Evidence that the stimulation of lipogenesis in the mammary glands of starved lactating rats re-fed with a chow diet is dependent on continued hepatic gluconeogenesis during the absorptive period. Effects of a gluconeogenic inhibitory, mercaptopicolinic acid, in vivo

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.

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.

Sequential changes in lipoprotein lipase activity and lipaemia induced by the Yoshida AH-130 ascites hepatoma in rats

The implantation of the Yoshida AH-130 ascites hepatoma to rats resulted in an exponential growth of the tumour cells followed by a late stationary phase. The tumour burden was accompanied by a dramatic decrease in body weight. Tumour growth was associated with a marked hypertriglyceridaemia during the period of exponential growth, while in the stationary phase the plasma triacylglycerol concentration was similar to that observed in the non-tumour-bearing animals. Similar increases were observed, following tumour inoculation, in the plasma concentrations of non-esterified fatty acids and glycerol, suggesting an intense lipolytic activity. These changes in lipaemia were associated with a marked decrease in LPL activity in white adipose tissue; in contrast, LPL activity was increased in the tumour-bearing animals in brown adipose tissue at day 6 following inoculation and in the heart during most of the period studied. Although the presence of the tumour did not induce any changes in blood lactate concentrations, it caused a decrease in circulating glucose; conversely, the tumour induced an important increase in the concentration of circulating ketone bodies, suggesting a metabolic adaptation of the tumour-bearing rats to glucose sparing and alternative fuel utilization. It may be suggested that the hyperlipidaemia present in the Yoshida AH-130 bearing rats is partly due to a decreased LPL activity in white adipose tissue which does not seem to be influenced by changes in insulin circulating concentrations.

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)

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.

Insulin resistance of glucose metabolism in isolated brown adipocytes of lactating rats. Evidence for a post-receptor defect in insulin action

The mechanism responsible for the insulin resistance described in vivo in brown adipose tissue (BAT) of lactating rats was investigated. The effect of insulin on glucose metabolism was studied on isolated brown adipocytes of non-lactating and lactating rats. Insulin stimulation of total glucose metabolism is 50% less in brown adipocytes from lactating than from non-lactating rats. This reflects a decreased effect of insulin on glucose oxidation and lipogenesis. However, the effect of noradrenaline (8 microM) on glucose metabolism was preserved in brown adipocytes from lactating rats as compared with non-lactating rats. The number of insulin receptors is similar in BAT of lactating and non-lactating rats. The insulin-receptor tyrosine kinase activity is not altered during lactation, for receptor autophosphorylation as well as tyrosine kinase activity towards the synthetic peptide poly(Glu4-Tyr1). The defect in the action of insulin is thus localized at a post-receptor level. The insulin stimulation of pyruvate dehydrogenase activity during euglycaemic/hyperinsulinaemic clamps is 2-fold lower in BAT from lactating than from non-lactating rats. However, the percentage of active form of pyruvate dehydrogenase is similar in non-lactating and lactating rats (8.6% versus 8.9% in the basal state, and 37.0% versus 32.3% during the clamp). A decrease in the amount of pyruvate dehydrogenase is likely to be involved in the insulin resistance described in BAT during lactation.

Effect of nutritional status on insulin sensitivity in vivo and tissue enzyme activities in the rat

The hyperinsulinaemic-glucose-clamp technique, in combination with measurement of glucose turnover in conscious unrestrained rats, was used to assess the effects of nutritional status on insulin sensitivity in vivo and glucose metabolism. Liver, heart and quadriceps skeletal-muscle glycogen content and activities of pyruvate dehydrogenase (PDH) and glycogen synthase were measured both basally and at the end of a 2.5 h glucose clamp (insulin 85 munits/h) in rats 6, 24 and 48 h after food withdrawal. Clamp glucose requirement and glucose turnover were unchanged by fasting. Activation of glycogen synthase and glycogen deposition in liver and skeletal muscle during the clamps were also not impaired in rats after a prolonged fast. By contrast with skeletal muscle, activation of cardiac-muscle glycogen synthase and glycogen deposition during the clamps were markedly impaired by 24 h of fasting and were undetectable at 48 h. Skeletal-muscle PDH activity fell with more prolonged fasting (6 h, 15.3 +/- 3.4%; 24 h, 4.7 +/- 0.7%; 48 h, 4.3 +/- 0.6% active; P less than 0.005), but at 24 and 48 h was stimulated by the clamp to values unchanged by the duration of fasting. Stimulation of cardiac PDH activity by the clamp was, however, impaired in rats fasted for 24 or 48 h. Basal hepatic PDH did not change significantly with fasting (6 h, 5.3 +/- 1.1%; 24 h, 4.6 +/- 0.7%; 48 h, 3.9 +/- 0.5%), and, although it could be partly restored at 24 h, very little stimulation occurred at 48 h. Hepatic pyruvate kinase and acetyl-CoA carboxylase activity were both stimulated by the clamps, and this was not impaired with more prolonged fasting. During the glucose clamps, blood concentrations of lactate, pyruvate and alanine were increased to a greater extent in rats fasted for 24 and 48 h than in rats studied 6 h after food withdrawal. The findings suggest that, although sensitivity to insulin of whole-body glucose disposal is unchanged with fasting, there may be qualitative differences in the metabolism of glucose.

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.

Regulation by insulin of glucose metabolism in mammary gland of anaesthetized lactating rats. Stimulation of phosphofructokinase-1 by fructose 2,6-bisphosphate and activation of acetyl-CoA carboxylase

The effect of insulin on glucose metabolism in mammary gland was studied by the euglycaemic/hyperinsulinaemic-clamp technique. Measurement of metabolite concentrations and enzyme activities in the mammary gland suggests two sites of action of insulin: phosphofructokinase-1 and acetyl-coA carboxylase. The increase in phosphofructokinase-1 activity could be linked to the 2-fold increase in fructose 2,6-bisphosphate concentration, since no change in maximal activity and in sensitivity of the enzyme toward fructose 6-phosphate was detected in vitro.

An 'in situ' perfusion system suitable for investigating mammary-tissue metabolism in the lactating rat. Hormonal regulation of acetyl-CoA carboxylase

A technique is described for the non-recirculating perfusion of inguinal/abdominal mammary tissue in situ in anaesthetized lactating rats. Tissue viability was maintained, without resort to infusion of vasoactive chemicals which may also be effectors of cellular metabolism, for at least 90 min. Total tissue adenine nucleotides (per mg of DNA) were somewhat decreased in perfused relative to non-perfused mammary tissue. DNA content (per g wet wt. of tissue) was diminished after 90 min of perfusion to approx. 65% of its value in control tissue. Adenylate energy-charge ratios were lower in perfused tissue in the absence of hormones than in control tissue. They were increased to control values by the presence of either insulin or isoprenaline in the perfusate. No changes occurred in flow rate of the perfusate that might account for these increases. In mammary tissue perfused without addition of hormones, acetyl-CoA carboxylase activities were similar to those measured in control tissue samples, although activity-ratio measurements implied some increase in the phosphorylation of this enzyme. Insulin or isoprenaline increased the activity of acetyl-CoA carboxylase, especially when this was measured at low concentrations of citrate. Confirming conclusions from previous experiments with mammary acini and explant preparations, insulin activated acetyl-CoA carboxylase in mammary tissue, but inhibition of its activity was not mediated by cyclic AMP.

Hepatic uptake of gluconeogenic substrates in late-pregnant and mid-lactating rats

Lactate uptake by liver is markedly increased in late-pregnant and mid-lactating rats without concomitant changes in its availability. Glycerol contribution to the liver 3-C unit uptake is only significant at term gestation (50% of lactate uptake) but almost negligible at mid-lactation (10% of lactate uptake). Pyruvate is only taken up by the liver of 15-day lactating rats. As a general trend, the livers of either pregnant or lactating rats are provided with an enhanced capacity to take up gluconeogenic substrates.

Tissue-specific changes in the ability of insulin and noradrenaline to activate pyruvate dehydrogenase in vivo during lactation in the rat

Changes are described in the total pyruvate dehydrogenase (PDH) activity, the proportion of PDH in the active state and its control by insulin and noradrenaline in vivo, in white adipose tissue, liver, skeletal muscle and mammary gland with pregnancy, lactation and on weaning. Lactation resulted in a decrease in total PDH in white adipose tissue and an increase in the mammary gland, whereas the proportion in the active state decreased in muscle and increased in the mammary gland. The ability of insulin to activate PDH of white adipose tissue was lost during lactation, whereas it was retained by the other tissues. The ability of noradrenaline to activate PDH was decreased in white adipose tissue but increased in liver during lactation. These various adaptations should limit the use of glucose and lactate carbon by adipose tissue and skeletal muscle during lactation and thereby facilitate their preferential utilization by the mammary gland.

The regulation of lipogenesis in vivo in the lactating mammary gland of the rat during the starved-refed transition. Studies wtih acarbose, a glucosidase inhibitor

Depression of carbohydrate digestion by oral administration of acarbose, a glucosidase inhibitor, led to a 75% inhibition of the re-activation of lipogenesis in vivo in the mammary gland of 18 h-starved lactating rats refed with 5 g of chow diet. Rates of [1-14C]glucose incorporation in vitro into lipid and CO2 in mammary-gland acini isolated from refed animals were elevated compared with acini from starved rats, but acarbose treatment completely prevented this stimulation. Gastric intubation of glucose led to a large stimulation of lipogenesis in the mammary gland of starved lactating rats, similar to that induced by refeeding with chow diet; this was dependent on the amount of glucose given and the time elapsed between glucose administration and injection of 3H2O for the measurement of lipogenesis. The switch-on of lipogenesis in the mammary gland of starved lactating rats, by refeeding or by intubation of glucose, was associated with a decrease in the ratio of [glucose 6-phosphate]/[fructose 1,6-bisphosphate] in the gland, indicative of an increase in phosphofructokinase activity. A time-course study revealed that the ratio decreased rapidly over the first 30 min of chow refeeding, after which a large surge in lipogenesis was seen. Acarbose, given 25 min after the onset of refeeding, led to a stepwise increase in the ratio, in parallel with the observed decrease in lipogenic activity. It is concluded that the control of lipogenesis in the mammary gland is closely linked to the availability of dietary carbohydrate. An important site of regulation of lipogenesis in the gland appears to be at the level of phosphofructokinase. A possible role of insulin in the regulation of phosphofructokinase activity, and the acute modulation of insulin-sensitivity in the gland during the starved-refed transition, are discussed.

Time course of changes in plasma glucose and insulin concentrations and mammary-gland lipogenesis during re-feeding of starved conscious lactating rats

Temporal changes in circulating insulin concentrations were measured during re-feeding of 18 h-starved lactating rats. Insulin concentrations rose rapidly over the first 20 min of re-feeding with 5 g of chow diet, and then sharply declined between 20-30 min and remained low for the rest of the 90 min experimental period. Lipogenic activity in the mammary gland also exhibited a peak during re-feeding, but there was a clear time lag between the insulin response and the lipogenic response. Blood-flow measurements failed to show any major increase to the tissue during this activation of lipogenesis. Acute suppression of insulin secretion at 30 min (after the initial surge) abolished the switch-on of lipogenesis, suggesting that the insulin-sensitivity of the gland may be acutely enhanced over this period of re-feeding.

Hepatic glycogen synthesis on carbohydrate re-feeding after starvation. A regulatory role for pyruvate dehydrogenase in liver and extrahepatic tissues

Glucose administration to 48 h-starved rats increased hepatic glucose, lactate, pyruvate and glycogen concentrations and re-activated PDH (pyruvate dehydrogenase complex) in kidney, but not in heart or liver. Dichloroacetate together with glucose re-activated PDH in all three tissues, decreased hepatic lactate and pyruvate concentrations and impaired glycogen resynthesis. Thus on re-feeding, delayed PDH re-activation is important for provision of precursors for hepatic glyconeogenesis.

Effect of lactation on insulin sensitivity of glucose metabolism in rat adipocytes

During lactation glucose metabolism in paraovarian adipocytes is characterized by a 40 and 80% decrease of glucose incorporation into CO2 and fatty acids in the presence of insulin. In contrast with the stimulation by insulin of glucose incorporation into lactate, glycerol remains unchanged. As a result, insulin sensitivity of total glucose metabolism (oxidation and lipid synthesis) is not altered in adipocytes from lactating rats.

Role of fructose 2,6-bisphosphate in mammary gland of fed, starved and re-fed lactating rats

The fructose 2,6-bisphosphate (Fru-2,6-P2) content and intracellular concentration of lactating mammary gland was measured in fed, starved and re-fed rats. There was little or no change on starvation, and about 1.5-fold rise on re-feeding, contrasting with estimated glycolytic changes of about 10-fold. The 6-phosphofructokinase (PFK-1) activity of mammary extracts was highly sensitive to added Fru-2,6-P2 under all conditions examined, and appeared to approach saturation at physiological concentrations of this effector. The activity of mammary PFK-1 measured under optimal and 'physiological' conditions suggested that this enzyme operates in vivo at about 24% of maximal rate, and is likely to be an important rate-limiting factor in mammary glycolysis.