Renal response to solute loading in infant rats: relation to anatomical development.

rh~~~~~, MARY &LEN. Renal response to solute loading in infant rats: relation to anatomical development. Am. J. Physiol. 2 19(4) : 10891097. 1970.--.Dehydrated infant rats, 10 and 20 days old, received acute urea and salt loads. Urea-loaded lo-day-old rats were unable to enhance urinary concentrating ability. In contrast, 20-day-old rats showed enhanced concentrating ability accompanied by a rise in urea and sodium concentrations in the papilla. Salt loading had no effect on these parameters at either age. The anatomy of the 20-day-old rat kidney was like that of the adult. The 1 O-day-old rat kidney differed in that: I) medullary interstitial material was more abundant, 2) loops of IIenle were composed predominantly of cuboidal as opposed to squamous epithelium, 3) specialized fornices of the renal pelvis were just beginning to appear, 4) outer medullary vasa recta (VR) bundles were less well developed. Inability of the 1 O-day-old rat to accumulate additional urea in the papilla and, thereby, to enhance concentrating ability may be due, in part, to these structural immaturities. Differences 1-3 above could retard urea recycling from collecting duct to loop of Henle by lengthening the diffusion path for urea. Buildup of high papilla urea concentration could also be retarded if lateral water movement from descending to ascending VR (4) were less effective, allowing more water to reach the papilla.