CHANGES IN GLOMERULAR CAPILLARY PERMEABILITY TO DEXTRANS (D) DURING DEVELOPMENT

Abstract

An increase in the permeability of the glomerular capillaries has been suggested to contribute to the rise in GFR occurring during maturation. This is difficult to reconcile with the observation that the urine of the newborn contains more protein than that of the adult. It is this inconsistency that prompted us to measure the clearance of polydisperse tritiated D (Einstein-Stokes radius (r) 18-58Å) in 6 newborn and 7 six-week old puppies. The separation of D according to molecular size was accomplished by fractionating samples of plasma and urine on a Sephacryl gel column. At both ages, the permeance of D molecules with radii less than 23Å was similar to that of ¹²⁵I iothalamate (r∼5Å), while larger molecules were restricted to a progressively greater degree. The relative clearance of D ranging in size from 24-40Å was 21 ± 1% (SE) greater in the newborn than in the more mature puppy. No significant differences between the two age groups were found in the relative clearances of D smaller than 23Å or larger than 41Å. Although these data are open to various interpretations, the simplest is that the number of pores per unit of glomerular capillary surface area is larger in the newborn than in the older pupppy. Electrical charge notwithstanding, the existence of a greater pore density would explain the presence of relatively large amounts of protein in the urine of the newborn, but it cannot be construed to account for the low GFR prevailing at early age.

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