Maturational development of glomerular ultrafiltration in the rat.

To ascertain the cause of low glomerular filtration rate in newborn and immature mammals, we measured glomerular pressures and flows directly in immature (30- to 45-day-old) euvolemic Munich-Wistar rats with surface glomeruli. As with total kidney GFR, single nephron (SN)GFR was found to be significantly lower than in adult rats, on average by 40% when corrected for kidney weight. Equality between efferent oncotic pressure and transglomeruler hydraulic pressure difference (deltaP) was usually achieved in immature rats, indicating that the glomerular capillary ultrafiltration coefficient is not a factor limiting SNGFR and GFR in immature rats. Although the average values for deltaP in immature rats were slightly, albeit significantly, lower than in adults, markedly lower values (79 +/- 5 vs. 136 +/- 10 nl/min per g kidney wt) for glomerular plasma flow rate (QA) proved to be the primary factor responsible for the lower SNGFR and GFR values in immature rats. Considerably higher values for afferent and efferent arteriolar resistances contributed to this low QA state in immature rats.