A quantitative analysis of the glomerular charge barrier in the rat

Modifying the ionic strength (I) is a gentle way to alter charge interactio ns, but it cannot be done for studies of the glomerular sieving of proteins in vivo. We therefore perfused 18 isolated rat kidneys with albumin soluti ons of different ionic strengths at a low temperature (cIPK) to inhibit tub ular uptake and protease activity. Four anionic proteins were studied, name ly albumin (Alb), orosomucoid (Oro), ovalbumin (Ova), and anionic horseradi sh peroxidase (aHRP), together with the neutral polymer Ficoll. With normal ionic strength of the perfusate (152 mM), the fractional clearance (theta) was 0.0018 +/- 0.0003 for Alb, 0.0033 +/- 0.0003 for Oro, 0.090 +/- 0.008 for Ova, and 0.062 +/- 0.002 for aHRP. These theta values were all lower th an for Ficoll of similar hydrodynamic size; e.g., theta (Ficoll 36 Angstrom ) was >20 times higher than theta for albumin. Low ionic strength (34 mM) i ncreased size selectivity as theta for anionic proteins and Ficoll fell, su ggesting a reduction in small-pore radius from 44 +/- 0.4 to 41 +/- 0.5 Ang strom, P < 0.01. In contrast, low I reduced the charge density of the membr ane, <omega>, to one-quarter of the 20-50 meq/l estimated at normal I. Thes e dynamic changes in omega seem to be due to volume alterations of the char ged gel, fluid shifts that easily are accounted for by the changes in elect roosmotic pressures. The finding that low ionic strength induces inverse ef fects on size selectivity and charge density strongly suggests that separat e structures of the glomerular wall are responsible for the two properties.