Mammary uptake and excretion of prostanoids in relation to mammary blood flow and milk yield during pregnancy-lactation and somatotropin treatment in dairy goats

Regulation of fluid flow through the mammary gland of dairy cows and its effect on milk production: a systematic review

Dairy milk consists of more than 85% water. Therefore, understanding the regulation of fluid absorption in the mammary gland is relevant to improving milk production. In recent decades, studies using different approaches, including blood flow, transmembrane fluid flow, tight junction, fluid flow of the paracellular pathway and functional mammary epithelial cell state, have been conducted aiming to investigate how mammary gland fluid absorption is regulated. However, the relationship between regulation mechanisms of fluid flow and milk production has not been studied systematically. The present review summarizes a series of key milk yield regulatory factors mediated by whole-mammary fluid flow, including milk, mammary blood flow, blood/tissue fluid-cell fluid flow and cell-alveolus fluid flow. Whole-mammary fluid flow regulates milk production by altering transporter activity, ion channels, local microcirculation-related factors, driving force of fluid transport (osmotic pressure or electrochemical gradient), cellular connection state and a cell volume sensitive mechanism. In addition, whole-mammary fluid flow plays important roles in milk synthesis and secretion. Knowledge gained from fluid flow-mediated regulatory mechanisms of the dairy mammary gland will lead to a fundamental understanding of lactation biology and will be beneficial for the improvement of dairy productivity. © 2017 Society of Chemical Industry.

Inhibition of local blood flow control systems in the mammary glands of lactating cows affects uptakes of energy metabolites from blood

To test the effect of mammary blood flow on net uptakes of milk precursors by the mammary glands, inhibitors of nitric oxide synthase (NOS) and cyclooxygenase (COX) were infused into the mammary circulation of 4 lactating cows. Inhibitors were infused in a 4×4 Latin square design, where treatments were infusion for 1 h of saline, NOS inhibitor (Nω-nitro-l-arginine methyl ester hydrochloride), COX inhibitor (indomethacin), or both NOS + COX inhibitors into one external iliac artery. Para-aminohippuric acid was also infused to allow for estimation of iliac plasma flow (IPF), of which approximately 80% flows to the mammary glands. Blood samples were collected before, during, and after inhibitor infusion from the contralateral external iliac artery and ipsilateral mammary vein. Inhibition of COX and NOS each produced a decrease in IPF, although the NOS effect was smaller and IPF continued to be depressed throughout the recovery period. The combination of COX and NOS inhibition produced a 50% depression in IPF and there was no carryover into the recovery period. Treatments that depressed IPF also increased arterial concentrations of acetate, β-hydroxybutyrate (BHBA), and glucose. Similarly, arteriovenous differences of acetate, BHBA, and glucose were all increased during IPF depression. To correct for a potential effect of arterial concentration, arteriovenous differences were normalized to arterial concentration, producing an extraction percentage. Inhibition of COX increased glucose extraction and tended to increase acetate and BHBA extraction. Dual inhibition only increased BHBA extraction and had no effect on mammary extraction of other metabolites. These extractions did not increase because clearances of glucose and TAG decreased as IPF decreased, and clearances of acetate and BHBA tended to decrease. Net uptake of TAG was depressed by dual NOS/COX inhibition, whereas uptakes of acetate, BHBA, and glucose were not affected by any of the treatments. To separate effects of flow from effects of arterial concentration, uptakes were regressed against IPF and arterial concentration simultaneously. According to the slopes of the regressions, a 10% decrease in IPF from the mean observed during saline infusion resulted in 3.8, 7.3, and 10.4% decreases in uptakes of acetate, glucose, and triacylglycerol, respectively. These findings indicate that mammary blood flow affects milk precursor uptake, and that clearance should not be assumed constant to predict mammary uptakes of milk precursors in situations where blood flow is changing.

Mammary remodeling in primiparous and multiparous dairy goats during lactation

Milk production is generally lower but lactation persistency higher in primiparous (PP) than in multiparous (MP) goats. This may be related to differences in development and maintenance of mammary gland function, but the underlying mechanisms are not well understood. The present study aimed to elucidate whether differences in lactational performance between PP and MP mammary glands are related to the time course of development and maintenance, not only of the mammary epithelial cell (MEC) population, but also of the mammary vasculature that sustains synthetic activity. Mammary biopsies were obtained from both mammary glands of 3 PP and 6 MP (>or=2 parity) dairy goats at parturition (d 1), d 10, 60, and 180 of lactation. Gene transcription relating to MEC turnover and vascular function was quantified by real-time reverse transcription-PCR, mammary morphology was characterized (quantitative histology), and cell turnover was determined (terminal deoxynucleotidyl transferase dUTP nick end labeling assay and Ki-67). Primiparous glands showed higher expression for the genes involved in angiogenesis; namely, vascular endothelial growth factor receptor 2, and angiopoietin 1 and 2 and their receptor, a few days after parturition (d 10). Primiparous glands also had higher rates of MEC proliferation in early lactation. It therefore appears that initiation of lactation is associated with development and growth of the mammary gland into early lactation, which continues for a longer period in PP compared with MP glands. In addition, MEC survival was found to be higher in PP glands throughout lactation, and MEC in PP glands underwent more extensive differentiation. This could explain the reported flatter lactation curve and higher lactation persistency in PP glands. Although some of the genes included in this study were differentially expressed in PP and MP glands during the course of lactation, it was not possible to identify any specific genomic factor(s) that could account for the differences between PP and MP glands with respect to mammary development and MEC survival during lactation. It remains to be established why parity number affects MEC and vascular development and survival during lactation, and, in particular, which regulatory mechanisms are involved.

Continuous lactation effects on mammary remodeling during late gestation and lactation in dairy goats

The present study aimed to 1) elucidate whether continuous milking during late gestation in dairy goats negatively affects mammary remodeling and hence milk production in the subsequent lactation, and 2) identify the regulatory factors responsible for changes in cell turnover and angiogenesis in the continuously lactating mammary gland. Nine multiparous dairy goats were used. One udder half was dried off approximately 9 wk prepartum (normal lactation; NL), and the other udder half of the same goat was milked continuously (continuous lactation; CL) until parturition or until the half-udder milk yields had dropped to below 50 g/d. Mammary biopsies were obtained from each udder half just before the NL gland was dried off (before dry period), within the first 2 wk after drying-off (early dry period, samples available only for NL glands), in the mid dry period, within the last 2 wk before parturition (late dry period), and at d 1 (the day of parturition), 3, 10, 60, and 180 of lactation. Mammary morphology was characterized in biopsies by quantitative histology, and cell turnover was determined by immunohistochemistry (terminal deoxynucleotidyl transferase dUTP nick end labeling and Ki-67). Transcription of genes encoding factors involved in mammary epithelial cell (MEC) turnover and vascular function was quantified by quantitative reverse transcription PCR. Results demonstrated that omitting the dry period was possible in goats but was not as easy as claimed before. Renewal of MEC was suppressed in CL glands, which resulted in a smaller MEC population in the subsequent lactation. At the time of parturition (and throughout lactation), the mammary glands subjected to CL had smaller alveoli, more fully differentiated MEC, and a substantially larger capillary fraction compared with NL glands. The continuously lactating gland thus resembled a normally lactating gland in an advanced stage of lactation. None of the studied genomic factors could account for these treatment differences. The described characteristics in CL glands compared with NL glands indicated a gland maintained in lactation for a longer period.