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

Differences in preterm and term milk fatty acid compositions may be caused by the different hormonal milieu of early parturition

INTRODUCTION

The hormonal milieus of pregnancy and lactation are driving forces of nutrient fluxes supporting infant growth and development. The decrease of insulin sensitivity with compensatory hyperinsulinemia with advancing gestation, causes adipose tissue lipolysis and hepatic de novo lipogenesis (DNL).

SUBJECTS AND METHODS

We compared fatty acid (FA) contents and FA-indices for enzyme activities between preterm (28-36 weeks) and term (37-42) milks, and between colostrum (2-5 days), transitional (6-15) and mature (16-56) milks. We interpreted FA differences between preterm and term milks, and their changes with lactation, in terms of the well known decrease of insulin sensitivity during gestation and its subsequent postpartum restoration, respectively.

RESULTS

Compared with term colostrum, preterm colostrum contained higher indices of DNL in the breast (DNL-breast) and medium chain saturated-FA (MCSAFA), and lower DNL-liver and monounsaturated-FA (MUFA). Preterm milk also had higher docosahexaenoic acid (DHA) in colostrum and transitional milk and higher arachidonic acid (AA) in mature milk. Most preterm-term differences vanished with advancing lactation. In both preterm and term milks, DNL-breast and MCSAFA increased with advancing lactation, while DNL-liver, MUFA, long chain SAFA and AA decreased. DHA decreased in term milk. MUFA was inversely related to MCSAFA in all samples, correlated inversely with PUFA in colostrum and transitional milks, but positively in mature milk. MCSAFA correlated inversely with PUFA in mature milk.

CONCLUSION

Higher maternal insulin sensitivity at preterm birth may be the cause of lower MUFA (a proxy for DNL-liver) and higher MCSAFA (a proxy for DNL-breast) in preterm colostrum, compared with term colostrum. Restoring insulin sensitivity after delivery may be an important driving force for milk FA-changes in early lactation.

Pre-clinical and behavioural toxicity profile of PAMAM dendrimers in mice

A toxicity study of different polyamidoamine (PAMAM) dendrimers in Swiss albino mice was performed with a clinical overview. In the current study, mice were treated with various G4 dendrimers (NH 2 and OH surface groups) at different dosage levels (low, medium and high) via the intraperitoneal route for 15 consecutive days, followed by a 15 day recovery period. The toxicity profile was investigated in mice for their general behaviour, feed intake, body weight, carbohydrate, lipid and protein metabolism, haematological parameters, histopathology and cell viability. No significant differences were observed in feed intake, body weight and organ weight among the various dendrimer-fed groups and control. In general, dendrimers have no significant effect on the carbohydrate and lipid metabolism. However, declined glucose levels in the high-NH 2 dose group indicates the interference of this dosage level with glucose metabolism. Although all the test groups showed activity in the normal range, the high-NH 2 group showed comparatively higher alanine aminotransferase and aspartate aminotransferase activity. Minor, but insignificant ( p > 0.05), differences were observed in the red blood cells, haematocrit value and haemoglobin of the treated group when compared to the control group in a dose-dependent manner, but these recovered during the recovery period. There was no effect on other haematological parameters. Histopathological evaluation of dendrimer-treated groups did not reveal any abnormalities in the low- and medium-dose groups, but at a high dose level, toxicity was observed in the liver and kidney. However, after the recovery period, toxicity in the high dose level was not found. A cell viability study of cationic dendrimers has shown reduced cytotoxicity after the complexation with a guest molecule such as indomethacin. This study showed that dendrimers are not toxic, in general. The high dose of G4-NH 2 displayed some toxicity; however, that too was recovered after some time.

Body weight and fat deposition in prolactin receptor-deficient mice

To explore the roles of the lactogens in adipose tissue development and function, we measured body weight, abdominal fat content, and plasma leptin concentrations in a unique model of lactogen resistance: the PRL receptor (PRLR)-deficient mouse. The absence of PRLRs in knockout mice was accompanied by a small (5-12%), but progressive, reduction in body weight after 16 weeks of age. Females were affected to a greater degree than males. The reduction in weight in female PRLR-deficient mice (age 8-9 months) was associated with a 49% reduction in total abdominal fat mass and a 29% reduction in fat mass expressed as a percentage of body weight. Lesser reductions were noted in male mice. Plasma leptin concentrations were reduced in females but not in males. That the reductions in abdominal fat may reflect in part the absence of lactogen action in the adipocyte is suggested by the demonstration of PRLR messenger RNA in normal mouse white adipose tissue. Nevertheless, steady state levels of PRLR messenger RNA in mature adipocytes are very low, suggesting that the effects of lactogens might be mediated by other hormones or cellular growth factors. Our observations suggest roles for the lactogens in adipose tissue growth and metabolism in pregnancy and postnatal life.

Subacute toxicity of alpha-ergocryptine in Sprague-Dawley rats. 2: metabolic and hormonal changes

The present study describes the metabolic changes observed in a dietary subacute toxicity experiment with the ergot alkaloid alpha-ergocryptine in Sprague-Dawley rats. The observed effects on metabolic and hormonal parameters were described separately from the general toxicological effects, in view of the important role of dopamine agonists in metabolism (e.g. ergot alkaloids in fescue toxicosis). The rats were fed 0, 4, 20, 100 or 500 mg ergocryptine/kg diet for 28-32 days (equal to 0, 0.36, 1.7, 8.9 and 60 mg ergocryptine/kg body weight/day for females and 0, 0.34, 1.4, 6.6 and 44 mg ergocryptine/kg body weight/day for males). Total cholesterol and high-density lipoprotein (HDL)-cholesterol were decreased dose dependently in females but the ratio HDL-cholesterol/total cholesterol was only decreased at 20 mg/kg body weight. Triglycerides and glucose concentrations were decreased in the highest dose groups of both sexes. Serum urea concentrations were increased in the 20, 100 and 500 mg/kg dose groups. Insulin, glucagon and liver glycogen were increased in the highest dose group at the end of the study, when the animals were allowed to eat prior to blood sampling and necropsy. Prolactin, T4 and FT4 were decreased in the 20, 100 and 500 mg/kg dose groups of both sexes. Follicle-stimulating hormone (FSH) was decreased in the 20, 100 and 500 mg/kg female dose groups and luteinizing hormone (LH) was increased in the 20, 100 and 500 mg/kg male dose groups. It is postulated that the observed effects on food intake, metabolism (lipid and carbohydrate) and hormonal parameters are due to an interaction of ergocryptine with central dopaminergic activities, which comprise a major functional component of a central regulatory system for metabolism.

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.

Regulation of milk lipid secretion and composition

Triacylglycerols make up 98% of the lipid content of milk, ranging in different species from 0 to 50% of the total milk volume. The fatty aid composition of the triacylglycerols depends on the species, the dietary fatty acid composition, and the carbohydrate-to-lipid ratio of the diet. The rate of lipid synthesis in the lactating mammary gland depends on the stage of mammary development and is decreased by fasting and starvation in ruminants and rodents but not in species that fast during lactation, such as seals and hibernating bears. Regulatory agents include insulin, prolactin, and non-esterified fatty acids. Dietary trans fatty acids may depress milk lipid synthesis under certain conditions. Evidence is presented that fatty acids may play a major regulatory role in acute changes in de novo mammary fatty acid synthesis, acting primarily on the activity of acetyl coenzyme A carboxylase.

Decreased sensitivity of very-low-density lipoprotein secretion to the inhibitory effect of insulin in cultured hepatocytes from lactating rats

Hepatocytes were prepared from 10-11-day lactating rat dams and from lactating dams which had been weaned for periods of either 1-2 days or 7 days. Hepatocytes from each group were cultured for periods of up to 48 h in a chemically defined medium. Compared with those from the 7-day weaned animals, hepatocytes from the lactating rats were resistant to the inhibitory effects of insulin on the secretion of very-low-density lipoprotein (VLDL) triacylglycerol (TAG). These differences persisted for up to 48 h in culture. Hepatocytes from the 1-2 day weaned animals remained relatively insulin-resistant in this respect. Similar differences in the response to insulin were not observed for the secretion of VLDL apolipoprotein B. TAG production increased and ketogenesis decreased in the hepatocytes from the lactating compared with those from the 7-day weaned rats. Insensitivity of the liver to the normal effects of insulin on the secretion of VLDL TAG may arise from a need to maintain an adequate flux of hepatic lipids to the lactating mammary gland in order to meet the large demand for milk-fat production.

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.

Coordinated regulation of hormone-sensitive lipase and lipoprotein lipase in human adipose tissue in vivo: implications for the control of fat storage and fat mobilization

The enzymes lipoprotein lipase (LPL, EC 3.1.1.34) and hormone-sensitive lipase (HSL, EC 3.1.1.3) apparently catalyze opposing functions in white adipose tissue: the former is concerned with fat storage, the latter with fat mobilization. We have studied their regulation in vivo in normal subjects in the postabsorptive state and after eating meals of different compositions, by measurement of arteriovenous concentration differences for triacylglycerol, non-esterified fatty acids and glycerol across a subcutaneous adipose depot. The two enzymes are regulated in a broadly reciprocal manner: in the overnight-fasted state, HSL is more active, but after a meal HSL is suppressed whilst LPL is activated. The movement of fatty acids in and out of adipose tissue appears to be driven by concentration gradients generated by regulation of these two enzymes, and also by activation, in the postprandial period, of the process of fatty acid esterification. The results show some interesting and perhaps unexpected features of metabolic regulation. Of the fatty acids generated by the action of LPL on circulating TAG, a large proportion is released directly into the venous plasma: close to 100% in the overnight-fasted state, and 50% or more at the peak of LPL action after a meal, making what appear reasonable assumptions. We suggest that this apparent 'inefficiency' of fat storage reflects the energetic cost of maintaining precise control over such a fundamental process. Although LPL is usually thought of as the enzyme regulating fat deposition, in fact the fatty acids and glycerol it releases from circulating TAG represent a substantial proportion of those released from adipose tissue, especially in the postprandial state. In addition, although HSL is considered the enzyme responsible for fat mobilization, suppression of its activity is essential to normal regulation of fat deposition. Thus, fat storage and fat mobilization during normal daily life are controlled by coordinated regulation of a number of enzymatic processes in white adipose tissue.

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)

Effect of starvation on lipoprotein lipase activity in different tissues during gestation in the rat

This study was addressed to determine whether the tissue-specific LPL activity response to fasting differs between nonpregnant and pregnant rats over the course of pregnancy. Fed and 24-h fasted rats were studied at days 12, 15 or 20 of gestation and were compared to virgin controls. In fed rats at days 15 and 20 of gestation LPL activity decreased in lumbar adipose tissue and the heart and liver, and increased in mammary gland tissue. Fasting decreased LPL activity in lumbar adipose tissue in 12 day pregnant and virgin rats and in mammary gland tissue in pregnant rats at 15 and 20 days of gestation and in virgin rats, whereas it increased LPL activity in heart tissue in rats at day 15 and 20 and in liver at day 20 of gestation. Plasma triacylglycerols were higher in 20 day pregnant rats than in the other groups when fed and this difference was even more noticeable in the fasting condition where the plasma beta-hydroxybutyrate level also reached the highest value in the 20 day pregnant rats. Since tissue LPL activity controls the hydrolysis and uptake of circulating triacylgylcerols, the present results indicate that in fed rats after the 15th day of gestation circulating triacylglycerols are preferentially taken up by the mammary gland instead of being taken up by adipose tissue and heart. However, after fasting, circulating triacylglycerols are driven to the heart and liver in the late pregnant rat, and become a major source for fatty acid oxidation, an effect that seems to be specially evident in the liver of the 20 day pregnant rat where there is an intense increase in LPL activity and the triacylglycerols become preferential substrates for ketone body production.

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.

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.

Regulation of lipoprotein lipase activity and mRNA in the mammary gland of the lactating mouse

We examined the effects of reproductive stage and fasting on lipoprotein lipase (LPL) activity and mRNA in the mouse mammary gland. Heparin-releasable and cell-associated LPL activity rose immediately after birth, followed 1-2 days later by an increase in LPL mRNA. Fasting decreased LPL activity in the mammary gland at all reproductive stages. During lactation, both milk and heparin-releasable LPL were substantially decreased by an overnight fast, whereas cell-associated LPL was less affected and LPL mRNA did not change. These studies indicate that the extracellular, heparin-releasable, fraction of mammary LPL activity responds most rapidly to alterations in physiological state, usually accompanied by smaller changes in cellular enzyme activity. Changes in the level of LPL mRNA were seen only during the transition from pregnancy to lactation, and these tended to follow, rather than precede, changes in enzyme activity. We conclude that in the mammary gland as in adipose tissue, LPL is regulated primarily at the translational and post-translational level.