Postglomerular capillary solute flux restricted by shape and charge in the dog

The permselectivity characteristics of the postglomerular (PG) microcirculation in dog kidney were investigated employing 3H-labeled cationic (DEAE) and anionic sulfated dextrans (dextran-SO4) ranging from 19 to 29 A Stokes-Einstein Radius. With the use of the multiple-indicator dilution (MID) technique, a bolus injection was made into the left renal artery and timed serial samples were obtained from renal venous and urine outflows. The injection solution contained 125I-labeled albumin (plasma reference), [14C]inulin and/or creatinine (glomerular and interstitial references), and a test [3H]dextran probe. A control run was carried out with tracer, then charge interaction was analyzed by repeating the MID run with excess unlabeled compound or after protamine sulfate infusion. After loading, renal vein recovery and mean transit time (t) were unchanged relative to [14C]inulin for [3H]dextran-SO4. But excess DEAE resulted in reduced recovery and decreased t for [3H]DEAE. After protamine sulfate, the renal vein and urine recoveries of [3H]dextran-SO4 decreased and the renal vein t increased. These findings demonstrate saturable anionic binding sites in the PG microcirculation. Under conditions where charge interaction was eliminated, the ratio of renal vein t for 125I-albumin to cationic or anionic dextran was always less than its ratio to neutral dextran, implying a larger apparent volume of distribution. We concluded that PG capillaries also limit solute flux on the basis of shape.