On the Mechanism of Inhibition in Fluid Reabsorption by the Renal Proximal Tubule of the Volume-Expanded Rat

Abstract

A B S T R A C T We undertook to determine the extent to which the inhibition in absolute proximal fluid reabsorption in response to expansion of extracellular volume with noncolloid-containing solutions is the result of concomitant reductions in postglomerular (efferent arteriolar) protein concentration. Selective elevation of efferent arteriolar oncotic pressure in volume-expanded rats (Ringer's 10% body weight) to levels slightly in excess of normal by microperfusion with 9-10% albumin-Ringer's solution nearly completely reversed the inhibition in absolute and fractional reabsorption in adjacent proximal tubules. In contrast, during similar microperfusion with a 6-7% albumin solution, no increase in proximal reabsorption was measured. We interpret these findings to indicate that the bulk of the inhibition in absolute proximal reabsorption in response to volume expansion with colloid-free solutions is causally mediated by the accompanying parallel decline in postglomerular vascular protein concentration.

Renal Response to Chronic Intravenous Salt Loading in the Rat

“…Recent studies from this laboratory have yielded evidence to indicate that the natriuresis which accompanies acute Ringer's loading in the rat is attended by marked expansion of plasma volume and corresponding dilution of pre-and postglomerular plasma protein concentration (2,3,5). In contrast, the results of the present study demonstrate that the natriuresis of chronic Ringer's loading is associated with only mild expansion of plasma volume and little reduction in pre-and postglomerular plasma protein concentration.…”

“…There is abundant evidence in the rat to indicate that the natriuresis that attends the acute expansion of extracellular volume with colloid-free solutions is associated with an increase in the rate of delivery of proximal tubule fluid to more distal segments of the nephron, an effect due both to a rise in glomerular filtration rate (GFR)1 and a fall in absolute proximal reabsorption (1)(2)(3)(4)(5). The latter change has been shown to be causally mediated, at least in part, by the accompanying dilutional reduction in postglomerular vascular protein concentration (3).…”

“…The latter change has been shown to be causally mediated, at least in part, by the accompanying dilutional reduction in postglomerular vascular protein concentration (3). Little is known, however, of the behavior of the proximal nephron during chronic, and, therefore, seemingly more physiologic alterations in extracellular volume.…”

Effect of increased peritubule protein concentration on proximal tubule reabsorption in the presence and absence of extracellular volume expansion.

“…For instance, Brenner, Troy, and Daugharty (1) found that capillary microperfusion with hyperoncotic albumin solution increased reabsorption by the proximal tubule in saline-loaded rats, and similarly Knox, Schneider, Willis, Strandhoy, and Ott (2) found that intrarenal infusion of hyperoncotic albumin solution increased proximal sodium reabsorption in saline-loaded dogs. It has been proposed that these changes in peri-…”

Blood flow dependence of postglomerular fluid transfer and glomerulotubular balance.

“…Previous investigators have shown under a variety of experimental conditions that changes in absolute proximal fluid reabsorption rate (APR) occur in a manner predictable from simultaneously measured levels of peritubular intracapillary oncotic and hydraulic forces (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Thus, reduction in the concentration of postglomerular plasma protein, induced by a variety of experimental maneuvers, such as intravenous infusion of large volume of colloid-free solutions, was associated with a decrease in APR (4,8).…”