Observations on the Mechanism of Decreased Tubular Reabsorption of Sodium and Water during Saline Loading*

Abstract

The effects of saline loading on tabular reabsorption were studied in the dog by maintaining a constant rate of urine flow by ureteral or aortic constriction during saline loading. Under these conditions of unchanged urine flow, saline loading was associated with a decreased glomerular filtra-tion, decreased urinary osmolality, and usually increased urinary electrolyte concentrations. Similar reductions in filtration rate by ureteral or aortic constriction in the absence of saline loading were associated with increases in urinary osmolality and decreases in electrolyte concentration. Decreased tabular reabsorption during saline loading was evident by (1) increased sodium and chloride excretion with a decreased filtered load of electrolyte and (2) an unchanged rate of urine flow in the presence of a reduced filtration rate. This decreased tabular reabsorption (in the presence of infusion of a mineralocorticoid) was associated with decreased urinary osmolality despite a constant infusion of vasopressin, no increase in solute excretion, and maintained or increased sodium excretion. To account for this combination of changes, we suggest that saline loading may result in a primary increase in medullary blood flow. As a consequence of the decreased medullary interstitial hypertonicity, passive water reabsorption from the descending limb of Henle''s loop could be decreased. The resulting increased rate of flow of fluid with a lower concentration of sodium may then lead to a decrease in the net reabsorption of sodium by the ascending limb and more distal portions of the nephron.