Osmotic Volume Flow in the Proximal Tubule of Necturus Kidney

Abstract

Volume changes due to osmotic flow in the distal portion of proximal tubules of Necturi were measured by the split oil drop technique. In agreement with previous findings no volume flow was induced by NaCI concentrations close to 60 raM. The tubule wall was found to be permeable to plasma electrolytes, which have an apparent reflection coefficient of 0.69. The mean apparent hydraulic conductivity was 0,33 X I0 -n cmS/dyne sec, comparable with other epithelia. A number of lipid-insoluble nonelectrolytes of widely varying molecular size had apparent reflection coefficients of about 0.5. In view of the insensitivity to molecular size it seems likely that apparent reflection coefficients determined from tubular volume changes depend primarily on the porosity of the intercellular barrier closest to the lumen and give little information about the subsequent fate of the test substances.T h e hydraulic conductivity of the proximal tubule of Necturus kidney m a y be determined by measuring the flow of solution induced by an osmotic pressure gradient. In previous experiments of this type we used mannitol to create the osmotic pressure difference since the rate of mannitol leakage out of the tubule h a d been found to be small (1, 2). These earlier results were based on sets of single observations since the stop flow perfusion method that was used yields only a single d a t u m for each tubule. T h e subsequent development of the split oil drop technique for the measurement of fluid movement by Gertz (3) has m a d e it possible to observe the time course of fluid movement and thus to make more accurate measurements. It was found that the isonatric point, at which there is no net N a movement, was 60 rnM NAG1, in agreement with previous observations. T h e tubule was therefore perfused with 60 inM NaC1 solutions, to which a series of lipid insoluble nonelectrolytes had been added, in order to 5x7

A parallel path model forNecturus proximal tubule

“…This approach was used because measurements of epithelial properties such as permeability, solute reflection coefficient and hydraulic conductivity, from the instantaneous determination of volume or solute flows into or out of the tubule lumen (time zero extrapolation method) reflect primarily the prop- erties of the barrier closest to the tubule lumen (i.e., the cell luminal membrane and tight junction in parallel). This point was emphasized by Bentzel et aL [7] in their determination of reflection coefficients and hydraulic conductivity [8], and by Solomon [33] in his analogue model of the results of Oken et al [30].…”

“…The reflection coefficient for NaC1 of the tubule epithelium was estimated as 0.69 by the time zero extrapolation method [7]. Since the reflection coefficient for the cellular path is 0.97 [38,41], the shunt estimate would be expected to be considerably less than 0.69.…”

“…Two factors are involved: first, the time zero extrapolation method [7,8] gives the Lp of only the luminal barrier;…”

Dimensions of polar pathways through rabbit gallbladder epithelium

“…In this respect gallbladder epithelium resembles the epithelium of kidney proximal tubules [5,23], stomach [1] and small intestine [33]. In all these tissues the anomalous permeability of relatively large hydrophilic solutes, such as sucrose, inulin and even bigger molecules, is explained by postulating the existence of some large aqueous pores in the epithelium.…”

Functional Organization of Proximal and Distal Tubular Electrolyte Transport

“…Arguing in favor of this model are results from perfusion studies in which transtubular water movement, induced by known transtubular osmotic gradients, is observed. If in such experiments osmotic equili bration by water movement is assumed, a value for hydraulic conduct ivity can be estimated [3,44]. The value so calculated also permits an estimate to be made of the osmotic gradient necessary to effect the reabsorption of some twothirds of the glomerular filtrate.…”