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DEVELOPMENTAL PHYSIOLOGY AND PREGNANCY

Mechanisms of neonatal increase in glomerular filtration rate

¹Department of Physiology, School of Medical Sciences, University of New South Wales, Sydney, Australia; and ²Department of Medical Cell Biology, Division of Integrative Physiology, University of Uppsala, Sweden

Submitted 14 November 2007 ; accepted in final form 11 July 2008

To investigate the mechanisms responsible for the neonatal increase in glomerular filtration rate (GFR), renal function studies (whole kidney and micropuncture) were carried out in anesthesized fetal sheep (133–140 days gestation; term = 150 days) and lambs (12–18 days). Fetuses were delivered and placed in a water bath (39.5°C), keeping the umbilical cord moist and intact. Lambs were studied on a thermostatically controlled heating pad. Animals were prepared for either blood flow studies or micropuncture measurements. Expected differences in blood composition and cardiovascular and renal function were observed between fetuses and lambs, and values obtained for most variables were similar to those measured in chronically catheterized unanesthetized animals. Fetal GFR was much lower than that of lambs (0.20 vs. 0.62 ml·min^–1·g kidney^–1, P < 0.001). Free-flow, stop-flow, and net filtration pressures (NFP) were lower in the fetuses than the lambs (NFP 20.8 vs. 23.8 mmHg, P < 0.001), as was the calculated ultrafiltration coefficient (0.014 vs. 0.022 ml·min^–1·g^–1·mmHg^–1, P < 0.001). Thus, we conclude that rises in both net filtration pressure and the ultrafiltration coefficient contribute to the large increase in GFR between fetal life and 2 wk after birth.

fetus; lamb; micropuncture; free-flow pressure; stop-flow pressure