The effect of phenylalanine on the electrical properties of proximal tubule cells in the frog kidney

Modulation of luminal L-alanine transport in proximal tubular cells of frog kidney induced by low micromolar Cd2+ concentration

The kidneys are recognized as a major target of cadmium-induced toxicity. However, all mechanisms that are involved in the early stages of cadmium nephrotoxicity, particularly considering low micromolar concentrations of cadmium ions (Cd²⁺) are still unknown. Therefore, the aim of this study was to investigate the effects of peritubular acute exposure to micromolar Cd²⁺ concentration (2.3 μmol/L) on the rapid depolarization and the rate of slow repolarization of peritubular membrane potential difference (PD), induced by luminal application of L-alanine in proximal tubular cells of frog kidney. The results showed that the luminal application of L-alanine rapidly depolarized the peritubular membrane PD of -42.00 ± 11.68 mV by 23.89 ± 4.15 mV with an average rate of slow repolarization of 5.64 ± 0.81 mV/min. Additionally, peritubular acute exposure to Cd²⁺ induced change in rapid depolarization of peritubular membrane PD of -53.33 ± 13.01 mV by 18.78 ± 3.31 mV (P < 0.01) after luminal application of L-alanine. Also, peritubular acute exposure to Cd²⁺ led to statistically significant decrease in the rate of slow repolarization of peritubular membrane PD (3.53 ± 0.35 mV/min; P < 0.05). In conclusion, these results suggest that peritubular acute exposure to low micromolar Cd²⁺ concentration decreased the rapid depolarization and the rate of slow repolarization of peritubular membrane PD induced by luminal application of L-alanine. This is followed by reduced luminal sodium-coupled transport of L-alanine and this change may be one of the possible mechanisms involved in the early stages of Cd²⁺-induced nephrotoxicity.

Regulation of glomerulotubular balance: II: impact of angiotensin II on flow-dependent transport

Underlying glomerulotubular balance (GTB) is the impact of axial flow to regulate Na(+) and HCO(3)(-) transport by modulating Na(+)-H(+) exchanger 3 (NHE3) and H-ATPase activity. It is not known whether the cascade of events following a change in flow relies on local angiotensin (ANG II) generation or receptor availability. Mouse tubules were microperfused in vitro at flows of 5 and 20 nl/min, and net fluid (J(v)) and HCO(3)(-) (J(HCO3)) absorption and cell height were measured. Na(+) (J(Na)) and Cl(-) (J(Cl)) absorption and changes in microvillous torque were estimated. Raising flow increased Na(+) and HCO(3)(-) reabsorption but did not change either Cl(-) transport or cell volume. Losartan reduced absolute Na(+) and HCO(3)(-) absorption at both low and high flows but did not affect fractional flow-stimulated transport. Compared with controls, in AT(1a) knockout (KO) mouse tubules, 53% of flow-stimulated Na(+) absorption was abolished, but flow-stimulated HCO(3)(-) absorption was retained at similar levels. The remaining flow-stimulated J(HCO3) was eliminated by the H-ATPase inhibitor bafilomycin. Inhibition of the AT(2) receptor by PD123319 increased both J(Na) and J(HCO3) but did not affect flow-mediated fractional changes. NHE3 expression at the protein level was reduced in AT(1a) KO mice kidneys. We conclude that 1) although the AT(1a) receptor is necessary for flow to impact NHE3, the effect on H(+)-ATPase is independent of AT(1a); 2) the small flow-mediated changes in cell volume suggest a coordinate flow effect on both luminal and basolateral transporters; and 3) there is no evidence of flow-dependent Cl(-) transport, and thus no evidence for convective paracellular Cl(-) transport in mouse tubules.

Absence of KCNQ1-dependent K+ fluxes in proximal tubular cells of frog kidney

The present study was designed to investigate the functional significance of KCNQ1-mediated K+ secretory fluxes in proximal tubular cells of the frog kidney. To this end, we investigated the effects on rapid depolarization and slow repolarization of the peritubular membrane potential after luminal addition of L-phenylalanine or L-alanine plus/minus KCNQ1 channel blockers. Perfusing the lumen with 10 mmol/L L-phenylalanine plus/minus luminal 293B, a specific blocker of KCNQ1, did not modify the rapid depolarization and the rate of slow repolarization. Perfusing the lumen with 10 mmol/L L-alanine plus/minus luminal HMR-1556, a more potent KCNQ1 channel blocker, did not also alter the rapid depolarization and the rate of slow repolarization. Pretreatment (1 h) of the lumen with HMR-1556 also failed to modify rapid depolarization and rate of slow repolarization upon luminal 10 mmol/L L-alanine. Perfusing the lumen with 1 mmol/L L-alanine plus/minus luminal HMR-1556 did not change the rapid depolarization and the rate of slow repolarization. The pretreatment (1 h) with luminal HMR-1556 did not modify the rapid depolarization and the rate of slow repolarization upon luminal 1 mmol/L L-alanine. The pretreatment (1 h) of the lumen with HMR-1556 did not change transference number for K+ of peritubular cell membrane. Finally, luminal barium blunted the rapid depolarization upon application of luminal 1 mmol/L L-alanine. RT-PCR showed that KCNQ1 mRNA was not expressed in frog kidney. In conclusion, the KCNQ1-dependent K+ secretory fluxes are absent in proximal tubule of frog kidney.

Flow-dependent transport in a mathematical model of rat proximal tubule

The mathematical model of rat proximal tubule has been extended to include calculation of microvillous torque and to incorporate torque-dependent solute transport in a compliant tubule. The torque calculation follows that of Du Z, Yan Q, Duan Y, Weinbaum S, Weinstein AM, and Wang T (Am J Physiol 290: F289-F296, 2006). In the model calculations, torque-dependent scaling of luminal membrane transporter density [either as an ensemble or just type 3 Na(+)/H(+) exchanger (NHE3) alone] had a relatively small impact on overall Na(+) reabsorption and could produce a lethal derangement of cell volume; coordinated regulation of luminal and peritubular transporters was required to represent the overall impact of luminal flow on Na(+) reabsorption. When the magnitude of torque-dependent Na(+) reabsorption in the model agrees with that observed in mouse proximal tubules, the model tubule shows nearly perfect perfusion-absorption balance at high luminal perfusion rates, but enhanced sensitivity of reabsorption at low flow. With a slightly lower coefficient for torque-sensitive transporter insertion, perfusion-absorption balance in the model tubule is closer to observations in the rat over a broader range of inlet flows. In simulation of hyperglycemia, torque-dependent transport attenuated the diuretic effect and brought the model tubule into closer agreement with experimental observation in the rat. The model was also extended to represent finite rates of hydration and dehydration of CO(2) and H(2)CO(3). With carbonic anhydrase inhibition, torque-dependent transport blunted the diuretic effect and enhanced the shift from paracellular to transcellular NaCl reabsorption. The new features of this model tubule are an important step toward simulation of glomerulotubular balance.

Role of KCNE1-dependent K+ fluxes in mouse proximal tubule

The electrochemical gradient for K+ across the luminal membrane of the proximal tubule favors K+ fluxes to the lumen. Here it was demonstrated by immunohistochemistry that KCNE1 and KCNQ1, which form together the slowly activated component of the delayed rectifying K+ current in the heart, also colocalize in the luminal membrane of proximal tubule in mouse kidney. Micropuncture experiments revealed a reduced K+ concentration in late proximal and early distal tubular fluid as well as a reduced K+ delivery to these sites in KCNE1 knockout (-/-), compared with wild-type (+/+) mice. These observations would be consistent with KCNE1-dependent K+ fluxes to the lumen in proximal tubule. Electrophysiological studies in isolated perfused proximal tubules indicated that this K+ flux is essential to counteract membrane depolarization due to electrogenic Na+-coupled transport of glucose or amino acids. Clearance studies revealed an enhanced fractional urinary excretion of fluid, Na+, Cl-, and glucose in KCNE1 -/- compared with KCNE1 +/+ mice that may relate to an attenuated transport in proximal tubule and contribute to volume depletion in these mice, as indicated by higher hematocrit values.

Effects of cumene hydroperoxide on cellular cation composition in frog kidney proximal tubular cells

Effects of cumene hydroperoxide were studied on the peritubular membrane potential and cellular cation composition in frog kidney proximal tubular cells. After perfusion of isolated frog kidneys for 30 min with 1.3x10(-4) mol l(-1) cumene hydroperoxide Ringer solution, the peritubular membrane potential gradually declined. The ouabain-like effects were demonstrated on cell Na and K activities after 1 h of perfusion with cumene hydroperoxide. The peritubular apparent transference number for potassium was decreased. Intracellular pH was not altered in the presence of cumene hydroperoxide. Intracellular free Ca(2+) concentration increased slowly and moderately. The concentration of the malondialdehyde in the kidney homogenates, measured as an index of lipid peroxidation, was increased. A previously observable effect of cumene hydroperoxide on the peritubular membrane potential was prevented by oxygen radical scavengers.

Regulation of an inwardly rectifying ATP-sensitive K+ channel in the basolateral membrane of renal proximal tubule

Functional coupling of Na+,K+-ATPase pump activity to a basolateral membrane (BLM) K+ conductance is crucial for sustaining transport in the proximal tubule. Apical sodium entry stimulates pump activity, lowering cytosolic [ATP], which in turn disinhibits ATP-sensitive K+ (KATP) channels. Opening of these KATP channels mediates hyperpolarization of the BLM that facilitates Na+ reabsorption and K+ recycling required for continued Na+,K+-ATPase pump turnover. Despite its physiological importance, little is known about the regulation of this channel. The present study focuses on the regulation of the BLM KATP channel by second messengers and protein kinases using membrane patches from dissociated, polarized Ambystoma proximal tubule cells. The channel is regulated by protein kinases A and C, but in opposing directions. The channel is activated by forskolin in cell-attached (c/a) patches, and by PKA in inside-out (i/o) membrane patches. However, phosphorylation by PKA is not sufficient to prevent channel rundown. In contrast, the channel is inhibited by phorbol ester in c/a patches, and PKC decreases channel activity (nPo) in i/o patches. The channel is pH sensitive, and lowering cytosolic pH reduces nPo. Increasing intracellular [Ca2+] ([Ca2+]i) in c/a patches decreases nPo, and this effect is direct since [Ca2+]i inhibits nPo with a Ki of approximately 170 nM in i/o patches. Membrane stretch and hypotonic swelling do not significantly affect channel behavior, but the channel appears to be regulated by the actin cytoskeleton. Finally, the activity of this BLM KATP channel is coupled to transcellular transport. In c/a patches, maneuvers that inhibit turnover of the Na+,K+-ATPase pump reduce nPo, presumably due to a rise in intracellular [ATP], although the associated cell depolarization cannot be ruled out as the possible cause. Conversely, stimulation of transport (and thus pump turnover) leads to increases in nPo, presumably due to a fall in intracellular [ATP]. These results show that the inwardly rectifying KATP channel in the BLM of the proximal tubule is a key element in the feedback system that links cellular metabolism with transport activity. We conclude that coupling of this KATP channel to the activity of the Na+,K+-ATPase pump is a mechanism by which steady state NaCl reabsorption in the proximal tubule may be maintained.

Apical and basolateral membrane conductances in the TBM cell line

Cultured cell lines present several advantages over whole organ or ex vivo isolated epithelium for the physiological and biochemical study of epithelial transport. We have developed a new technique allowing for simultaneous intracellular and transepithelial electrophysiological measurements in the epithelium formed by a cultured cell line grown on thin collagen membranes. This technique was applied to the TBM 18/23 (toad bladder origin) cell line. The transepithelial and basolateral membrane potentials were -30 +/- 11 and -72 +/- 8 (SD) mV (n = 36), respectively. With the use of the effect of amiloride, which partially blocked the apical membrane conductance, and circuit analysis, the apical and basolateral membrane conductances were estimated to 0.7 +/- 0.1 and 2.8 +/- 0.4 mS/cm2, respectively. A sodium-selective conductive pathway was demonstrated in the apical membrane, and a barium-sensitive K(+)-selective conductance was shown to be present in the basolateral membrane. The basolateral membrane conductance was not modified by sudden inhibition of sodium transport by amiloride, but it was significantly reduced after a long-term decrease of Na+ transport. The cultured TBM cell line appears to be a convenient model to investigate the regulation of membrane ionic conductances in tight epithelia.

Potassium-selective channels in the basolateral membrane of single proximal tubule cells of frog kidney

The membrane potential of proximal tubule cells is dominated by the potassium conductance of the basolateral membrane. In the present paper the nature of this conductance is investigated by the patch-clamp technique. Only one type of K channel was found in the basolateral membranes of freshly isolated proximal cells. In cell-attached patches, the current/voltage relationship is markedly non-linear with much larger inward (30 pS) than outward (approximately 6 pS) conductances, even in the presence of roughly symmetrical K concentrations. Thus the channels show inward rectification. The determination of the conductance for outward current flow is complicated since the current/voltage curves show an area of negative conductance. Nevertheless, taking the conductance for outward current flow and the density of the channels it is possible to account for all of the previously reported potassium conductance of amphibian proximal tubule cells. The open probability of the channels was found not to depend upon the membrane potential. However, the non-linearity of the current/voltage relationships will confer upon the channel the same voltage dependence as that seen in intact proximal tubules, i.e. the conductance decreases with depolarisation. Incubation of cells in Ringer with no substrates or in the presence of alanine and/or glucose showed no change in the activity of the channels. These findings suggest that, although these channels may represent the basolateral conductance of frog proximal tubule cells, they are not involved in the well-established coupling between transport rate and potassium conductance.

pHi-dependent membrane conductance of proximal tubule cells in culture (OK): differential effects on K(+)- and Na(+)-conductive channels

Confluent monolayers of the established opossum kidney cell line were exposed to NH4Cl pulses (20 mmol/liter) during continuous intracellular measurements of pH, membrane potential (PDm) and membrane resistance (R'm) in bicarbonate-free Ringer. The removal of extracellular NH4Cl leads to an intracellular acidification from a control value of 7.33 +/- 0.08 to 6.47 +/- 0.03 (n = 7). This inhibits the absolute K conductance (gK+), reflected by a decrease of K+ transference number from 71 +/- 3% (n = 28) to 26 +/- 6% (n = 5), a 2.6 +/- 0.2-fold rise of R'm, and a depolarization by 24.2 +/- 1.5 mV (n = 52). In contrast, intracellular acidification during a block of gK+ by 3 mmol/liter BaCl2 enhances the total membrane conductance, being shown by R'm decrease to 68 +/- 7% of control and cell membrane depolarization by 9.8 +/- 2.8 mV (n = 17). Conversely, intracellular alkalinization under barium elevates R'm and hyperpolarizes PDm. The replacement of extracellular sodium by choline in the presence of BaCl2 significantly hyperpolarizes PDm and increases R'm, indicating the presence of a sodium conductance. This conductance is not inhibited by 10(-4) mol/liter amiloride (n = 7). Patch-clamp studies at the apical membrane (excised inside-out configuration) revealed two Na(+)-conductive channels with 18.8 +/- 1.4 pS (n = 10) and 146 pS single-channel conductance. Both channels are inwardly rectifying and highly selective towards Cl-. The low-conductive channel is 4.8 times more permeable for Na+ than for K+. Its open probability rises at depolarizing potentials and is dependent on the pH of the membrane inside (higher at pH 6.5 than at pH 7.8).

Influence of mepacrine, indomethacin, and nordihydroguaiaretic acid on the electrical properties of frog renal proximal tubules

In proximal renal tubules of the frog kidney, stimulation of sodium-coupled transport leads to a depolarization of the peritubular cell membrane, followed by partial repolarization. These alterations of the potential difference across the peritubular cell membrane (PDpt) are in part the result of altered peritubular potassium conductance. The repolarization has been blunted by the phospholipase A2 inhibitor mepacrine, but not by the cyclooxygenase inhibitor indomethacin. In the present study the effect of mepacrine, indomethacin and the lipoxygenase inhibitor nordihydroguaiaretic acid on the electrical properties of proximal renal tubules has been tested in the presence and absence of stimulated sodium-coupled transport. In the absence of inhibitors, addition of 10 mmol/l phenylalanine to the luminal perfusate leads to a rapid depolarization and partial repolarization of the peritubular cell membrane, a decrease of the luminal cell membrane resistance (Ra) and a small increase of the cellular core resistance (Rc). Removal of phenylalanine leads to rapid hyperpolarization, increase of Ra and decline Rc. Mepacrine (100 mumols/l) depolarizes the cell membrane and increases the peritubular cell membrane resistance (Rb), Rc and the intracellular pH. In the presence of mepacrine, phenylalanine leads to a sustained depolarization and a transient decrease of Ra. Indomethacin (10 mumol/l) does not significantly modify PDpt, the lumped resistance of both cell membranes (Rm) or Rc in the presence or absence of phenylalanine. Nordihydroguaiaretic acid (50 mumols/l) does not alter significantly PDpt, Ra, Rb or Rc prior to phenylalanine.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell swelling, co-transport activation and potassium conductance in isolated perfused rabbit kidney proximal tubules

1. Isolated, perfused rabbit proximal tubules were used to study the effects of activation of the apical membrane sodium co-transporters, and of the effects of osmotically induced cell swelling, upon cell volume, basolateral membrane potential and apparent partial conductance of potassium. 2. Activation of electrogenic apical sodium co-transport caused a depolarization of the basolateral membrane and a reduction of the basolateral apparent potassium transference number. This was followed by a spontaneous partial recovery of potential and increase in apparent potassium transference number. 3. Stimulation of apical sodium co-transport led to a sustained increase in cell volume. 4. A sustained increase in cell volume (of similar magnitude to that seen after activation of apical membrane sodium co-transporters) was also caused by reduction of bath and perfusate osmolality by removal of 89 mmol l-1 mannitol from both lumen and bath solutions. 5. This reduction in bath and perfusate osmolality also led to a basolateral membrane hyperpolarization and an increase in basolateral apparent potassium transference number. 6. These observations support the possibility that some of the partial recovery of basolateral membrane potential (Vb1) during apical sodium co-transport stimulation is due to a cell volume sensitive change in basolateral potassium conductance.

Stretch-activated channels in the basolateral membrane of single proximal cells of frog kidney

Epithelial cells are capable of regulating their volume in response to osmotic swelling or shrinkage. In the present paper a channel is described which may be involved in such a volume-regulatory response. Channels were studied in cell-attached patches of the basolateral membrane of cells isolated from frog kidneys using the patch-clamp technique. The open probability of the channels is increased by the application of negative pressure to the rear of the patch pipette or by bathing the cells in hypotonic fluid. In addition, the channels are voltage-sensitive, such that depolarisation increases the open probability. The channels have a conductance of 25 pS with amphibian Ringer as the pipette solution and appear not to discriminate between potassium and sodium. Replacement of chloride by gluconate as the dominant anion in the pipette solution did not affect the current/voltage relationship, suggesting that the channels are cation-non-selective. Inward currents are observed at the resting membrane potential with either potassium or sodium as the dominant cation in the pipette solution: this obviates the channels serving a role as the route for solute exit from the cell during a volume-regulatory decrease response and suggests that they may act as the transduction mechanism sensing changes in cell volume.

Electrogenic transport of neutral and dibasic amino acids in a cultured opossum kidney cell line (OK)

A study has been made of electrogenic cellular uptake of amino acids resulting in the depolarization of cell membrane potential (PDm) in confluent monolayers of an established opossum kidney (OK) cell line using conventional and pH-selective microelectrodes. Apical superfusion of neutral and dibasic amino acids rapidly depolarized the cell membrane, while application of acidic amino acids had no effect on PDm. The depolarization in response to L-phenylalanine and L-arginine was stereoselective, dose-dependent and saturable. 10 mmol/l of L-phenylalanine reduced PDm by 4.8 +/- 0.4 mV (n = 51) in a completely sodium-dependent way and the concentration necessary for half-maximal depolarization (C1/2) was about 1.5 mmol/l. On the other hand, the C1/2 for L-arginine was about 0.02 mmol/l. The maximal depolarization produced by L-arginine (measured at 10 mmol/l) amounted to 6.8 +/- 1.2 mV (n = 10) and this was not affected when extracellular sodium was replaced by choline (6.3 +/- 1.2 mV; n = 10). The depolarizations induced by L-phenylalanine and L-arginine were significantly additive (p less than 0.001). The intracellular pH of OK cells was 7.09 +/- 0.03 (n = 11) and did not change during L-arginine application. We conclude that (1) carrier-mediated uptake of neutral and dibasic amino acids into OK cells is at least partially electrogenic. (2) L-Phenylalanine is transported by a Na+-symport. (3) In contrast, L-arginine depolarizes PDm independently of extracellular sodium. (4) Electrogenic uptake of acidic amino acids is not detectable in OK cells.

Acetazolamide and transient responses of basolateral membrane potential of rabbit kidney proximal tubules perfused in vitro

1. A study was made of the partial recovery of basolateral membrane potential that follows some depolarizing manoeuvres in cells of isolated perfused segments of rabbit proximal convoluted tubules. 2. Peritubular application of 10(-4) M-acetazolamide (a known inhibitor of the basolateral sodium-bicarbonate co-transporter) caused a hyperpolarization of both the basolateral membrane potential (Vbl) and the transepithelial potential (Vte). 3. Activation of electrogenic apical sodium co-transport caused a depolarization of the basolateral membrane followed by partial recovery of potential, and a sustained transepithelial hyperpolarization. The partial recovery of basolateral membrane potential was significantly smaller in the presence of 10(-4) M-acetazolamide applied to the peritubular fluid, although the magnitude of the initial depolarization was not significantly altered. 4. Addition to the bath of 0.5 mM-barium, a potassium conductance blocker, caused a transepithelial and basolateral membrane depolarization followed by partial recovery of potential. The partial recovery of basolateral membrane potential was significantly smaller in the presence of 10(-4) M-acetazolamide applied to the peritubular fluid, although the magnitude of the initial depolarization was again not significantly altered. 5. Increases in bath potassium concentration from 5 to 20 mM led to transepithelial and basolateral membrane depolarization followed by partial recovery of potentials. In paired experiments the partial recovery of basolateral potential was significantly reduced when 10(-4) M-acetazolamide was present in the bath. 6. These observations are consistent with the hypothesis that the basolateral sodium-bicarbonate co-transporter plays a role in the recovery of Vbl following these depolarizing manoeuvres.

Microelectrode characterization of the basolateral membrane of rabbit S3 proximal tubule

The purpose of this study was to characterize the basolateral membrane of the S3 segment of the rabbit proximal tubule using conventional and ion-selective microelectrodes. When compared with results from S1 and S2 segments, S3 cells under control conditions have a more negative basolateral membrane potential (Vbl = -69 mV), a higher relative potassium conductance (tK = 0.6), lower intracellular Na+ activity (ANa = 18.4 mM), and higher intracellular K+ activity (AK = 67.8 mM). No evidence for a conductive sodium-dependent or sodium-independent HCO3- pathway could be demonstrated. The basolateral Na-K pump is inhibited by 10(-4) M ouabain and bath perfusion with a potassium-free (0-K) solution. 0-K perfusion results in ANa = 64.8 mM, AK = 18.5 mM, and Vbl = -28 mV. Basolateral potassium channels are blocked by barium and by acidification of the bathing medium. The relative K+ conductance, as evaluated by increasing bath K+ to 17 mM, is dependent upon the resting Vbl in both S2 and S3 cells. In summary, the basolateral membrane of S3 cells contains a pump-leak system with similar properties to S1 and S2 proximal tubule cells. The absence of conductive bicarbonate pathways results in a hyperpolarized cell and larger Na+ and K+ gradients across the cell borders, which will influence the transport properties and intracellular ion activities in this tubule segment.

A stretch-activated K+ channel sensitive to cell volume

The role of K+ channels in cell osmoregulation was investigated by using the patch-clamp technique. In cell-attached patches from Necturus proximal tubule, the short-open-time K+ channel at the basolateral membrane could be stretch-activated by pipette suction, where a negative pressure of 6 cm H2O (588.6 Pa) was sufficient to increase the open probability of the channel by a factor of 4.0 +/- 0.8 (n = 7 tubules). A 50% reduction in bath osmolarity increased cell volume by 66 +/- 10% and increased the K+-channel open probability by a factor of 5.8 +/- 1.4 (n = 7) in the same cell-attached patches that were activated by pipette suction. A kinetic analysis indicates one open state and at least two closed states for this epithelial K+ channel. Both suction and swelling shorten the longest time constant of the closed-time distribution by a factor of 3, without significant effect on either the mean open time or the shorter closed-state time constant. The similar effect of suction and swelling is consistent with the hypothesis that stretch-activated K+ channels mediate the increase in macroscopic K+ conductance that occurs during osmoregulation of amphibian proximal tubules. Calculations based on a simple model indicate that small increments in cell volume could produce statistically significant increases in K+-channel activity.

Voltage dependence of the basolateral membrane conductance in the Amphiuma collecting tubule

The basolateral potassium conductance of cells of most epithelial cells plays an important role in the transcellular sodium transport inasmuch as the large negative equilibrium potential of potassium across this membrane contributes to the electrical driving force for Na+ across the apical membrane. In the present study, we have attempted to establish the I-V curve of the basolateral membrane of the Amphiuma collecting tubule, a membrane shown to be K+ selective. Transepithelial I-V curves were obtained in short, isolated perfused collecting tubule segments. The "shunt" conductance was determined using amiloride to block the apical membrane Na+ conductance. In symmetrical solutions, the "shunt" I-V curve was linear (conductance: 2.2 +/- 0.3 mS.cm-2). Transcellular current was calculated by subtracting the "shunt" current from the transepithelial current in the absence of amiloride. Using intracellular microelectrodes, it was then possible to measure the basolateral membrane potential simultaneously with the transcellular current. The basolateral conductance was found to be voltage dependent, being activated by hyperpolarization: conductance values at -30 and -80 mV were 3.6 +/- 1.0 and 6.6 +/- 1.0 mS.cm-2, respectively. Basolateral I-V curves were thus clearly different from that predicted by the "constant field" model. These results indicate that the K+-selective basolateral conductance of an amphibian collecting tubule shows inward ("anomalous") rectification. Considering the electrogenic nature basolateral Na-K-pump, this may account for coupling between pump-generated potential and basolateral K+ conductance.

Na transport stimulation by novobiocin: intracellular ion concentrations and membrane potential

Microelectrodes and electron microprobe analysis were employed to study the effect of novobiocin on membrane potential and intracellular electrolyte concentrations in the frog skin epithelium. In both species investigated (Rana esculenta and Rana temporaria), novobiocin (1 mM, outer bath) caused a stimulation of transepithelial Na transport, a depolarization of apical membrane potential, a fall in the apical fractional resistance, and an increase in the intracellular Na concentration. The rise in the Na concentration was accompanied by an equivalent fall in the K concentration. All effects of novobiocin were fully reversible by subsequent application of amiloride. The depolarization as well as the Na increase suggests that the natriferic effect of novobiocin is due to a stimulation of the apical Na influx. Combining both measurements it was possible to calculate the effect of novobiocin on the Na permeability of the apical membrane directly. In Rana esculenta novobiocin increased the permeability from 4.5 to 23.2 nm/s. In Rana temporaria the increase was significantly smaller, from 8.7 to 16.9 nm/s. The transport rate as measured by the short-circuit current showed a non-linear dependence on the apical Na permeability. In the range of transport rates normally encountered, however, the current was a linear function of the Na permeability consistent with the view that the apical membrane is rate-limiting in transepithelial Na transport.

Trans- and paracellular K+ transport in diluting segment of frog kidney

In frog diluting segment transepithelial K+ net flux (JKte) occurs via trans- and paracellular transport routes. Inhibition of transcellular K+ transport discloses JKte across the shunt-pathway. By means of K+-sensitive microelectrodes we have measured secretory JKte induced by an acute K+ load, in the diluting segment of the isolated and doubly-perfused frog kidney. Transcellular K+ transport was inhibited by blocking the luminal K+ permeability either directly by barium or indirectly by the diuretic drug amiloride (via intracellular acidification induced by inhibition of Na+/H+ exchange), by the Na+/K+ pump inhibitor ouabain or by inducing an acute acid load. All experimental maneouvers led to a reduction of secretory JKte to about 50% of the control JKte. The apparent permeability coefficient for K+ of this nephron portion after inhibition of transcellular secretory JKte was reduced to a similar extent. We conclude: In frog diluting segment the ratio of trans- over paracellular JKte is close to unity. This ratio represents a minimum estimate because inhibition of the transcellular K+ pathway by barium, amiloride or an acute acid load may have been incomplete. Acidosis and/or amiloride exert large antikaliuretic effects due to the inhibition of the luminal K+ permeability.

Fused cells of frog proximal tubule: I. Basic membrane properties

Proximal tubular cells of the frog (Rana esculenta) kidney were fused within an isolated tubule portion to giant cells according to the polyethylene-glycol fusion method. Cell membrane potentials (Vm) were measured while cells were superfused with various experimental solutions. Rapid concentration step-changes of different ions allowed to calculate the respective transference numbers (tion). In some experiments the specific cell membrane resistances (Rm) were evaluated by measuring Vm induced by short current pulses injected into the cell with a second electrode. The experiments reveal: i) Fused cells of the proximal tubule exhibit a Vm of -49.5 +/- 1.6 mV (n = 65). ii) Addition of glucose to the perfusate yields a transient depolarization, consistent with a rheogenic Na/glucose cotransport system. iii) In absence of organic substrates the whole cell membrane conductance is made up of K+ and HCO3-. iv) There is a positive relationship between Vm and tK+ and a negative relationship between Vm and tHCO3-. v) HCO3--induced Vm changes are attenuated or abolished when Na+ is replaced with choline+, consistent with a rheogenic Na+/HCO3- cotransport system. vi) Replacement of Na+ by choline+ depolarizes Vm and increases Rm by about 50%; addition of 3 mmol/liter Ba2+ to the Na+-free perfusate increases Rm by about 58% compared to the initial control value. vii) There is no measurable cell membrane Cl- conductance. We conclude that fused cells of proximal tubule exert both luminal and peritubular membrane properties. In absence of organic substrates the cell membrane potential is determined by the HCO3- and K+ transport systems.

On the nature of delayed repolarization during sustained sodium coupled transport in frog proximal tubules

In proximal tubules of the frog kidney, stimulation of coupled transport of sodium with phenylalanine leads to depolarization of the cell membrane, followed by repolarization within a few minutes. The repolarization is due to a delayed increase of potassium conductance at the peritubular cell membrane. The present study was designed to test for the role of depolarization, of calmodulin and of arachidonic acid metabolites for the delayed increase of potassium conductance. To this end, the potential difference across the peritubular cell membrane of proximal convoluted tubules (PDpt) has been recorded continuously during exposure of the lumen to phenylalanine or during galvanic current injection into a neighbouring cell. During control conditions, PDpt averages -68.6 +/- 1.0 mV (n = 45). Phenylalanine leads to a depolarization of the peritubular cell membrane by +31.5 +/- 1.3 mV (n = 20), followed by a repolarization by -12.9 +/- 1.1 mV (n = 20) within 3 min. Injection of currents from 10 to 80 nAmps leads to a depolarization by +0.83 +/- 0.01 mV/nAmps which is again followed by repolarization. A linear correlation is observed between the magnitude of depolarization (dep) and repolarization (rep) within 3 min: rep (mV) = -(0.24 +/- 0.01) dep (mV) +(2.45 +/- 0.12) mV (r = 0.90). Thus, depolarization is capable to trigger delayed repolarization. The extent of repolarization is a function of the magnitude of depolarization. The possible involvement of calmodulin or arachidonic acid metabolites has been tested for by inducing sodium coupled transport in the presence of 100 mumol/l mepacrine, 10 mumol/l indomethacin or 10 mumol/l trifluoperazine.

Properties of single K+ channels in the basolateral membrane of rabbit proximal straight tubules

The basolateral membrane of rabbit straight proximal tubules, which were cannulated and perfused on one side, was investigated with the patch clamp technique. Properties of inward and outward directed single K+ channel currents were studied in cell-attached and inside-out oriented cell-excised membrane patches. In cell-attached patches with NaCl Ringer solution both in pipette and bath, outward K+ currents could be detected after depolarization of the membrane patch by about 20-30 mV. The current-voltage (i/V) relationship could be fitted by the Goldman-Hodgkin-Katz (GHK) current equation, with the assumption that these channels were mainly permeable for K+ ions. A permeability coefficient PK of (0.17 +/- 0.04).10(-12) cm3/s was obtained, the single channel slope conductance at infinite positive potential g(V infinity) was 50 +/- 12 pS and the single channel conductance at the membrane resting potential g(Vbl) was 12 +/- 3 pS (n = 4). In cell-excised patches, with NaCl in the pipette and KCl in the bath, the data could also be fitted to the GHK equation and yielded PK = (0.1 +/- 0.01).10(-12) cm3/s, g(V infinity) = 40 +/- 4 pS and g(Vbl) = 7 +/- 1 pS (n = 8). In cell-attached patches with KCl in the pipette and NaCl in the bath, inward K+ channels occurred at clamp potentials less than or equal to 60 mV, whereas outward K+ channel current was detected at more positive voltages. The current-voltage curves showed slight inward rectification.(ABSTRACT TRUNCATED AT 250 WORDS)

Volume regulation in the early proximal tubule of the Necturus kidney

The ability of early proximal tubule cells of the Necturus kidney to regulate volume was evaluated using light microscopy, video analysis and conventional microelectrodes. Necturus proximal tubule cells regulate volume in both hyper- and hyposmotic solutions. Volume regulation in hyperosmotic fluids is HCO3- dependent and is associated with a decrease in the relative K+ conductance of the basolateral cell membrane and a decrease in the resistance ratio, Ra/Rbl. Volume regulation in hyposmotic solutions is also dependent upon the presence of HCO3- but is also inhibited by 2 mM Ba2+ in the basolateral solution. Hyposmotic regulation is accompanied by an increase in the relative K+ conductance of the basolateral cell membrane and an increase in Ra/Rbl. Neither hypo- nor hyposmotic regulation have any affect on the depolarization of the basolateral cell membrane potential induced by HCO3- removal. We conclude that volume regulation in the early proximal tubule of the kidney involves both HCO3(-)-dependent transport systems and the baso-lateral K+ conductance.

The effect of cyanide on apparent potassium conductance across the peritubular cell membrane of frog proximal tubules

To test for the effect of cyanide on frog proximal renal tubules the potential difference across the peritubular cell membrane (PDpt) has been recorded continuously before and during peritubular application of 1 mmol/l cyanide using conventional microelectrodes. Before application of cyanide PDpt amounts to -61.5 +/- 2.2 mV in the absence of luminal substrate. Cyanide depolarizes the peritubular cell membrane by +18.8 +/- 2.3 mV/10 min in the presence and by +4.5 +/- 0.9 mV/10 min in the absence of luminal substrate. The rapid depolarization of the cell membranes to addition of glucose to luminal perfusate is not significantly influenced by exposure to cyanide, whereas the influence of altered peritubular potassium concentration (from 3 to 9 mmol/l) is significantly reduced from +15.2 +/- 1.7 mV to +8.7 +/- 1.8 mV. Following exposure to cyanide the lumped resistance of the luminal and peritubular cell membranes increases significantly by 36 +/- 7%/6 min, and the cellular core resistance significantly by 14 +/- 6%/6 min. As a result, cyanide markedly decreases the peritubular potassium conductance, depolarizes the cell membranes and reduces the driving force for sodium coupled transport processes. Thus cyanide fully mimics the effects of ouabain, although cyanide in contrast to ouabain is expected to deplete the cells from ATP. In conclusion ATP/ADP is not likely to play a major role in the regulation of sodium coupled transport processes and peritubular potassium conductance in amphibian proximal tubules.

Current-voltage relations of sodium-coupled sugar transport across the apical membrane of Necturus small intestine

The current-voltage (I-V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane of Necturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, psi mc, and that across the entire epithelium, psi ms, when the latter was varied over the range +/- 200 mV, under steady conditions in the presence of galactose and after the current across the apical membrane carried by the cotransporter, ImSNa, is blocked by the addition of phloridzin to the mucosal solution. ImSNa was found to be strongly dependent upon psi mc over the range -50 mV less than psi mc less than EmSNa where EmSNa is the "zero current" or "reversal" potential. Over the range of values of psi mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so that ImSNa = gmSNa (EmSNa - psi mc) where gmSNa is the contribution of this mechanism to the conductance of the apical membrane and is "near constant." In several instances ImSNa "saturated" at large hyperpolarizing or depolarizing values of psi mc. The values of EmSNa determined in the presence of 1, 5, and 15 mM galactose strongly suggest that if the Na-galactose cotransporters are kinetically homogeneous, the stoichiometry of this coupled process is unity. Finally, the shapes of the observed I-V relations are consistent with the predictions of a simple kinetic model which conforms with current notions regarding the mechanico-kinetic properties of this cotransport process.

Influence of glucose absorption on ion activities in cells and submucosal space in goldfish intestine

Mucosal glucose addition evokes in goldfish intestinal epithelium a fast depolarization of the mucosal membrane potential (delta psi mc = 12 mV) followed by a slower repolarization (delta psi mc = -7 mV). The intracellular sodium activity, aiNa+, rises from 13.2 +/- 2.4 meq/l by 6.7 +/- 0.5 meq/l within 5 min, aiCl- rises about 3 meq/l above the control value of 37.7 +/- 2.2 meq/l, while aiK is constant (97.7 +/- 7.4 meq/l). The potassium activity measured in the submucosal interstitium near the basal side of the cells (asK+) is 5.2 +/- 0.2 meq/l in non-absorbing tissue compared to 4.2 meq/l in the bathing solution and shows a transient increase due to glucose absorption (1.1 +/- 0.1 meq/l). In chloride-free media asK+ = 4.2 +/- 0.1 meq/l and psi mc hyperpolarizes by -13 mV. The depolarization due to glucose absorption increases (delta psi mc = 14.1 +/- 1.4) and the repolarization (delta psi repolmc) disappears. In addition, aiNa+ rises from 16.3 +/- 2.4 meq/l by 9.9 +/- 1.5 meq/l within 5 min, aiK+ remains constant and equal to the value in chloride containing solutions (88.5 +/- 2.8 meq/l); asK+ increases transiently (1.1 +/- 0.1 meq/l). Serosal Ba2+ (5 mM) depolarizes psi mc (+14.2 +/- 1.0 mV) and abolishes the repolarization. Increased serosal or mucosal potassium activity depolarizes psi mc and abolishes the repolarization. These effects are discussed in terms of changes of ion activities, the basolateral potassium conductance, the influence of intracellular Ca2+, the functional state of the Na/K-pump, and modulation of membrane permeabilities by extracellular potassium.

Influence of potassium depletion on potassium conductance in proximal tubules of frog kidney

In order to test for the contribution of intracellular potassium activity to the link of sodium/potassium-ATPase activity and potassium conductance, studies with conventional and potassium selective microelectrodes were performed on proximal tubules of the isolated perfused frog kidney. The peritubular transference number for potassium (tk), i.e., the contribution of peritubular slope potassium conductance to the slope conductance of the cell membranes (luminal and peritubular), was estimated from the influence of peritubular potassium concentration on the potential difference across the peritubular cell membrane (PDpt). During control conditions, PDpt is -65 +/- 1 mV, intracellular potassium activity (Ki) 57 +/- 2 mmol/l and tk 0.41 +/- 0.05. The resistance in parallel of the luminal and peritubular cell membranes (Rm) is 44 +/- 4 k omega cm, the resistance of the cellular cable (Rc) 137 +/- 13 M omega/cm. When the cells are exposed 10 min to potassium free perfusates (series I), PDpt increases by -28 +/- 3 mV within 2 min and then decreases gradually to approach the control value within 10 min. Ki decreases by 22 +/- 3 mmol/l and Rc increases by 35 +/- 10%. After a transient decrease, Rm increases by 36 +/- 9%. Readdition of peritubular potassium leads to a transient increase of PDpt, a gradual decrease of Rm and Rc as well as a gradual increase of Ki. tk recovers only slowly to approach 65 +/- 8% of control value within 3 and 79 +/- 10% within 6 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of barium on the effects of phenylalanine in proximal tubules

The present study was designed to further test for the role of peritubular potassium conductance in the repolarization of peritubular cell membrane during sustained stimulation of sodium coupled transport by phenylalanine. To this end the potential difference across the peritubular cell membrane (PDpt) has been recorded continuously, while 10 mmol/l phenylalanine (Phe) were added to the luminal perfusate, both in the presence or absence of peritubular or luminal barium (1 mmol/l). In the absence of phenylalanine and barium, PDpt amounts to -65.5 +/- 2.2 mV. Phe leads to a rapid depolarization of the peritubular cell membrane by +36.2 +/- 2.2 mV within 30 s, followed by an almost complete repolarization by -28.9 +/- 2.6 mV within 7 min. In the presence of barium in peritubular perfusate, the depolarization following Phe is +24.3 +/- 2.6 mV and the repolarization almost abolished (-4.3 +/- 0.9 mV). In the presence of barium in luminal perfusate, Phe leads to a depolarization by +35.7 +/- 2.4 mV followed by a repolarization of -17.0 +/- 3.2 mV within 7 min. It is concluded that the repolarization during sustained stimulation of sodium coupled transport is in large part due to alterations of peritubular potassium conductance.

The effect of phenylalanine on intracellular pH and sodium activity in proximal convoluted tubule cells of the frog kidney

The present study was performed to test the influence of sodium coupled transport of neutral substrates on intracellular pH and sodium activity in proximal tubules of the amphibian kidney. To this end, kidneys of rana esculenta have been isolated and perfused both through the portal vein (peritubular capillaries) and the aorta (luminal perfusate). The potential difference across the peritubular membrane of proximal tubule cells has been reduced with conventional (PDpt) as well as with sodium (PDna) and hydrogen ion (PDh) selective microelectrodes continuously before, during, and after the luminal application of 10 mmol/l phenylalanine, replacing 10 mmol/l raffinose. PDh and PDna allowed the calculation of intracellular pH (pHi) and sodium activity (Nai), respectively. In the absence of phenylalanine in the tubule lumen, PDpt approximates -57.5 +/- 2.3 mV (n = 27), pHi 7.73 +/- 0.04 (n = 14, extracellular pH 7.77), and Nai 13.3 +/- 0.9 mmol/l (n = 13, extracellular sodium activity 74 mmol/l). Within 1 min the luminal application of phenylalanine leads to a depolarisation of PDpt by +32 +/- 2 mV, as well as an increase of pHi by 0.24 +/- 0.04 and of Nai by 5.2 +/- 1.0 mmol/l. At 8 min from luminal application of phenylalanine, Nai plateaus 5 +/- 1 mmol/l above control value, PDpt increases again to a value of +12 +/- 2 mV below and pHi decreases to a value 0.04 +/- 0.07 above their respective control values. All changes are fully reversed after removal of phenylalanine from the tubule lumen.(ABSTRACT TRUNCATED AT 250 WORDS)

Ouabain decreases apparent potassium-conductance in proximal tubules of the amphibian kidney

According to a previous study from this laboratory, the electrochemical gradient for potassium across the peritubular cell membrane of proximal tubules in the isolated perfused frog kidney increases following the application of ouabain. In order to test, if this phenomenon were due to a decrease of potassium conductance, the effects of ouabain on cell membrane resistances and the sensitivity of the peritubular cell membrane potential difference (PDpt) to step changes of peritubular potassium and bicarbonate concentration were studied. In the absence of ouabain, PDpt averaged -60 +/- 3 mV (n = 25). A step increase of peritubular potassium concentration from 3 to 18 mmol/l (pH 8.07) depolarizes PDpt (delta PDk) by +24 +/- mV (n = 8). An increase of bicarbonate from 20 to 40 mmol/l (pH 8.07) hyperpolarizes PDpt (delta PDb) by -2.8 +/- 0.4 mV (n = 9). The resistance of the luminal and peritubular cell membranes in parallel (Rm) amounts to 45 +/- 9 k omega cm (tubule length) (n = 4) and the voltage divider ratio (VDR) to 1.4 +/- 0.2 (n = 7). The resistance of the cellular cable (cellular core, Rc) approaches 131 +/- 37 M omega/cm (n = 4). Peritubular application of 0.1 mmol/l ouabain leads to a gradual decline of PDpt (t1/2 approx. 30 min), to an increase of Rm, a decrease of delta PDk and an increase of delta PDb. VDR and Rc are not changed significantly. The data point to a functional link between the sodium/potassium ATPase and the potassium conductance of the peritubular cell membrane.