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

Leucine-Rich Repeat Kinase 2 (Lrrk2)-Sensitive Na+/K+ ATPase Activity in Dendritic Cells

Leucine-rich repeat kinase 2 (Lrrk2) has been implicated in the pathophysiology of Parkinson's disease. Lrrk2 is expressed in diverse cells including neurons and dendritic cells (DCs). In DCs Lrrk2 was shown to up-regulate Na+/Ca2+-exchanger activity. The elimination of Ca2+ by Na+/Ca2+ -exchangers requires maintenance of the Na+ gradient by the Na+/K+ -ATPase. The present study thus explored whether Lrrk2 impacts on Na+/K+ -ATPase expression and function. To this end DCs were isolated from gene-targeted mice lacking Lrrk2 (Lrrk2-/-) and their wild-type littermates (Lrrk2+/+). Na+/K+ -ATPase activity was estimated from K+ induced, ouabain sensitive, current determined by whole cell patch clamp. Na+/K+ -ATPase α1 subunit transcript and protein levels were determined by RT-qPCR and flow cytometry. As a result, the K+ induced current was significantly smaller in Lrrk2-/- than in Lrrk2+/+ DCs and was completely abolished by ouabain (100 μM) in both genotypes. The K+ induced, ouabain sensitive, current in Lrrk2+/+ DCs was significantly blunted by Lrrk2 inhibitor GSK2578215A (1 μM, 24 hours). The Na+/K+ -ATPase α1 subunit transcript and protein levels were significantly lower in Lrrk2-/- than in Lrrk2+/+ DCs and significantly decreased by Lrrk2 inhibitor GSK2578215A (1 μM, 24 hours). In conclusion, Lrrk2 is a powerful regulator of Na+/K+ -ATPase expression and activity in dendritic cells.

Regulation of Voltage-Gated K+ Channel Kv1.5 by the Janus Kinase JAK3

The tyrosine kinase Janus kinase 3 (JAK3) participates in the regulation of cell proliferation and apoptosis. The kinase further influences ion channels and transport proteins. The present study explored whether JAK3 contributes to the regulation of the voltage-gated K(+) channel Kv1.5, which participates in the regulation of diverse functions including atrial cardiac action potential and tumor cell proliferation. To this end, cRNA encoding Kv1.5 was injected into Xenopus oocytes with or without additional injection of cRNA encoding wild-type JAK3, constitutively active (A568V)JAK3, or inactive (K851A)JAK3. Voltage-gated K(+) channel activity was measured utilizing dual electrode voltage clamp, and Kv1.5 channel protein abundance in the cell membrane was quantified utilizing chemiluminescence of Kv1.5 containing an extracellular hemagglutinin epitope (Kv1.5-HA). As a result, Kv1.5 activity and Kv1.5-HA protein abundance were significantly decreased by wild-type JAK3 and (A568V)JAK3, but not by (K851A)JAK3. Inhibition of Kv1.5 protein insertion into the cell membrane by brefeldin A (5 μM) resulted in a decline of the voltage-gated current, which was similar in the absence and presence of (A568V)JAK3, suggesting that (A568V)JAK3 did not accelerate Kv1.5 protein retrieval from the cell membrane. A 24 h treatment with ouabain (100 µM) significantly decreased the voltage-gated current in oocytes expressing Kv1.5 without or with (A568V)JAK3 and dissipated the difference between oocytes expressing Kv1.5 alone and oocytes expressing Kv1.5 with (A568V)JAK3. In conclusion, JAK3 contributes to the regulation of membrane Kv1.5 protein abundance and activity, an effect sensitive to ouabain and thus possibly involving Na(+)/K(+) ATPase activity.

Energy-sensitive regulation of Na+/K+-ATPase by Janus kinase 2

Janus kinase 2 (JAK2) contributes to intracellular signaling of leptin and erythropoietin, hormones protecting cells during energy depletion. The present study explores whether JAK2 is activated by energy depletion and regulates Na(+)/K(+)-ATPase, the major energy-consuming pump. In Jurkat cells, JAK2 activity was determined by radioactive kinase assay, phosphorylated JAK2 detected by Western blotting, ATP levels measured by luciferase assay, as well as Na(+)/K(+)-ATPase α1-subunit transcript and protein abundance determined by real-time PCR and Western blotting, respectively. Ouabain-sensitive K(+)-induced currents (Ipump) were measured by whole cell patch clamp. Ipump was further determined by dual-electrode voltage clamp in Xenopus oocytes injected with cRNA-encoding JAK2, active (V617F)JAK2, or inactive (K882E)JAK2. As a result, in Jurkat T cells, JAK2 activity significantly increased following energy depletion by sodium azide (NaN3) or 2,4- dinitro phenol (DNP). DNP- and NaN3-induced decrease of cellular ATP was significantly augmented by JAK2 inhibitor AG490 and blunted by Na(+)/K(+)-ATPase inhibitor ouabain. DNP decreased and AG490 enhanced Ipump as well as Na(+)/K(+)-ATPase α1-subunit transcript and protein abundance. The α1-subunit transcript levels were also enhanced by signal transducer and activator of transcription-5 inhibitor CAS 285986-31-4. In Xenopus oocytes, Ipump was significantly decreased by expression of JAK2 and (V617F)JAK2 but not of (K882E)JAK2, effects again reversed by AG490. In (V617F)JAK2-expressing Xenopus oocytes, neither DNP nor NaN3 resulted in further decline of Ipump. In Xenopus oocytes, the effect of (V617F)JAK2 on Ipump was not prevented by inhibition of transcription with actinomycin. In conclusion, JAK2 is a novel energy-sensing kinase that curtails energy consumption by downregulating Na(+)/K(+)-ATPase expression and activity.

The effect of acute hypoxia on short-circuit current and epithelial resistivity in biopsies from human colon

BACKGROUND AND AIMS

In isolated colonic mucosa, decreases in short-circuit current (ISC) and transepithelial resistivity (RTE) occur when hypoxia is either induced at both sides or only at the serosal side of the epithelium. We assessed in human colon biopsies the sensitivity to serosal-only hypoxia and mucosal-only hypoxia and whether Na, K-ATPase blockade with ouabain interacts with hypoxia.

MATERIALS AND METHODS

Biopsy material from patients undergoing colonoscopy was mounted in an Ussing chamber for small samples (1-mm2 window). In a series of experiments we assessed viability and the electrical response to the mucolytic, dithiothreitol (1 mmol/l). In a second series, we explored the effect of hypoxia without and with ouabain. In a third series, we evaluated the response to a cycle of hypoxia and reoxygenation induced at the serosal or mucosal side while keeping the oxygenation of the opposite side.

RESULTS

1st series: Dithiothreitol significantly decreased the unstirred layer and ISC but increased RTE. 2nd series: Both hypoxia and ouabain decreased ISC, but ouabain increased RTE and this effect on RTE prevailed even during hypoxia. 3rd series: Mucosal hypoxia caused lesser decreases of ISC and RTE than serosal hypoxia; in the former, but not in the latter, recovery was complete upon reoxygenation.

CONCLUSIONS

In mucolytic concentration, dithiothreitol modifies ISC and RTE. Oxygen supply from the serosal side is more important to sustain ISC and RTE in biopsy samples. The different effect of hypoxia and Na, K-ATPase blockade on RTE suggests that their depressing effect on ISC involves different mechanisms.

Tissue-specific ionomotive enzyme activity and K+ reabsorption reveal the rectum as an important ionoregulatory organ in larval Chironomus riparius exposed to varying salinity

A role for the rectum in the ionoregulatory homeostasis of larval Chironomus riparius was revealed by rearing animals in different saline environments and examining: (1) the spatial distribution and activity of keystone ionomotive enzymes Na(+)-K(+)-ATPase (NKA) and V-type H(+)-ATPase (VA) in the alimentary canal, and (2) rectal K(+) transport with the scanning ion-selective electrode technique (SIET). NKA and VA activity were measured in four distinct regions of the alimentary canal as follows: the combined foregut and anterior midgut, the posterior midgut, the Malpighian tubules and the hindgut. Both enzymes exhibited 10-20 times greater activity in the hindgut relative to all other areas. When larvae were reared in either ion-poor water (IPW) or freshwater (FW), no significant difference in hindgut enzyme activity was observed. However, in larvae reared in brackish water (BW), NKA and VA activity in the hindgut significantly decreased. Immunolocalization of NKA and VA in the hindgut revealed that the bulk of protein was located in the rectum. Therefore, K(+) transport across the rectum was examined using SIET. Measurement of K(+) flux along the rectum revealed a net K(+) reabsorption that was reduced fourfold in BW-reared larvae versus larvae reared in FW or IPW. Inhibition of NKA with ouabain, VA with bafilomycin and K(+) channels with charybdotoxin diminished rectal K(+) reabsorption in FW- and IPW-reared larvae, but not BW-reared larvae. Data suggest that the rectum of C. riparius plays an important role in allowing these larvae to cope with dilute as well as salinated environmental conditions.

K(+) transport in Malpighian tubules of Tenebrio molitor L: is a K(ATP) channel involved?

The presence of ATP-regulated K(+) (K(ATP)) channels in Tenebrio molitor Malpighian tubules was investigated by examining the effect of glibenclamide on both fluid secretion and basolateral membrane potentials (V(bl)). Glibenclamide, a K(ATP) channel blocker, slowed fluid secretion of Tenebrio tubules. In low bath K(+) concentration (5 mmol l(-1)), glibenclamide either hyperpolarized or depolarized V(bl), resembling the effect seen with Ba(2+). Subsequent addition of 6 mmol l(-1) Ba(2+) caused a further hyper- or depolarization of V(bl). In control Ringer (50 mmol l(-1) KCl, 90 mmol l(-1) NaCl), glibenclamide had no visible effect on V(bl). The effect of ouabain was investigated in low bath [K(+)] in the presence of Ba(2+). V(bl) responded by a small but significant hyperpolarization from -51+/-4 mV to -56+/-4 mV (n=16, P<0.001) in response to 1 mmol l(-1) ouabain. Repeating the experiments in the presence of both glibenclamide and Ba(2+) resulted in a depolarization of V(bl) when ouabain was added. In low bath [K(+)] (high Na(+)), the Na(+)/K(+)-ATPase is expected to function at a high rate. In the presence of Ba(2+), replacing Na(+) by K(+) rapidly depolarized V(bl), but this was followed by a repolarization. Repeating the experiments in the presence of glibenclamide markedly reduced the depolarizing effect and abolished the repolarization, with a gradual decrease in the sensitivity of V(bl) to the surrounding [K(+)]. These results suggest the presence of K(ATP) channels in the basolateral membrane. Glibenclamide had no visible effect on V(bl) in high K(+) or in the absence of Ba(2+), indicating that other highly conductive K(+) channels may mask the effect on K(ATP) channels. This is the first demonstration of the presence of K(ATP) channels in an insect epithelium.

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.

An intracellular ATP-activated, calcium-permeable conductance on the basolateral membrane of single renal proximal tubule cells isolated from Rana temporaria

1. The following study describes the properties of a non-selective cation channel, which has a unit conductance below the resolving power of the single channel technique, located on the basolateral membrane of single proximal tubule cells isolated from frog kidney. The conductance was examined using cell-attached, inside-out and outside-out patches. Due to the small single channel magnitude, macroscopic patch currents were measured. 2. Addition of 2 mM ATP to the intracellular surface of excised patches activated an outwardly rectifying conductance (MCANS): outward (Gout) and inward (Gin) conductances increased by 46.8 +/- 6.7 and 11.6 +/- 2.1 pS, respectively (n = 29). MCANS was more selective for cations than anions, with a cation:anion selectivity ratio of 10.1 +/- 1.7 (n = 7), but did not discriminate between Na+ and K+. It was more selective for Ca2+ over Na+, with a Ca2+:Na+ selectivity ratio of 4. 49 +/- 0.69 (n = 7). 3. In cell-attached patches addition of 100 microM strophanthidin to the bath increased both Gout and Gin. However this increase in conductance was absent in the presence of Gd3+, which inhibits MCANS. 4. These data suggest that single proximal tubule cells isolated from frog kidney contain an ATP-activated, non-selective cation conductance. The conductance does not discriminate between Na+ and K+, but is more selective for Ca2+ over Na+. Considering the prevailing electrochemical gradients for these ions, functional activation of the conductance would be expected to lead to a rise in intracellular Ca2+. MCANS is linked to the activity of the Na+, K+-ATPase and may therefore provide a link between the ATPase and K+ channel activity in the basolateral membrane and form an integral part of the pump-leak mechanism in transporting epithelia.

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.

Two K(+)-selective conductances in single proximal tubule cells isolated from frog kidney are regulated by ATP

1. The whole-cell and single channel patch clamp techniques were used to identify K(+)-selective conductances in single proximal tubule cells isolated from frog kidney and to examine their ATP sensitivity. Whole-cell currents were inhibited by the K+ channel inhibitors Ba2+ and quinidine in a dose-dependent manner. Addition of Ba2+ alone, quinidine alone, or both inhibitors together revealed two separate conductances, one of which was blocked by both Ba2+ and quinidine (GBa)1, the other being sensitive to quinidine alone (Gquin). 2. With Na(+)-containing Ringer solution in the bath and K(+)-containing Ringer solution in the pipette, both currents were selective for K+ over Na+. The K+ : Na+ selectivity ratio of GBa was around 50:1, while that of Gquin was 4:1. In symmetrical KCl solutions GBa showed inward rectification, while Gquin demonstrated outward rectification. 3. In the absence of pipette ATP, both GBa and Gquin ran down over 10 min. However, when 2 mM ATP was included in the pipette GBa increased, while Gquin remained unchanged. 4. Single channel studies demonstrated that a basolateral K+ channel shared several of the characteristics of GBa. It was inhibited by both Ba2+ and quinidine, underwent run-down in excised patches in the absence of ATP, and was activated by ATP. 5. We conclude that cells of the frog proximal tubule contain at least two distinct K(+)-selective conductances, both of which are regulated by ATP, and which may be involved in pump-leak coupling.

Cross-talk between ATP-regulated K+ channels and Na+ transport via cellular metabolism in frog skin principal cells

Isolated frog skin epithelium, mounted in an Ussing chamber and bathed in standard NaCl Ringer solution, recycles K+ across the basolateral membrane of principal cells through an inward-rectifier K+ channel (Kir) operating in parallel with a Na+-K+-ATPase pump. Here we report on the metabolic control of the Kir channel using patch clamping, short-circuit current measurement and enzymatic determination of cellular (ATP (ATPi). 2. The constitutively active Kir channel in the basolateral membrane has the characteristics of an ATP-regulated K+ channel and is now classed as a KATP channel. In excised inside-out patches the open probability (Po) of KATP channels was reduced by ATPi with half-maximum inhibition at an ATPi concentration of 50 microM. 3. ATPi measured (under normal Na+ transport conditions) with luciferin-luciferase was 1.50 +/- 0.23 mM (mean +/- S.E.M.; range, 0.4-3.3 mM n = 11). Thus the KATP channel would be expected to be inactive in intact cells if ATPi was the sole regulator of channel activity. KATP channels which were inactivated by 1 mM ATPi in excised patches could be reactivated by addition of 100 microM ADP on the cytosolic side. When added alone, ADP blocks this channel with half-maximal inhibition at [ADPi] > 5 mM. 4. Sulphonylureas inhibit single KATP channels in cell-attached patches as well as the total basolateral K+ current measured in frog skin epithelia perforated with nystatin on the apical side. 5. Na+-K+-ATPase activity is a major determinant of cytosolic ATP. Blocking the pump activity with ouabain produced a time-dependent increase in ATPi and reduced the open probability of KATP channels in cell-attached membranes. 6. We conclude that the ratio of ATP/ADP is an important metabolic coupling factor between the rate of Na+-K+ pumping and K+ recycling.

Effects of urea on K+ fluxes and cell volume in perfused rat liver

Exposure of the perfused rat liver to a perfusate made hyperosmotic by the presence of 200 mmol l-1 glucose led, as expected, to marked, transient hepatocellular shrinkage followed by volume-regulatory net K+ uptake. However, even after this volume-regulatory K+ uptake had ceased, the liver cells are still slightly shrunken. Withdrawal of glucose from the perfusate resulted in marked transient cell swelling, net K+ release from the liver and restoration of cell volume. However, when the Krebs-Henseleit perfusate was made hyperosmotic by the presence of urea (20-300 mM), there was no immediate decrease in liver mass, yet a slight and persistent cell shrinkage developing 2 min after the onset of exposure to urea. Surprisingly, urea induced concentration-dependent net K+ efflux from the liver and removal of urea net K+ reuptake from the inflowing perfusate. The urea (200 mM)-induced net K+ release resembled that observed following a lowering of the influent [NaCl]: making the perfusate hypoosmotic (245 mosmol l-1, by reducing influent [NaCl] by 30 mM) gave roughly the same K+ response as hyperosmotic exposure (505 mosmol/l) resulting from the presence of 200 mM urea. The urea-induced K+ efflux was not inhibited in the presence of ouabain (1 mM), or in Ca(++)-free perfusion, but was modified in the presence of quinidine (1 mM) or Ba++ (1 mM). The direction in which the liver was perfused had no effect on the urea-induced net K+ release.(ABSTRACT TRUNCATED AT 250 WORDS)

Peritubular Na-K exchange ion pump in maleate-treated frog kidney proximal tubular cells

1. After perfusion of isolated frog kidneys for 1 hr with 10(-3) or 10(-2) M maleate Ringer, the peritubular membrane potential gradually declined in a dose-dependent manner. 2. The ouabain-like effects of maleate on cell Na and K activities were dose-dependent and smaller than the effects of zero K or 10(-4) M ouabain. Intracellular pH was not altered in the presence of 10(-2) M maleate. 3. The driving force for Na entry into the cell was reduced, respectively, to 81.4 and 58.4% (of control) in the presence of 10(-3) and 10(-2) M maleate. 4. There was no histochemically detectable inhibition of proximal tubule Na-K ATPase activity during 3 hr of perfusion with 10(-2) M maleate.

Intracellular potassium activity in mammalian proximal tubule: effect of perturbations in transepithelial sodium transport

Intracellular potassium activity (alpha Ki) was measured in control conditions in mid-cortical rabbit proximal convoluted tubule using two methods: (i) by determination of the K+ equilibrium potential (EK) using Ba(2+)-induced variations in the basolateral membrane potential (VBL) during transepithelial current injections and (ii) with double-barrel K-selective microelectrodes. Using the first method, the mean VBL was -48.5 +/- 3.2 mV (n = 16) and the mean EK was -78.4 +/- 4.1 mV corresponding to alpha Ki of 68.7 mM. With K-selective microelectrodes, VBL was -36.6 +/- 1.1 mV (n = 19), EK was -64.0 +/- 1.1 mV and alpha Ki averaged 40.6 +/- 1.7 mM. While these last EK and VBL values are significantly lower than the corresponding values obtained with the first method (P less than 0.001 and P less than 0.01, respectively), the electrochemical driving force for K transport across the basolateral membrane (microK = VBL-EK) is not significantly different for both techniques (30.1 +/- 3.3 mV for the first technique and 27.6 +/- 1.8 mV for ion-selective electrodes). This suggests an adequate functioning of the selective barrel but an underestimation of VBL by the reference barrel of the double-barrel microelectrode. Such double-barrel microelectrodes were used to measure temporal changes in alpha Ki and microK in different experimental conditions where Na reabsorption rate (JNa) was reduced. alpha Ki was shown to increase by 12.2 +/- 2.7 (n = 5) and 14.1 +/- 4.4 mM (n = 5), respectively, when JNa was reduced by omitting in the luminal perfusate: (i) 5.5 mM glucose and 6 mM alanine and (ii) glucose, alanine, other Na-cotransported solutes and 110 mM Na. In terms of the electrochemical driving force for K exit across the basolateral membrane, microK, a decrease of 5.4 +/- 2.0 mV (P less than 0.05, n = 5) was measured when glucose and alanine were omitted in the luminal perfusate while microK remained unchanged when JNa was more severely reduced (mean change = -1.7 +/- 2.1 mV, NS, n = 5). In the latter case, this means that the electrochemical driving force for K efflux across the basolateral membrane has not changed while both the active influx through the Na-K pump and the passive efflux in steady state are certainly reduced. If the main pathway for K transport is through the basolateral K conductance, this implies that this conductance must have decreased in the same proportion as that of the reduction in the Na-K pump activity.

Apical and basal membrane ion transport mechanisms in bovine retinal pigment epithelium

1. Intracellular voltage recordings using conventional and double-barrelled chloride-selective microelectrodes have been used to identify several transport mechanisms at the apical and basolateral membranes of the isolated bovine retinal pigment epithelium (RPE)-choroid preparation. Intracellular recordings were obtained from two cell populations, melanotic (pigmented) and amelanotic (non-pigmented). The electrical properties of these two populations are practically identical. For melanotic cells the average apical resting membrane potential (VA) is -61 +/- 2 mV (mean +/- S.E.M., n = 49 cells, thirty-three eyes). For these cells the ratio of apical to basolateral membrane resistance (a) was 0.22 +/- 0.02. The mean transepithelial voltage and resistance were 6 +/- 1 mV and 138 +/- 7 omega cm2, respectively. 2. The apical membrane, which faces the distal retina, contains a Ba(2+)-inhibitable K+ conductance and a ouabain-inhibitable, electrogenic Na(+)-K+ pump. In addition it contains a bumetanide-sensitive mechanism, the putative Na(+)-K(+)-Cl- cotransporter. The basolateral membrane contains a DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid)-inhibitable chloride channel. The relative conductances of the apical and basolateral membranes to K+ and Cl- are TK approximately 0.9 and TCl approximately 0.7, respectively. 3. The ouabain-induced fast phase of apical membrane depolarization (0-30 s) was used to calculate the equivalent resistances of the apical (RA) and basolateral (RB) cell membranes, as well as the paracellular or shunt resistance (RS). They are: 3190 +/- 400, 17920 +/- 2730 and 2550 +/- 200 omega (mean +/- S.E.M., n = 9 tissues), respectively. From these data the equivalent electromotive forces (EMF) at the apical (EA) and basolateral (EB) membranes were also calculated. They are: -69 +/- 5.0 and -24 +/- 5.0 mV, respectively. 4. Intracellular Cl- activity (aiCl) was measured using double-barreled ion-selective microelectrodes. In the steady state aiCl = 61 +/- 4.0 mM and the Nernst potential ECl = -13.5 +/- 1.5 mV (mean +/- S.E.M., n = 4). 5. In the intact eye or in retina, RPE-choroid preparations it has been shown that the transition between light and dark alters the K+ concentration in the extracellular (or subretinal) space between the photoreceptors and the apical membrane of the RPE. These light-induced changes in subretinal [K+]o were qualitatively simulated in vitro by altering apical K+ between 5 and 2 mM. This produced a sequence of voltage changes at the apical and basolateral membranes that had three operationally distinct phases. Phase 1 is generated by the combination of an apical membrane K+ diffusion potential and inhibition of the electrogenic Na(+)-K+ pump.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Cobalt activates potassium conductance in the plasma membrane of cultured renal epithelioid (MDCK)-cells

Cobalt has been shown to stimulate sodium transport across the distal nephron of the newt kidney. The mechanism of this action remained elusive. The present study has been performed to test for effects of cobalt on electrical properties of cultured subconfluent kidney (MDCK)-cells: cobalt (10 microM) leads to a rapid, sustained and reversible hyperpolarization of the cell membrane, paralleled by an increase of the potassium selectivity and a decrease of the resistance. Thus, cobalt increases the potassium conductance of the cell membrane. The half-maximal effect is elicited by approx. 1 microM. At extracellular calcium concentration reduced to less than 0.1 microM, cobalt (10 microM) leads to a transient hyperpolarization, which can be elicited only once. Thus, cobalt enhances the potassium conductance in a calcium dependent way. At higher concentrations (100 microM) cobalt hyperpolarizes the cell membrane only transiently even in the presence of extracellular calcium. Furthermore 100 microM cobalt interferes with ATP-induced hyperpolarization, which is known to result from calcium mediated activation of K+ channels. Thus, 100 microM cobalt may inhibit ATP-stimulated calcium entry into the cell.

Membrane potential of rat calvaria bone cells: dependence on temperature

The membrane potentials of bone cells derived from calvaria of new born rats was shown to be strongly dependent on temperature. When we lowered the temperature from 36 degrees C to 26 degrees C, cells with spontaneous resting membrane potentials (MP) of -80 to -50 mV depolarized (mean amplitude 8 mV; n = 33), and the membrane resistance increased by approximately 80% (n = 20). The temperature response depended on the actual MP, the reversal potential being in the range of -80 to -90 mV. With the application of ouabain (0.1-1 mmol/liter; n = 12), cells depolarized. Simultaneously, the reversal potential of the temperature response was shifted towards more positive values and approached the actual MP level of the cells. Consequently, the depolarization amplitudes induced by lowering temperature were reduced at spontaneous MP levels. The rise of the membrane resistance during cooling was unaffected. When the extracellular chloride concentration was reduced from 133 to 9 mmol/liter, temperature-dependent depolarizations persisted at spontaneous MP values (n = 5). The findings indicate that the marked effects of temperature changes on the MP of bone-derived cells are mainly determined by changes of the potassium conductance.

Cadmium enhances potassium conductance in cultured renal epitheloid (MDCK) cells

The kidney is a main target organ for cadmium toxicity. The present study has been performed to test for effects of cadmium on electrical properties of cultured subconfluent kidney (MDCK) cells. Cadmium leads to a rapid, sustained and reversible hyperpolarization of the cell membrane, paralleled by an increase of the potassium selectivity and a decrease of the resistance. Thus, cadmium increases the potassium conductance of the cell membrane. The half maximal effect is elicited congruent to 0.2 microM, a concentration encountered during chronic cadmium intoxication. At extracellular calcium concentration reduced to less than 0.1 microM, 5 microM cadmium leads to a transient hyperpolarization, which can be elicited only once. High concentrations (50 microM) of cadmium lead to a sustained hyperpolarization even at extracellular calcium concentrations of less than 0.1 microM. According to fluorescence measurements cadmium leads to an increase of intracellular calcium activity, which is sustained at 1 mM and transient at less than 1 microM extracellular calcium activity. In conclusion, cadmium at low concentrations enhances the potassium conductance in a calcium dependent way. The observations suggest that cadmium enhances intracellular calcium both by recruitment from intracellular stores and by modification of calcium transport across the cell membrane. At high concentrations cadmium enhances the potassium conductance independently from enhanced intracellular calcium activity.

Electrophysiological consequences of retinal hypoxia

Experiments on cats show that electrical activity of the inner (proximal) retina is unaffected during systemic hypoxia as long as arterial oxygen tension (PaO2) is above 40 mm Hg. This is due to effective regulation of inner retinal tissue PO2 by the retinal circulation. In contrast, some electrical signals generated in the outer (distal) retina begin to change when PaO2 falls below 70-80 mmHg. The outer retinal responses are generated by the retinal pigment epithelium, but their susceptibility to hypoxia results primarily from their dependence on photoreceptors. Photoreceptor metabolism is sensitive to hypoxia because of the high oxygen consumption of photoreceptors and their reliance on the choroidal circulation, which cannot regulate PO2 in the outer retina. Retinal electrophysiology and oxygen distribution are altered by acutely elevated intraocular pressure just as by hypoxia. These results raise the question as to how inner retinal function can be preserved when outer retinal function is altered. The explanations proposed relate to (1) differences in conditions of light adaptation in different studies, (2) the possible inappropriateness of the previous measurements in the inner retina for revealing photoreceptor dysfunction, and (3) a possible preservation of photoreceptor electrical responses when their metabolism is altered. Comparison of cat and human studies suggests that the human retina is affected in much the same way during hypoxia as the cat retina, but further experiments are required for an understanding of the role of hypoxia in human disease.

Membrane potential measurements of transitional cells from the crista ampullaris of the gerbil. Effects of barium, quinidine, quinine, tetraethylammonium, cesium, ammonium, thallium and ouabain

Transitional cells of the crista ampullaris were impaled with microelectrodes in order to record the membrane potential (PD) and to investigate membrane properties. In control solution the PD was -87 +/- 1 mV (n = 103). This value is not significantly different from -83 +/- 2 mV (n = 24) measured in Cl- free solution. [Cl-] steps from 150 to 15 mmol/l (n = 24) depolarized the membrane by about 2 mV, indicating a minor Cl- conductance. The transference number for K+ was 0.75 +/- 0.01 (n = 79) obtained from the PD responses to K+ steps from 3.6 to 25 mmol/l. The cell membrane depolarized and the amplitude of PD responses to [K+] steps was reduced by Ba2+ (2.10(-6) to 10(-3) mol/l), quinidine (10(-3) mol/l), quinine (10(-3) mol/l), Rb+ (20 mmol/l), Cs+ (20 mmol/l), NH4+ (20 mmol/l) and Tl+ (0.5 mmol/l), whereas tetraethylammonium (TEA, 20 mmol/l) had no effect. The dose-response curve for Ba2+ in the presence of 3.6 mmol/l K+ was shifted to the right by approximately three decades in the presence of 25 mmol/l K+ and by a factor of about 4 in the presence of 135 mmol/l gluconate as a substitute for Cl-. Transitional cells were depolarized by ouabain, suggesting the presence of (Na+ + K+)-ATPase.

Coordinated regulation of intracellular K+ in the proximal tubule: Ba2+ blockade down-regulates the Na+,K+-ATPase and up-regulates two K+ permeability pathways

To avoid large changes in cell K+ content and volume during variations in Na+,K+-ATPase activity, Na+-transporting epithelia must adjust the rate of K+ exit through passive permeability pathways. Recent studies have shown that a variety of passive K+ transport mechanisms may coexist within a cell and may be functionally linked to the activity of the Na+,K+-ATPase. In this study, we have identified three distinct pathways for passive K+ transport that act in concert with the Na+,K+-ATPase to maintain intracellular K+ homeostasis in the proximal tubule. Under control conditions, the total K+ leak of the tubules consisted of discrete Ba2+-sensitive (approximately 65%), quinine-sensitive (approximately 20%), and furosemide-sensitive (approximately 10%) pathways. Following inhibition of the principal K+ leak pathway with Ba2+, the tubules adaptively restored cell K+ content to normal levels. This recovery of cell K+ content was inhibited, in an additive manner, by quinine and furosemide. Following adaptation to Ba2+, the tubules exhibited a 30% reduction in Na+-K+ pump rate coupled with an increase in K+ leak by means of the quinine-sensitive (approximately 70%) and furosemide-sensitive (approximately 280%) pathways. Thus, the proximal tubule maintains intracellular K+ homeostasis by the coordinated modulation of multiple K+ transport pathways. Furthermore, these results suggest that, like Ba2+, other inhibitors of K+ conductance will cause compensatory changes in both the Na+-K+ pump and alternative pathways for passive K+ transport.

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.

The effect of hypoosmolarity on the electrical properties of Madin Darby canine kidney cells

The present study has been performed to test for the effect of hypotonic extracellular fluid on the electrical properties of Madin Darby canine kidney (MDCK)-cells. The volume of suspended MDCK-cells is 1,892 +/- 16 fl (n = 8) in isotonic (298.7 mosmol/l) extracellular fluid. Exposure of the cells to hypotonic (230.7 mosmol/l) extracellular fluid is followed by cellular swelling to 2,269 +/- 18 fl (n = 4) and subsequent volume regulatory decrease to 2,052 +/- 22 fl (n = 4) within 512 s. Volume regulatory decrease is abolished by quinidine (1 mmol/l) and by lipoxygenase inhibitor nordihydroguaiaretic acid (50 mumol/l). The potential difference across the cell membrane averages -53.6 +/- 0.9 mV (n = 49) in isotonic extracellular perfusates. Reduction of extracellular osmolarity depolarizes the cell membrane by +25.7 +/- 0.8 mV (n = 67), reduces the apparent potassium selectivity of the cell membrane, from 0.55 +/- 0.07 (n = 9) to 0.09 +/- 0.01 (n = 26), and increases the apparent chloride selectivity from close to zero to 0.34 +/- 0.02 (n = 21). Potassium channel blocker barium (1 mmol/l) depolarizes the cell membrane by +15.2 +/- 1.1 mV (n = 13). In the presence of barium, reduction of extracellular osmolarity leads to a further depolarization by +14.0 +/- 1.4 mV (n = 12). Addition of chloride channel blocker anthracene-9-COOH (1 mmol/l) leads to a hyperpolarization of the cell membrane by -6.7 +/- 2.2 mV (n = 11). In the presence of anthracene-9-COOH, reduction of the extracellular osmolarity leads to a depolarization by +22.4 +/- 1.7 mV (n = 11).(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage dependence of the Na-K ATPase: measurements of ouabain-dependent membrane current and ouabain binding in oocytes of Xenopus laevis

Ouabain- or dihydroouabain(DHO)-sensitive membrane currents and binding of 3H-ouabain were investigated under voltage-clamp conditions in full-grown prophase-arrested oocytes of Xenopus laevis. (1) The ouabain-sensitive current is outwardly directed and usually exhibits a maximum at about +20 mV. The occurrence of the maximum is not affected by application of blockers for passive K+ currents. In the presence of Ba2+ as a K+ channel blocker, the KI value for inhibition of the Na-K ATPase by ouabain is reduced by an order of magnitude, the number of binding sites is not affected. In K+-free solution (which inhibits the normal reaction cycle of the Na-K ATPase), addition of DHO has no significant effect on the remaining currents. (2) The voltage-dependence of the ouabain-sensitive current can be modulated. Reduction of extracellular Na+ increases the pump current at the resting potential and reduces the positive slope of the I-V curve. Simultaneously, the number of binding sites for ouabain is reduced by about 25%. Seasonal variations of an unknown factor affect the negative slope. (3) Modulation of the voltage-clamp conditions has no effect on the number of binding sites for ouabain, on current inhibition by ouabain, or on ouabain binding at different concentrations of the inhibitor. It is concluded that the ouabain-sensitive current is not significantly affected by passive permeabilities and that its current-voltage dependence reflects the voltage dependence of current generated by the Na-K pump. Since ouabain binding, also at non-maximal binding concentrations, is not affected by membrane potential, steps that affect the probability of the E2P state should be voltage-insensitive.

Base induced hyperpolarization of the cell potential in HCO3- free perfused Necturus renal proximal tubules

Short-term peritubular alkalinization from 7.5 to 8.5 hyperpolarized (-8.8 mV) the basolateral membrane potential (V1) in HCO3- free Herpes buffered Necturus renal proximal tubule cells. This sustained base induced hyperpolarization (BIH) was associated with an increase in the peritubular apparent transference number for potassium (tK+). The apparent transference number for potassium (tK+) was estimated at pH 7.5 and 8.5 by raising peritubular K+ from 2.5 to 10 mmol/l. tK+ increased linearly as V1 hyperpolarized, whereas tK+ measured in the presence of peritubular Ba2+ at pH 7.5 and 8.5 was nearly zero. However, the BIH persisted in the presence of barium at the peritubular, luminal or both sides of the epithelium. Moreover this BIH was also accompanied by a small hyperpolarization (-0.4 mV) of the transepithelial membrane potential (V3) in the absence or presence of peritubular and/or luminal Ba2+. Therefore we conclude that BIH must originate from additional mechanisms other than an increase in peritubular or luminal potassium conductance.

Electrophysiology of cell volume regulation in proximal tubules of the mouse kidney

The present study has been designed to test for the influence of cell swelling on the potential difference and conductive properties of the basolateral cell membrane in isolated perfused proximal tubules. During control conditions the potential difference across the basolateral cell membrane (PDbl) is -65 +/- 1 mV (n = 74). Decrease of peritubular osmolarity by 80 mosmol/l depolarizes the basolateral cell membrane by +7.8 +/- 0.5 mV (n = 42). An increase of bath potassium concentration from 5 to 20 mmol/l depolarizes the basolateral cell membrane by +25 +/- 1 mV (n = 11), an increase of bath bicarbonate concentration from 20 to 60 mmol/l hyperpolarizes the basolateral cell membrane by -3.2 +/- 0.5 mV (n = 13). A decrease of bath chloride concentration from 79.6 to 27 mmol/l hyperpolarizes the basolateral cell membrane by -1.8 +/- 0.7 mV (n = 6). During reduced bath osmolarity, the influence of altered bath potassium concentration on PDbl is decreased (delta PDbl = +16 +/- 2 mV, n = 11), the influence of altered bicarbonate concentration on PDbl is increased (delta PDbl = -6.0 +/- 0.8 mV, n = 13), and the influence of altered bath chloride concentration on PDbl is unaffected (delta PDbl = -1.8 +/- 0.6 mV, n = 6). Barium depolarizes the basolateral cell membrane to -28 +/- 2 mV (n = 16). In the presence of 1 mmol/l barium, decrease of peritubular osmolarity by 80 mosmol/l leads to a transient hyperpolarization of the basolateral cell membrane by -5.9 +/- 0.5 mV (n = 16).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Fused cells of frog proximal tubule: II. Voltage-dependent intracellular pH

Experiments were performed in intact proximal tubules of the doubly perfused kidney and in fused proximal tubule cells of Rana esculenta to evaluate the dependence of intracellular pH (pHi) on cell membrane potential applying pH-sensitive and conventional microelectrodes. In proximal tubules an increase of the K+ concentration in the peritubular perfusate from 3 to 15 mmol/liter decreased the peritubular cell membrane potential from -55 +/- 2 to -38 +/- 1 mV paralleled by an increase of pHi from 7.54 +/- 0.02 to 7.66 +/- 0.02. The stilbene derivative DIDS hyperpolarized the cell membrane potential from -57 +/- 2 to -71 +/- 4 mV and led to a significant increase of the K+-induced cell membrane depolarization, but prevented the K+-induced intracellular alkalinization. Fused proximal tubule cells were impaled by three microelectrodes simultaneously and cell voltage was clamped stepwise while pHi changes were monitored. Cell membrane hyperpolarization acidified the cell cytoplasm in a linear relationship. This voltage-induced intracellular acidification was reduced to about one-third when HCO-3 ions were omitted from the extracellular medium. We conclude that in proximal tubule cells pHi depends on cell voltage due to the rheogenicity of the HCO-3 transport system.

Extra- and intracellular hydrogen ion-selective microelectrode based on neutral carriers with extended pH response range in acid media

A series of new neutral hydrogen ion carriers suitable for application in H+-selective microelectrodes is presented. One carrier (ETH 1907) proves to be superior to tridodecylamine currently very much in use. Microelectrodes based on ETH 1907 in an optimized membrane composition exhibit a linear dynamic response function from pH 2 to 9 extended into the acidic range, a response time less than or equal to 5 s, and a resistance of about 35 G omega for a tip diameter of about 1 micron. This makes the electrode suitable for measurements at normal physiological intracellular pH as well as in acid physiological media. Measurements using this microelectrode in proximal tubule cells of isolated perfused frog kidney are presented.

Electrophysiological study of transport systems in isolated perfused pancreatic ducts: properties of the basolateral membrane

In order to study the mechanism of pancreatic HCO3- transport, a perfused preparation of isolated intra- and interlobular ducts (i.d. 20-40 microns) of rat pancreas was developed. Responses of the epithelium to changes in the bath ionic concentration and to addition of transport inhibitors was monitored by electrophysiological techniques. In this report some properties of the basolateral membrane of pancreatic duct cells are described. The transepithelial potential difference (PDte) in ducts bathed in HCO3(-)-free and HCO3(-)-containing solution was -0.8 and -2.6 mV, respectively. The equivalent short circuit current (Isc) under similar conditions was 26 and 50 microA . cm-2. The specific transepithelial resistance (Rte) was 88 omega cm2. In control solutions the PD across the basolateral membrane (PDbl) was -63 +/- 1 mV (n = 314). Ouabain (3 mmol/l) depolarized PDbl by 4.8 +/- 1.1 mV (n = 6) within less than 10 s. When the bath K+ concentration was increased from 5 to 20 mmol/l, PDbl depolarized by 15.9 +/- 0.9 mV (n = 50). The same K+ concentration step had no effect on PDbl if the ducts were exposed to Ba2+, a K+ channel blocker. Application of Ba2+ (1 mmol/l) alone depolarized PDbl by 26.4 +/- 1.4 mV (n = 19), while another K+ channel blocker TEA+ (50 mmol/l) depolarized PDbl only by 7.7 +/- 2.0 mV (n = 9). Addition of amiloride (1 mmol/l) to the bath caused 3-4 mV depolarization of PDbl. Furosemide (0.1 mmol/l) and SITS (0.1 mmol/l) had no effect on PDbl. An increase in the bath HCO3- concentration from 0 to 25 mmol/l produced fast and sustained depolarization of PDbl by 8.5 +/- 1.0 mV (n = 149). It was investigated whether the effect of HCO3- was due to a Na+-dependent transport mechanism on the basolateral membrane, where the ion complex transferred into the cell would be positively charged, or whether it was due to decreased K+ conductance caused by lowered intracellular pH. Experiments showed that the HCO3- effect was present even when the bath Na+ concentration was reduced to a nominal value of 0 mmol/l. Similarly, the HCO3- effect remained unchanged after Ba2+ (5 mmol/l) was added to the bath. The results indicate that on the basolateral membrane of duct cells there is a ouabain sensitive (Na+ + K+)-ATPase, a Ba2+ sensitive K+ conductance and an amiloride sensitive Na+/H+ antiport. The HCO3- effect on PDbl is most likely due to rheogenic anion exit across the luminal membrane.

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.

pH-dependent electrical properties and buffer permeability of the Necturus renal proximal tubule cell

Necturus kidneys were perfused with Tris-buffered solutions at three different pH values, i.e. 7.5, 6.0 and 9.0. A significant drop in fluid absorption occurred at pH 6.0, whereas pH 9.0 did not increase volume flow significantly. When acute unilateral, i.e. either in the lumen or the peritubular capillaries, and bilateral pH changes were elicited in both directions from 7.5 to 9.0 at a constant Tris-butyrate buffer concentration, both peritubular membrane potential difference V1 and transepithelial potential difference V3 hyperpolarized, independently of the side where the change in pH was brought about. Acid perfusions at pH 6.0 caused a similar response but of opposite sign. Analysis of the potential changes shows that pH influences not only the electromotive force and resistance of the homolateral membrane, but also the electrical properties of the paracellular path. Interference of pH with Na, Cl or K conductance was assessed. Any appreciable role for sodium or chloride was excluded, whereas the potassium transference number (tK) of the peritubular membrane increased 16% in alkaline pH. However, this increase accounts only for 19 to 36% of the observed hyperpolarization. Since changes in Tris-butyrate buffer concentration at constant pH do not affect V1 or V3 considerably, the hyperpolarization in pH 9 cannot be explained by an elevation in internal pH only, or by a Tris-H+ ion diffusion potential only. The role of the permeability of the buffers: bicarbonate, butyrate and phosphate, in determining electrical membrane parameters was evaluated. Transport numbers of the buffer anions ranked as follows: tHCO3 greater than tbutyrate greater than tphosphate. It is concluded that modulation of membrane potential by extracellular pH is mediated primarily by a change in peritubular cell membrane tK and additionally by membrane currents carried by buffer anions.

Basolateral membrane K permselectivity and regulation in bullfrog cornea epithelium

In the isolated bullfrog cornea epithelium, under short-circuit conditions the regulation of the K permeability of the basolateral membrane was studied with conventional and K-selective microelectrodes in Cl-free Ringers. In Cl-free Ringers, the transcellular current is less than 1 microA/cm2, allowing estimation of the basolateral membrane electromotive force from measurements of the membrane voltage (Vsc). The apparent basolateral membrane K conductance was determined from measurements of the effects of single ion substitutions of K for Na on the Vsc. An increase of K from 2.5 to 25 mM on the stromal side depolarized the membrane voltage by 29 mV, whereas additional increases to 56 and 100 mM resulted in depolarizations consistent with a Nernstian prediction. In the range between 25 and 56 mM K, these decreases in membrane voltage were smaller after either decreasing the stromal-side pH from 8.1 to 7.2 or substitution of sulfate with gluconate. In contrast, preincubation with 0.1 mM ouabain did not change the membrane voltage depolarizations over any of the K ranges between 2.5 and 100 mM. Equivalent circuit analysis, based on the effects of nystatin on the electrical parameters, was used to validate the changes in the apparent basolateral membrane K conductance following increases in [K], substitution of SO4 with gluconate and Na:K pump inhibition. An increase in the [K] to 120 mM decreased the basolateral membrane resistance nearly three-fold, whereas gluconate substitution resulted in a 2.5-fold increase of the basolateral membrane resistance. This resistance increased an additional 2-fold after exposure to 5 mM Ba.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Potassium channels in the basolateral membrane of the rectal gland of Squalus acanthias. Regulation and inhibitors

The present study examines the influences of pH and Ca2+ and several putative inhibitors on the basolateral K+ channel of the rectal gland of Squalus acanthias. Excised membrane patches were examined using the patch clamp technique. It is shown that reduction of the calcium activity on the cytosolic side to less than 10(-9) mol/l has no detectable inhibitory effect on this channel. Conversely, increase in calcium activity to some 10(-3) mol/l reduced the activity of this channel. Variations in cytosolic pH had only a moderate effect on the current amplitude: alkalosis by one pH unit increased and acidosis reduced the single current amplitude by some 15%. Several inhibitors were tested in excised patches when added to the cytosolic side. Ba2+ (approximately 5 X 10(-3) mol/l), quinine (approximately 10(-3) mol/l), quinidine (approximately 10(-4) mol/l), lidocaine (approximately equal to 1 mmol/l), tetraethylammonium (approximately 10 mmol/l), Cs+ (approximately 10 mmol/l), and Rb+ (approximately 20 mmol/l) all blocked this K+ channel reversibly. We conclude that the basolateral K+ channel of the rectal gland is distinct from other epithelial K+ channels inasmuch as it is not stimulated by Ca2+ directly, but that it is qualitatively similar to many other known K+ channels with respect to its sensitivity towards blockers.

Electrical properties of Ehrlich ascites tumor cells

The cell membrane potential (PD) of Ehrlich ascites tumor cells was measured continuously at 37 degrees C with conventional microelectrodes during rapid alterations of extracellular fluid composition. At extracellular electrolyte composition mimicking the in vivo situation PD is -56.7 +/- 0.7 mV and the apparent membrane resistance is 62.2 +/- 2.2 M omega. Increasing extracellular potassium concentration from 5.4 to 20.0 mmol/l depolarizes the cell membrane by +18.4 +/- 0.5 mV. Thus, the transference number for potassium (tk, apparent slope potassium conductance over slope membrane conductance) is 0.53 +/- 0.01. A significant correlation is observed between tk and PD: tk = -(0.014 +/- 0.001) [1/mV] X PD [mV] -(0.243 +/- 0.051). 0.7 mmol/l barium depolarizes the cell membrane by +28.2 +/- 0.7 mV, increases the apparent membrane resistance by a factor of 2.6 +/- 0.1 and abolishes the apparent potassium conductance. Reduction of extracellular sodium concentration from 141 to 21 mmol/l depolarizes the cell membrane by +3.1 +/- 1.3 mV. Similarly, 0.1 mmol/l amiloride depolarizes the cell membrane by +3.3 +/- 0.7 mV. Reduction of extracellular chloride concentration from 128 to 67 mmol/l hyperpolarizes the cell membrane by -2.5 +/- 0.2 mV. 1 mmol/l anthracene-9-COOH does not significantly alter PD. Temporary omission of glucose from the extracellular fluid has no appreciable effect on PD. In conclusion, PD of Ehrlich ascites tumor cells is in the range of other mammalian epithelial cells and is generated mainly by potassium diffusion, while the conductances to sodium and chloride appear to be small.

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.

Short-term effects of PTH on cultured rat osteoblasts: changes in membrane potential

The introduction of parathyroid hormone [bPTH (1-34)], 10(-8) M, into the medium of cultured rat osteoblasts results in rapid (less than 1 min) depolarization of the osteoblast membranes. Conventional and pH-sensitive microelectrodes were used to assess the mechanism underlying this change. PTH depolarized cell membrane independently of steady-state membrane potential (Vm). Blocking K+ conductance (Ba2+) and Ca2+-dependent K+ conductance (quinine) depolarized Vm by +13.1 +/- 4.6 (n = 6) and +14.8 +/- 6.7 mV (n = 6), respectively, and both abolished the effect of PTH on Vm. The rate of depolarization was reduced in low-Ca2+ medium. PTH inhibited low Na+-induced cell hyperpolarization, but intracellular pH was not altered by hormone addition. PTH-induced depolarization occurred even when the Na+-K+ pump was blocked with ouabain. A second slower response was seen in cells having a Vm lower than -60 mV, with an increase in negativity 5-15 min after hormone application. The results indicate that PTH rapidly modifies Vm by changes of K+ conductance, which may be the first step in hormonal stimulus-response coupling, and induces delayed, long-term changes in cell status.

Effects of ouabain and temperature on cell membrane potentials in isolated perfused straight proximal tubules of the mouse kidney

In isolated perfused segments of the mouse proximal tubule, the potential difference across the basolateral cell membrane (PDbl) was determined with conventional microelectrodes. Under control conditions with symmetrical solutions it amounted to -62 +/- 1 mV (n = 118). The potential difference across the epithelium (PDte) was -1.7 +/- 0.1 mV (n = 45). Transepithelial resistance amounted to 1.82 +/- 0.09 k omega cm (n = 28), corresponding to 11.4 +/- 0.6 omega cm2. Increasing bath potassium concentration from 5 to 20 mmol/l depolarized PDbl by +24 +/- 1 mV (n = 103), and PDte by +1.6 +/- 0.1 mV (n = 19). Thus, the basolateral cell membrane is preferably conductive to potassium. Rapid cooling of the bath perfusate from 38 degrees C to 10 degrees C led to a transient hyperpolarization of PDbl from -60 +/- 1 to -65 +/- 1 mV (n = 21) within 40 s followed by gradual depolarization by +18 +/- 1% (n = 14) within 5 min. The transepithelial resistance increased significantly from 1.78 +/- 0.11 k omega cm to 2.20 +/- 0.21 k omega cm (n = 15). Rapid rewarming of the bath to 38 degrees C caused a depolarization from -61 +/- 2 mV (n = 17) to -43 +/- 2 mV (n = 16) within 15 s followed by a repolarization to -59 +/- 2 mV (n = 10) within 40 s. Ouabain invariably depolarized PDbl. During both, sustained cooling or application of ouabain, the sensitivity of PDbl to bath potassium concentration decreased in parallel to PDbl pointing to a gradual decrease of potassium conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical properties of Madin-Darby-canine-kidney cells. Effects of extracellular sodium and calcium

In incompletely confluent Madin Darby canine kidney cells continuous measurements of the potential difference across the cell membrane (PD) were made with conventional microelectrodes during rapid changes of extracellular sodium and/or calcium concentration. During control conditions PD averages -50.6 +/- 0.7 mV. Reduction of extracellular sodium concentration from 131.8 to 17.8 mmol/l leads to a reversible hyperpolarization of the cell membrane to -65.3 +/- 1.1 mV. This hyperpolarization is not significantly reduced by omission of glucose or presence of amiloride (1 mmol/l) in the perfusates. Instead, 1 mmol/l amiloride depolarizes the cell membrane by +5.2 +/- 0.4 mV. 1 mmol/l barium depolarizes the cell membrane to -31.3 +/- 1.1 mV. Step increases of extracellular potassium concentration from 5.4 to 10 and 20 mmol/l depolarize the cell membrane by +5.5 +/- 0.5 mV and +16.5 +/- 1.8 mV respectively. In the presence of barium, the depolarizing effect of increasing extracellular potassium concentration and of amiloride is almost abolished. Reduction of extracellular sodium concentration in the presence of barium, however, leads to a transient hyperpolarization of the cell membrane. During this transient hyperpolarization, increasing extracellular potassium concentration depolarizes the cell membrane despite the continued presence of barium. Omission of extracellular calcium (EDTA) depolarizes the cell membrane by +36.7 +/- 3.2 mV. In the absence of extracellular calcium, the hyperpolarizing effect of reduced extracellular sodium concentration is markedly reduced (-4.5 +/- 1.2 mV). 2 mumol/l A23187 in the presence of extracellular calcium hyperpolarizes the cell membrane to -72.5 +/- 0.6 mV.(ABSTRACT TRUNCATED AT 250 WORDS)