Lumen-positive Potential Difference Research Articles

Free-flow potential profile along rat kidney proximal tubule. 1974.

The transepithelial electrical potential difference across rat renal proximal tubule was reinvestigated, using improved techniques. To diminish tip potential artefacts the microelectrodes were filled with HCO 3 - Ringer's solution instead of 3 molar KCI. The error of the potential measurements with HCO 3 Ringer's microelectrodes was tested and was found to be ≤0.5 mV. A significant electrical potential profile was detected along the proximal tubular lumen under free flow. From near zero at the glomerulum the potential difference rose to -1.5 mV, lumen negative, in the first tubular loop at approximately 0.1 to 0.3 mm of tubular length. It decreased then rapidly, changed sign and attained a maximum of ca.2.0 mV, lumen positive, at I mm of tubular length, after which it declined gradually to +1.6 mV in the last accessible loop. The mean of 85 punctures in intermediate and late loops was +1.8, S.D. ±0.33 mV, range +1.0 to +3.2 mV On the basis of perfusion experiments described in the subsequent paper, the lumen-negative potential difference across early loops can be explained as an active transport potential. It is caused by the presence of glucose and amino acids in the glomerular filtrate, which increase the rate of active Na + absorption over that of active HCO 3 absorption. The lumen-positive potential difference in intermediate and late loops is explained as the sum of a membrane diffusion potential arising from the shift in intratubular Cl - and HCO 3 concentrations and a small lumen-positive active transport potential from H + secretion/HCO 3 -absorption.

The relationship between distal tubular proton secretion and dietary potassium depletion: evidence for up-regulation of H+-ATPase

Dietary potassium depletion is associated with elevated plasma bicarbonate concentration and enhanced bicarbonate reabsorption in the distal tubule. The relationship between distal proton secretion and potassium status was investigated by in vivo microperfusion of the superficial distal tubule. Experiments were performed on anaesthetized rats that had been maintained on either a low-potassium or control diet for 3-5 weeks prior to experimentation. The distal tubules were perfused at 10 nl/min with either a standard or a barium chloride-containing solution, and the late distal tubular transepithelial potential difference (Vte) and pH of the luminal fluid were recorded using a double-barrelled voltage and ion-sensitive microelectrode. In control rats, the Vte was -40.7+/-2.4 mV and the tubular fluid pH was 6.44+/-0.07; in potassium-depleted animals, the Vte was -15.0+/-1.4 mV and the pH was 6.76+/-0.03. The pH values in both groups of animals were significantly lower than would be predicted from the Vte and systemic pH for passive H+ distribution, indicating active proton secretion. Moreover, in hypokalaemic rats, this difference from predicted pH was significantly greater than in control animals (control = 0.27+/-0.06 vs. low-potassium = 0.46+/-0.03; P<0.01), suggesting enhanced active proton secretion. During perfusion with a solution containing BaCl2, the late distal tubule Vte became lumen positive in potassium-depleted rats, contrasting with an increased lumen negativity in potassium-replete controls. The barium-induced lumen-positive potential difference observed in the hypokalaemic rats was abolished by intravenous administration of acetazolamide. These data are consistent with enhanced electrogenic proton secretion (H+ -ATPase) during dietary potassium deprivation.

Influence of Dietary Sodium and Other Factors on Plasma Aldosterone Concentrations and in Vitro Properties of the Lower Intestine in House Sparrows (Passer Domesticus)

ABSTRACT House sparrows ( Passer domesticus) had plasma aldosterone concentrations of about 180pgml−1 while maintained on a low-sodium diet (LS, 0.1mequiv Na+ ingested per day), 135pgml−1 on a sodium intake of 0.9mequivday−1 (high-salt diet, HS) and 45pgml−1 on a Na+ intake of 3.8mequivday−1 (high-salt diet with saline drinking water, HSS). The plasma concentration of aldosterone changed to the LS or the HS level within 1 day of switching from the HS or the LS diet, respectively. Neither dehydration (22h, 14.5% loss in body mass) nor brief periods of stress (1–5min of handling) caused a change in circulating levels of aldosterone. The electrical properties of the lower intestine acclimated to the different sodium intakes with a time course similar to that of the changes in aldosterone levels. On the LS diet, the lower intestine generated an electrical potential difference (PD) of 5mV (lumen negative) and a short-circuit current (Isc) of about 50 μA cm−2; these were consistently inhibited by amiloride (resulting in a lumen-positive PD) and were stimulated by glucose or amino acids (leucine and lysine) in about half of the tissues. In HS birds, the PD and Isc were abolished and the effects of glucose and amino acids were reduced, but amiloride still caused a significant change in transmural PD (to a mucosa-positive value). These properties resemble those of the chicken coprodeum more than they do those of chicken colon, although the tissues tested were from the mid-region of the large intestine and their histology resembled that of colon. Sparrows tested immediately upon capture from the wild had plasma aldosterone levels not significantly different from those of birds on the LS diet, which is consistent with the known diet of this species. However, Isc was higher and tissue resistance was lower in wild birds compared with low-salt birds in the laboratory, perhaps indicating the influence of other hormones in addition to aldosterone.

Evidence for electroneutral sodium chloride transport in rat proximal convoluted tubule.

One- to two-thirds of NaCl absorption in the late proximal convoluted tubule (no luminal organic solutes present) is inhibited by cyanide and thus is dependent on active transport. To examine whether this active transport-dependent NaCl transport is electrogenic or electroneutral, the effect of cyanide on transepithelial potential difference (PD) was measured in the rat proximal convoluted tubule microperfused in vivo. In the presence of an ultrafiltrate-like luminal perfusate containing glucose and alanine, cyanide addition caused the transepithelial PD to change from -0.44 +/- 0.04 to -0.05 +/- 0.03 mV (P less than 0.001). In the presence of a late proximal tubular fluid (high chloride, low bicarbonate, no organics), the transepithelial PD was 1.23 +/- 0.06 mV and was unchanged at 1.19 +/- 0.05 mV after cyanide addition (NS). To eliminate the possibility that an effect of cyanide on a putative acidification-dependent lumen-positive PD was concealing an effect on an electrogenic sodium transport-dependent lumen-negative PD, the above studies were repeated in the presence of acetazolamide. Cyanide did not affect the transepithelial PD (1.17 +/- 0.05 vs. 1.07 +/- 0.06 mV, NS). We conclude that, although cyanide-inhibitable NaCl transport is electrogenic in the presence of luminal organic solutes, it does not generate a transepithelial PD in their absence and therefore is electroneutral.

Effect of vasopressin on electrical potential difference and chloride transport in mouse medullary thick ascending limb of Henle's loop.

Medullary thick ascending limbs of Henle's loop of the Swiss-Webster mouse were perfused in vitro with an isotonic perfusate and a Ringer's bathing medium. In five studies, addition of a supramaximal concentration of synthetic arginine vasopressin (AVP) to the bathing medium resulted in an increase in electrical potential difference (PD) from 5.0 +/- 1.5 mV, lumen positive, to 10.7 +/- 1.4 mV (P < 0.001). When AVP was removed, the PD returned to 2.6 +/- 0.9 mV (P < 0.001), then increased again to 6.9 +/- 1.7 mV (P < 0.01) when AVP was added a second time. A significant, but submaximal, increase in PD of 2.3 +/- 0.6 MV (P < 0.05) was observed in five medullary thick ascending limbs when AVP was added to the bathing medium at a concentration of 10 microunits/ml. This increase was approximately one-third of the response observed at a concentration of 100 microunits/ml in the same tubule. No further increment in PD was observed in five medullary thick ascending limbs when the AVP concentration was increased from 100 to 1,000 microunits/ml. In seven thick ascendcing limbs, the effect of AVP on PD was reproduced by the addition of 8-[p-chlorophenylthio]-cyclic 3',5'-adenosine monophosphate to the bathing medium at a final concentration of 0.1 mM. AVP increased unidirectional chloride flux from lumen to bath from 29.3 +/- 3.2 to 69.8 +/- 6.2 peq/cm per s (P < 0.001) in spite of an increase in the lumen positive PD from 1.6 +/- 0.5 mV to 7.0 +/- 0.6 mV (P < 0.001). Unidirectional chloride flux from bath to lumen was not affected by AVP. In another series of experiments, net chloride flux increased from 15.6 +/- 3.0 to 41.7 +/- 5.3 peq/cm per s (P < 0.05) after addition of AVP. The effect of AVP on hydraulic water permeability (Lp) was examined by adding raffinose to the bathing medium in both the presence and the absence of AVP. The calculated Lp of 16 +/- 2 nm/s per atm in the absence of AVP, although very low, was significantly different from zero (P < 0.01). However, the Lp did not increase significantly when AVP was added to the bathing medium. These results suggest that AVP has a second site of action in the kidney to increase chloride transport by the medullary thick ascending limb in addition to its well-known effect on the water permeability of the collecting tubule. The former effect would contribute to urinary concentrating ability by increasing the axial osmotic gradient in the renal medulla.

Electrophysiological study of isolated perfused human collecting ducts: Ion dependency of the transepithelial potential difference.

Cortical and outer medullary collecting duct segments were dissected from human kidneys and perfused in vitro. The transepithelial potential difference was measured and found to be lumen positive +6.8 +/- 0.6 mV (n= 20). This lumen-positive potential difference was inhibited by ouabain and furosemide but not by acetazolamide. Replacement of chloride in bath and perfusion fluids caused a reversible decrease of the potential difference to near zero. We conclude from these studies: (a) the lumen-positive potential difference is dependent upon the presence of chloride ion suggesting the existence of an active electrogenic chloride reabsorptive process in the human collecting duct and (b) it is possible to examine human renal physiology directly using in vitro microperfusion of tubule segments.

Effect of inhibitors and diuretics on electrical potential differences in rat kidney proximal tubule.

Active transport potentials were studied across early loops of rat proximal tubule during luminal perfusion and peritubular superfusion with HCO3- Ringer's solution of identical ionic composition. From the effects of the carbonic anhydrase inhibitor acetazolamide and of ouabain it is concluded 1. that the lumen-positive active transport potential indicates an excess of active H+ secretion/HCO3- absorption over active Na+ absorption and 2. that the lumen-negative active transport potential, which develops in the presence of glucose (and/or aminoacids) in the tubular lumen, indicates stimulation of active Na+ absorption. Ouabain did not abolish the lumen-positive potential difference suggesting that active H+/HCO3- transport and active Na+ transport may be to some extent independent. Among the diuretics tested the mercurial diuretic mersalyl acted primarily on Na+ transport, and furosemide acted on HCO3- transport, whereas the effect of ethacrynic acid appeared to be unspecific.

Free-flow potential profile along rat kidney proximal tubule.

The transepithelial electrical potential difference across rat rena proximal tubule was reinvestigated, using improved techniques. To diminish tip potential artefacts the microelectrodes were filled with HCO3-Ringer's solution instead of 3 molar KCl. The error of the potential measurements with HCO3-Ringer's microelectrodes was tested and was found to be ≤0.5 mV. A significant electrical potential profile was detected along the proximal tubular lumen under free flow. From near zero at the glomerulum the potential difference rose to −1.5 mV, lumen negative, in the first tubular loop at approximately 0.1 to 0.3 mm of tubular length. It decreased then rapidly, changed sign and attained a maximum of ca. 2.0 mV, lumen positive, at 1 mm of tubular length, after which it declined gradually to +1.6 mV in the last accessible loop. The mean of 85 punctures in intermediate and late loops was+1.8, S.D.±0.33 mV, range+1.0 to+3.2 mV. On the basis of perfusion experiments described in the subsequent paper, the lumen-negative potential difference across early loops can be explained as an active transport potential. It is caused by the presence of glucose and amino acids in the glomerular filtrate, which increase the rate of active Na+ absorption over that of active HCO3 − absorption. The lumen-positive potential difference in intermediate and late loops is explained as the sum of a membrane diffusion potential arising from the shift in intratubular Cl− and HCO3 − concentrations and a small lumen-positive active transport potential from H+ secretion/HCO3 − absorption.

Active transport potentials, membrane diffusion potentials and streaming potentials across rat kidney proximal tubule.

To explain the origin of the free-flow potential profile along rat kidney proximal tubule we have measured the transepithelial potential difference in early and late proximal tubular loops under well defined perfusion conditions. During elimination of all transeptithelial concentration differences through luminal and peritubular perfusion with simple HCO2-Ringer's solution a lumen-positive potential difference was observed. It averaged +1.0±0.39 mV in early loops and +0.2±0.31 mV in late loops. It disappeared upon poisoning with cyanide and appears to be generated by active H+ secretion/HCO3− absorption. Plasma-like concentrations of glucose or amino acids shifted the potential difference into lumennegative direction by up to −1.3 mV in early loops and −0.3 mV in late loops. This potential shift is caused by induction of active Na+ absorption through cotransport with the respective organic solute. Besides active transport potentials the tubule is able to generate membrane diffusion potentials from transepithelial ion concentration gradients and streaming potentials from osmotic concentration gradients. Whereas membrane diffusion potentials account for about 80% of the free-flow potential difference in intermediate and late tubular loops, streaming potentials are too small to play a role under free-flow conditions.