Differential Na fluxes with artificial and native fluids in clip models of volume expansion

Abstract

Earlier we demonstrated significant differences in proximal tubular Na transport inhibition during saline volume expansion (VE) depending on whether artificial (AF) or native harvested tubular fluid (HTF) was used. In the present experiments the shrinking-drop technique was used to measure volume flux (Jv) in the early- and late-clip models of VE with AF alternating with HTF. In early-clip rats, with AF, Jv (x 10(4) mm3.mm-2.s-1) during the nonexpanded period was 2.92 +/- 0.105; during subsequent VE, it was 2.20 +/- 0.180 and 1.97 +/- 0.149 with HTF (for the latter two P greater than 0.3). In late-clip rats they were 2.83 +/- 0.135, 1.99 +/- 0.157, and 1.50 +/- 0.171 (for the latter two P less than 0.001), respectively. With AF, transport was inhibited equally in both models during VE. There were no significant differences in the electrolyte composition of AF and HTF during shrinkage as measured by microprobe analysis. The 47% inhibition of Jv with HTF but not AF during VE in the late-clip but not the early-clip rats strongly implies the presence of factors other than physical forces during VE, which inhibit proximal tubular Na transport. Studies of tubular transport during VE need to be performed using both AF and HTF.

Acute Volume Expansion and Salt-Loading Studies in Rats

“…To determine whether the additional sodium transport inhibition in the proximal tubules is responsible for the synergistic natriuresis reported earlier [14] during combined AcVE and ANP treatment, volume and sodium fluxes were measured in the proximal tubules. Four NS rats (weight 222 ± 10 g) were prepared for micropuncture experiments as described previously [11]. The left kidney was freed of adherent fat and connective tissue, decapsulated and placed in a plastic-coated metal cup for micropuncturing.…”

“…The modified shrinking drop technique was used to measure volume (Jv) and Na flux as described previously [11]. Briefly, shrinking droplets were continuously recorded by video camera (Newvychip, Javelin, USA) onto a super VHS Video Recorder (Panasonic) equipped with a time-lapse facility.…”

“…It is an often overlooked observation that in the experiments investigating physical factors for the natriuresis of AcVE, some 20 min or later into the experiments, natriuresis increased despite manipulations to prevent the drop of peritubular albumin levels [10]. Renal artery snare experiments [11] and fluid transfer studies between volume expanded and non-expanded rats [12] have shown that factors other than physical are responsible for the Na transport inhibition in the proximal tubules in AcVE.…”

Proximal tubular cell electrolytes during volume expansion in the rat.

“…Thus it can be deduced that the increase in [Cl-] found in the present experiments may reflect some transepithelial depolarization of the proximal tubular epithelium. This is not entirely unexpected since even though [Na+]i, [Rb+]i and cell dry weight are unchanged and the effect of the tubular factor on transepithelial Na+ and H20 transport is abolished by the reduced perfusion pressure (Reddy et al 1990), our previous work demonstrated that there was still a 20% reduction in Na+ and H20 transport in these kidneys (Reddy et al 1990). This could be caused by circulating factors.…”

“…Support for this view comes from previous studies where a proportion of the reduction in transepithelial transport seen in volume expansion could be attributed to a 'tubular factor'. This 'factor' appears in proximal tubular fluid during volume expansion and its action is transferable to non-volume-expanded kidneys (Gyory & Willis, 1983;Reddy et al 1990;Reddy et al 1991). In experiments where perfusion pressure was reduced in the experimental kidney at the time of volume expansion, the effect of the tubular factor was no longer demonstrable (Reddy et al 1990).…”