Accelerated reabsorption in the proximal tubule produced by volume depletion

Abstract

A B S T R A C T The renal response to chronic depletion of extracellular volume was examined using the techniques of micropuncture. Depletion of salt and water was produced by administration of furosemide to rats maintained on a sodium-free diet. There was a marked fall in body weight, plasma volume, and glomerular filtration rate. The intrinsic reabsorptive capacity of the proximal tubule, measured by the split-droplet technique, was greatly enhanced. The acceleration of proximal fluid reabsorption could not be accounted for by changes in filtration rate, tubular geometry, or aldosterone secretion. The half-time of droplet reabsorption in the distal tubule was not altered by sodium depletion.An increase in the reabsorption of fluid in the proximal tubule, as demonstrated directly in the present experiments, provides an explanation for a variety of clinical phenomena associated with volume depletion. INTRODUCTIONIt is now well established that expansion of the extracellular fluid leads to renal excretion of salt and water in large part through the mechanism of decreased reabsorption in the proximal tubule. It is also well-known that depletion of extracellular volume regularly induces retention of salt and water, but the mechanism is not so precisely established. There is reason to think that, in addition to a fall in glomerular filtration rate, there is an accelerated reabsorption of sodium in the proximal tubules resulting in decreased delivery of filtrate to the distal nephron. Thus far, however, the evidence has been

The influence of the extracellular fluid volume on the tubular reabsorption of uric acid.

“…Previous studies, employing an identical model of volume contraction in the rat, have demonstrated impressive increases in the reabsorption of salt and water in the proximal tubule (6). Similarly, volume expansion with isotonic saline has repeatedly been demonstrated to result in a decreased fractional reabsorption of sodium in the proximal tubule (11,12).…”

Renal Response to Chronic Intravenous Salt Loading in the Rat

“…In support of the former possibility was the finding of no significant weight gain and a proportionately greater decline in systemic protein concentration (39%) than hematocrit. Given this profound degree of hypoproteinemia without weight gain, a possibility not excluded in their study is that intravascular volume depletion may actually have occurred in experimental rats, a stimulus recently shown to increase cortical nephron filtration fraction (26) and favor enhanced rates of absolute proximal reabsorption (26)(27)(28).…”

A study of the intrarenal recycling of urea in the rat with chronic experimental pyelonephritis.

“…The unusually avid reabsorption of water and sodium presumably is a reflection of the severe extracellular fluid volume contraction associated with the extreme degrees of weight loss brought about by dehydration (11,13,32,33).…”