Effect of plasma sodium elevation on renal sodium reabsorption

Abstract

The effect of elevation of the plasma sodium concentration (P_Na) on the tubular reabsorption of sodium (T_Na) was studied in 38 dogs infused with hypertonic saline or Ringer's solution. An aortic clamp was used to regulate the glomerular filtration rate (GFR), so that several comparable levels of GFR could be obtained in one dog before and after hypertonic loading. It was found that T_Na increases progressively as P_Na is elevated, independently of changes in GFR. These results are not due to changes in the plasma bicarbonate-to-chloride ratio; nor to "saturation" of previously "unsaturated" distal tubules; nor influenced by altered aldosterone or antidiuretic hormone activity. Although the mechanism of the increase in T_Na is not definitely established by the present experiments, a possible explanation is that it is simply the result of the increased filtered sodium which occurs when P_Na is elevated.