Effect of lactose content and milking interval on mammary blood flow

Pharmacologic inhibition of mTORC1 mimics dietary protein restriction in a mouse model of lactation

Background

Understanding the mechanisms of N utilization for lactation can lead to improved requirement estimates and increased efficiency, which modern dairy diets currently fail to maximize. The mechanistic target of rapamycin complex 1 (mTORC1) is a central hub of translation regulation, processing extra- and intra-cellular signals of nutrient availability and physiological state, such as amino acids and energy. We hypothesized that dietary amino acids regulate lactation through mTORC1, such that inhibition of mTORC1 will lead to decreased lactation performance when amino acids are not limiting. Our objectives were to assess lactation performance in lactating mice undergoing dietary and pharmacologic interventions designed to alter mTORC1 activity.

Methods

First lactation mice (N = 18; n = 6/treatment) were fed an adequate protein diet (18% crude protein), or an isocaloric protein-restricted diet (9% crude protein) from the day after parturition until lactation day 13. A third group of mice was fed an adequate protein diet and treated with the mTORC1 inhibitor rapamycin (4 mg/kg every other day) intraperitoneally, with the first two groups treated with vehicle as control. Dams and pups were weighed daily, and feed intake was recorded every other day. Milk production was measured every other day beginning on lactation day 4 by the weigh-suckle-weigh method. Tissues were collected after fasting and refeeding.

Results

Milk production and pup weight were similarly decreased by both protein restriction and rapamycin treatment, with final production at 50% of control (P = 0.008) and final pup weight at 85% of control (P < 0.001). Mammary phosphorylation of mTORC1’s downstream targets were decreased by protein restriction and rapamycin treatment (P < 0.05), while very little effect was observed in the liver of rapamycin treated mice, and none by protein restriction.

Conclusions

Overall, sufficient supply of dietary amino acids was unable to maintain lactation performance status in mice with pharmacologically reduced mammary mTORC1 activity, as evidenced by diminished pup growth and milk production, supporting the concept that mTORC1 activation rather than substrate supply is the primary route by which amino acids regulate synthesis of milk components.

Triennial Lactation Symposium: A local affair: How the mammary gland adapts to changes in milking frequency

Regular removal of milk from the mammary gland is critical to maintaining milk secretion. Early studies in rodents demonstrated that changes in milking frequency influenced mammary blood flow, as well as mammary cell number and activity. Later studies in ruminants confirmed those observations and that the response was regulated locally within the mammary gland. In addition, it was discovered that increased milking frequency (IMF) during early lactation stimulated an increase in milk production that partially persisted through late lactation, indicating long-term effects on mammary function. The local mechanisms regulating the mammary response to IMF are poorly understood, although several have been proposed. To gain insight into the mechanisms underlying the mammary response to IMF, and to identify genes associated with the response, we used a functional genomics approach and conducted experiments on dairy cows exposed to unilateral frequent milking [UFM; twice daily milking (2X) of the left udder half and 4-times daily milking (4X) of the right udder half]. Across multiple experiments, we were unable to detect an effect of UFM on mammary cell proliferation or apoptosis. We have, however, identified distinct transcriptional signatures associated with the mammary response to milk removal and to UFM during early lactation. Sequential sampling of mammary tissue revealed that when UFM was imposed during early lactation, at least 2 sets of genes were coordinately regulated with changes in differential milk production of 4X vs. 2X udder halves. Moreover, some genes were persistently differentially expressed in 4X vs. 2X udder halves after UFM and were associated with the persistent increase in milk yield. We conclude that a coordinated transcriptional response is associated with the increase in milk yield elicited by IMF during early lactation and that the 2 sets of differentially expressed genes may be a marker for the autocrine up-regulation of milk production. Moreover, we propose that we have identified a novel form of imprinting associated with persistent alteration of mammary function, which we term "lactational imprinting."

Regulation of blood flow in the mammary microvasculature

Although milk yield of cows and goats is known to be closely related to the total flow of blood through the udder, a number of studies suggest that milk yield can vary independently. No studies have attempted to measure the proportion of total flow that is nutritive. Within the mammary gland, capillary networks form a basket-like architecture surrounding each alveolus. Notably, flow in individual capillaries is not constant and varies among capillaries. Capillary flow (measured by intravital microscopy) was decreased by oxytocin, which generally increased total flow in the mammary artery, suggesting that the proportion of total flow that is nutritive can vary. In addition to classic metabolic regulators (e.g., carbon dioxide and oxygen) of tissue blood flow, the mammary gland produces a number of vasodilatory compounds, including parathyroid hormone-related protein, insulin-like growth factor-I, prostacyclin, nitric oxide, and endothelin. All of these compounds have been shown to alter mammary blood flow. Mammary tissue also contains kallikrein and angiotensin-converting enzyme, which convert circulating kinins and angiotensin, respectively, into potent vasoactive compounds. A number of these compounds are produced by epithelial cells themselves, providing a mechanism for the functioning epithelium to control its own blood supply and, hence, nutrient flow for milk synthesis. In this review, we examine the nature of the mammary microcirculation, its behavior under different conditions, and some of the regulatory features of the mammary microvasculature.