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 bel ieved to exceed non-pregnant values. This notion is primarily based up on standard inulin clearances. However, the inulin clearance requires continuous infusion of inulin usually dissolved in saline. Since salin e infusion per se in pregnancy may influence the GFR, in the present s tudy the effect of saline infusion upon the GFR in pregnant as compare d with cyclic rats was investigated using various methods. The standar d inulin clearance was compared using the standard Cr-51-EDTA method w hich does not require saline infusion. Clearance of inulin dissolved i n glucose (5% in distilled water) instead of saline was also tested, w hile the Cr-51-EDTA method was employed using additional fluid infusio n 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 infu sion. The distribution volume of Cr-51-EDTA was measured in nephrectom ized rats (pregnant and cyclic) with and without saline or glucose inf usions. 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 rep lacement of saline by glucose during the measurements did not show a s ignificant increase of GFR in pregnant rats. The volume of distributio n per gram body weight of Cr-51-EDTA after saline infusion, but not af ter 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.