rapid fat gain was a depressed rate of fatty acid oxidation. Calculations of protein and carbohydrate intake over the refeeding period showed that the simplest explanation for the decrease in fatty acid oxidation is fat sparing. This is possible because of the large increase in dietary carbohydrate and protein intake during the refeeding period when metabolic rates are still depressed. The increased carbohydrate and protein may adequately compensate for the increasing energy requirements of the ER rats over the refeeding period affording rats the luxury of storing the excess dietary fat energy.

Interleukin-15 mediates reciprocal regulation of adipose and muscle mass: a potential role in body weight control

Interleukin (IL)-15 is a cytokine which is highly expressed in skeletal muscle. Cell culture studies have indicated that IL-15 may have an important role in muscle fiber growth and anabolism. However, data concerning the metabolic effects of this cytokine in vivo are lacking. In the present study, IL-15 was administered to adult rats for 7 days. While IL-15 did not cause changes in either muscle mass or muscle protein content, it induced significant changes in the fractional rates of both muscle protein synthesis and degradation, with no net changes in protein accumulation. Additionally, IL-15 administration resulted in a 33% decrease in white adipose tissue mass and a 20% decrease in circulating triacylglycerols; this was associated with a 47% lower hepatic lipogenic rate and a 36% lower plasma VLDL triacylglycerol content. The decrease in white fat induced by IL-15 was in adipose tissue. No changes were observed in the rate of lipolysis as a result of cytokine administration. These findings indicate that IL-15 has significant effects on both protein and lipid metabolism, and suggest that this cytokine may participate in reciprocal regulation of muscle and adipose tissue mass.

Ethanol intake during lactation. I. Effects On dams' metabolism and pups' body weight gain

Wistar lactating rats (8 pups per dam) had free access to either tap water (control group, C) or one of three concentrations of ethanol (E) in the drinking water: 5% (E5), 10% (E10), and 20% (E20). All animals received normal rat chow ad libitum and were killed on day 12 of lactation. Intake of both 10% and 20% ethanol solutions decreased food intake, dams' body weight, and pups' body weight gain as compared with findings in the C group. The relative weights (g/100g b.w.) of the mammary glands (MG) and of the parametrial white adipose tissue depot were decreased only in E20 as compared with findings in the C group. Protein and lipid content of these tissues were not altered in any of the ethanol groups. In comparison with the C group, the lipogenesis rate was increased in the MG (135. 6%) and liver (120.2%) in E5 and the MG (58.1%) and parametrial white adipose tissue depot (147.0%) in E20. No modifications in lipogenesis rate were noted in E10. The malic enzyme activity was decreased in the MG in E10 (25.3%) and E20 (26.4%) and in the liver in E20 (45.7%). In E5, however, it was increased in the liver (23. 9%). The activity of ATP-citrate lyase in the liver was decreased in E20 (56.7%), while it was increased by 37.5% in E5 and 34.2% in E10. Blood glucose concentration of dams was not affected by ethanol ingestion. However, plasma triacylglycerol concentration was higher in E10 (17.9%) and E20 (13.3%) than in the C group, and plasma protein was lower in E20 (15.7%) than in C. We concluded that alcohol intake during lactation increased the MG lipogenesis rate; although at the highest dose, this metabolic alteration was not enough to allow normal pups' growth. However, the low dose of ethanol (5%), despite having altered dams' metabolism, did not affect pups' body weight gain.

A high dietary lipid intake during pregnancy and lactation enhances mammary gland lipid uptake and lipoprotein lipase activity in rats

Rats fed a diet with high fat concentration produce larger amounts of milk with a higher lipid concentration than rats fed a lower fat diet. This investigation was designed to study the relationship between dietary fat intake, mammary gland lipid uptake and lipogenesis in rat dams fed, during pregnancy and lactation, one of two purified diets, with equal energy density, containing 2.5 (LL) or 20 g fat/100 g diet (HL). Milk lipid concentration and fatty acid composition were determined at d 14 of lactation. Mammary gland lipogenesis, lipoprotein lipase (LPL) activity and the uptake of [1-(14)C]triolein by the mammary gland and its transfer to the pups was measured. The intestinal absorption of oral (14)C-lipid, (14)CO(2) production and the amount of (14)C-lipid transferred to the pups (milk clot + pups carcass) were significantly higher in the HL group than in the LL group (P < 0.05). Mammary gland lipogenesis was 75% lower and LPL activity was 30% higher in the HL group (P < 0.05). Medium-chain fatty acids (C6-C14) excretion was 46% lower and that of long-chain fatty acids was 142% (P < 0.001) higher in the HL group than in the LL group. The higher milk lipid excretion in the rats fed a high-fat diet resulted from a larger uptake of dietary lipid by the mammary gland, indicated by a larger transfer of (14)C-lipid to the pups and by a higher LPL activity in the mammary gland.

Leptin administration to lactating rats is unable to induce changes in lipid metabolism in white adipose tissue or mammary gland

During lactation in the rat, despite hyperphagia, there are no changes in either the plasma levels or the gene expression of leptin. Removal of the litter, however, results in an important increase in the circulating concentration of leptin. Administration of leptin to lactating rats resulted in no changes in the in vivo lipogenic rate and lipoprotein lipase (LPL) activity in either adipose tissue or mammary gland, although there was an increase in insulin levels as a consequence of leptin administration. Conversely, litter removal resulted in an important decrease of LPL activity and lipogenic rate in the mammary gland while an increase in these parameters took place in adipose tissue. It is concluded that leptin is not the signal responsible for the changes in lipid metabolism that take place both in adipose tissue and mammary gland following litter removal.

Short-term effects of leptin on lipid metabolism in the rat

In this study, we have examined the short-term effects of leptin on lipid metabolism in the rat. Acute leptin administration induced hypertriglyceridaemia (31% increase in plasma triacylglycerols) which was not associated with changes in lipoprotein lipase activity in white adipose tissue. Surprisingly, leptin administration did not induce any changes in the lipogenic rate in either white adipose tissue or liver. Leptin administration caused a decreased tissue uptake of exogenous 14C-triacylglycerols. These data suggest that leptin induces important changes in lipid uptake in adipose tissue and skeletal muscle which could be responsible for the observed hypertriglyceridaemia.

The role of pyruvate dehydrogenase, phosphofructo-1-kinase and acetyl-CoA carboxylase in the regulation of fatty acid synthesis in the lactating rat mammary gland during the starved to re-fed transition

Re-feeding 24-h-starved lactating rats resulted in a rapid (within 0.5 h) restoration of glucose uptake by the mammary gland and a slower (within 3 h) restoration of fatty acid synthesis. The rapid reactivation of glucose uptake (82% of fed value within 0.5 h of re-feeding) correlated with a rapid reactivation of 6-phosphofructo-1-kinase (6-PF-1-K) and glycolysis (as determined by a 97% decrease in the [fructose-6-phosphate]/[fructose-1,6-bisphosphate] ratio). This could not be fully explained by a fall (29%) in the tissue concentration of its allosteric inhibitor, citrate. The delayed reactivation of pyruvate dehydrogenase (PDH) correlated very closely with the delayed reactivation of fatty acid synthesis and explained the continued output of pyruvate and lactate within the first 0.5 h of re-feeding. PDH reactivation preceded the reactivation of acetyl-CoA carboxylase (ACC), which did not occur significantly until 1.5 h of re-feeding. ACC reactivation correlated with a decrease in the tissue concentration of citrate and a second late phase of 6-PF-1-K activation. It is clear that the important regulatory steps 6-PF-1-K, PDH and ACC, are reactivated asynchronously in the lactating mammary gland in response to re-feeding starved rats and that PDH is more important than ACC in the regulation of fatty acid synthesis.

Lipid metabolism in tumour-bearing mice: studies with knockout mice for tumour necrosis factor receptor 1 protein

The implantation of the Lewis lung carcinoma (a fast-growing mouse tumour that induces cachexia) to both wild-type and gene-deficient mice for the tumour necrosis factor (TNF) receptor type I protein (Tnfr1(0)/Tnfr1(0)), resulted in a considerable loss of carcass (26%) and white (77%) and brown adipose (37%) tissue weights in the wild-type mice, while it induced much less marked effects in the gene-deficient mice. Tumour burden also inflicted an important decrease in total lipoprotein lipase (LPL) activity in epididymal white adipose tissue (50%) in the wild-type mice while no changes were observed in the knockout mice. In addition, all tumour-bearing animals were clearly hypertriglyceridaemic (80% increase in circulating triacylglycerols in wild-type and 36% in knockout mice). It is concluded that although TNF seems to be to some extent responsible for adipose waste, LPL changes and hyperlipaemia (via receptor I), the role of other cytokines (alone or in combination with TNF) in promoting changes in lipid metabolism during cancer cachexia cannot be discarded.

Anti-TNF treatment does not reverse the abnormalities in lipid metabolism of the obese Zucker rat

Because obesity, insulin resistance, and hyperlipidemia are often associated, and recent evidence suggests that the cytokine tumor necrosis factor-alpha (TNF) may influence the activity of insulin in various target tissues, the present study was designed to see whether TNF was also associated with the changes in lipid metabolism that lead to hyperlipidemia in the obese model of the Zucker rat. A polyclonal goat anti-rat TNF antibody was subcutaneously administered to Zucker rats for 4 days to block TNF actions. The results indicate that none of the alterations in lipid metabolism seen in the obese animals were reversed by the anti-TNF treatment. This was the case for the lipogenic rate in liver and adipose tissue, the disposal of an exogenous [14C]triolein load, adipose tissue lipoprotein lipase activity, and the hypertriglyceridemia. Measurements of lipolysis in adipose tissue slices from the anti-TNF-treated animals also did not show any significant effect of the treatment. In conclusion, TNF does not seem to be involved in the abnormalities of lipid metabolism observed in the obese Zucker rat.

Lipid metabolism in rats bearing the Yoshida AH-130 ascites hepatoma

Rats bearing the Yoshida AH-130 ascites hepatoma showed important changes in lipid metabolism. The presence of this rapidly growing tumour induced a significant reduction in the intestinal absorption of an oral [14C]triolein load but without changes in whole body oxidation of the tracer to CO2. Both white (WAT) and brown (BAT) adipose tissue lipoprotein lipase (LPL) activities were increased at day 4 of tumour growth, changes that seem to be related with those observed in [14C]lipid accumulation; however, heart LPL activity was increased at day 7 but there was no change at day 4. In addition, there was a marked hyperlipemia in the tumour-bearing animals, whereas the blood ketone body concentrations were lower in these animals in comparison with the corresponding pair-fed group. The in vivo lipogenic rate was increased in liver of the tumour-bearing animals (day 4); conversely, it was decreased in WAT and skeletal muscle (day 4) and IBAT (day 7) of the AH-130-bearing rats. It may be suggested that the increased liver lipogenic rate associated with tumour burden is the main factor contributing to the hyperlipidaemia present in the Yoshida AH-130 bearing rats.

Effects of ethanol intake on lipid metabolism in the lactating rat

Female rats receiving alcohol (20%) in drinking water during lactation (AL) were compared to pair-fed animals (PF) and normal controls (C) fed ad lib. All animals were killed on the 12th day of lactation. When compared to C rats, food intake decreased in both AL and PF groups, and this effect was followed by a lower body weight and mammary gland (MG), liver, and parametrial adipose tissue weights. Mammary glands triacylglyceride concentration (TG) was much lower in PF than in AL, although in the latter, values did not reach those of C, and had higher liver TG concentration than any of the other groups. Both PF and AL rats had lower plasma TG, glycerol, and free fatty acid concentrations and higher beta-hydroxybutyrate concentration than C rats. When compared to C rats, the rate of lipogenesis in MG was higher in both PF and AL rats, whereas in liver it was higher in PF and lower in AL rats, and in adipose tissue it was higher in PF and unchanged in AL rats. The appearance of 14C lipids 4 h after oral [14]triolein in both MG and liver was lower in AL and PF rats and only lower in adipose tissue of AL rats as compared to the c rats. Lipoprotein lipase and hormone-sensitive lipase activities were lower in MG in both PF and AL rats than in C, whereas in adipose tissue the activity of lipoprotein lipase did not differ between AL and C rats and the activity of HSL was lower in the former. These findings therefore show that in spite of reduced uptake of orally administered triglycerides due to decreased LPL activity, maternal alcohol feeding during lactation in the rat preserves the mammary gland triglyceride content thanks to enhanced lipogenetic activity. On the other hand, it causes liver triglycerides accumulation, probably as a result of the decreased rate of triglycerides released into circulation, and these changes are not caused by the reduced food intake of the animals.

Acute effects of endotoxin (lipopolysaccharide) on tissue lipid metabolism in the lactating rat. The role of delivery of intestinal glucose

The aim of this study was to compare the effects of endotoxin on lipid metabolism and, in particular, lipogenesis in virgin and lactating rats. Intraperitoneal administration of bacterial endotoxin (lipopolysaccharide, LPS; 3 mg/kg body wt.) to fed virgin rats caused a 4-fold increase in lipogenic rate in liver in vivo. The stimulatory effect was not seen when glucose (6 mmol) was administered either orally or intraperitoneally to increase the basal rate. In contrast, the rate of lipogenesis in interscapular brown adipose tissue was inhibited, after LPS, and this was relieved by intraperitoneal glucose. In the lactating rat there were no significant changes in hepatic lipogenesis after the administration of endotoxin. However, LPS decreased the lipogenic rate in mammary gland of lactating rats and intraperitoneal glucose administration, but not oral, was able to restore the rate. In both virgin and lactating rats, LPS decreased glucose removal from the intestinal tract. In lactating rats, LPS induced a rise in blood concentrations of lactate, and plasma triacylglycerols and non-esterified fatty acids, similar to those in endotoxin-treated virgin rats. The administration of LPS did not decrease the accumulation of radioactivity in lipid in either liver or in mammary gland after injection of 3H-oleate. In contrast, LPS decreased the accumulation of radioactivity in mammary gland after injection of 3H-chylomicrons and increased it in liver and plasma. These changes were accompanied by a decrease in mammary gland activity of lipoprotein lipase. Intraperitoneal glucose partially reversed these changes in chylomicron disposition.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of tri-iodothyronine administration on the disposal of oral [1-14C]triolein, lipoprotein lipase activity and lipogenesis in the rat during lactation and on removal of the litter

The effect of tri-iodothyronine (T3) administration on the utilization of dietary [14C]lipid by the mammary gland and adipose tissue of lactating and litter-removed rats was studied. (1) After an oral load of [1-14C]triolein, the lactating rats treated with T3 (50 micrograms/100 g body wt.) over 24 h showed an increase in 14CO2 production and a decrease in the total [14C]lipid transferred through the mammary gland that was paralleled by a decrease in tissue lipoprotein lipase (LPL) activity. (2) T3 administration decreased plasma prolactin in the lactating rats. Prolactin replacement in T3-treated rats restored LPL activity in the mammary gland, but did not increase the amount of dietary [14C]lipid transferred to the milk. (3) Chronic T3 administration (4 days) to lactating rats did not affect pup growth or the lipogenic rate in the mammary gland. (4) The administration of T3 to litter-removed rats inhibited the increase of LPL activity in white adipose tissue and decreased the accumulation of dietary [14C]lipid. This decrease was accompanied by increased 14CO2 production and [14C]lipid accumulation in skeletal muscle and heart. (5) It is concluded that hyperthyroidism depresses LPL activity in mammary gland and white adipose tissue, but not in muscle. The increased accumulation of [14C]lipid in muscle and increased production of 14CO2 in lactating and in litter-removed rats treated with T3 is in part due to the decreased total LPL in mammary gland and adipose tissue respectively, which are therefore less able to compete with muscle for the available plasma triacylglycerols.

Bromocriptine treatment increases lipolysis and steady-state levels of G proteins in adipocytes from lactating rats

An increase in the rate of lipogenesis and a decrease in the lipolytic response to catecholamines can be observed in adipocytes after weaning or litter removal. Bromocriptine treatment of lactating rats also produces an increase in lipogenesis but fails to decrease the lipolytic response of adipocytes to catecholamines seen after weaning or litter removal. No changes in total number or affinity of beta-adrenergic receptors or adenosine A1 receptors were detected by bromocriptine treatment using radioligand binding assays. However, we observed an increase in the amount of radioactivity from [32P]NAD+ incorporated into alpha-Gs (192 +/- 26%) and alpha-Gi (178 +/- 33%) by cholera- and pertussis-toxin-catalyzed ADP-ribosylation, respectively, with the same treatment, Immunoblotting using RM/1 and AS/7 antisera, which specifically recognize alpha-Gs and alpha-Gi 1,2, respectively, confirmed the increase in the steady-state levels of these G-protein alpha-subunits. The increase in the steady-state levels of alpha-Gs may account, at least in part, for the increased lipolytic response of adipocytes to catecholamines in bromocriptine-treated rats.

Regulation of rat mammary-gland uptake of orally administered [1-14C]triolein by insulin and prolactin: evidence for bihormonal control of lipoprotein lipase activity

The effects of insulin deficiency (streptozotocin-induced) or insulin plus prolactin deficiency on the disposal of orally administered [1-14C]triolein between oxidation to 14CO2, uptake by mammary gland and transfer to suckling pups were studied. Insulin deficiency decreased mammary-gland total weight (by 40%), but caused no change in total tissue DNA. A greater proportion of the [1-14C]triolein was oxidized to 14CO2 (120% increase) in the insulin-deficient rats, and there was a tendency for total transfer of [14C]lipid to mammary gland and suckling pups to be decreased. A parallel decrease in total mammary-tissue lipoprotein lipase activity occurred. Combined hormone deficiency caused more dramatic changes in all parameters measured. Replacement of insulin (24 h) in insulin-deficient rats decreased 14CO2 production, increased the uptake of [14C]lipid into the mammary gland and tended to increase total lipoprotein lipase activity. In contrast, administration of prolactin to insulin-deficient rats had no effect on any of the parameters measured. Replacement of insulin (24 h) in the combined hormone-deficient rats increased uptake of [14C]lipid and lipoprotein lipase approx. 3-fold, whereas prolactin again had no significant effects. Replacement of both hormones increased (6-fold) transfer of [14C]lipid to the pups, but did not increase overall uptake of [14C]lipid (mammary gland, milk clot and pups) above the value for insulin alone. It is concluded that insulin is the primary regulator of triacylglycerol uptake and of lipoprotein lipase activity in the lactating mammary gland of the rat and that the action of prolactin on these processes is indirect. Prolactin, but not insulin, appears to have a direct effect on milk lipid transfer to pups.

Nutritional status, suckling behavior, and prolactin release during lactation

This experiment was conducted to determine whether suckling-induced prolactin (PRL) values in chronically food-restricted lactating rats resulted from the inability of their litters to suckle vigorously. Rats were randomly assigned to be fed ad lib (AL) or to be chronically restricted (CR) to 50% of AL before and during pregnancy and lactation. Rats nursed their own litter (own) or a litter of the opposite dietary treatment group (other) for 90-min periods at early or peak lactation. Blood was collected regularly through a catheter implanted several days earlier. Suckling contributed to differences in PRL between AL and CR dams in early lactation as demonstrated by an interaction between dietary treatment and type of litter. However, tests of AL own vs. AL other and CR own vs. CR other were not significant. This is probably because the inattentive behavior of CR dams attenuated the PRL response in this group. At peak lactation, PRL values were related to recovery time from catheter implantation; therefore, determinants of PRL release change over the course of lactation.

Effects of triiodothyronine administration on dietary [14C]triolein partitioning between deposition in adipose tissue and oxidation to [14C]CO2 in ad libitum-fed or food-restricted rats

Refeeding a chow meal containing [1-14C]triolein to food-restricted rats results in increased accumulation of [14C]lipid in carcass and epididymal adipose tissue and lower oxidation to [14C]CO2 compared to ad libitum-fed rats (Biochem. J. 285, 773-778, 1992). In the present experiments the effects of treatment with triiodothyronine (T3) for three days on lipid accumulation in refed food-restricted rats has been examined. T3 decreased accumulation of [14C]lipid in carcass and epididymal adipose tissue (32 and 77%, respectively) of food-restricted rats on refeeding the chow-[1-14C]triolein meal. This decreased accumulation of [14C]lipid was accompanied by increased [14C]CO2 production (77%) and decreased heparin-elutable lipoprotein lipase activity in the epididymal fat pad (90%) and subcutaneous adipose tissue (80%). Accumulation of [14C]lipid in the latter did not decrease significantly. In contrast, T3 treatment of ad libitum-fed rats increased [14C]lipid deposition in carcass (44%) and in subcutaneous adipose tissue (240%) on refeeding, when compared to untreated ad libitum rats. Lipoprotein lipase activity in the two adipose tissue depots of the refed ad libitum+T3 rats, however, decreased. Thus, the effects of T3 on [14C]lipid deposition are adipose-tissue-depot-specific and depend on the previous dietary intake (over 14 days) of the rat. T3-treatment increased the lipoprotein lipase activity released from perfused hearts to a similar extent in both food-restricted and ad libitum-fed rats compared to the corresponding untreated groups. The rates of lipogenesis in-vivo in liver, epididymal and subcutaneous adipose tissue of food-restricted rats refed chow were not altered by T3. It is concluded that the increased deposition of dietary lipid in the food-restricted rat can be partially reversed by treatment with T3, suggesting that the low-T3 state associated with this condition may be in part responsible.

Effects of feeding medium-chain triacylglycerols on maternal lipid metabolism and pup growth in lactating rats

To study the effects of medium-chain triacylglycerols (MCT) on maternal lipid metabolism and pup growth, MCT (200 g/kg) were incorporated into a commercial chow diet and fed to lactating rats for 8-10 d. The results were compared with similar diets containing sunflower oil (polyunsaturated fatty acids; PUFA), tristearin (saturated fatty acid) or triolein (monounsaturated fatty acid). There was decreased food and energy intake with the MCT diet and this was accompanied by decreased (35%) pup growth. All the high-fat diets inhibited lipogenesis in vivo in the lactating mammary gland, the order of effectiveness being PUFA > triolein > tristearin > MCT. Only the MCT diet increased the rate of hepatic lipogenesis (180%). Experiments feeding an MCT meal containing [1-14C]octanoate indicated that very little (3-4%) of the C was present in mammary gland lipid, unlike the findings with [1-14C]triolein meal (40%). The major portion (65%) of the absorbed [1-14C]octanoate was oxidized to 14CO2. There was no evidence for adaptation of the mammary gland to increased dietary lipid uptake on the triolein or MCT diets. It is concluded that the decreased pup growth on the MCT diet is due in part to the decreased energy intake and to the inability of dietary medium-chain fatty acids to provide substrates for milk lipid synthesis.

Effects of exogenous insulin or vanadate on disposal of dietary triacylglycerols between mammary gland and adipose tissue in the lactating rat: insulin resistance in white adipose tissue

The effects of exogenous insulin or vanadate (an insulin mimetic) on the disposal of dietary [14C]lipid between oxidation to 14CO2, deposition in adipose tissue or uptake by mammary gland and transfer to suckling pups were studied in virgin and lactating rats. After an oral load of [1-14C]triolein, virgin rats treated with a supraphysiological dose of insulin over 24 h showed a decrease (58%) in 14CO2 production and increased accumulation of [14C]lipid in carcass and white adipose tissue. There was a 2.5-fold increase in lipoprotein lipase activity in the latter. Chronic vanadate administration (12 days) had no effect on these parameters. In lactating rats, the stimulation of the deposition of [14C]lipid in adipose tissue by exogenous insulin was about 10% of that in virgin rats. In prolactin-deficient lactating rats there was no stimulation of [14C]lipid deposition in adipose tissue by insulin. However, both insulin and vanadate treatment increased the accumulation of [14C]lipid in mammary gland to the values seen in the mammary glands plus pups of normal lactating rats. Lipoprotein lipase activity in the gland was also restored to normal values. It is concluded that in lactation there is resistance to insulin stimulation of dietary lipid deposition in adipose tissue, and that this is not due to circulating prolactin. In addition, exogenous insulin plays a role in the regulation of lipoprotein lipase and hence of dietary lipid uptake into lactating mammary gland.

Lipid metabolism in cachectic tumor-bearing rats at different stages of tumor growth

Rates of lipogenesis and lipoprotein lipase (LPL) activity were measured in liver, adipose tissue, heart, and tumor at several stages during 10 days of palpable growth of a transplantable Leydig cell tumor in rats. This model showed the same characteristics as human cancer cachexia, including anorexia, weight loss, and muscle wasting. Comparison with pair-fed controls showed that the rate of loss of body fat was greater than could be explained by anorexia alone. The rate of lipogenesis tended to decrease during the later stages of tumor growth, particularly in the liver, where there was a statistically significant reduction on Days 5 and 10. This may be largely attributable to decreased availability of substrates caused by decreasing food intake and increasing glucose uptake by the tumor. There was a significant decrease in plasma glucose concentration by Day 10. In contrast, LPL activity in adipose tissue was depressed from the earliest stage of tumor growth, and this is likely to be a major cause of lipid depletion in cancer. There was no difference in adipose tissue LPL activity between the fed and postabsorptive states in the tumor-bearing rats, indicating that the normal response to nutrient intake was impaired. Thus, treatment of cancer cachexia should concentrate on normalizing the metabolic response to nutrient ingestion.

Pyrimidine nucleotide synthesis in the rat mammary gland: changes in the lactation cycle and effects of diabetes

Measurements have been made of the activities of the enzymes of the de novo and salvage pathways of pyrimidine synthesis (carbamoyl phosphate synthetase II (glutamine) (EC 6.3.5.5); dihydroorotate dehydrogenase (EC 1.3.99.11); the overall activity of Complex II (orotate phosphoribosyl pyrophosphate transferase (EC 2.4.2.10) and orotidine 5-phosphate decarboxylase (EC 4.1.1.23); uracil phosphoribosyltransferase (EC 2.4.2.9)) in the mammary gland of rats at different stages of the lactation cycle and the effects of diabetes on the activity of these enzymes in lactation have been studied. From a consideration of the changes in enzyme activities and the changes in the tissue concentration of phosphoribosyl pyrophosphate, an activator of the de novo pathway and substrate for both the de novo and salvage routes, it is concluded that the de novo pathway is the major route of pyrimidine synthesis in mammary tissue. Diabetes decreases the activity of the enzymes of the de novo pathway; the effects are particularly marked for Complex II. The present results on pyrimidine synthesis are compared to the pattern for purine synthesis previously published.

Effects of lactation on the regulation of hepatic metabolism in the rat and sheep: adrenergic receptors and cyclic AMP responses

The number and coupling efficiency of beta-adrenoceptors in liver membranes and intact hepatocytes of lactating and non-lactating female rats were compared to assess whether or not alterations in this signalling system could contribute towards the changed pattern of hepatic metabolism during lactation. In view of the different adaptations of hepatic metabolism to lactation in ruminants, the adrenergic receptor profile of sheep liver membranes was also determined. Post-receptor responses at two stages 'down-stream' of cyclic AMP generation were also evaluated in rat hepatocytes in response to the beta-adrenergic agonist isoprenaline. No changes in the number of affinity of hepatic beta-adrenoceptors were found in sheep or rats when lactating and non-lactating individuals were compared. Sheep liver was found to have a much greater concentration of beta-adrenoceptors than rat liver, and a much higher ratio of beta:alpha 1. The sensitivity and responsiveness of cyclic AMP generation in response to isoprenaline were similar in hepatocytes prepared from lactating and non-lactating rats, although the response to saturating concentrations of glucagon was diminished in hepatocytes from lactating rats. The activity ratio of cyclic AMP-dependent protein kinase (PK-A) also reacted similarly (in respect of both responsiveness and sensitivity) to isoprenaline in these two groups of hepatocytes. Contrastingly, the sensitivity of rat hepatocyte phosphorylase activity to beta-adrenergic stimulation was greatly diminished during lactation.

Refeeding meal-fed rats increases lipoprotein lipase activity and deposition of dietary [14C]lipid in white adipose tissue and decreases oxidation to 14CO2. The role of undernutrition

Meal-fed (3 h) rats had a decreased food intake, body weight and carcass fat compared with rats fed ad libitum. On refeeding a chow meal containing [1-14C]triolein, the production of 14CO2 was lower (45%) and the accumulation of carcass [14C]lipid higher (37%) in the meal-fed rats. There was higher lipoprotein lipase activity and greater accumulation of [14C]lipid in the epididymal and subcutaneous adipose-tissue depots of the meal-fed rats. In contrast, heparin-releasable lipoprotein lipase was not increased in perfused hearts of meal-fed rats on refeeding. Return of meal-fed rats to feeding ad libitum reversed these changes before the restoration of body weight or carcass fat. Evidence is presented that decreased dietary intake rather than meal pattern is an important determinant of the alterations in adipose lipid metabolism in the meal-fed rat in response to a meal.

Effects of litter removal on the lipolytic response and the regulatory components of the adenylate cyclase in adipocytes isolated from lactating rats

The effects of litter removal on the status of different components of the hormone-sensitive adenylate cyclase system were analysed in plasma membranes of rat adipocytes. These effects were correlated with the decreased lipolytic response of adipose tissue. No changes in total number of A1 adenosine receptors or their affinity were detected in response to litter removal. In contrast, beta-adrenergic receptors showed a decrease (35%) in total number of receptors, without any significant change in their affinity. The status of alpha-GS and alpha-Gi, the alpha-subunits of G proteins which mediate stimulation and inhibition respectively of adenylate cyclase, were probed by cholera- and pertussis-toxin-catalysed ADP-ribosylation respectively and by immunoblot. Associated with litter removal, decreases of 63% and 62% in the incorporation of [alpha 32P]ADP-ribose catalysed by cholera toxin and pertussis toxin into alpha-Gs and alpha-Gi respectively were detected. Immunoblotting using RM/1 (anti-alpha-Gs) and AS/7 (anti-alpha-Gi) antisera also showed decreases in the levels of alpha-Gs (52%) and alpha-Gi (55%) in adipocyte membranes from litter-removed rats compared with lactating rats. Alterations in the status of hormone-sensitive adenylate cyclase components, such as those described herein, may be biochemical mechanism(s) by which adipose tissue shows a decreased lipolytic response during recovery from lactation.

Comparison of effects of platelet-activating factor and tumour necrosis factor-alpha on lipid metabolism in adrenalectomized rats in vivo

The acute metabolic effects of platelet-activating factor (PAF) and tumour necrosis factor-alpha (TNF-alpha) were compared in sham-operated and adrenalectomized rats. PAF caused hyperglycaemia in sham-operated rats, whereas with TNF-alpha there was a slight decrease in blood glucose. Both PAF and TNF-alpha resulted in marked hypoglycaemia in the adrenalectomized rats. Plasma insulin was depressed (about 50%) by PAF and TNF-alpha in sham-operated rats, whereas in the adrenalectomized rats the already low plasma insulin concentration was not significantly altered. Liver glycogen content was the same in control and treated sham-operated rats, but was considerably decreased (about 50%) in the adrenalectomized rats. In sham-operated rats, PAF and TNF-alpha increased plasma non-esterified fatty acids and triacylglycerols, suggesting increased lipolysis, whereas in adrenalectomized rats there was no significant increase in non-esterified fatty acids with PAF, although it still occurred with TNF-alpha. This suggests that the lipolytic effect of TNF-alpha may be direct, whereas that of PAF is indirect, possibly via increased catecholamines in the sham-operated rats. The stimulation (about 3-fold) of hepatic fatty acid synthesis in vivo by PAF and TNF-alpha in sham-operated rats was still evident in the adrenalectomized rats, although the absolute increase was smaller. PAF, but not TNF-alpha increased (100%) sterol synthesis in adrenalectomized rats. It is concluded that PAF and TNF-alpha can increase hepatic lipogenesis in vivo in the absence of adrenal hormones and in the presence of a low plasma insulin.

Metabolism of meadowfoam oil fatty acids in mice

Meadowfoam oil is unusual because over 95% of the fatty acids are 20- and 22-carbon aliphatic acids with cis double bonds located principally at the 5- and/or 13-position. Since little information is available on the metabolism of the 5c-20:1 and 5c,13c-22:2 fatty acids, an exploratory study in mice was conducted to investigate the metabolism of purified samples of the free fatty acids isolated from meadowfoam oil, and to determine the effect of meadowfoam oil on weight gain and tissue lipid composition. Mice fed diets containing 5% by wt of the purified 5c-20:1 or 5c,13c-22:2 for 6 days exhibited no apparent physiological problems. Total liver lipids from mice fed the purified fatty acid diets contained mean values of 2.0% 5c-20:1 and 2.1% 5c,13c-22:2; total heart lipids contained 1.7% 5c-20:1 and 10.7% 5c,13c-22:2. Liver total phospholipids from mice fed a 5% meadowfoam oil diet for 19 wk contained 1.4% 5c-20:1 and 1.9% 5c,13c-22:2. There was no evidence of desaturation, elongation or retroconversion. Weight gain for mice fed the meadowfoam oil diet for 19 wk was similar to mice fed corn oil, and was higher than for mice fed hydrogenated cottonseed oil. Considering the high 5c-20:1 and 5c,13c-22:2 content of the diets, the percentages of these fatty acids in mouse tissue lipids from both the short- and long-term studies were low. Weight gain was surprisingly good since the meadowfoam oil diet was essential fatty acid-deficient. Results of this initial investigation suggest that the 5c-20:1 and 5c,13c-22:2 fatty acids were utilized primarily for energy.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid metabolism in the obese Zucker rat. Disposal of an oral [14C]triolein load and lipoprotein lipase activity

Oxidation in vivo of [14C]triolein to 14CO2 was significantly lower in obese (fa/fa) Zucker rats as compared with their lean (+/?) controls. In response to a 24 h starvation period, both lean and obese rats showed an enhanced rate of [14C]triolein oxidation. There were, however, no changes in the rate of intestinal absorption of [14C]triolein between the lean and obese animals. Conversely, the total tissular [14C]lipid accumulation was significantly higher in white adipose tissue, carcass and plasma in the obese animals, whereas that of brown adipose tissue was lower. This was associated with a marked hyperinsulinaemia and hypertriglyceridaemia in the fa/fa animals. Starvation dramatically decreased [14C]lipid accumulation in white adipose tissue of the lean Zucker rats, but had no effect in the obese rats. The lipogenic rate of the obese rats was significantly higher than that of lean rats in liver, white adipose tissue, skeletal muscle and carcass. Lipoprotein lipase activity (per g of tissue) was significantly lower in both white and brown adipose tissue of obese versus lean rats; however, total activity was higher in both tissues. Starvation significantly lowered perigenital-adipose-tissue lipoprotein lipase activity in the lean groups, and had no effect in the obese ones. These results demonstrate that the tissue capacity of exogenous lipid uptake is involved, but cannot be the only factor influencing the maintenance of obesity in these animals. Thus, in the adult fa/fa rat, the large increase in obesity is not solely dependent on a deviation of energy-producing substrate metabolism towards the storage of lipids in white fat. Other factors, such as a low rate of oxidation, a high lipogenic rate and decreased brown-adipose-tissue activity are involved in the perseverance of the obesity syndrome.

Metabolic effects of platelet-activating factor in rats in vivo. Stimulation of hepatic glycogenolysis and lipogenesis

1. The effects of platelet-activating factor (1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphocholine; PAF) on hepatic metabolism in vivo in rats were studied. 2. PAF stimulated synthesis of hepatic lipid (saponified and non-saponified) in a dose-dependent fashion and caused hypertriglyceridaemia. There was no effect of PAF on lipogenesis in isolated hepatocytes. 3. High doses of PAF also decreased hepatic glycogen. 4. All doses of PAF decreased plasma insulin, and this was accompanied by hyperglycaemia, except at the lowest dose. 5. The selective PAF-receptor antagonist L659.989 prevented the stimulation of lipogenesis, but indomethacin did not.

Reciprocal changes in amino acid metabolism in mammary gland and liver of the lactating rat on starvation and refeeding as indicated by the tissue accumulation of alpha-amino[1-14C]isobutyrate

Measurements of the tissue accumulation in vivo and in vitro by hepatocytes and mammary-gland acini of alpha-amino[1-14C]isobutyrate ([1-14C]AIB) were compared in virgin and lactating rats. The results indicate the existence of a reciprocal relationship between mammary gland and liver for AIB accumulation that is dependent on the lactational and the nutritional state of the rat. This suggests that amino acids are preferentially directed to the mammary gland during active lactation.

Pyruvate dehydrogenase activities and rates of lipogenesis during the fed-to-starved transition in liver and brown adipose tissue of the rat

The percentages of pyruvate dehydrogenase complex (PDH) in the active form (PDHa) in two lipogenic tissues (liver and brown adipose tissue) in the fed state were 12.0% and 13.4% respectively. After acute (0.5 h) insulin treatment, PDHa activities had increased by 77% in liver and by 234% in brown fat. Significant decreases in PDHa activities were observed in both tissues by 5 h after the removal of food. The patterns of decline in PDHa activities in the two lipogenic tissues were similar in that the major decreases in activities were observed within the first 7 h of starvation. The significant decreases in PDHa activities observed after starvation for 6 h were accompanied by decreased rates of lipogenesis. Hepatic and brown-fat PDHa activities after acute (30 min) exposure to exogenous insulin were less in 6 h-starved than in fed rats, but the absolute increases in PDHa activities over the 30 min exposure period were similar in fed and 6 h-starved rats. Increases in PDHa activities were paralleled by increases in lipid synthesis in both tissues. Re-activation of PDH in response to insulin treatment or chow re-feeding after 48 h starvation occurred more rapidly in brown adipose tissue than in liver. The results are discussed in relation to the importance of the activity of the PDH complex as a determinant of the total rate of lipogenesis during the fed-to-starved transition and after insulin challenge or re-feeding.

Lipid metabolism during the initiation of lactation in the rat. The effects of starvation and tumour growth

1. The effects of starvation post partum (24 h) and tumour growth pre partum on the initiation of lactation in the rat were studied. 2. Tumour growth decreased food intake at 24 h, but not at 2 days post partum. 3. Pup growth rate increased with hyperphagia; starvation and tumour burden decreased pup growth, and starvation decreased maternal body weight. 4. Starvation decreased gastrointestinal-tract mass; tumour growth decreased gastrointestinal-tract and mammary-gland mass. 5. Mammary-gland DNA-synthesis rate was high immediately post partum, but decreased by day 3 of lactation; starvation and tumour burden decreased this rate, and also decreased gastrointestinal-tract DNA-synthesis rate. 6. Arteriovenous differences for glucose and lactate across the mammary gland did not change with time, nor were they affected by the tumour. Starvation decreased arterial glucose and lactate, and the gland extracted less glucose but produced lactate. 7. Mammary-gland lipogenesis was sensitive to starvation and to tumour growth. 8. In contrast with the gradual development of mammary-gland lipogenic enzyme activities, lipoprotein lipase activity was high in the gland by 2 days post partum; starvation or tumour burden decreased the activity. 9. The mammary gland is sensitive post partum to decreased food intake, and to tumour presence. The effects of the latter are apparently independent of hypophagia.

Polymyxin B diminishes blood flow to brown adipose tissue and lactating mammary gland in the rat. Possible mechanism of its action to decrease the stimulation of lipogenesis on refeeding

Polymyxin B, a cyclic decapeptide antibiotic, increased blood glucose and lactate, and inhibited the stimulation of lipogenesis in interscapular brown adipose tissue and lactating mammary gland of starved-refed virgin and lactating rats respectively. Lipogenesis was not inhibited in white adipose tissue or liver. The antibiotic increased the haematocrit. The relative blood flow to brown adipose tissue and lactating mammary gland was decreased by polymyxin B, and this was accompanied by a decrease in tissue ATP content. In vitro polymyxin B did not affect glucose utilization or conversion into lipid, nor the stimulation by insulin of these processes in brown-adipose-tissue slices. Treatment of rats in vivo with polymyxin B resulted in decreased utilization of glucose in vitro in brown-adipose-tissue slices. Similarly, acini from mammary glands of polymyxin B-treated lactating rats had decreased rates of conversion of [1-14C]glucose to lipid. It is concluded that the effects of polymyxin B may be brought about by decreases in tissue blood flow. The possibility that these effects are secondary to inhibition of glucose utilization cannot be ruled out.

Interleukin-1 and lipid metabolism in the rat

Intravenous administration of a single dose (20 micrograms) of recombinant interleukin-1-beta to virgin, lactating and litter-removed rats rapidly decreased intestinal lipid absorption in all groups. In vivo, oxidation of [14C]triolein to 14CO2 was also significantly decreased by interleukin-1. In addition, the cytokine decreased [14C]lipid accumulation in the mammary gland of lactating rats and in the adipose tissue of virgin and litter-removed rats. The decrease in lipid uptake in the interleukin-treated rats was accompanied by hypertriglyceridaemia; however, there was no significant decrease in tissue lipoprotein lipase activity, except in heart from lactating rats. In contrast, interleukin-1 administration had no effect on lipogenesis in liver, white or brown adipose tissue of virgin rats fed on glucose. These results suggest that interleukin-1 profoundly affects lipid metabolism by delaying intestinal absorption and decreasing tissue uptake.

Re-examination of the putative roles of insulin and prolactin in the regulation of lipid deposition and lipogenesis in vivo in mammary gland and white and brown adipose tissue of lactating rats and litter-removed rats

1. The effects of various treatments to alter either plasma prolactin (bromocryptine administration or removal of litter) or the metabolic activity of the mammary gland (unilateral or complete teat sealing) on the disposal of oral [14C]lipid between 14CO2 production and [14C]lipid accumulation in tissues of lactating rats were studied. In addition, the rates of lipogenesis in vivo were measured in mammary gland, brown and white adipose tissue and liver. 2. Bromocryptine administration lowered plasma prolactin, but did not alter [14C]lipid accumulation in mammary gland or in white and brown adipose tissue. 3. In contrast, complete sealing of teats results in no change in plasma prolactin, but a 90% decrease in [14C]lipid accumulation in mammary gland and a 4-fold increase in white and brown adipose tissue. The rate of lipogenesis in mammary gland was decreased by 95%, but there was no change in the rate in white and brown adipose tissue. Unilateral sealing of teats resulted in a decrease in [14C]lipid accumulation in white adipose tissue. 4. Removal of the litter for 24 h (low prolactin) produced a similar pattern to complete teat sealing, except that there was a 6-fold increase in lipogenesis in white adipose tissue. Re-suckling for 5 h increased plasma prolactin, but did not alter the response seen in litter-removed lactating rats. 5. Changes in lipoprotein lipase activity and in plasma insulin paralleled the reciprocal changes in [14C]lipid accumulation in white and brown adipose tissue and in mammary gland. 6. It is concluded that the plasma insulin is more important than prolactin in regulating lipid deposition in adipose tissue during lactation, and that any effects of prolactin must be indirect.

Comparison of the flux of carbon to hepatic glycogen deposition and fatty acid and cholesterol synthesis on refeeding rats fed ad libitum or meal-fed rats with a chow-diet meal

Meal-fed rats and rats fed ad libitum had similar rates of hepatic glycogen deposition on refeeding with a chow meal. In contrast, the rate of hepatic lipid synthesis (cholesterol plus fatty acids) was 6-fold higher on refeeding in the meal-fed group compared with the 'ad libitum' group. There were no significant differences in the gastrointestinal or hepatic contents of glucose or lactate between the two groups. It is suggested that in the meal-fed group exogenous glucose may be directly converted into glycogen, whereas the substrate for lipid synthesis is a C3 unit.