Micropuncture Study of the Effect of Acute Reductions in Glomerular Filtration Rate on Sodium and Water Reabsorption by the Proximal Tubules of the Rat*

Although it has been established that sodium re- absorption by the renal tubules occurs by an active transport process and that water follows passively in proportion to sodium (1-3), the factors regu- lating the rate of reabsorption under various con- ditions are poorly understood. Micropuncture data in the rat (4, 5) and dog (6) indicate that under normal, nondiuretic conditions, approximately 65% of the glomerular filtrate is reabsorbed in the first two-thirds of the proximal convoluted tubule. When the filtered load of sodium is in- creased acutely in rats by hypertonic saline infu- sion, fractional reabsorption of the glomerular filtrate by the proximal tubules is unchanged (7, 8), indicating that an absolute increase in rate of reabsorption occurs. Direct information is lack- ing on the effect of acute reductions in filtered so- dium on tubular reabsorption. Clearance experi- ments in the dog suggest, however, that an adaptive reduction in sodium transfer occurs, since urine formation does not cease and significant amounts of sodium continue to be excreted even when the glomerular filtration rate (GFR) has been reduced by 40% (9-11).