ABSTRACT
The developing fetal and neonatal gastrointestinal (GI) tract is influenced by many growth factors, including epidermal growth factor (EGF), insulin-like growth factor (IGF), transforming growth factor (TGF), and erythropoietin (Epo). Granulocyte colony-stimulating factor (G-CSF), typically regarded as a hematopoietic growth factor, might also be included because it exists in high concentrations in amniotic fluid, colostrum, and human milk, and because granulocyte CSF receptors (G-CSF-R) are abundantly expressed on the villous enterocytes of the developing intestine. As a first step toward understanding whether the effects of G-CSF on the GI tract were local or systemic, we sought to determine whether recombinant human G-CSF (rhG-CSF) administered enterally to suckling mice, is absorbed into the circulation, and if so, whether the G-CSF-R is essential for this absorption. We enterally administered rhG-CSF to suckling mice, selecting a daily dose based on the amount of G-CSF normally swallowed by the fetus and neonate (3 ng), or in other mice, a dose of G-CSF 100 times larger (300 ng). Pups were tested at either 5-7 days of age, or at 14-16 days of age. C57BL/6 x 129SvJ mice were used. Some mice had a targeted null mutation in the G-CSF-R gene, producing a non-functional G-CSF-R protein. At intervals following the enteral G-CSF dosing, G-CSF concentrations in plasma were measured by specific ELISA. The bioavailability of G-CSF was invariably <1%, regardless of the dose of rhG-CSF given, the age of the pups, or whether they had a functional G-CSF-R. After enteral administration of rhG-CSF to suckling mice, only minimal quantities of G-CSF are absorbed into the circulation, and the G-CSF-R is not essential for this absorption.
