Dual effects of saline loading on renal tubular sodium reabsorption in the dog

Abstract

Saline (0.9–1.5%) was administered at increasing rates to hydropenic dogs. Constriction of one renal artery performed early in the course of the infusion and maintained thereafter produced unilateral reductions in glomerular filtration rate (GFR) up to 50%. Since GFR increased bilaterally, the filtered load obtained late in the infusion on the constricted side was often comparable to that noted earlier on the control side. At comparable filtered loads sodium reabsorption was approximately 15% lower, later in the infusion. It was proposed that a saline infusion, apart from its effects on filtered load, progressively inhibits sodium reabsorption by decreasing the percentage of the sodium load reabsorbed. These data virutally excluded a fixed transport maximum for sodium. On the basis of filtered sodium, T^c_h2o, U_osm, and solute excretion data it was suggested that during saline loading sodium reabsorption in the proximal tubule and ascending limb generally remains proportional to the filtered load while simultaneously there develops an inhibition in sodium reabsorption which may exert a major effect at a site distal to the ascending limb.