Postnatally, glucagon acutely lowers plasma amino acid (AA) concentrations by stimulating hepatic AA catabolism, but its fetal actions remain unclear. This study tested whether a 2 h fetal glucagon infusion would stimulate hepatic AA catabolism and inhibit placental AA transfer. Late-gestation pregnant sheep (0.9 gestation) underwent surgical, vascular catheterization and received fetal glucagon (n = 8) or vehicle infusions (n = 8) in a crossover design with a 48 h washout period. Nutrient uptake and utilization were assessed during each infusion, and fetal liver and placental tissue were collected post-infusion under hyperglucagonemic (n = 4) or vehicle (n = 4) conditions. Glucagon receptor was identified in fetal hepatocyte and trophoblast cells. Glucagon reduced fetal plasma AA concentrations by 20% (p = 0.0103) and increased plasma glucose by 47% (p = 0.0152), leading to a three-fold rise in fetal plasma insulin (p = 0.0459). Hepatic gene expression associated with AA catabolism and gluconeogenesis increased (p < 0.0500) following glucagon infusion, and hepatic metabolomic analysis showed enrichment in AA metabolism pathways. However, placental AA transfer was unaffected by 2 h fetal glucagon infusions. In conclusion, a 2 h glucagon infusion stimulates hepatic glucose production and enhances AA catabolism in the fetal liver, lowering plasma AA concentrations. The primary acute effects of fetal glucagon are hepatic, as placental AA transfer is unchanged.

Fetal growth restriction (FGR) is associated with elevated fetal glucagon and reduced maternal placental lactogen (hPL) plasma concentrations. In sheep, fetal glucagon inhibits placental lactogen secretion into maternal circulation. To investigate whether glucagon similarly regulates hPL secretion by human trophoblast, we examined glucagon receptor (GCGR) concentrations and glucagon’s effect on hPL secretion using BeWo choriocarcinoma and primary human trophoblast (PHT) cells. We hypothesized treating trophoblast cells with glucagon would inhibit hPL secretion. GCGR localization and quantification was determined by immunohistochemistry (IHC), RNAscope, and western blotting. GCGR localized to the basal (fetal-facing) membrane of the syncytiotrophoblast, with minimal localization in the maternal-facing microvillous membrane. hPL and human chorionic gonadotropin (hCG) expression and secretion were quantified by qPCR and ELISA. In BeWo cells, forskolin did not change GCGR concentrations and increased human chorionic gonadotropin alpha CGA mRNA (CGα; P<0.0001). Unexpectedly, it decreased chorionic somatomammotropin 1 mRNA (CSH1; P<0.0001) and reduced both hPL (P<0.0001) and hCG (P=0.0012) secretion. Glucagon also reduced hPL secretion (P=0.0018) in non-forskolin treated BeWo cells. In PHTs, GCGR concentrations remained unchanged over duration of culture. PHT cells increased hCG (P=0.0140) with time in culture. hPL secretion followed a biphasic pattern, and glucagon inhibited both CSH1 mRNA (P<0.0100) and hPL secretion during the latter secretory phase (P=0.0313). These findings confirm GCGR localizes on the fetal-facing membrane of PHTs and demonstrate that glucagon inhibits hPL secretion in human trophoblasts, consistent with prior sheep studies. Our results link fetal glucagon to placental endocrine adaptations in pregnancies affected by placental insufficiency and FGR.