Transporters dynamically expressed at the mammary gland transport critical nutrients into the breast milk of nursing mothers to meet the nutritional demands of the suckling infant. However, xenobiotics may interact with these transporters to potentially alter the nutrient composition of milk and compromise neonatal nutrition. The aim of the present study was to quantitatively evaluate the constitutive expression of various nutrient transporters in whole mammary gland tissue and mammary epithelial organoids (MEO) isolated from female Sprague-Dawley rats at various stages of pregnancy, lactation, and involution. Furthermore, the studys aim was to determine if appropriately cultured mammary epithelial organoids (MEO) maintain in vivo transporter expression to lay down critical groundwork for the development of an in vitro screening tool assessing xenobiotic-nutrient transporter interactions. The following transporters were evaluated using quantitative real-time reverse transcription polymerase chain reaction (QRT-PCR): multidrug resistance protein (Mdr) 1a, 1b; multidrug resistance-like protein (Mrp) 1; organic cation transporter (Oct) 1; organic cation/carnitine transporter (Octn) 1, 2, and 3; concentrative nucleoside transporter (Cnt) 1, 2, and 3; equilibrative nucleoside transporter (Ent) 1, 2, and 3; nucleobase transporter (Ncbt) 1 and 2; oligopeptide transporter (Pept) 1 and 2; methotrexate carrier (Mtx) 1; divalent metal transporter (Dmt) 1; and the milk protein ?-casein. Transporter expression patterns in MEO differed from whole tissue for ?-actin, Mdr1a, Mdr1b, Oct1, Octn3, Ent3, Cnt1, Cnt3, Ncbt1, Pept2, Mtx1, and ?-casein. This brings into question whether whole mammary gland tissue is truly appropriate for an understanding of transporter expression in the mammary epithelium. Nevertheless, four general transporter expression patterns emerged in isolated MEO: decline throughout lactation (Mdr1a, Mdr1b, Mrp1 & Dmt1), increase throughout lactation (Cnt1 & Octn3), increase in early lactation (Oct1, Octn2, Ent1, Cnt2, Cnt3, Pept2 & Mtx1) and constant expression throughout lactation (Octn1, Ent2, Ent3, Ncbt1, Ncbt2 & Pept1). These expression patterns will provide insight into the critical windows of nutrient delivery to the breast milk to provide adequate nutritional stimuli to the suckling infant. Furthermore, MEO cultured in an extracellular matrix-rich environment maintained transporter expression at the mRNA level, which underscores the potential of the primary MEO in vitro model system as a screening tool for xenobiotic-transporter interactions at the mammary gland. Transporter expression patterns in MEO were unique for each transporter evaluated. This information accompanied by an in vitro screening tool may allow for predictions of xenobiotic interference with breast milk composition to help safeguard infant health.

Transporters dynamically expressed at the mammary gland transport critical nutrients into the breast milk of nursing mothers to meet the nutritional demands of the suckling infant. However, xenobiotics may interact with these transporters to potentially alter the nutrient composition of milk and compromise neonatal nutrition. The aim of the present study was to quantitatively evaluate the constitutive expression of various nutrient transporters in whole mammary gland tissue and mammary epithelial organoids (MEO) isolated from female Sprague-Dawley rats at various stages of pregnancy, lactation, and involution. Furthermore, the studys aim was to determine if appropriately cultured mammary epithelial organoids (MEO) maintain in vivo transporter expression to lay down critical groundwork for the development of an in vitro screening tool assessing xenobiotic-nutrient transporter interactions. The following transporters were evaluated using quantitative real-time reverse transcription polymerase chain reaction (QRT-PCR): multidrug resistance protein (Mdr) 1a, 1b; multidrug resistance-like protein (Mrp) 1; organic cation transporter (Oct) 1; organic cation/carnitine transporter (Octn) 1, 2, and 3; concentrative nucleoside transporter (Cnt) 1, 2, and 3; equilibrative nucleoside transporter (Ent) 1, 2, and 3; nucleobase transporter (Ncbt) 1 and 2; oligopeptide transporter (Pept) 1 and 2; methotrexate carrier (Mtx) 1; divalent metal transporter (Dmt) 1; and the milk protein ?-casein. Transporter expression patterns in MEO differed from whole tissue for ?-actin, Mdr1a, Mdr1b, Oct1, Octn3, Ent3, Cnt1, Cnt3, Ncbt1, Pept2, Mtx1, and ?-casein. This brings into question whether whole mammary gland tissue is truly appropriate for an understanding of transporter expression in the mammary epithelium. Nevertheless, four general transporter expression patterns emerged in isolated MEO: decline throughout lactation (Mdr1a, Mdr1b, Mrp1 & Dmt1), increase throughout lactation (Cnt1 & Octn3), increase in early lactation (Oct1, Octn2, Ent1, Cnt2, Cnt3, Pept2 & Mtx1) and constant expression throughout lactation (Octn1, Ent2, Ent3, Ncbt1, Ncbt2 & Pept1). These expression patterns will provide insight into the critical windows of nutrient delivery to the breast milk to provide adequate nutritional stimuli to the suckling infant. Furthermore, MEO cultured in an extracellular matrix-rich environment maintained transporter expression at the mRNA level, which underscores the potential of the primary MEO in vitro model system as a screening tool for xenobiotic-transporter interactions at the mammary gland. Transporter expression patterns in MEO were unique for each transporter evaluated. This information accompanied by an in vitro screening tool may allow for predictions of xenobiotic interference with breast milk composition to help safeguard infant health.