Effects of pregnancy on glomerular dynamics: micropuncture study in the rat.

Glomerular dynamics were investigated by micropuncture in 15-day pregnant Munich-Wistar rats. Female non-pregnant rats were used as control. In hydropenia, single nephron glomerular filtration rate (SNGFR) averaged 37.2 nl/min/g kidney weight in pregnant rats, and 26.2 nl/min/g kidney weight in controls (P less than 0.0005). During pregnancy, mean glomerular plasma flow (GPF) rose from 63.0 to 104.5 nl/min (P less than 0.0005), and glomerular capillary hydrostatic pressure (PG) from 44.8 to 50.6 mm Hg (P less than 0.0005). This rise in PG accounted for an increase in effective filtration pressure (EFP). Mean EFP at the efferent end of the glomerulus rose from near zero to 9.7 mm Hg, indicating filtration pressure disequilibrium. After extracellular fluid volume expansion with saline, GPF and EFP were still greater during pregnancy. A filtration pressure disequilibrium occurred also in non-pregnant rats, allowing calculation of definite values of the ultrafiltration coefficient (Kf). During pregnancy, mean Kf was reduced from 0.0332 to 0.0285 nl/(sec X mm Hg) (P less than 0.005). SNGFR was moderately, but not significantly, increased. These results show that during pregnancy glomerular dynamics is characterized by a rise in GPF and EFP, and by a reduction in Kf. Following these changes, a plasma-flow dependent rise in SNGFR occurs in hydropenia. After expansion, instead, SNGFR is unmodified because the dependence of ultrafiltration on plasma-flow declines while the influence of Kf increases at high values of GPF.