Fetal hypoxemia on a molecular level: adaptive changes in the hypothalamic-pituitary-adrenal (HPA) axis and the lungs

The development of diseases in later life, such as diabetes type II, hypertension and cardiovascular disease, is linked to abnormal intrauterine conditions that reduce birth weight. Obviously, fetal development can be disturbed so profoundly, that fetal programming is changed permanently. We have examined the effects of hypoxia, or more precisely hypoxemia, on the fetal hypothalamic-pituitary-adrenal (HPA) axis and lungs using molecular biology techniques in order to elucidate the underlying mechanisms. Chronically catheterized fetal sheep were subjected to a hypoxemia (48 h) without change in arterial pH or paCO2. Major changes occurred, although the degree of hypoxemia was just moderate. There was a transient increase in the fetal plasma ACTH-concentrations with an upregulation of the cortisol-concentrations, which was more pronounced in the older, hypoxemic fetuses (134-136 days of gestation) than in the younger, hypoxemic animals (126-130 days of gestation; term is 145 days). There was an unique, differential regulation for pro-opiomelanocortin messenger RNA (mRNA), the precursor molecule of e.g. ACTH, in the pars distalis and pars intermedia of the pituitary gland. This finding supported the increased bioactivity besides the increased concentrations for ACTH. Simultaneously, there was an increase in the mRNAs of the ACTH-receptor and of the steroid-synthesizing enzymes in the fetal adrenal gland of the older, hypoxemic fetuses. No changes in the fetal plasma androstenedione-concentrations were observed. Clearly, there was a selective increase of the cortisol-synthesis. Growth and maturation of the fetal lung might also have been affected, because of the increase in surfactant-protein A mRNA in the older, hypoxemic animals and the decrease in the insulin-like growth factor-I and its binding protein-5 mRNA in the younger, hypoxemic fetuses. In summary, even a moderate degree of hypoxemia was shown to affect the different levels of fetal organism profoundly, offering a pathophysiological basis for changes in fetal development.