Altered bile acid physiology during lactation in the rat

Differential abundance of IGF1, bile acids, and the genes involved in their signaling in the dominant follicle microenvironment of lactating cows and nulliparous heifers

It is well documented that incidence of fertility problems is high in lactating cows but not in heifers of the same genetic merit. Understanding the metabolic and molecular differences between fertile heifers and relatively infertile lactating cows will help us understand the pathogenesis of infertility in dairy cows. Follicular waves in lactating cows (30-50 days in milk; n = 12) and heifers (n = 10) were synchronized by ultrasound-guided follicle ablation. Follicular fluid and granulosa cells of the dominant follicle were collected by ultrasound-guided aspiration along with blood sampling on Day 6 after synchronization. Dominant and subordinate follicles were larger in lactating cows than in heifers. Metabolic stress in lactating cows was evidenced by lower glucose and higher ß-hydroxy butyric acid compared with heifers. Insulin-like growth factor 1 signaling was reduced in the dominant follicle in lactating cows through reduced insulin-like growth factor 1 concentrations in plasma and follicular fluid of the dominant follicle, and reduced expression of pregnancy-associated plasma protein A (PAPPA) in their granulosa cells. We also found increased levels of total bile acids in the follicular fluid of the dominant follicle of lactating cows compared with heifers. Granulosa cells of the dominant follicle had higher expression of SLC10A2 and GPBAR1 (bile acid transporter and receptor, respectively) in lactating cows. These novel data are indicative of increased bile acid signaling within the dominant follicles of lactating cows compared with heifers. Overall, we demonstrate in the present study the metabolic, endocrine, and molecular differences within the microenvironment of the dominant follicles in lactating cows and heifers. These differences in follicular microenvironment may contribute toward abnormal ovarian function in lactating dairy cows.

ABCG5/ABCG8-independent biliary cholesterol excretion in lactating rats

Lactation is associated with increased expression of bile acid transporters and an increased size and hydrophobicity of the bile acid pool in rats. ATP-binding cassette (ABC) transporters multidrug resistance protein 2 (Mdr2), Abcb11 [bile salt export pump (Bsep)], and Abcg5/Abcg8 heterodimers are essential for the biliary secretion of phospholipids, bile acids, and cholesterol, respectively. We investigated the expression of these transporters and secretion of their substrates in female control and lactating Sprague Dawley rats and C57BL/6 mice. Expression of Abcg5/Abcg8 mRNA was decreased by 97 and 60% by midlactation in rats and mice, respectively; protein levels of Abcg8 were below detection limits in lactating rats. Mdr2 mRNA expression was decreased in lactating rats and mice by 47 and 59%, respectively. Despite these changes in transporter expression, basal concentrations of cholesterol and phospholipid in bile were unchanged in rats and mice, whereas increased Bsep mRNA expression in early lactation coincided with an increased basal biliary bile acid concentration in lactating mice. Following taurocholate infusion, coupling of phospholipid and taurocholate secretion in bile of lactating mice was significantly impaired relative to control mice, with no significant changes in maximal secretion of cholesterol or bile acids. In rats, taurocholate infusion revealed a significantly impaired coupling of cholesterol to taurocholate secretion in bile in lactating vs. control animals. These data reveal marked utilization of an Abcg5/Abcg8-independent mechanism for basal biliary cholesterol secretion in rats during lactation, but a dependence on Abcg5/g8 for maximal biliary cholesterol secretion.

Increased cholesterol 7alpha-hydroxylase expression and size of the bile acid pool in the lactating rat

Maximal bile acid secretory rates and expression of bile acid transporters in liver and ileum are increased in lactation, possibly to facilitate increased enterohepatic recirculation of bile acids. We determined changes in the size and composition of the bile acid pool and key enzymes of the bile acid synthetic pathway [cholesterol 7alpha-hydroxylase (Cyp7a1), sterol 27-hydroxylase (Cyp27a1), and sterol 12alpha-hydroxylase (Cyp8b1)] in lactating rats relative to female virgin controls. The bile acid pool increased 1.9 to 2.5-fold [postpartum (PP) days 10, 14, and 19-23], compared with controls. A 1.5-fold increase in cholic acids and a 14 to 20% decrease in muricholic acids in lactation significantly increased the hydrophobicity index. In contrast, the hepatic concentration of bile acids and small heterodimer partner mRNA were unchanged in lactation. A 2.8-fold increase in Cyp7a1 mRNA expression at 16 h (10 h of light) demonstrated a shift in the diurnal rhythm at day 10 PP; Cyp7a1 protein expression and cholesterol 7alpha-hydroxylase activity were significantly increased at this time and remained elevated at day 14 PP but decreased to control levels by day 21 PP. There was an overall decrease in Cyp27a1 mRNA expression and a 20% decrease in Cyp27a1 protein expression, but there was no change in Cyp8b1 mRNA or protein expression at day 10 PP. The increase in Cyp7a1 expression PP provides a mechanism for the increase in the bile acid pool.

Increased expression of ileal apical sodium-dependent bile acid transporter in postpartum rats

The expression and activity of the apical ileal sodium-dependent bile acid transporter (asbt) was examined in the small intestine of control, pregnant, and lactating postpartum rats 2, 12, and 21 days after delivery. Western blot analysis of brush border membrane vesicles (BBMV) prepared from different regions of the small intestine demonstrated that expression of asbt was maximal in the most distal segments for all experimental groups, was not substantially affected in pregnant and 2-day postpartum rats, and was significantly increased in 12- and 21-day postpartum rats. Analysis of mRNA suggested that asbt protein was regulated at the posttranscriptional level in postpartum rats. Increased expression of asbt protein postpartum was maximal (approximately 2-fold) in the proximal region of the ileum, consistent with a 60% increase in taurocholate (TC) transport in BBMV from the proximal ileum in 14- to 21-day postpartum rats relative to control rats. Absorption of TC, determined from the intact proximal ileum using an intestinal loop model, demonstrated a 30% increase in TC uptake per unit weight of tissue in 14- to 21-day postpartum rats relative to control rats. Together with the marked increase in intestinal mass observed at peak lactation, these data indicate a significant increase in asbt-mediated reclamation of bile acids in the intestine of lactating rats.

Enhancement of cytosolic and microsomal glutathione S-transferase activities in liver and small intestine from female rats during lactation

The influence of lactation on hepatic and intestinal glutathione S-transferase activities in mother rats was studied. Cytosolic and microsomal activities were assessed 7, 14 and 21 days after delivery, using 1-chloro-2,4-dinitrobenzene as substrate. Cytosolic and microsomal activities from liver and small intestine determined 7 days post partum did not differ from those of virgin female rats. The hepatic cytosolic activity was significantly increased with respect to that of virgin females 14 days after delivery and tended to revert to the control value on day 21 of lactation, whereas the intestinal activity was increased on day 14 and remained augmented even 21 days post partum. Although the respective microsomal activities showed percent increases higher than those of the cytosolic enzymes, they both exhibited a similar pattern of stimulation in response to lactation.

Cellular energy metabolism and regulation

Consistent with the increased demand for nutrients imposed by lactation and growth, those tissues directly involved in the digestion, absorption, and processing of the required additional nutrients show response to these states. During lactation, the rumen, upper intestine, and liver increase in size, and more energy is spent on Na+,K+ transport and on protein turnover. The massive endocrine influences during lactation suggest that the metabolism of other tissues besides these and mammary tissue would be influenced, but evidence is rather sparse. Ion transport and protein metabolism in some muscles may indeed be increased. Although substrate cycles characteristically account for a substantially smaller portion of the energy expenditure in the intact animal than do ion transport and protein turnover, stage of lactation influences some of these cycles, particularly the triacylglycerol fatty acid cycle. The needs for additional quantitative in vivo measurements of metabolic conversions and for mechanistic model description of metabolic events in nonmammary tissues are discussed.