Functional characteristics of peritubular capillary membrane in rat kidney

The permeability characteristics of the peritubular capillary membrane in the rat kidney were investigated on the basis of the transport of hippuran, inulin, myoglobin, horseradish peroxidase, albumin, and gamma-globulin from peritubular capillary blood to renal hilar lymph. Data obtained in a previous investigation on single-nephron plasma flow, filtration fraction, net driving force, and fluid reabsorption along the peritubular capillary were also used. The data were analyzed in a computer-based model taking into account the transport both by diffusion and by convection. The results show that the membrane contains a few large pores through which the plasma proteins leak out into the renal interstitium and a system of several smaller pores responsible for the fluid reabsorption. The mean equivalent radius of the large pores was estimated from the larger molecular probes to be approximately 180 A (range 150-225 A), and the corresponding total pore area over pore length was estimated at 3 X 10(-4) cm (range 6 X 10(-4) to 1 X 10(-4) cm). The small-pore system was analyzed from the transport of hippuran, inulin, and myoglobin and from fluid reabsorption and showed a pore radius of somewhat below 20 A and pore areas over pore length of 50 cm. Here, the fluid reabsorption and the transport of hippuran turned out to be a sensitive marker of the pore area and the transport of inulin and myoglobin of the pore radius.