Temporal Changes in Phosphate Concentrations and Tissue Non-Specific Alkaline Phosphatase Activity in Bovine Utero-Placental Tissues and Fetal Fluids.

Phosphate is critical for pregnancy maintenance, placental, and fetal development. Although the mechanisms regulating postnatal phosphate homeostasis are well described, those regulating phosphate availability during gestation in cattle remain poorly understood. Tissue non-specific alkaline phosphatase (TNSALP, encoded by ALPL), is a postnatal regulator of phosphate availability, yet its role in the regulation of utero-placental phosphate availability in ruminants remains poorly understood. This study characterized phosphate abundance and TNSALP activity in bovine utero-placental tissues and fetal fluids, and the expression of ALPL mRNA, and TNSALP protein and activity localization in bovine utero-placental tissues during early-mid gestation. Fetal fluids and utero-placental tissues were collected (n=3-10 per group; range: 48-133 days). Phosphate concentrations and TNSALP activity were quantified spectrophotometrically. Expression of ALPL mRNA was quantified using qPCR. TNSALP protein and activity localization were determined using histological staining. Phosphate concentrations increased in allantoic fluid and decreased in amniotic fluid as gestation progressed (P ≤ 0.0001). Phosphate abundance did not change in utero-placental tissues throughout gestation. Gestational day affected TNSALP activity in allantoic fluids, amniotic fluids, and endometria (P ≤ 0.001), but not placentomes. Gestational day affected expression of ALPL mRNA in endometria and placentomes (P ≤ 0.01). TNSALP protein and enzymatic activity localized to the endometrial luminal and glandular epithelia, endometrial and placental vasculature, and the caruncular-cotyledonary epithelial interface. Collectively, these findings establish a spatiotemporal profile of phosphate abundance and TNSALP activity in bovine utero-placental tissues and fetal fluids and demonstrate TNSALP is localized to regions associated with maternal-fetal nutrient exchange.