The Effect of an Expansion of Extracellular Fluids on Net Na Flux in the Jejunum of Rats

Abstract

Isotonic expansion of extracellular fluids diminishes the reabsorption of sodium in the proximal tubule. If this effect is mediated through some mechanism extrinsic to the kidney, net sodium flux in other epithelial structures, such as the jejunum, should also be influenced. In the rat, expansion of E.C.F. produces in all experiments a diminished reabsorption of sodium and water, which appears during the first period after the infusion of saline is started and becomes statistically significant in the following periods. Because of the experimental conditions, net fluxes are often nverted, giving rise to a secretion. At the same time, the sodium concentration of the solution perfused increases during its transit through the jejunum. Net potassium flux is unchanged. Aldosterone has no influence on the phenomenon whereas angiotensin may have an additive effect. A similar response of two epithelial structures as far apart as the jejunum and the proximal tubule is highly suggestive of a common mechanism extrinsic to the kidney. The experimental model chosen has excluded the role of a suppression of aldosterone and angiotensin. There is lso no support for the involvement of a sodium-potassium exchange system. The present model offers the opportunity to study without any major technical problem the possible role of general, humoral and physical factors, all of which have been proposed as responsible for the diminished reabsorption of sodium in the proximal tubule after E.C.F. expansion.

A Study of Intercellular Spaces in the Rabbit Jejunum during Acute Volume Expansion and after Treatment with Cholera Toxin

“…Cholera toxin and expansion of extracellular fluid volume are both known to reverse net reabsorption of fluid in the small intestine and to cause fluid secretion (1)(2)(3)(4). Several explanations have been proposed to explain the mechanisms of these effects.…”

Intestinal transport of water and electrolytes during extracellular volume expansion in dogs

“…Moreover, the anatomy of the kidney makes it extremely difficult to carry out direct measurement of tubular transepithelial fluxes under anything approaching physiologic conditions. For this reason, and working on the assumption that a hormone which inhibits sodium transport by the kidney might also inhibit transport by other epithelial membranes, investigators have turned to sodium transport by toad bladder (11), frog skin (12,13), and intestinal epithelium (14,15). Two recent reports have shown that saline loading decreases net sodium absorption from the small intestine of the rat (14) and cat (15).…”

“…For this reason, and working on the assumption that a hormone which inhibits sodium transport by the kidney might also inhibit transport by other epithelial membranes, investigators have turned to sodium transport by toad bladder (11), frog skin (12,13), and intestinal epithelium (14,15). Two recent reports have shown that saline loading decreases net sodium absorption from the small intestine of the rat (14) and cat (15). In the studies carried out on rats, it was shown that net negative flux of salt and water occurred during saline loading, but unidirectional fluxes were not carried out to determine the mechanisms which determine the net transepithelial transport.…”

Variations in gastric emptying of liquid elicited by acute blood volume changes in awake rats

“…Increasing evidence suggests that the gastrointestinal (GI) tract plays an active role in the acute management of blood volume (BV) imbalances, since acute BV changes modify the GI motor function (1-9) as well as its permeability to salt and water (10)(11)(12)(13)(14)(15).…”

“…BV expansion decreases small bowel salt and water absorption and increases secretion (12)(13)(14) while BV retraction increases salt and water absorption and decreases secretion (11)(12)(13)(14)(15). These opposite responses appear to be regulatory in that they tend to correct a fluid imbalance (10,11,13).…”