Localization of tubular uptake segment of filtered Cystatin C and Aprotinin in the rat kidney D. Baran, O. Tenstad and K. Aukland Department of Physiology, University of Bergen, Bergen, Norway Received 16 June 2005, revision requested 11 July 2005, revision received 15 December 2005, accepted 20 December 2005 Correspondence: D. Baran, Department of Biomedicine, Section of Physiology, University of Bergen, Jonas Lies vei 91, N-5009 Bergen, Norway. E-mail: dadash.baran@biomed. uib.no Abstract Aim: The renal tubular uptake of 125 I-Aprotinin (*Ap) is on average located more superficially than its filtration site, causing transfer of some of *Ap filtered in deep to more superficial cortical zones. 125 I-Cystatin C (*Cy) showed less uptake in deep cortical zones than Ap, suggesting a longer and/or a more superficial tubular uptake site. To test that hypothesis and to quantify the outward transfer of the filtered polypeptides, we estimated the tubular uptake pattern of the tracers in perfusion fixed rat kidneys after intravenous injection of *Cy and *Ap. Methods: Autoradiographs were made from 10 lm thick slices of Microfil nephron casts from outer (OC), middle (MC) and inner (IC) cortical zones to quantify cortical border-crossing *Ap transfer. Single nephron glomerular filtration rate (snGFR) was estimated as the zonal uptake of *Ap corrected for *Ap transfer, divided by its time-integrated plasma concentration and the zonal number of glomeruli. Results: *Ap and *Cy uptake fell exponentially along the proximal convo- luted tubule (PCT), indicating an uptake proportional to luminal concen- tration. Uptake in IC exceededx that in MC and OC nephrons. The per cent PCT length with *Cy uptake (67.2  1.6) exceeded that of *Ap (54.6  1.8). The zonal border-crossing PCT length (29–34% of total PCT) from deep to more superficial cortical zones transferred 4–6% more *Cy than *Ap. Conclusion: Greater tubular uptake length of *Cy than of *Ap causes more cortical border-crossing of *Cy. The zonal snGFR estimated from Aprotinin uptake corrected for border-crossing agreed well with that obtained with the Hanssen ferrocyanide technique. Keywords glomerular filtration rate, polypeptides, renal tubular topo- graphy. Amino acids and polypeptides are freely filtered in the glomeruli and completely absorbed in the proximal tubules. Aprotinin, a basic polypeptide (Ap, MW 6.5 kDa, pI 10.5) is filtered in glomeruli without any steric restrictions, completely taken up by, and retained in the proximal tubular cells for more than 20 min after intravenous (i.v.) injection (Tenstad et al. 1994b, 1996). Total and local glomerular filtration rate (GFR) can therefore be estimated as cortical uptake of Aprotinin labelled by two different radioactive tracers ( 131 I- or 125 I-Ap) divided by their time-integrated plasma concentrations before and after acute interven- tions (Tenstad et al. 1994a, Wang et al. 1995, Treeck & Aukland 1997). The uptake of Ap is completed in the first two-third of proximal convoluted tubular length, located on average about 10% of cortical thickness more superficial than its parent glomerulus (Baran et al. 2003a,b). Because of a more superficial uptake site for Ap than its filtration site, some of Ap filtered in deep will be recovered in more superficial Acta Physiol 2006, 186, 209–221 Ó 2006 Scandinavian Physiological Society, doi: 10.1111/j.1748-1716.2006.01530.x 209