Sequential changes in brown adipose tissue composition, cytochrome oxidase activity and GDP binding throughout pregnancy and lactation in the rat

The sequential appearance of changes in interscapular brown adipose tissue composition, cytochrome oxidase activity and GDP binding was studied throughout pregnancy and lactation in the rat. Brown adipose tissue was hypertrophied during pregnancy because of progressive lipid accumulation, whereas its mitochondrial component and GDP binding to brown fat mitochondria were unchanged. In early lactation (day 5) there was a decrease in the overall GDP binding to brown fat only because of the lower mitochondrial protein content. In late stages of lactation (days 10 and 15), the amount of tissue and its mitochondrial protein content were minimal and the GDP binding per mitochondrial protein decreased substantially. Scatchard analysis in day-15-lactating rats indicated a large decrease in GDP binding sites without any changes in affinity. It is concluded that the diminished thermogenic activity of brown fat in lactation is attained through changes at different structural levels of the tissue occurring in a characteristic sequential trend; first a reduction in its mitochondrial component, and only later, at mid-lactation, a decrease in the specific mitochondrial proton conductance pathway activity.     

Iodothyronine 5''-deiodinase activity and thyroid hormone content in brown adipose tissue during the breeding cycle of the rat.

Brown adipose tissue iodothyronine 5'-deiodinase activity is significantly lower in 17-day pregnant rats compared with virgin controls and remains low during late pregnancy and lactation. It fully recovers with abrupt weaning, but only partially with spontaneous weaning. Even though this profile of changes is remarkably in step with the known pattern of modifications in brown fat thermogenesis during the breeding cycle, the lowered iodothyronine 5'-deiodinase activity appearing between days 15 and 17 of pregnancy occurs earlier than the reduction in brown adipose tissue thermogenesis. Brown fat 3,3',5-tri-iodothyronine content is also reduced in late pregnant, early and mid-lactating rats, most probably as a consequence of the lowered 5'-deiodination of thyroxine in situ. Acute insulin treatment increases brown fat iodothyronine 5'-deiodinase activity in virgin animals as well as in late-pregnant and lactating rats, despite the lowered basal enzyme activity levels in the latter groups. Thus an impaired response to insulin in brown fat does not appear to be a factor leading to the lowered iodothyronine 5'-deiodinase activity during late pregnancy and lactation.     

Fatty acid biosynthesis in rabbit mammary gland during pregnancy and early lactation

The pattern of fatty acids synthesized by mammary-gland explants from rabbits during pregnancy and early lactation has been studied. From day 12 to day 18 of pregnancy, long-chain (C(14:0)-C(18:1)) fatty acids were the major products. From day 18 to day 21 of pregnancy there was an increase of up to 12-fold in the rate of fatty acid synthesis per unit wet weight of tissue that was almost exclusively caused by the synthesis of octanoic fatty acid and decanoic fatty acid, which are characteristic of rabbit milk. These medium-chain fatty acids were mainly incorporated into triglycerides. From day 22 to day 27 of pregnancy there was little change in the rate of fatty acid synthesis and the proportions of fatty acids synthesized were essentially the same as those synthesized by the lactating gland, i.e. 80-90% octanoic acid plus decanoic acid. About 2-4 days before parturition a second lipogenic stimulus occurred, although the pattern of fatty acids synthesized did not change.     

Evidence for a differential physiological modulation of brown fat iodothyronine 5'-deiodinase activity in the perinatal period

Brown adipose tissue iodothyronine 5'-deiodinase increases progressively in fetuses from the day 17 of pregnancy on, it reaches peak values on the 20th day of gestation and declines in the last days of fetal life as well as during the first day of life. Birth of premature fetuses causes a sudden drop in the enzyme activity. Postmaturity is associated to a decrease in brown fat 5'-deiodinase similar to that found after birth in fetuses born at term. In the first hours of life brown fat iodothyronine 5'-deiodinase is essentially insensitive to the cold-stimulus. Present data indicates that, differently from adult rats, brown fat iodothyronine 5'-deiodinase activity during the perinatal period is dissociated from the thermogenic activity of the tissue. It is suggested that factors different from the action of the sympathetic nervous system may play a main role in brown fat iodothyronine 5'-deiodinase activity modulation in the fetal and neonatal life.     

Influence of feed intake during pregnancy and lactation on fat body reserve mobilisation, plasma leptin and reproductive function of primiparous lactating sows.

From day 23 of pregnancy, 24 gilts received either a medium (M, n = 16) or a high (H, n = 8) level of feeding calculated to meet 115% or 190% of energy for maintenance, respectively. During lactation, all H sows were fed ad libitum (H-AL) whereas M sows were fed either ad libitum (M-AL, n = 8) or were restricted (M-RE, n = 8) to the amount of feed ingested by H-AL sows. Increased feed intake during pregnancy increased live weight, backfat thickness, and estimated body lipid and protein on days 4 and 25 of lactation (P < 0.05). It also resulted in lower feed intake and higher lipid mobilisation during lactation (P < 0.05) without a detrimental influence on milk production. Activities of malic enzyme and glucose-6-phosphate dehydrogenase from neck fat samples were higher in H than M sows on day 4 (P < 0.05). They decreased during lactation in H sows (P < 0.05). Mean diameter of adipocytes decreased during lactation in the 3 groups (P < 0.05) but did not differ between groups on days 4 and 25. Plasma leptin on days 4, 11, 18 and 25 was higher in H than in M sows (P < 0.05) but was not influenced by lactational feed intake. Neither measured characteristics of gonadotrophin secretion on day 22, nor of ovarian activity on day 26, were significantly influenced by the level of feeding during pregnancy or lactation.     

PPAR-gamma,TNF-alpha messenger RNA levels and lipase activity in the pregnant and lactating rat

Dramatic alternations in maternal metabolism occur during gestation and lactation, especially glucose and fat metabolism. For example, in rats, the amount of body fat mass increases during gestation, then decreases just prior to delivery, and remains low after parturition. To investigate the factors involved in such changes in maternal fat mass, messenger ribonucleic acid (mRNA) levels of adipocytokines, peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and tumor necrosis factor-alpha (TNF-alpha), were examined in the intraabdominal adipose tissue of non-pregnant rats, pregnant rats and postpartum rats. We also examined the issue of whether apoptosis, which could be promoted by PPAR-gamma and TNF-alpha, is involved in any of the changes in maternal fat mass The activity of lipoprotein lipase (LPL) and hormone sensitive lipase (HSL) in adipose tissue was also measured. PPAR-gamma and TNF-alpha mRNA levels remained constant during the gestational and postpartum periods. Apoptosis was not detected at any time as evidenced by DNA laddering and in situ staining. LPL activity was significantly increased at day 5 and remained elevated until day 14 of gestation. HSL activity was significantly increased at day 10 of gestation and then decreased after delivery, at day 10 of lactation. In conclusion, during the gestational and postpartum period of rats, changes in maternal fat mass did not directly correlate with the levels of expression of PPAR-gamma and TNF-alpha mRNA. Apoptosis also does not appear to influence on fat mass change. The changes in LPL and HSL activities during gestation suggest that these enzymes might be regulators of maternal adipose tissue level.     

High-protein diet during gestation and lactation affects mammary gland mRNA abundance, milk composition and pre-weaning litter growth in mice

We evaluated the effect of a high-protein diet (HP) on pregnancy, lactational and rearing success in mice. At the time of mating, females were randomly assigned to isoenergetic diets with HP (40% w/w) or control protein levels (C; 20%). After parturition, half of the dams were fed the other diet throughout lactation resulting in four dietary groups: CC (C diet during gestation and lactation), CHP (C diet during gestation and HP diet during lactation), HPC (HP diet during gestation and C diet during lactation) and HPHP (HP diet during gestation and lactation). Maternal and offspring body mass was monitored. Measurements of maternal mammary gland (MG), kidney and abdominal fat pad masses, MG histology and MG mRNA abundance, as well as milk composition were taken at selected time points. HP diet decreased abdominal fat and increased kidney mass of lactating dams. Litter mass at birth was lower in HP than in C dams (14.8 v. 16.8 g). Dams fed an HP diet during lactation showed 5% less food intake (10.4 v. 10.9 g/day) and lower body and MG mass. On day 14 of lactation, the proportion of MG parenchyma was lower in dams fed an HP diet during gestation as compared to dams fed a C diet (64.8% v. 75.8%). Abundance of MG α-lactalbumin, β-casein, whey acidic protein, xanthine oxidoreductase mRNA at mid-lactation was decreased in all groups receiving an HP diet either during gestation and/or lactation. Milk lactose content was lower in dams fed an HP diet during lactation compared to dams fed a C diet (1.6% v. 2.0%). On days 14, 18 and 21 of lactation total litter mass was lower in litters of dams fed an HP diet during lactation, and the pups' relative kidney mass was greater than in litters suckled by dams receiving a C diet. These findings indicate that excess protein intake in reproducing mice has adverse effects on offspring early in their postnatal growth as a consequence of impaired lactational function.     

Diet-induced obesity impairs mammary development and lactogenesis in murine mammary gland

We have developed a mouse model of diet-induced obesity that shows numerous abnormalities relating to mammary gland function. Animals ate approximately 40% more calories when offered a high-fat diet and gained weight at three times the rate of controls. They exhibited reduced conception rates, increased peripartum pup mortality, and impaired lactogenesis. The impairment of lactogenesis involved lipid accumulation in the secretory epithelial cells indicative of an absence of copius milk secretion. Expression of mRNAs for beta-casein, whey acid protein, and alpha-lactalbumin were all decreased immediately postpartum but recovered as lactation was established over 2-3 days. Expression of acetyl-CoA carboxylase (ACC)-alpha mRNA was also decreased at parturition as was the total enzyme activity, although there was a compensatory increase in the proportion in the active state. By day 10 of lactation, the proportion of ACC in the active state was also decreased in obese animals, indicative of suppression of de novo fatty acid synthesis resulting from the supply of preformed fatty acids in the diet. Although obese animals consumed more calories in the nonpregnant and early pregnant states, they showed a marked depression in fat intake around day 9 of pregnancy before food intake recovered in later pregnancy. Food intake increased dramatically in both lean and obese animals during lactation although total calories consumed were identical in both groups. Thus, despite access to high-energy diets, the obese animals mobilized even more adipose tissue during lactation than their lean counterparts. Obese animals also exhibited marked abnormalities in alveolar development of the mammary gland, which may partially explain the delay in differentiation evident during lactogenesis.     

Protein metabolism in the mouse during pregnancy and lactation.

Protein synthesis was measured in vivo in the whole body and in a number of individual tissues in mice at various stages of pregnancy and lactation. The absolute rate of protein synthesis in the whole body increased from 640 mg/day in virgin mice to 1590 mg/day by day 18 of pregnancy, and to 2100 mg/day by day 15 of lactation. Large proportions of these increments were contributed by the rapidly growing foetuses and placentae in the pregnant animals and by protein synthesis in the mammary glands during lactation. In addition, a substantial stimulation of growth and protein synthesis was also observed in the liver and the gastrointestinal tract. Gastrocnemius muscle showed no changes in protein metabolism, indicating that in the well-fed mouse this tissue is not required to play a role as a protein reserve during pregnancy and lactation.     

Impaired basal and noradrenaline-induced iodothyronine 5'-deiodinase activity in brown adipose tissue from pregnant and lactating rats

Basal iodothyronine 5'-deiodinase activity is lowered in brown fat from 20-day pregnant, 5 and 15-day lactating rats when compared with virgin controls. Acute noradrenaline treatment caused a seven fold increase in 5'-deiodinase activity in brown fat from virgin control rats. Late pregnant and lactating rats showed a reduction in noradrenaline-induced 5'-deiodinase activity in brown adipose tissue and the maximum impairment was observed in 15-day lactating rats. Lowered 5'-deiodinase activity in brown fat during late pregnancy and lactation correlates with the known reduction in the thermogenic activity of the tissue during these situations and agrees with the proposal that the rate of 3,5,3'-triiodothyronine generated in situ because of thyroxine 5'-deiodination could be an essential event related to thermogenesis in brown fat. Even though the relationship between 3,5,3'-triiodothyronine generation in the tissue and the specific thermogenic mechanisms of brown fat is unknown, present results indicate a close link between the thermogenic and 5'-deiodinase activities in physiological situations when brown adipose tissue needs to adapt to a low activity, such as that of the breeding cycle.     

Metabolic adaptations during lactogenesis. Fatty acid synthesis in rabbit mammary tissue during pregnancy and lactation

1. Mammary tissue was obtained from rabbits at various stages of pregnancy and lactation and used for tissue-slice incubations (to measure the rate of fatty acid synthesis and CO(2) production) and to determine relevant enzymic activities. A biphasic adaptation in fatty acid synthetic capacity during lactogenesis was noted. 2. The first lactogenic response occurred between day 15 and 24 of pregnancy. Over this period fatty acid synthesis (from acetate) increased 14-fold and the proportions of fatty acids synthesized changed to those characteristic of milk fat (77-86% as C(8:0)+C(10:0) acids). 3. The second lactogenic response occurred post partum as indicated by increased rates of fatty acid synthesis and CO(2) production (from acetate and glucose) and increased enzymic activities. 4. Major increases in enzymic activities between mid-pregnancy and lactation were noted for ATP citrate lyase (EC 4.1.3.8), acetyl-CoA synthetase (EC 6.2.1.1), acetyl-CoA carboxylase (EC 6.4.1.2), fatty acid synthetase, glucose 6-phosphate dehydrogenase (EC 1.1.1.49), and 6-phosphogluconate dehydrogenase (EC 1.1.1.44). Smaller increases in activity occurred with glycerol 3-phosphate dehydrogenase (EC 1.1.1.8) and NADP(+)-isocitrate dehydrogenase (EC 1.1.1.42) and the activity of NADP(+)-malate dehydrogenase (EC 1.1.1.40) was negligible at all periods tested. 5. During pregnancy and lactation there was a close temporal relationship between fatty acid synthetic capacity and the activities of ATP citrate lyase (r=0.94) and acetyl-CoA carboxylase (r=0.90).     

Serum progesterone and corpus luteum function in pregnant pigtailed monkeys (Macaca nemestrina)

Corpus luteum (CL) function and control during pregnancy and early lactation in the pigtailed macaque was investigated. Peripheral concentrations of progesterone (P) on day 10 of pregnancy were 12.98 ± 2.21 ng/ml and decreased progressively to 7.96 ± 1.27 ng/ml by day 21 of pregnancy. The concentration of P increased around day 27 of gestation and reached peak levels of 18.48 ± 2.45 ng/ml on day 37, there-after gradually decreasing to a nadir at about midgestation. Ten days before parturition P concentrations increased again (P < 0.05). Concentrations of P decreased from 6.62 ± 1.48 ng/ml on the day of delivery to 2.16 ± 0.43 ng/ml on day 2 of lactation and remained low thereafter. Ovariectomy on day 35 did not affect the normal course of gestation or the patterns of P secretion during pregnancy. However, in these ovariectomized animals, in spite of suckling, P was not detectable after parturition. In intact monkeys, serum concentrations of P in the uteroovarian vein at days 80 and 159 of pregnancy were higher relative to the uterine vein. Incubation studies utilizing ³H-cholesterol as a substrate revealed that the CL were capable of synthesizing P on days 35 and 159 of gestation. Histologically, the CL contained active luteal cells at late pregnancy.Low serum concentrations of chorionic gonadotropin were detected on day 10 of gestation; concentrations of this hormone reached high levels between days 18 and 24 and the titers were nondetectable after day 40 of pregnancy. Luteinizing hormone was present in constant amounts in the circulation during pregnancy and lactation.These data suggest that the CL of pregnancy in the pigtailed monkey is functional or capable of functioning during various stages of pregnancy. However, the fetoplacental unit is the primary source of P during the latter 4.5 months of gestation. As in other primates, a functional CL is not required for maintenance of pregnancy after implantation nor for lactation. Thus, the physiological significance of CL function during pregnancy is unclear.     

Ribosomal RNA turnover and the level of ribonuclease activity in the liver of the pregnant rat.

Studies were undertaken on the turnover of ribosomal RNA and on ribonuclease activity in the liver of the pregnant rat in an attempt to explain the accumulation of liver RNA which occurs during the latter half of pregnancy. Between the 15th and 20th day of gestation the rate constant of degradation, biological half-life and daily rate of synthesis of ribosomal RNA were calculated to be 0.0887, 7.81 days and 6.21 mg per liver per 100g body weight respectively. Corresponding values in non-pregnant rats were 0.123, 5.68 days and 3.47 mg per liver per 100g body weight. The increase in RNA was therefore associated with an increase in its rate of synthesis and a decrease in its rate of breakdown. From the 14th day of pregnancy there was a decrease in alkaline ribonuclease activity and a marked increase in the level of alkaline ribonuclease inhibitor. The activity of acid ribonuclease was found to increase and that of acid phosphatase to decrease during this period.     

Hormonal induction and regulation of lactose synthetase in mouse mammary gland

The augmentation of lactose synthetase activity during late pregnancy and lactation was measured by using both a tissue-culture assay and a cell-free assay. The results indicated at least a 100-fold augmentation in specific activity between late pregnancy and lactation. The cell-free assay indicated that the activities of both subunits of this enzyme had increased to 20-30% of the value during lactation by the last day of pregnancy. The tissue-culture assay, however, showed activities only 3-4% of the maximum at the time of parturition. This suggests that not all the enzyme present in the tissue before lactation commenced was active. Since at all stages of pregnancy and lactation the B subunit, alpha-lactalbumin (which is also a milk protein), was rate-limiting, it is suggested that the rate of lactose synthesis may be linked to the rate of milk-protein synthesis. Both subunits of lactose synthetase could be induced in tissue culture by the hormones insulin+hydrocortisone+prolactin. Of the three hormones, prolactin appeared to be the ;trigger' that induced the synthesis of these proteins if the tissue had been stimulated previously by insulin+hydrocortisone.     

Oxidative stress status of highly prolific sows during gestation and lactation

Elevated oxidative stress is reported to be associated with pregnancy complications in highly prolific sows. Oxidative DNA damage and the antioxidant status were determined in blood samples collected during the course of gestation and lactation in multiparous sows. Blood samples were drawn from sows (n = 5) on days 30, 60, 90 and 110 of gestation (G30, G60, G90 and G110, respectively), on day 3, 10 and 18 of lactation (L3, L10 and L18, respectively) and on day 5 of postweaning (W5). Lymphocytes were isolated from the fresh blood and cryopreserved in each time point. Lymphocyte DNA damage was analyzed by alkaline single-cell gel electrophoresis (comet assay) to determine the single- and double-strand brakes and endogenous antioxidant concentrations using an HPLC system with UV detection. The comet assay showed elevated (P < 0.05) DNA damage (between 38% and 47%) throughout the gestational and lactational periods than during early gestation (G30; 21%). Plasma retinol concentration was reduced (P < 0.05) at the end of gestation (G110) compared with G30. Plasma α-tocopherol concentrations also showed a similar trend as to retinol. This study indicates that there is an increased systemic oxidative stress during late gestation and lactation, which are not fully recovered until the weaning compared with the G30, and that antioxidant nutrients in circulation substantially reduced in the mother pig at G110.     

Lipoprotein lipase mRNA expression in brown adipose tissue: translational and/or posttranslational events are involved in the modulation of enzyme activity

Lipoprotein lipase mRNA abundance in rat brown adipose tissue increases during the first 24 h of cold exposure. Lipoprotein lipase mRNA levels do not change in brown fat throughout pregnancy and lactation, whereas enzyme activity is significantly lowered. After 5 h of acute cold or noradrenaline administration there is a 2-fold increase in lipoprotein lipase mRNA abundance, whereas lipoprotein lipase activity is stimulated to more than 6-fold the basal values. It is concluded that translational and/or posttranslational mechanisms are involved in the noradrenergic modulation of lipoprotein lipase activity in brown fat.     

Essential fatty acid interconversion during gestation in the rat

The synthesis of arachidonic acid has been investigated in fetal and pregnant rat liver microsomes in the course of the gestation. The delta 5-desaturase activity decreased 2-3 times in rat liver between the 19th and 22nd day of the pregnancy. During this period the delta 5-desaturate activity increased 3-fold in the fetal liver, exceeding the activity of the maternal liver. In contrast, the activity of the fetal delta 6-desaturase was in the same range as in pregnant rat liver and the liver of control animals and did not change between these two stages of the gestation. The elongation rate of linoleic acid in fetal liver was 2-3 times lower than in maternal liver but this increased during the pregnancy. The fatty acid activate rate was always higher than the activity of the desaturases. At the 19th day, the activity of the delta 5-desaturase was apparently the rate limiting step of arachidonic acid synthesis in fetal liver. We did not find any delta 5- and delta 6-desaturase activities or linoleic acid elongation in the placenta microsomes.     

Energy Metabolism During Late Gestation and Lactation in Muciparous Sows in Relation to Backfat Thickness and the Interval from Weaning to First Oestrus

Ten crossbred, fourth or fifth parity sows were divided into 2 groups - high (H) and low (L) - according to their backfat thickness 9 days before parturition. Body weight, backfat thickness and litter weight were recorded repeatedly during a 5 week lactation period. The length of the interval from weaning to first oestrus was also noted. All sows were fed a commercial diet (11.9 MJ/kg, 14.5% crude protein). During gestation, daily food intake was 2.2 kg/sow, while during lactation it was 3.0 kg/sow plus 0.4 kg/piglet. Blood samples were drawn on day 9 before parturition and on days 2,7,14 and 21 of lactation. The samples were analysed to determine concentrations of glucose, urea nitrogen, creatinine, triglycerides, free fatty acids and beta-hydroxybutyric acid. In both groups, concentrations of free fatty acids and urea nitrogen were low on day 9 before parturition while those of triglycerides were high, indicating anabolism regardless of backfat thickness. During the first week of lactation, concentrations of free fatty acids increased in the H-group but not in the L-group, and concentrations of urea nitrogen were higher in the H-group. These differences, together with the greater loss of weight observed in the H-group, indicate that catabolism of maternal fat and protein depots was more pronounced in the Η-group than in the L-group during this time. On day 14 of lactation, both groups showed equally low concentrations of free fatty acids, decreasing creatinine concentrations and stable triglyceride and urea nitrogen concentrations. Furthermore, weight loss during the second and third weeks of lactation was low in both groups. These facts, taken together, indicate that the catabolic rate was decreasing in both groups during this period. No differences in return to oestrus interval were noted between the groups. The present study indicates that under a restricted feeding regime the catabolic rate during the first week of lactation is higher in sows with higher backfat thickness in late gestation. As lactation progresses, a more balanced metabolism is achieved regardless of backfat thickness, which may tend to reduce differences in return to oestrous interval.     

Effects of perinatal maternal food restriction on pituitary-gonadal axis and plasma leptin level in rat pup at birth and weaning and on timing of puberty

The effects of maternal 50% food restriction (FR) during the last week of gestation and/or lactation on pituitary-gonadal axis (at birth and weaning), on circulating levels of leptin (at weaning), and on the onset of puberty have been determined in rats at birth and at weaning. Maternal FR during pregnancy has no effect at term on the litter size, on the basal level of testosterone in male pups, and on the drastic surge of circulating testosterone that occurs 2 h after birth. At weaning, similar retardation of body growth is observed in male and female pups from mothers exposed to FR. This undernutrition induces the most drastic effects when it is performed during both gestation and lactation or during lactation alone. Drastic retardation of testicle growth with reduction of cross-sectional area and intratubular lumen of the seminiferous tubules is observed in male pups from mothers exposed to undernutrition during both gestation and lactation or during lactation alone. Maternal FR during the perinatal period reduces circulating levels of FSH in male pups without affecting LH and testosterone concentrations. Maternal FR does not affect circulating levels of LH, estradiol, and progesterone in female pups. Female pups from mothers exposed to FR during both gestation and lactation show a significant increase of plasma FSH as well as a drastic retardation of ovarian growth. The follicular population was also altered. The number of antral follicles of small size (vesicular follicles) was increased, although the number of antral follicles of large size (graafian follicles) was reduced. Maternal FR occurring during both late gestation and lactation (male and female pups), during lactation alone (male and female pups), or during late gestation (female pups) induces a drastic reduction of plasma leptin and fat mass in pups at weaning. The onset of puberty is delayed in pups of both sexes from mothers exposed to FR during lactation and during both gestation and lactation. In conclusion, these data demonstrate that a perinatal growth retardation induced by maternal FR has long-term consequences on both size and histology of the genitals, on plasma gonadotropins and leptin levels, on fat stores at weaning, and on the onset of puberty.     

Reproduction in mice: liver enlargement in mice during pregnancy and lactation

The mechanism of liver enlargement during pregnancy was investigated in the C57BL/6J strain of mice. The C57BL/6J female exhibited a two-fold increase in liver mass during pregnancy. After the completion of lactation the size of the liver was reduced. Liver growth was accomplished with no increase in hepatocyte number and without an increase in total liver DNA content. During the early stages of liver expansion in pregnant females, DNA synthesis could be turned on by partial hepatectomy. However, during the last few days of gestation DNA synthesis and liver growth in response to partial hepatectomy were inhibited. During lactation this inhibition of growth was maintained, but inhibition of DNA synthesis was partially lifted. DNA synthesis and liver growth in response to partial hepatectomy were normal after the termination of lactation. Because of the limited scope of this investigation the full implications of these findings are not yet certain.