Effect of osmotic diuresis on intrarenal solutes in diabetes insipidus and hydropenia

Abstract

Changes in composition of renal solutes were studied in normal and in diabetes insipidus (D.I.) dogs subjected to varying degrees of solute (mannitol) loading. In both D.I. and hydropenic states, solute loading induced decreases in concentration of medullary solutes, and consequently decreases in corticomedullary gradients of solute concentration. The maximal level of papillary osmolality was associated with excretion of maximally hypertonic urine (nonloaded hydropenic states). The minimal level of papillary osmolality was not associated with excretion of hypotonic urine (D.I. states), but with excretion of slightly hypertonic urine during massive solute loading in normal hydropenic states. Hypertonicity of renal papillae (with respect to systemic plasma) even during excretion of hypotonic urine, together with a 7.6% vol of distribution of PAH in the papillae, indicates that final urine occupies only a small fraction of the renal papillary mass. Except for differences in location of peak concentrations, profiles of sodium and urea roughly paralleled those of total solute under all conditions. Decreases in papillary concentrations of sodium, induced by solute loading, paralleled those of osmolality in all cases. Decreases in papillary urea paralleled those of total solute (and Na) during loading in hydropenic dogs, but not in D.I. dogs. In the latter, concentration of papillary urea remained essentially fixed while those of Na and total solute decreased. In contrast to tissue concentrations of sodium and urea those of potassium increased in going from cortex to papilla under all conditions. The significance of these findings is discussed.