Tissue-specific effects of rapid tumour growth on lipid metabolism in the rat during lactation and on litter removal.

1. The effect of tumour burden on lipid metabolism was examined in virgin, lactating and litter-removed rats. 2. No differences in food intake or plasma insulin concentrations were observed between control animals and those bearing the Walker-256 carcinoma (3-5% of body wt.) in any group studied. 3. In virgin tumour-bearing animals, there was a significant increase in liver mass, blood glucose and lactate, and plasma triacylglycerol; the rate of oxidation of oral [14C]lipid to 14CO2 was diminished, and parametrial white adipose tissue accumulated less [14C]lipid compared with pair-fed controls. 4. These findings were accompanied by increased accumulation of lipid in plasma and decreased white-adipose-tissue lipoprotein lipase activity. 5. In lactating animals, tumour burden had little effect on the accompanying hyperphagia or on pup weight gain; tissue lipogenesis was unaffected, as was tissue [14C]lipid accumulation, plasma [triacylglycerol] and white-adipose-tissue and mammary-gland lipoprotein lipase activity. 6. On removal (24 h) of the litter, the presence of the tumour resulted in decreased rates of lipogenesis in the carcass, liver and white and brown adipose tissue, decreased [14C]lipid accumulation in white adipose tissue, but increased accumulation in plasma and liver, increased plasma [triacylglycerol] and decreased lipoprotein lipase activity in white adipose tissue. 7. The rate of triacylglycerol/fatty acid substrate cycling was significantly decreased in white adipose tissue of virgin and litter-removed rats bearing the tumour, but not in lactating animals. 8. These results demonstrate no functional impairment of lactation, despite the presence of tumour, and the relative resistance of the lactating mammary gland to the disturbance of lipid metabolism that occurs in white adipose tissue of non-lactating rats with tumour burden.     

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.     

The utilization of ketone bodies by the interscapular brown adipose tissue of the rat

The activities of 3-oxo acid-CoA transferase (EC 2.8.3.5, 13-15 micromol/min per g) and acetoacetyl-CoA thiolase (EC 2.3.1.9, 18-21 micromol/min per g) in interscapular brown adipose tissue of the rat are comparable to the activities reported for heart and kidney. The incorporation of D-3-hydroxy[3-14C]butyrate into lipid in vivo was about 30-fold higher in interscapular brown adipose tissue than in white adipose tissue of virgin rats. In lactating rats, the mammary gland was the major site of ketone body incorporation into lipid and incorporation of D-3-hydroxy-[3-14C]butyrate into lipid in brown adipose tissue was lower than in virgin rats. After an oral load of medium chain triacylglycerol, which inhibits lipogenesis in lactating mammary gland, the incorporation of ketone bodies into lipid was decreased in mammary gland but increased in brown adipose tissue. The rate of oxidation of D-3-hydroxy[3-14C]butyrate by brown adipose tissue slices in vitro was higher than the rate of incorporation into lipid.     

Tissue-specific effects of starvation and refeeding on the disposal of oral [1-14C]triolein in the rat during lactation and on removal of litter.

1. The effects of starvation and refeeding on the disposal of oral [14C]triolein between 14CO2 production and 14C-lipid accumulation in tissues of virgin rats, lactating rats and lactating rats with pups removed were studied. 2. Starvation (24 h) increased 14CO2 production in lactating rats and lactating rats with pups removed to values found in virgin rats. This increase was accompanied by decreases in 14C-lipid accumulation in mammary gland and pups of lactating rats and in white and brown adipose tissue of lactating rats with pups removed. 3. Short-term (2 h) refeeding ad libitum decreased 14CO2 production in lactating rats and lactating rats with pups removed, and restored the 14C-lipid accumulation in mammary glands plus pups and in white and brown adipose tissue respectively 4. Insulin deficiency induced with mannoheptulose inhibited the restoration of 14C-lipid accumulation in white adipose tissue on refeeding of lactating rats with pups removed, but did not prevent the restoration of 14C-lipid accumulation in mammary gland. 5. Changes in the activity of lipoprotein lipase in mammary gland and white adipose tissue paralleled the changes in 14C-lipid accumulation in these tissues. 6. It is concluded that 14C-lipid accumulation in mammary gland may not be affected by changes in plasma insulin concentration and that it is less sensitive to starvation than is lipogenesis or lactose synthesis. This has the advantage that the milk lipid content can still be maintained from hepatic very-low-density lipoprotein for a period after withdrawal of food. The major determinant of the disposal of oral 14C-triolein appears to be the total tissue activity of lipoprotein lipase. When this is high in mammary gland (fed lactating rats) or white adipose tissue (fed lactating rats with pups removed), less triacylglycerol is available for the muscle mass and consequently less is oxidized.     

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.     

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.     

Effects of lactation on L-leucine metabolism in the rat. Studies in vivo and in vitro.

1. The turnover rate of L-[1-14C]leucine was increased by 35% in lactating rats compared with virgin rats. Starvation or removal of pups (24 h) returned the value to that of the virgin rat. 2. Incorporation of L-[U-14C]leucine into lipid and protein of mammary glands of lactating rats in vivo increased 7-fold and 6-fold respectively compared with glands of virgin rats. Lactation caused no change in the incorporation of L-[U-14C]leucine into hepatic lipid and protein. 3. The production of 14CO2 from L[l-14C]leucine (in the presence of glucose) was similar in isolated acini from glands of fed (chow) and starved lactating rats. Feeding with a 'cafeteria' diet caused a slight decrease, and removal of pups a large decrease, in the oxidative decarboxylation of leucine. 4. Oxidation of L-[2-14C]leucine to 14CO2 was increased about 3-fold in acini from starved lactating rats or lactating rats fed on a 'cafeteria' diet compared with rats fed on a chow diet. Insulin decreased the formation of 14CO2 in all three situations. 5. Incorporation of L-[U-14C]- and [2-14C]-leucine into lipid was decreased in acini from starved lactating rats and lactating rats fed on a 'cafeteria' diet. Insulin tended to increase the conversion of [2-14C]leucine into lipid, but this was significant only in the case of the acini from 'cafeteria'-fed rats. 6. Experiments with (-)-hydroxycitrate indicate that the major route for conversion of leucine carbon into lipid in acini is via citrate translocation from the mitochondria. 7. The physiological implications of these findings are discussed.     

Expression, activity, and localization of hormone-sensitive lipase in rat mammary gland during pregnancy and lactation

We examined the presence of hormone-sensitive lipase (HSL) in mammary glands of virgin, pregnant (12, 20, and 21 days), and lactating (1 and 4 days postpartum) rats. Immunohistochemistry with antibody against rat HSL revealed positive HSL in the cytoplasm of both alveolar epithelial cells and adipocytes. In virgin rats, immunoreactive HSL was observed in mammary adipocytes, whereas diffuse staining was found in the epithelial cells. Positive staining for HSL was seen in the two types of cells in pregnant and lactating rats. However, as pregnancy advanced, the staining intensity of immunoreactive HSL increased in the epithelial cells parallel to their proliferation, attaining the maximum during lactation. An immunoreactive protein of 84 kDa and a HSL mRNA of 3.3. kb were found in the rat mammary gland as in white adipose tissue. Both HSL protein and activity were lower in mammary glands from 20 and 21 day pregnant rats than from those of virgin rats, although they returned to virgin values on days 1 and 4 of lactation. Mammary gland HSL activity correlated negatively to plasma insulin levels. Immunoreactive HSL and HSL activity were found in lactating rats' milk. The observed changes indicate an active role of HSL in mammary gland lipid metabolism.     

Influence of starvation/refeeding transition on lipogenesis and NADPH producing systems in adipose tissue, mammary gland and liver at mid-lactation

Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and malic enzyme are enzymes involved in NADPH synthesis. Their specific activities and glucose utilization by isolated cell systems have been measured in adipose tissue and mammary gland from mid-lactating rats during starvation/refeeding transition. Starvation for 24 h produced a 75-90% decrease in the specific activities of these NADPH producing systems in mammary gland. Acinis isolated from the gland of starved rats had a lower production of CO2, fatty acids and triacylglycerols from (1-14C)glucose and (6-14C)-glucose than did gland from control rats. The activities of these enzymes in adipose tissue were very low and did not undergo any measurable alteration with starvation. The ability of adipocytes from well fed lactating rats to synthesize fatty acids from (1-14C)glucose was completely blocked. However, starvation is accompanied by a marked decrease in glucose incorporation into triacylglycerols. All the variations observed "in vivo" and "in vitro" in mammary gland returned almost to normal values by refeeding the starved lactating rats.     

Enzyme and protein turnover in adipose tissue in pregnancy and lactation

1. The relative rates of synthesis of fatty acid synthetase and the pyruvate dehydrogenase complex were measured in adipose tissue in virgin, late pregnant and early lactating rats after injection of l-[2,3-³H]alanine. The relative rate of synthesis of fatty acid synthetase decreased approximately 4-fold between 2 days prepartum and 2 days postpartum. The relative rate of synthesis of the pyruvate dehydrogenase complex did not change. 2. The fractional rate of total adipose tissue protein synthesis was measured by constant infusion with l-[U-¹⁴C]tyrosine. Total protein synthesis did not differ in virgin and 2-day lactating rats. The half-life of adipose tissue protein in virginn rats determined by decay of ¹⁴C label from protein after injection of NaH¹⁴CO₃ was 86.9 ± 6.7 h. This is in close agreement witht the half-life (82.5 ± 20 h) calculated from the fractional rate of protein synthesis determined by the constant infusion method.     

Studies on the mode of uptake of blood triglycerides by the mammary gland of the lactating goat. The uptake and incorporation into milk fat and mammary lymph of labelled glycerol, fatty acids and triglycerides

1. The mode of uptake of the precursors of milk fat by the mammary gland of the lactating goat has been examined by infusing radioactive fatty acids, glucerol or doubly labelled triglycerides into the mammary artery or jugular vein of animals surgically prepared to permit samples of arterial and venous blood to be withdrawn without disturbance to the animal. 2. Acetate was taken up by the mammary gland and incorporated into milk fat. The decrease in the specific radioactivity of blood acetate across the gland was evidence of acetate production, but there was no significant release of labelled lipid from the mammary gland. 3. When labelled long-chain fatty acids or glycerol were infused into the lactating goat, there was extensive transfer of radioactivity into milk in spite of the absence of net uptake of substrate by the mammary gland. The decrease in the specific radioactivity of each substrate across the mammary gland, however, showed that both fatty acids and glycerol were simultaneously taken up and released by mammary tissue. 4. The infusion of chylomicra and triglyceride emulsions labelled with (3)H and (14)C revealed that both glycerol and fatty acids were released during triglyceride uptake by mammary tissue. Changes in the (3)H/(14)C ratio during the transfer of triglyceride from blood into milk showed that at least 80% of the triglyceride was hydrolysed during uptake, but the potential re-utilization of both products of hydrolysis for triglyceride synthesis in mammary tissue implied that only a minimum value could be obtained from the change in the ratio. 5. The time-course of the transfer of (3)H and (14)C into milk and lymph were closely similar after the infusion of [2-(3)H]glycerol tri[1-(14)C]oleate or of a mixture of [2-(3)H]glycerol and [1-(14)C]oleate. 6. The results were consistent with the hypothesis that plasma triglycerides are extensively or completely hydrolysed during mammary uptake.     

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 liverin 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 intestina 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 of³H-oleate. In contrast, LPS decreased the accumulation of radioactivity in mammary gland after injection of²H-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. It is concluded that the inhibitory effect of LPS on mammary gland lipogenesis and uptake of exogenous lipid is primarily due to sensitivity of this tissue to the rate of delivery of glucose from the intestine.     

The effect of dietary fat on lipogenesis in mammary gland and liver from lactating and virgin mice

1. Virgin and lactating C(3)H mice maintained on laboratory chow were transferred to a high-fat (15% corn oil) or a fat-free diet 3 days before being killed. 2. The linoleate content of liver, mammary gland and milk was decreased in lactating mice given the fat-free diet but was increased in those fed on the high-fat diet. Changes in linoleate content and mammary gland followed a similar but much less marked trend in virgin animals. 3. Hepatic fatty acid synthesis in lactating and virgin mice fed on the fat-free diet was higher than in corresponding animals fed on either the chow or the high-fat diet. The lipogenic capacity of livers from mice fed on either the chow or the high-fat diet was greater in lactating than in virgin animals. These changes in hepatic lipogenic capacity were accompanied by alterations in the specific activities of certain enzymes involved in fat synthesis. 4. Mammary gland from virgin and lactating animals showed no such adaptation to dietary fat. Results indicate that fatty acid synthesis in neither mammary-gland parenchymal cells nor mammary-gland adipose cells can be influenced by dietary fat in the same way as in the hepatocyte.     

Effect of thyroidectomy on pathways of glucose metabolism in lactating rat mammary gland

1. Assessment of the overall metabolic changes in lactating mammary gland after thyroidectomy has been made by measurement of the incorporation of (14)C from specifically labelled glucose, pyruvate and acetate into (14)CO(2) and (14)C-labelled lipid in the experimental rats and in sham-operated control animals. 2. Thyroidectomy depressed the oxidation of (14)C-labelled substrates, an effect still apparent when the control rats were pair-fed with thyroidectomized rats; however, the ratio of oxidation of [1-(14)C]glucose/oxidation of [6-(14)C]glucose was unaltered. In parallel with these studies it was revealed that the activities of hexokinase, glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and NADP-linked isocitrate dehydrogenase were all lower in the thyroidectomized group than in the pair-fed control group. 3. Thyroidectomy also lowered the incorporation of (14)C-labelled substrates into (14)C-labelled lipid, an effect further studied by measurement of the activities of citrate-cleavage enzyme and acetate thiokinase. Restricting the food intake of the control rats to that of the thyroidectomized group lowered the activity of citrate-cleavage enzyme, but no further depression was observed on thyroidectomy. The oxidized and reduced nicotinamide nucleotide content of mammary tissue was shown to be decreased in the thyroidectomized rats compared with the control rats.     

Regulation of cholesterol synthesis in the liver and mammary gland of the lactating rat.

The activity of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase; EC 1.1.1.34) in the lactating mammary gland of rats killed between 10:00 and 14:30 h was 2-3 times that in the livers of the same animals. In contrast, after injection of 3H2O in vivo, the rate of appearance of 3H in the cholesterol of the gland was much lower than that in the liver. In the mammary gland of virgin and non-lactating animals, the activity of HMG-CoA reductase was less than 10% of that of the lactating gland. The activity of HMG-CoA reductase in the lactating mammary gland was significantly (P less than 0.005) lower at midnight than at mid-day, and appeared to show an inverse relationship to the activity of the liver enzyme. However, there was no corresponding change in the incorporation of 3H into the gland cholesterol. Withdrawal of food for 6h had no effect on the activity of HMG-CoA reductase in the lactating mammary gland, but resulted in a significant decrease (P less than 0.005) in that of the liver. Starvation of lactating rats for 24h produced a significant decrease (P less than 0.005) in the activity of the enzyme in both organs. There was also a significant decline in the rate at which 3H2O was incorporated in vivo into the cholesterol of both organs (liver, P less than 0.05; gland, P less than 0.005). Giving a high-fat palatable diet together with chow to lactating animals led to a decline in HMG-CoA reductase activity in the mammary gland, but not in liver. This decrease in the gland was not accompanied by a corresponding decline in the apparent rate of cholesterol synthesis.     

Alterations in the activities of rat tissue hexose monophosphate dehydrogenases in response to premature weaning and dietary restriction at mid-lactation

Summary The activities of the hexose monophosphate dehydrogenases increased in adipose tissue, remained unchanged in liver and decreased in mammary gland following the weaning of rats at mid-lactation (day 14). When dietary intake was restricted at mid-lactation, the activities of the hexose monophosphate dehydrogenases increased in adipose tissue, decreased in liver, but were unaltered in mammary gland. Premature weaning on day 14 postpartum resulted in maternal increases in both plasma insulin and glucose, which peaked at day 16. The plasma insulin levels decreased from day 14 to day 18 postpartum in the normal lactating rat, and a similar trend was observed for animals on a restricted dietary intake. Daily food consumption in the lactating rat decreased from 50 g to 20 g after premature weaning. The live weight of pups raised on dams given a restricted food intake from day 14 had decreased by day 17 postpartum, whereas an increase in daily live weight gain was recorded for the litters from the lactating controls. The results demonstrate that the activities of the hexose monophosphate dehydrogenases are regulated differentially between tissues of the lactating rat.     

The pattern of fatty acid synthesis in lactating rabbit mammary gland studied in vivo

The biosynthesis of fatty acids has been studied in lactating rabbits at 6h after intravenous injection of sodium [1-(14)C]acetate. The specific radioactivities of the individual fatty acids (C(6:0) to C(14:0)) and the proportions of these fatty acids synthesized were similar in mammary tissue and milk. Hexanoic acid had the highest specific radioactivity, and the C(8:0)-C(14:0) fatty acids had similar specific radioactivities, which were about five times those of C(16) and C(18) acids. No radioactivity was detected in fatty acids of chain length C(14) in these tissues were similar to those of the long-chain fatty acids in the milk and mammary gland. The results show that the C(4:0)-C(14:0) fatty acids are synthesized within the mammary gland rather than by fatty acid uptake from circulating blood or by oxidation of long-chain fatty acids within the gland. We conclude that de novo synthesis of esterified fatty acids in vivo by this tissue has a high degree of chain-length specificity.     

Amino acid metabolism and protein synthesis in lactating rats fed on a liquid diet.

1. Amino acid metabolism was studied in control virgin rats, lactating rats and virgin rats protein-pair-fed with the lactating rats (high-protein virgin rats). 2. Urinary excretion of nitrogen and urea was higher in lactating than in control virgin rats, and in high-protein virgin rats it was higher than in lactating rats. 3. The activities of urea-cycle enzymes (units/g) were higher in high-protein virgin than in lactating rats, except for arginase. In lactating rats the activities of carbamoyl-phosphate synthase, ornithine carbamoyltransferase and argininosuccinate synthase were lower than in control virgin rats. When the liver size is considered, the activities in lactating rats were similar to those in high-protein virgin rats, except for arginase. 4. N-Acetylglutamate content was higher in high-protein virgin rats than in the other two groups. 5. The rate of urea synthesis from precursors by isolated hepatocytes was higher in high-protein virgin rats than in the other two groups. 6. The flooding-dose method (L-[4-3H]phenylalanine) for measuring protein synthesis was used. The absolute synthesis rates of mammary gland, liver and small-intestinal mucosa were higher in lactating rats than in the other two groups, and in high-protein virgin rats than in control virgin rats 7. These results show that the increased needs for amino acids during lactation are met by hyperphagia and by a nitrogen-sparing mechanism.     

Effects of inhibition of protein synthesis by cycloheximide on lipogenesis in mammary gland and liver of lactating rats.

1. Administration of cycloheximide (an inhibitor of protein synthesis) to lactating rats raised the concentrations of amino acids, and in particular, the branched-chain amino acids (valine, leucine and isoleucine) in blood, liver and mammary gland. 2. Inhibition of protein synthesis increased the incorporation in vivo of L-[U-14C]leucine into lipids of mammary gland and liver. 3. Cycloheximide treatment caused no immediate change in the overall rate of lipogenesis in vivo (measured with 3H2O) in mammary gland but increased the rate in liver 3-fold; this latter effect also occurred in livers of virgin rats. 4. The increased rate of hepatic lipogenesis was not accompanied by significant changes in the plasma insulin concentration or the activity of acetyl-CoA carboxylase. 5. Although cycloheximide decreased the entry of total triacylglycerol into the circulation it did not alter the rate of secretion of newly synthesized saponifiable lipid. 6. Cycloheximide slightly stimulated lipogenesis from endogenous substrates in isolated hepatocytes, but this effect was abolished when lactate was the exogenous substrate. 7. Administration of cycloheximide to virgin rats decreased liver glycogen and increased the hepatic content of glucose 6-phosphate, pyruvate and lactate. 8. It is concluded that (a) there is no short-term link between the rate of protein synthesis and lipogenesis in the lactating mammary gland and (b) the increased rate of hepatic lipogenesis in cycloheximide-treated rats is mainly due to stimulation of glycogenolysis, glycolytic flux and consequent increased availability of pyruvate.     

Connexins 26, 32 and 43 are expressed in virgin, pregnant and lactating mammary glands

Gap junctions (GJ) are formed by a number of homologous proteins termed connexins. Here expression of connexins Cx26, Cx32 and Cx43, was evaluated by immunofluorescence (IF) in mammary glands from virgin, pregnant and lactating rats. Cx26, Cx32 and Cx43 labeling was detected in epithelial parenchymal cells at all functional stages. Cx26 and Cx32 labeling was very low in glands from virgin animals, somewhat greater in glands from pregnant animals and significantly higher (in number and size) in lactating animals. In the last ones, Cx26 and Cx32 punctate labeling was localized to the basal and lateral membranes of alveolar epithelial cells and collecting ductules. Cx43 punctate labeling was restricted to the periphery of alveoli towards the basal pole of epithelial cells at all functional stages, and it enlarged slightly during lactation. At this localization, Cx43 may form GJ between myoepithelial cells and/or between epithelial and myoepithelial cells. Cx43 was also found to be steadily expressed in the connective tissue which surrounds and invades each parenchymal lobe, at all functional stages. At this localization, Cx43 may couple fibroblasts and/or adipose cells. IF studies in sections from lactating mice showed the same distribution of connexins. Immunoblots confirmed specificity of labeling and the presence of Cx32 and Cx43 in the mammary gland. The increase in connexin expression detected during pregnancy and lactation may be important for epithelial cell differentiation and secretion in the mammary gland.