Mechanism of Inhibition of Proximal Tubule Fluid Reabsorption after Exposure of the Rat Kidney to the Physical Effects of Expansion of Extracellular Fluid Volume

Abstract

A B S T R A C T The natriuresis and concomitant decline in absolute proximal reabsorption (APR) that occur in rats in response to saline loading are blunted markedly when renal perfusion pressure is reduced immediately before, but not after, the volume load. To ascertain the mechanism responsible for these differences between early clamp (EC) vs. late clamp (LC), intracapillary and interstitial determinants of peritubular capillary uptake of APR were measured in seven LC and seven EC Munich-Wistar rats before and after isotonic saline loading (8% body wt). With volume expansion in LC animals, we observed a marked decline in APR (averaging 11 ± 1 nl/min), associated with large increases in urinary sodium excretion rate, which averaged 8±2 ,ueq/min. In EC, the changes in urinary sodium excretion rate (+1±0 ,eq/min) and APR (-3± 1 nl/min) with volume expansion were smaller in magnitude. Since peritubular capillary reabsorption coefficient and mean peritubular transcapillary hydraulic pressure difference did not change with saline loading in LC, the marked fall in APR was attributed primarily to a measured large decline in mean peritubular transcapillary oncotic pressure difference (AII). Despite an equivalent mean fall in AH with volume expansion in EC, near-constancy of APR was found to be associated with a simultaneous and equivalent decline in mean peritubular transcapillary hydraulic pressure difference (a consequence of decreased mean peritubular capillary hydraulic pressure), which effectively offset the fall in All. These Portions of these studies were presented at

Blood flow dependence of postglomerular fluid transfer and glomerulotubular balance.

“…Although most of such studies emphasize the number of open capillary channels available for fluid transfer as an effector mechanism for blood flow dependency of permeability coefficient, another possibility exists, namely that the diameter of the already patent vascular channels may vary in accordance with the level of inflow. In this respect, Koch Jensen and Steven (47,48) demonstrated by a morphometric method that the diameter of the peritubular capillary in renal superficial cortex can indeed change in response ' On the basis of previous (1,15) The observed blood flow dependence of APR is reminiscent of plasma flow dependence of GFR in this species of animals and lead us to consider that this EABF dependence of K, may play an important role in the so-called glomerulotubular balance, a term used to describe changes in GFR that are accompanied by directionally similar and roughly proportional changes in the rate of fluid reabsorbed by the proximal tubule. Fig.…”

“…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).…”

“…Conversely, systemic or direct intraperitubular capillary infusion of hyperoncotic solutions was shown to accompany a rise in APR (4,13,18). In other studies, intracapillary hydraulic pressure, a force opposing peritubular capillary uptake of APR, has also been demonstrated to be an important modulator of APR (1,3,12,15). However, under certain other circumstances, particularly when renal blood flow changes, the prevailing APR is not readily explainable solely on the basis of these intracapillary forces (20)(21)(22)(23)(24)(25)(26)(27).…”

“…Direct measure-ments of time-averaged hydraulic pressures were recorded in single capillaries of surface glomeruli (PcC), proximal tubules (PT), efferent arterioles (PE), and distalmost branches of the peritubular capillaries (Pci). By the same device, subcapsular space hydraulic pressure was also measured and values taken to represent cortical interstitial hydraulic pressure (PI) as described in detail previously (1,2,20).…”

Importance of Efferent Arteriolar Vascular Tone in Regulation of Proximal Tubule Fluid Reabsorption and Glomerulotubular Balance in the Rat

“…As with fluid movement across extrarenal capillary beds, rates of filtration and absorption of fluid across renal glomerular and peritubular capillaries are governed by local imbalances in transcapillary hydraulic and colloid osmotic pressures (the so-called Starling forces) (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) (21)(22)(23)(24)(25)(26)(27)(28); both hormone systems are capable of altering glomerular and postglomerular Starling forces substantially (18)(19)(20)(29)(30)(31)(32)(33)(34). The present study was therefore designed to evaluate the roles played by these endoge-I)ous renal vasoactive substances in mediating the close coupling betweeni CFR and APR that occurs in response to partial aortic constriction in the Munich-Wistar rat.…”