Effect of lactation on lipolysis in rat adipose tissue

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.

Implementation of a healthy diet to lactating rats attenuates the early detrimental programming effects in the offspring born to obese dams. Putative relationship with milk hormone levels

Lactation is a critical period of development and alterations in milk composition due to maternal diet or status may affect infant growth. We aimed to evaluate in rats whether improving maternal nutrition during lactation attenuates early imprinted adverse metabolic effects in the offspring born to obese dams. Three groups were studied: Control (C) dams, fed with standard diet; Western diet (WD) dams, fed with WD one month prior to gestation and during gestation and lactation; and Reversion (Rev) dams, fed as WD-dams, but moved to a standard diet during lactation. Macronutrient content, insulin, leptin and adiponectin levels were determined in milk. Phenotypic traits and circulating parameters in dams and their offspring were determined throughout lactation. Results showed that, at weaning, WD-dams displayed lower body weight and greater plasma insulin and non-esterified fatty acids levels than C-dams, and signs of hepatic steatosis. Milk from WD-dams showed lower protein content and insulin, leptin, and adiponectin levels during the entire or the late lactation. Rev-dams retained excess body fat content, but milk composition and most circulating parameters were not different from controls at late lactation and showed higher leptin mRNA levels in mammary gland than WD-dams. The offspring of WD-dams, but not that of Rev-dams, displayed higher body weight, adiposity, and circulating leptin and glucose levels than controls at weaning. In conclusion, dietary improvement during lactation prevents early adverse effects in offspring associated with maternal intake of an obesogenic diet, that may be related with the normalization of milk hormone levels.

Reduction of litter size during lactation in rats greatly influences fatty acid profiles in dams

This study aimed to determine in lactating rats how fatty acid profiles are affected by litter size. On day 2 after parturition, litters of lactating rats were adjusted to a normal litter size of 9 pups/dam (NL) or to a small litter of 4 pups/dam (SL), and dams were studied at day 21 of lactation. Plasma glucose, insulin, and docosahexaenoic acid (DHA) concentrations were higher in SL than in NL dams, whereas the concentrations of most other fatty acids, triacylglycerols (TAG), and non-esterified fatty acids were lower in the SL dams. In the liver, the concentration of TAG was lower in SL than in NL dams as was the concentration of most fatty acids, with the exception of stearic acid (STA), arachidonic acid (ARA), and DHA concentrations that were higher in SL. Both plasma and liver Δ⁹ desaturase indices were lower in SL than in NL dams, whereas both Δ⁵ and Δ⁶ desaturase indices were higher in SL dams. In the liver, the expression of acetyl CoA carboxylase was lower in SL than in NL dams, and among the different adipose tissue depots, only mesenteric adipose tissue showed a higher concentration of most fatty acids in SL than in NL dams. It is proposed that reduction of litter size during lactation decreases liver lipogenesis de novo, although the synthesis of long-chain polyunsaturated fatty acids from their corresponding precursors increases, and lipolytic activity in mesenteric adipose tissue decreases probably as result of increased insulin responsiveness.

Maternal protein reserves and their influence on lactational performance in rats

To determine the contribution of tissue protein reserves to lactational performance, multiparous female Sprague-Dawley rats were mated, caged individually and offered a diet high in protein (215 g crude protein (N x 6.25; CP)/kg dry matter (DM); H) ad lib. until day 12 of gestation. Subsequently half the rats continued to receive diet H while the remainder were offered a diet low in protein (65 g CP/kg DM; L) until parturition. This treatment aimed to produce a difference in carcass protein at parturition. On day 1 of lactation females were allocated to either diet H or a low-protein diet (90 g CP/kg DM; L2) offered until day 13 of lactation, giving four lactation treatment groups HH, HL2, LH and LL2. Groups of females were slaughtered on days 2 and 12 of gestation and days 1 and 13 of lactation and carcass and major organs were analysed. Weight gain of standardized litters was used as an indicator of lactational performance. Maternal carcass protein contents at parturition were 43.5 (SE 1.2) and 38.7 (SE 0.8) g (P < 0.01) for diets H and L respectively. During lactation there was little change in carcass protein content of HH rats while LH rats appeared to replenish their depleted reserves. Food intake or lactational performance did not differ between these two groups. HL2 and LL2 rats lost carcass protein with HL2 rats losing more than LL2 rats (P < 0.05). Intake and lactational performance were reduced compared with that on diet H (P < 0.05) but for the first 6 d of lactation were both greater (P < 0.05) for diet HL2 than for diet LL2. All four groups showed a considerable loss of body fat during lactation which was not affected by diet. The ability of HL2 rats to catabolize more protein and consume more food allowed them to sustain a greater lactational performance. Previous maternal protein depletion had no influence on lactational performance as long as an adequate supply of dietary protein was provided.

Progesterone can either increase or decrease weight gain and adiposity in ovariectomized Syrian hamsters

We examined the effects of estradiol and progesterone on weight gain, food intake, and carcass composition in Syrian hamsters (Mesocricetus auratus). In ovariectomized (OVX) hamsters injections of 5 micrograms/day estradiol benzoate (EB) alone decreased weight gain and adiposity, whereas treatment with progesterone alone (1 or 5 mg/day) resulted in increased weight gain and adiposity. However, concurrent treatment with progesterone and EB reduced body fat content to levels significantly below those of hamsters treated with EB alone. In a second experiment, 17 beta-estradiol and progesterone were administered via subcutaneous Silastic capsules in doses which produce physiological levels of steroids. Implants of estradiol significantly decreased body weight gain and fat content. As in the first experiment, these effects of estradiol were exaggerated by concurrent progesterone administration. Implants of progesterone alone did not affect body weight or fatness in OVX hamsters. These data indicate that estradiol and progesterone interact to decrease body lipid stores in hamsters, whereas in rats progesterone reverses the adiposity-reducing actions of estradiol. This species difference in responses to progesterone could help to explain why rats increase, whereas hamsters decrease, their body lipid stores during pregnancy, when circulating progesterone levels are elevated.

Cyclical mastopathy: a critical review of therapy

Because of a recently completed trial which demonstrated that dietary fat reduction improved the symptoms of cyclical mastopathy, we have carried out a critical review of other studies of treatment for this disorder to determine what, if any, mechanisms of action were shared by effective treatments. Therapies were classified as 'definitely effective' if their effectiveness was demonstrated in at least one randomized placebo-controlled study of women with cyclical mastopathy, 'probably effective' if their effectiveness in relieving breast symptoms was demonstrated in at least one study in women with premenstrual syndrome, and 'probably not effective' if they had not been shown to improve breast symptoms in at least two studies in women with premenstrual syndrome. Bromocriptine, danazol, evening primrose oil, tamoxifen and reduction of dietary fat intake were classified as definitely effective and norethisterone and Bellergal as probably effective. Several therapeutic manoeuvres, including reduction of methylxanthine ingestion and the administration of vitamin E or diuretics, have not been adequately evaluated in women with cyclical breast symptoms. A review of the published reports of the physiological effects of therapies that were definitely or probably effective revealed that these agents acted either to alter serum prolactin levels or lipid metabolism, or both. As evidence exists that prolactin influences lipid metabolism it is postulated that cyclical mastopathy may be a disorder of lipid metabolism.

Effects of lactation and removal of pups on the rate of triacyglycerol/fatty acid substrate cycling in white adipose tissue of the rat

The rate of the triacylglycerol/fatty acid substrate cycle was measured in vivo in adipose tissue of virgin and lactating rats with pups removed. The rate decreased by 70% in adipose tissue of lactating rats and increased 9-fold on removal of the pups. Similar differences in cycling rate were seen in adipose tissue incubated in vitro in the presence of isoprenaline.

Effects of pregnancy and lactation on lipolysis of ewe adipocytes induced by beta-adrenergic stimulation

During pregnancy and lactation, three phases of beta-adrenergic stimulation of lipolysis appeared in adipocytes isolated from omental adipose tissue of ewes: During the first 90 days of pregnancy, there was a fall in the maximal lipolytic effect of isoproterenol. The maximum response to beta-lipolytic stimulus was reduced by 38% on day 90 (815 nmol/2 h/10(6) cells, compared to 1304 for controls) without any change in system sensitivity. Between days 90 and 120 of pregnancy (switch in adipose metabolism), this lipolytic effect increased progressively and became significantly larger (34%) than in non-pregnant ewes around parturition. The system sensitivity was then higher (0.05-0.11 vs. 0.21 microM) and remained so throughout lactation. During the first 3 weeks of lactation, this stimulated rate was twice as great as in controls.

Amino acid metabolism in hepatocytes isolated from lactating rats

Parenchymal hepatocytes isolated from lactating rats had similar rates of amino acid incorporation into protein, but increased rates of urea formation compared to hepatocytes from non-lactating rats. The increased urea formation may be due to increased amino acid transport and degradation. The liver contributes to the increased utilization of amino acids during lactation.

Adenosine and the control of lipolysis in rat adipocytes during pregnancy and lactation

The rate of noradrenaline-stimulated lipolysis is lower in fat-cells from lactating than from pregnant rats; this difference is eliminated by the addition of adenosine deaminase [Aitchison, Clegg & Vernon (1982) Biochem. J. 202, 243-247]. The activity of 5'-nucleotidase, and hence the capacity of the cells to synthesize adenosine, was the same in fat-cells and also stromal cells of adipose tissue from pregnant, lactating and male rats. The response and sensitivity of fat-cells to the anti-lipolytic effects of adenosine were measured by incubating cells in the presence of noradrenaline, adenosine deaminase (to remove endogenous adenosine) and various concentrations of the adenosine analogue N6-phenylisopropyladenosine (PIA). PIA caused a greater inhibition of the rate of noradrenaline-stimulated lipolysis in adipocytes from lactating than from pregnant rats. The concentration of PIA required to inhibit by 50% the rate of noradrenaline-stimulated lipolysis fell from over 100 nM for fat-cells from pregnant rats to 30 nM for fat-cells from lactating rats. The decreased rate of noradrenaline-stimulated lipolysis during lactation was not due to the smaller mean cell volume of adipocytes during this state.

Lipolysis in rat adipocytes during pregnancy and lactation. The response to noradrenaline

1. Lipolysis has been measured in parametrial adipocytes from virgin, pregnant and lactating rats. 2. The basal rate and the maximal rate of lipolysis, the latter measured in the presence of noradrenaline and theophylline, remained constant between the three experimental categories, with the exception of a significant transient increase in the basal rate at parturition. 3. The noradrenaline-stimulated lipolysis rate rose above the virgin rate during pregnancy and fell below it during lactation; inclusion of adenosine deaminase in incubations abolished these differences in response to noradrenaline. 4. Cyclic AMP phosphodiesterase activity was lower in adipocytes during pregnancy and lactation than in virgin animals.

Metabolic cooperativity between epithelial cells and adipocytes of mice

We have demonstrated that glycogen and lipid synthesis in adipocytes is modulated by the lactational state and that this modulation in mammary adipocytes requires the presence of the adjacent epithelial cells. Glycogen and lipid synthesis from [14C]glucose was measured in mammary fat pads cleared of epithelium, in abdominal fat pads, and in adipocytes from both sources and from intact mammary gland of mature virgin, pregnant, and lactating mice. Accumulation of glycogen, the activity of glycogen synthase, and the lipogenic rate in abdominal and mammary adipocytes remained high during pregnancy but decreased to insignificant levels by early lactation. The depressant effects of lactation were observed solely in those mammary adipocytes isolated from intact glands. The presence of mammary epithelial cells was also required to effect the stimulated lipogenesis in mammary adipocytes during pregnancy, We conclude that the metabolic activity of adipocytes is modulated both during pregnancy and lactation to channel nutrients to the mammary epithelial cell. The fact that the changes occur in mammary adipocytes only when epithelial cells are present indicates that local as well as systemic factors are operating in these modulations.

Diet, pregnancy, and lactation: effects on adipose tissue, lipoprotein lipase, and fat cell size

Adipose tissue fat cell size and lipoprotein lipase (LPL) activity were determined in the retroperitoneal and subscapular depots of nonpregnant, pregnant, and postpartum rats fed either a standard laboratory diet or a high-fat diet containing 55% fat by weight. High-fat feeding for 20 days increased, in nonpregnant rats, fat cell size and LPL activity two- to threefold in both depots. In pregnant rats at term, fat cell size was increased and LPL activity was depressed in both dietary groups. Twenty days postpartum, both retroperitoneal fat cell size and LPL activity were decreased in proportion to the size of the litter. Rats not allowed to lactate had fat cell sizes and LPL activity that were not significantly different than in nonpregnant controls. Fat cell size and LPL activity in rats nursing four pups were reduced to 77% and 36% of control, respectively. Those nursing a normal-sized litter of eight pups demonstrated a further reduction of fat cell size to 38% and of LPL activity to 2% of nonpregnant control values. High-fat feeding and obesity did not prevent the fat loss and decreased LPL activity associated with lactation; fat cell size was decreased to 61% and LPL activity to 3% of control values. Values for the subscapular depot followed essentially the same pattern as that observed for the retroperitoneal depot. Mammary LPL activity was increased more than tenfold in animals nursing four or eight pups compared with values at term, whereas no activity was detected in rats not allowed to lactate.

Lipogenesis in interscapular brown adipose tissue of virgin, pregnant and lactating rats. The effects of intragastric feeding

Lipogenesis in brown adipose tissue of virgin rats increased 8--10-fold after intragastric feeding with glucose or medium-chain triacylglycerol, and this increase was prevented by short-term insulin deficiency. Brown adipose tissue increased in weight during pregnancy, regressed during lactation and hypertrophied again on weaning; the rate of lipogenesis paralleled these changes. Glucose did not increase brown-adipose-tissue lipogenesis at mid-lactation.

Control of glucose metabolism in isolated acini of the lactating mamary gland of the rat. Effects of oleate on glucose utilization and lipogenesis

Oleate (1mM) had only small inhibitory effects on glucose utilization and lipogenesis in acini isolated from rat mammary gland. Esterification of [1-14C]oleate was unaffected by insulin but were decreased by 60% by acetoacetate (2mM). Glycerol (1mM), but not insulin, relieved this inhibition. These experiments provide further support for the role of acetoacetate in regulating substrate utilization by the gland.

Effects of lactation of ketogenesis from oleate or butyrate in rat hepatocytes

1. Rates of ketogenesis from endogenous butyrate or oleate were measured in isolated hepatocytes prepared from fed rats during different reproductive states [virgin, pregnant, early-lactating (2-4 days) and peak-lactating (10-17 days)]. In the peak-lactation group there was a decrease (25%) in the rate of ketogenesis from butyrate, but there were no differences in the rates between the other groups. Wth oleate, the rate of ketogenesis was increased in the pregnant and in the early-lactation groups compared with the virgin group, whereas the rate was 50% lower in the peak-lactation group. 2. Experiments with [1-(14)C]oleate indicated that these differences in rates of ketogenesis were not due to alterations in the rate of oleate utilization, but to changes in the amount of oleoyl-CoA converted into ketone bodies. 3. Although the addition of carnitine increased the rates of ketogenesis from oleate in all groups of rats, it did not abolish the differences between the groups. 4. Measurements of the accumulation of glucose and lactate showed that hepatocytes from rats at peak lactation had a higher rate of glycolytic flux than did hepatocytes from the other groups. After starvation, the rate of ketogenesis from oleate was still lower in the peak-lactation group compared with the control group. This suggests that the alteration in ketogenic capacity in the former group is not merely due to a higher glycolytic flux. 5. It is concluded that livers from rats at peak lactation have a lower capacity to produce ketone bodies from long-chain fatty acids which is due to an alteration in the partitioning of long-chain acyl-CoA esters between the pathways of triacylglycerol synthesis and beta-oxidation. The physiological relevance of this finding is discussed.