Glomerular albumin filtration: a comparison of micropuncture studies in the isolated perfused rat kidney with in vivo experimental conditions.

Glomerular albumin filtration was investigated in the isolated perfused rat kidney and compared with in vivo experiments. Applying micropuncture techniques, we obtained samples from the glomerulus or adjacent early proximal convoluted tubules (EPT) of cortical nephrons and analyzed them for albumin using ultramicrodisc electrophoresis. By determining albumin in glomerular filtrate, we could calculate the sieving coefficient (EPTalb/Palb) directly. The control in vivo value was 0.27 +/- 0.05 X 10(-3) (N = 11). In the isolated perfused rat kidney, the sieving coefficient was 2.1 +/- 0.8 X 10(-3), N = 18 (30 g/liter albumin in perfusate) and 2.3 +/- 0.8 X 10(-3), N = 13 (50 g/liter albumin in perfusate), which is approximately eight times the control in vivo value. With elevated renal venous pressure (20 cm H2O), it increased further to 5.4 +/- 1.6 X 10(-3), N = 8. In all experiments, GFR and proximal transit times were similar to the in vivo controls. Although no major morphologic changes could be detected in any instance, the albumin filtration was greatly elevated. These data confirm the role of a functional barrier in the prevention of glomerular albumin filtration.