Influence of peritubular protein on solute absorption in the rabbit proximal tubule. A specific effect on NaCl transport.

Abstract

A B S T R A C T The effect of removal of peritubular protein on the reabsorption ofvarious solutes and water was examined in isolated rabbit proximal convoluted tubules (PCT) perfused in vitro. In 22 PCT perfused with ultrafiltrate (UF) and bathed in serum, volume absorption (JV) was 1.44 nl/mm per min and potential difference (PD) was -3.6 mV. When these same PCT were bathed in a protein-free UF, JV was reduced 38% without a change in PD. Simultaneous measurements of total CO2 net flux (JTCO2) and glucose efflux (JG) showed that <2% of the decrease in JV could be accounted for by a reduction in JTCO2 and JG, suggesting that removal of peritubular protein inhibited sodium chloride transport (JNacl). Therefore, in eight additional PCT, JNaCl was measured, in addition to PD, JV, JG, and JTCO2. In these PCT, the decrease in total solute transport induced by removal of bath protein was 201.7±37.5 posmol/mm per min. JG decreased slightly (9.1±3.9 posmol/mm per min); NaHCO3 transport did not change (9.2±6.6 posmol/mm per min); but JNac1 decreased markedly (160.6±35.7 posmol/mm per min). 80% of the decrease in JV could be accounted for by a decrease in JNacl. In 13 additional PCT perfused with simple NaCl solutions, a comparable decrease in JV and JNaci was observed when peritubular protein was removed without an increase in TCO2 backleak.

ACUTE VOLUME EXPANSION: RAT PROXIMAL TUBULAR Na TRANSPORT INHIBITION BY AUTOLOGOUS TUBULAR FLUID WITHOUT CHANGES IN Na PERMEABILITY

“…Thus, it is quite possible that, although paraceilular permeability was unchanged, the cellular entry step could be affected by VE in a way so as to reduce Na entry into the cell leading to an apparem reduction in pump activity itself as measured by Ac^a-The results presented here do, however, prove that paracellular permeability changes through increased backleak do not account for the reduction in measured Na transport with HTF under our volume-expanded conditions. Similar lack of change in measured (Imai and Kokko, 1972) and indirectly derived (Berry and Cogan, 1981) P^^, was found in the rabbit isolated tubule on removal of albumin from the bath as a model for volume expansion. Those authors argued that because, under net volume flux conditions (1 -* b), net flux is only a small proportion of b -* 1 backflux, small but unmeasurable changes in Pfsj^ (reflecting paracellular backflux) could still account for the reduced net Na transport they found.…”

“…In the rabbit in vitro isolated tubule preparation reducing bath protein concentrations, a model frequently equated with volume expansion experiments, did not result in a change of transepithelial p.d. although both Na and volume fluxes were decreased (Berry and Cogan, 1981;Imai and Kokko, 1972). Allhough we did not measure transepithelial p.d.…”

“…Wesson et ai, 1950) or both luminal and peritubular sides (Wesson et ai, 1950), whereas others have found tbat peritubular hypotonieity in the Necturus (Bentzel, 1974;Bentzel and Reezek, 1978;Berry and Boulpaep, 1975;Boulpaep, 1972) and in the rat (Seely, 1973) increased PN;, while hypertonicity decreased it. Most, but not all (Berry and Cogan, 1981;Imai and Kokko, 1972), of these studies employed indirect methods to assess pNa. such as electrophysiological means (Berry and Boulpaep, 1975;Boulpaep, 1972;Seely, 1973;Ussing and Windhager, 1964;Windhager et ai, 1967) or the expanding droplet technique (Bentzel, 1974;Bentzel and Reczek, 1978;Berry and Boulpaep, 1975;Boulpaep, 1972).…”

“…Others, however, have shown that it is an increase in osmolality that leads to an increase in frogskin or Necturus proximal tubular Na permeability (Ussing and Windhager, 1964;Windhager, Boulpaep and Giebisch, 1967). Others, still, have found no change in paracellular permeability in the rabbit in vitro isolated tubule preparation following removal of proteinoncotic forces from the bath (Imai and Kokko, 1972;Berry and Cogan, 1981).…”

Effects of extracellular fluid volume and plasma bicarbonate concentration on proximal acidification in the rat.

“…These authors therefore postulated that the decreased fractional bicarbonate reabsorption seen in volume-expanded states was related to changes in filtered load rather than to changes in peritubular protein concentration. In agreement with this proposal, Berry and Cogan using the isolated perfused rabbit PCT, found that bath protein removal did not inhibit bicarbonate absorption despite a significant depression in NaCl and volume absorption (18).…”