The glomerular filtration rate during pregnancy: Saline infusion enhances the glomerular filtration rate in the pregnant rat

The glomerular filtration rate (GFR) of pregnant rats is generally believed to exceed non-pregnant values. This notion is primarily based upon standard inulin clearances. However, the inulin clearance requires continuous infusion of inulin usually dissolved in saline. Since saline infusion per se in pregnancy may influence the GFR, in the present study the effect of saline infusion upon the GFR in pregnant as compared with cyclic rats was investigated using various methods. The standard inulin clearance was compared using the standard Cr-51-EDTA method which does not require saline infusion. Clearance of inulin dissolved in glucose (5% in distilled water) instead of saline was also tested, while the Cr-51-EDTA method was employed using additional fluid infusion with either saline or 5% glucose in distilled water in an identical manner as compared with the inulin method. The GFR was also studied in conscious rats using Cr-51-EDTA clearance with and without fluid infusion. The distribution volume of Cr-51-EDTA was measured in nephrectomized rats (pregnant and cyclic) with and without saline or glucose infusions. The results show a significant increase of the GFR in pregnant rats as compared with cyclic rats only when saline was infused during the measurement; thus, GFR measurements without fluid infusion or replacement of saline by glucose during the measurements did not show a significant increase of GFR in pregnant rats. The volume of distribution per gram body weight of Cr-51-EDTA after saline infusion, but not after glucose infusion, was significantly increased as compared with the values obtained without additional infusion. It is concluded that the increase of the GFR seen in pregnant rats when either the Cr-51-EDTA method or the inulin method is accompanied by saline loading is rather due to infusion of saline in the pregnant animal and not a result of the pregnant condition per se.