THE EFFECTS OF SALT RESTRICTION ON THE RENAL CON- CENTRATING OPERATION IN NORMAL, HYDROPENIC

In the presence of antidiuretic hormone, the elaboration of a concentrated urine by the absorp-tion of solute-free water depends upon the osmotic gradient established between collecting duct fluid and the hypertonic medullary interstitium (1-3). The quantity of solute deposited within the me-Iuilla depends, in large part, upon the rate at which sodium is presented to the ascending limb of the loop of Henle (2, 4, 5). Considerable evidence indicates that prolonged salt restriction results in a decreased extracellular fluid volume and glomerular filtration rate (GFR) coincident with a sharp reduction in sodium excre-tion (6-8). It might be anticipated that such changes could affect medullary salt supply and thereby influence the renal concentrating opera-tion. The reported effects of salt restriction on the concentrating operation appear to vary de-pending upon the species studied. Experiments in the dog have demonstrated that dietary salt restriction reduces maximal urine osmolality (9, 10), whereas similar studies in man (4) and the rat (11) have failed to reveal any such alteration. The present studies were undertaken in order to elucidate further the effects of prolonged salt restriction on the concentrating operation in nor-mal man under conditions of maximal hydropenia. METHODS Three experimental protocols were employed in maxi-mally hydropenic subjects free of cardiovascular or re-nal disease. In the experiments of Group I, a solut