Methodologic considerations in the study of glomerular ultrafiltration.

Abstract

Over the past decade numerous studies have detailed the dynamics of glomerular ultrafiltration. Observations in a unique strain of Munich-Wistar rats, which afford the direct measurement of intraglomerular capillary hydrostatic pressure, have determined that under normal physiologic conditions these animals are in a state of filtration equilibrium. Both mathematical models based on this observation and experimental evidence indicate that under these circumstances the primary determinant of glomerular filtration rate (GFR) is the rate of glomerular plasma flow. In addition, this state of filtration equilibrium provides a condition in which changes in the product of glomerular surface area and glomerular capillary hydraulic conductivity (ultrafiltration coefficient) have no influence on glomerular filtration rate. Similarly, changes in glomerular hydrostatic pressure have a relatively small influence on GFR. These conclusions represent a marked alteration of the previously held concepts of the determinants of GFR. Unfortunately, studies in other strains of rats and in other animal species, particularly the dog, have led to the conclusion that filtration disequilibrium is the normal physiologic condition. The validity of this conclusion, however, is made uncertain by methodologic necessities that may lead to erroneous physiologic measurements of the determinants of GFR. In the present Editorial Review we examine the data reflecting on the applicability of filtration equilibrium to mammalian species. In addition, we summarize two new in vitro techniques for the further study of the dynamics of glomerular ultrafiltration.