Atrial natriuretic peptide receptors are present and functional by midgestation in fetal sheep

This study examined the ontogeny of the mRNA for three atrial natriuretic peptide (ANP) receptors in the ovine fetal kidney and the effect of systemic ANP infusion in the very immature ovine fetus. mRNA was isolated from the kidneys of 60-, 100-, and 140-day fetuses (n = 4 at each age). Northern blots [5 micrograms poly(A)+ RNA per track] were probed for the guanylate cyclase (GC)-A, GC-B, and clearance receptors, using beta-actin as a control for variations in loading. The results were quantitated using laser densitometry. Levels of clearance receptor mRNA were significantly higher in 140-day than 60-day fetal kidneys (P < 0.05), whereas levels of mRNA for the GC-A and GC-B receptors remained steady. We propose that binding of ANP to an increased number of C receptors in the late-gestation fetal kidney could explain the previously documented increase in total ANP receptor number in late-gestation ovine kidneys without increased ANP biological activity. Systemic ANP infusion into four fetuses of approximately 74 days gestation resulted in a natriuresis and diuresis, indicating the presence of functional ANP receptors in the ovine kidney early in gestation.