Glomerular ultrafiltration in the pseudopregnant rat.

Whole kidney and single nephron indices of glomerular ultrafiltration were measured by clearance and micropuncture techniques in anesthetized virgin, 9-day pregnant, and 9-day pseudopregnant Munich-Wistar rats. Whole kidney glomerular filtration rate (GFR) and single nephron glomerular filtration rate (SNGFR) were elevated in pregnant and pseudopregnant rats compared with virgins (0.78 +/- 0.05, 0.75 +/- 0.06 vs. 0.57 +/- 0.03 ml/min, P less than 0.005 and P less than 0.001; 32.1 +/- 2.5, 30.0 +/- 2.8 vs. 22.1 +/- 2.0 nl/min, P less than 0.01 and P less than 0.05, respectively). Total renal plasma flow rate (RPF) and single glomerular plasma flow rate (QA) were also increased in pregnant and pseudopregnant rats compared to virgins (3.05 +/- 0.19, 2.90 +2- 0.24 vs. 2.28 +/- 0.21 ml/min, P less than 0.01 and P less than 0.05; 109.0 +/- 15.8, 100.4 +/- 12.8 vs. 68.0 +/- 6.9 nl/min, both P less thn 0.05). There was little difference in the other determinants of ultrafiltration among the three groups. Plasma volume was measured in separate experiments and was higher in pregnant and pseudopregnant rats compared with virgins (9.4 +/- 0.2, 9.8 +/- 0.4 vs. 8.4 +/- 0.3 ml, P less than 0.01 and P less than 0.05, respectively). The gestational increase in GFR in the rat occurs as the result of increased RPF, which is due to both plasma volume expansion and renal vasodilation. Since the changes in renal hemodynamics seen in pseudopregnancy were almost identical to those occurring in pregnant rats, the early stimulus to increased GFR must be maternal and not fetoplacental in origin.