Cloning, expression, and localization of a rat hepatocyte inwardly rectifying potassium channel

Defective bicarbonate reabsorption in Kir4.2 potassium channel deficient mice impairs acid-base balance and ammonia excretion

The kidneys excrete the daily acid load mainly by generating and excreting ammonia but the underlying molecular mechanisms are not fully understood. Here we evaluated the role of the inwardly rectifying potassium channel subunit Kir4.2 (Kcnj15 gene product) in this process. In mice, Kir4.2 was present exclusively at the basolateral membrane of proximal tubular cells and disruption of Kcnj15 caused a hyperchloremic metabolic acidosis associated with a reduced threshold for bicarbonate in the absence of a generalized proximal tubule dysfunction. Urinary ammonium excretion rates in Kcnj15- deleted mice were inappropriate to acidosis under basal and acid-loading conditions, and not related to a failure to acidify urine or a reduced expression of ammonia transporters in the collecting duct. In contrast, the expression of key proteins involved in ammonia metabolism and secretion by proximal cells, namely the glutamine transporter SNAT3, the phosphate-dependent glutaminase and phosphoenolpyruvate carboxykinase enzymes, and the sodium-proton exchanger NHE-3 was inappropriate in Kcnj15-deleted mice. Additionally, Kcnj15 deletion depolarized the proximal cell membrane by decreasing the barium-sensitive component of the potassium conductance and caused an intracellular alkalinization. Thus, the Kir4.2 potassium channel subunit is a newly recognized regulator of proximal ammonia metabolism. The kidney consequences of its loss of function in mice support the proposal for KCNJ15 as a molecular basis for human isolated proximal renal tubular acidosis.

Differential expression of TRPM7 in rat hepatoma and embryonic and adult hepatocytes

TRPM7 channels are implicated in cellular survival, proliferation, and differentiation. However, a profile of TRPM7 activity in a specific cell type has not been determined from embryonic to terminally differentiated state. Here, we characterized TRPM7 expression in a spectrum of rat liver cells at different developmental stages. Using the whole-cell patch clamp technique, TRPM7-like Na+ currents were identified in RLC-18 cells, a differentiated, proliferating hepatocellular line derived from day 17 embryonic rat liver. Currents were outwardly rectifying, enhanced in divalent-free solutions, and inhibited by intracellular Mg2+. Reverse transcription – polymerase chain reaction (RT–PCR) revealed that RLC-18 cells express both TRPM6 and TRPM7. However, mean currents were reduced almost 80% by 1 mmol/L 2-aminoethoxyphenylborate (2-APB) and were abolished in RLC-18 cells heterologously expressing a dominant negative TRPM7 construct, suggesting that TRPM7 is the major current carrier in these cells. Functional comparison showed that relative to terminally differentiated adult rat hepatocytes, currents were 1.8 and 3.9 times higher in, respectively, RLC-18 and WIF-B cells, a rat hepatoma – human fibroblast cross. Our results demonstrate that plasma membrane TRPM7 channels are more highly expressed in proliferating cells as compared with terminally differentiated and nondividing rat hepatocytes and suggest that downregulation of this channel is associated with hepatocellular differentiation.

Potassium-dependent activation of Kir4.2 K⁺ channels

The inwardly rectifying potassium channel Kir4.2 is sensitive to changes in the extracellular potassium concentration ([K(+)](o)). This form of regulation is manifest as a slow (tens of minutes) increase in the whole-cell currents when [K(+)](o) is increased. Here we have investigated the mechanism of K(o)(+) sensitivity of Kir4.2 expressed in Xenopus oocytes. Using two-electrode voltage clamp we found that the sensitivity is specific for the homomeric form of the channel and is completely abolished when coexpressed with Kir5.1. Furthermore, unlike Kir1.1, there is no coupling between the intracellular pH sensitivity and K(o)(+) sensitivity, as is evident by introducing a mutation (K66M), which greatly decreases the pH(i) sensitivity while the K(o)(+) sensitivity remains unchanged. K(o)(+)-dependent activation of Kir4.2 does not involve an increase in the surface expression of the channel, nor is there a difference in the open probability between high and low [K(+)] as determined through patch-clamp measurements. We also found that there is an inverse relationship between the rates of both activation and deactivation and [K(+)](o). Using a kinetic model we argue that Kir4.2 exists in at least three states at the plasma membrane: a deactivated state, an intermediate unstable state and an active state, and that [K(+)](o) affects the rate of transition from the intermediate state to the active state.

Kir5.1 underlies long-lived subconductance levels in heteromeric Kir4.1/Kir5.1 channels from Xenopus tropicalis

The inwardly-rectifying potassium channel subunit Kir5.1 selectively co-assembles with members of the Kir4.0 subfamily to form novel pH-sensitive heteromeric channels with unique single channel properties. In this study, we have cloned orthologs of Kir4.1 and Kir5.1 from the genome of the amphibian, Xenopus tropicalis (Xt). Heteromeric XtKir4.1/XtKir5.1 channels exhibit similar macroscopic current properties to rat Kir4.1/Kir5.1 with a faster time-dependent rate of activation. However, single channel analysis of heteromeric XtKir4.1/XtKir5.1 channels reveals that they have markedly different long-lived, multi-level subconductance states. Furthermore, we demonstrate that the XtKir5.1 subunit is responsible for these prominent subconductance levels. These results are consistent with a model in which the slow transitions between sublevel states represent the movement of individual subunits. These novel channels now provide an excellent model system to determine the structural basis of subconductance levels and contribution of heteromeric pore architecture to this process.

Mg2+- and MgATP-inhibited and Ca2+/calmodulin-sensitive TRPM7-like current in hepatoma and hepatocytes

Although understood to be ubiquitously expressed, the functional identification and significance of Mg(2+)-inhibited, nonspecific cation currents has been established in only a few cell types. Here we identified an outwardly rectifying nonspecific cation current in quiescent rat hepatocytes and the proliferating and polarized rat hepatoma, WIF-B. Under whole cell recording conditions in which cells were bathed and dialyzed with Na-gluconate solutions, the latter Ca(2+) and Mg(2+) free, current reversed close to 0 mV, was time independent, and was greater than 10 times higher at +120 mV compared with -120 mV. Outward current at -120 mV developed slowly, from 17.7 +/- 10.3 pA/pF at patch rupture to 106.6 +/- 15.6 pA/pF at 12 min in WIF-B cells, and 4.9 +/- 2.7 to 20.6 +/- 5.6 pA/pF in rat hepatocytes. The nonspecific TRP channel inhibitor, 2-aminoethoxyphenylborate (2-APB), inhibited current (IC(50) = 72 +/- 13 microM) and caused apoptotic cell death in WIF-B cells. Rat hepatocyte survival was more resistant to 2-APB. Dialysis of WIF-B cells with physiological concentrations of Mg(2+) and Mg-ATP, but not ATP alone, inhibited current development, suggesting that Trpm7 rather than Trpm6 underlies this current. RT-PCR demonstrated that both Trpm6 and Trpm7 are expressed at similar levels in both cell types, suggesting that the functional differences noted are not transcript dependent. Intracellular Ca(2+) (IC(50) = 125 +/- 35 nM) also inhibited current development, and this could be partially relieved by the calmodulin and Ca(2+)/calmodulin-dependent kinase inhibitors W-7, staurosporine, KN-93, or calmodulin kinase II (CaMKII) inhibitory peptide. To summarize, our results show that in addition to their established Mg(2+) sensitivity, Trpm7-like channels are inhibited by cytosolic Ca(2+) in a CaMKII-dependent manner and may support hepatocellular survival during proliferation.

Amiodarone pretreatment of organ donors exerts anti-oxidative protection but induces excretory dysfunction in liver preservation and reperfusion

The continuous shortage of organs necessitates the use of marginal organs from donors with various diseases, including arrhythmia-associated cardiac failure. One of the most frequently used anti-arrhythmic drugs is amiodarone (AM), which is given in particular in emergency situations. Apart from its anti-arrhythmic actions, AM provides anti-oxidative properties in cardiomyocytes. Thus, we were interested in whether AM donor pretreatment affects the organ quality and function of livers procured for preservation and transplantation. Donor rats were pretreated with AM (5 mg/kg of body weight) 10 minutes before flush-out of the liver with a cold (4 degrees C) histidine-tryptophan-ketoglutarate solution (n = 8). Livers were then stored for 24 hours at 4 degrees C before ex situ reperfusion with a 37 degrees C Krebs-Henseleit solution for 60 minutes in a nonrecirculating system. At the end of reperfusion, tissue samples were taken for histology and Western blot analysis. Animals with vehicle only (0.9% NaCl) served as ischemia/reperfusion controls (n = 8). Additionally, livers of untreated animals (n = 8) not subjected to 24 hours of cold ischemia served as sham controls. AM pretreatment effectively attenuated lipid peroxidation, stress protein expression, and apoptotic cell death. This was indicated by an AM-mediated reduction of malondialdehyde, heme oxygenase-1, and caspase-3 activation. However, AM treatment also induced mitochondrial damage and hepatocellular excretory dysfunction, as indicated by a significantly increased glutamate dehydrogenase concentration in the effluate and decreased bile production. In conclusion, AM donor pretreatment exerts anti-oxidative actions in liver preservation and reperfusion. However, these protective AM actions are counteracted by an induction of mitochondrial damage and hepatocellular dysfunction. Accordingly, AM pretreatment of donors for anti-arrhythmic therapy should be performed with caution.

MUPP1 complexes renal K+ channels to alter cell surface expression and whole cell currents

We previously found that the Ca(2+)-sensing receptor (CaR) interacts with and inactivates the inwardly rectifying K(+) channel Kir4.2 that is expressed in the kidney cortex and that has a COOH-terminal PDZ domain. To identify potential scaffolding proteins that could organize a macromolecular signaling complex involving the CaR and Kir4.2, we used yeast two-hybrid cloning with the COOH-terminal 125 amino acids (AA) of Kir4.2 as bait to screen a human kidney cDNA library. We identified two independent partial cDNAs corresponding to the COOH-terminal 900 AA of MUPP1, a protein containing 13 PDZ binding domains that is expressed in the kidney in tight junctions and lateral borders of epithelial cells. When expressed in human embryonic kidney (HEK)-293 cells, Kir4.2 coimmunoprecipitates reciprocally with MUPP1 but not with a Kir4.2 construct lacking the four COOH-terminal amino acids, Kir5.1, or the CaR. MUPP1 and Kir4.2 coimmunoprecipitate reciprocally from rat kidney cortex extracts. Coexpression of MUPP1 with Kir4.2 in HEK-293 cells leads to reduced cell surface expression of Kir4.2 as assessed by cell surface biotinylation. Coexpression of MUPP1 and Kir4.2 in Xenopus oocytes results in reduced whole cell currents compared with expression of Kir4.2 alone, whereas expression of Kir4.2DeltaPDZ results in minimal currents and is not affected by coexpression with MUPP1. Immunofluorescence studies of oocytes demonstrate that MUPP1 reduces Kir4.2 membrane localization. These results indicate that Kir4.2 interacts selectively with MUPP1 to affect its cell surface expression. Thus MUPP1 and Kir4.2 may participate in a protein complex in the nephron that could regulate transport of K(+) as well as other ions.

Expression of inwardly rectifying potassium channel subunits in native human retinal pigment epithelium

Previously, we demonstrated that the inwardly rectifying K(+) (Kir) channel subunit Kir7.1 is highly expressed in bovine and human retinal pigment epithelium (RPE). The purpose of this study was to determine whether any of the 14 other members of the Kir gene family are expressed in native human RPE. Conventional reverse transcription-polymerase chain reaction (RT-PCR) analysis indicated that in addition to Kir7.1, seven other Kir channel subunits (Kir1.1, Kir2.1, Kir2.2, Kir3.1, Kir3.4, Kir4.2 and Kir6.1) are expressed in the RPE, whereas in neural retina, all 14 of the Kir channel subunits examined are expressed. The identities of RT-PCR products in the RPE were confirmed by DNA sequencing. Real-time RT-PCR analysis showed, however, that transcripts of these channels are significantly less abundant than Kir7.1 in the RPE. Western blot analysis of the Kir channel subunits detected in the RPE by RT-PCR revealed the expression of Kir2.1, Kir3.1, Kir3.4, Kir4.2, Kir6.1, and possibly Kir2.2, but not Kir1.1, in both human RPE and neural retina. Our results indicate that human RPE expresses at least five other Kir channel subtypes in addition to Kir7.1, suggesting that multiple members of the Kir channel family may function in this epithelium.

Interaction of the Ca2+-sensing receptor with the inwardly rectifying potassium channels Kir4.1 and Kir4.2 results in inhibition of channel function

The Ca(2+)-sensing receptor (CaR), a G protein-coupled receptor, is expressed in many epithelial tissues including the parathyroid glands, kidney, and GI tract. Although its role in regulating PTH levels and Ca(2+) metabolism are best characterized, it may also regulate salt and water transport in the kidney as demonstrated by recent reports showing association of potent gain-of-function mutations in the CaR with a Bartter-like, salt-wasting phenotype. To determine whether this receptor interacts with novel proteins that control ion transport, we screened a human adult kidney cDNA library with the COOH-terminal 219 amino acid cytoplasmic tail of the CaR as bait using the yeast two-hybrid system. We identified two independent clones coding for approximately 125 aa from the COOH terminus of the inwardly rectifying K(+) channel, Kir4.2. The CaR and Kir4.2 as well as Kir4.1 (another member of Kir4 subfamily) were reciprocally coimmunoprecipitated from HEK-293 cells in which they were expressed, but the receptor did not coimmunoprecipitate with Kir5.1 or Kir1.1. Both Kir4.1 and Kir4.2 were immunoprecipitated from rat kidney extracts with the CaR. In Xenopus laevis oocytes, expression of the CaR with either Kir4.1 or Kir4.2 channels resulted in inactivation of whole cell current as measured by two-electrode voltage clamp, but the nonfunctional CaR mutant CaR(R796W), and that does not coimmunoprecipitate with the channels, had no effect. Kir4.1 and the CaR were colocalized in the basolateral membrane of the distal nephron. The CaR interacts directly with Kir4.1 and Kir4.2 and can decrease their currents, which in turn could reduce recycling of K(+) for the basolateral Na(+)-K(+)-ATPase and thereby contribute to inhibition of Na(+) reabsorption.

Modulation of hepatocellular swelling-activated K+currents by phosphoinositide pathway-dependent protein kinase C

K+channels participate in the regulatory volume decrease (RVD) accompanying hepatocellular nutrient uptake and bile formation. We recently identified KCNQ1 as a molecular candidate for a significant fraction of the hepatocellular swelling-activated K+current ( IKVol). We have shown that the KCNQ1 inhibitor chromanol 293B significantly inhibited RVD-associated K+flux in isolated perfused rat liver and used patch-clamp techniques to define the signaling pathway linking swelling to IKVolactivation. Patch-electrode dialysis of hepatocytes with solutions that maintain or increase phosphatidylinositol 4,5-bisphosphate (PIP2) increased IKVol, whereas conditions that decrease cellular PIP2decreased IKVol. GTP and AlF4−stimulated IKVoldevelopment, suggesting a role for G proteins and phospholipase C (PLC). Supporting this, the PLC blocker U-73122 decreased IKVoland inhibited the stimulatory response to PIP2or GTP. Protein kinase C (PKC) is involved, because K+current was enhanced by 1-oleoyl-2-acetyl- sn-glycerol and inhibited after chronic PKC stimulation with phorbol 12-myristate 13-acetate (PMA) or the PKC inhibitor GF 109203X. Both IKVoland the accompanying membrane capacitance increase were blocked by cytochalasin D or GF 109203X. Acute PMA did not eliminate the cytochalasin D inhibition, suggesting that PKC-mediated IKVolactivation involves the cytoskeleton. Under isotonic conditions, a slowly developing K+current similar to IKVolwas activated by PIP2, lipid phosphatase inhibitors to counter PIP2depletion, a PLC-coupled α1-adrenoceptor agonist, or PKC activators and was depressed by PKC inhibition, suggesting that hypotonicity is one of a set of stimuli that can activate IKVolthrough a PIP2/PKC-dependent pathway. The results indicate that PIP2indirectly activates hepatocellular KCNQ1-like channels via cytoskeletal rearrangement involving PKC activation.

Contribution of a time-dependent and hyperpolarization-activated chloride conductance to currents of resting and hypotonically shocked rat hepatocytes

Hepatocellular Cl- flux is integral to maintaining cell volume and electroneutrality in the face of the many transport and metabolic activities that describe the multifaceted functions of these cells. Although a significant volume-regulated Cl- current (VRAC) has been well described in hepatocytes, the Cl- channels underlying the large resting anion conductance have not been identified. We used a combination of electrophysiological and molecular approaches to describe potential candidates for this conductance. Anion currents in rat hepatocytes and WIF-B and HEK293T cells were measured under patch electrode-voltage clamp. With K+-free salts of Cl- comprising the major ions externally and internally, hyperpolarizing steps between -40 and -140 mV activated a time-dependent inward current in hepatocytes. Steady-state activation was half-maximal at -63 mV and 28-38% of maximum at -30 to -45 mV, previously reported hepatocellular resting potentials. Gating was dependent on cytosolic Cl-, shifting close to 58 mV/10-fold change in Cl- concentration. Time-dependent inward Cl- currents and a ClC-2-specific RT-PCR product were also observed in WIF-B cells but not HEK293T cells. All cell types exhibited typical VRAC in response to dialysis with hypertonic solutions. DIDS (0.1 mM) inhibited the hepatocellular VRAC but not the inward time-dependent current. Antibodies against the COOH terminus of ClC-2 reacted with a protein between 90 and 100 kDa in liver plasma membranes. The results demonstrate that rat hepatocytes express a time-dependent inward Cl- channel that could provide a significant depolarizing influence in the hepatocyte.

Inhibitory effects of berberine on ion channels of rat hepatocytes

AIM: To examine the effects of berberine, an isoquinoline alkaloid with a long history used as a tonic remedy for liver and heart, on ion channels of isolated rat hepatocytes.

METHODS: Tight-seal whole-cell patch-clamp techniques were performed to investigate the effects of berberine on the delayed outward potassium currents (I_K), inward rectifier potassium currents (I_K1) and Ca²⁺ release-activated Ca²⁺ currents (I_CRAC) in enzymatically isolated rat hepatocytes.

RESULTS: Berberine 1-300 μmol/L reduced I_K in a concentration-dependent manner with EC₅₀ of 38.86 ± 5.37 μmol/L and n_H of 0.82 ± 0.05 (n = 8). When the bath solution was changed to tetraethylammonium (TEA) 8 mmol/L, I_K was inhibited. Berberine 30 μmol/L reduced I_K at all examined membrane potentials, especially at potentials positive to +60 mV (n = 8, P < 0.05 or P < 0.01 vs control). Berberine had mild inhibitory effects on I_K1 in rat hepatocytes. Berberine 1-300 μmol/L also inhibited I_CRAC in a concentration-dependent fashion. The fitting parameters were EC₅₀ = 47.20 ± 10.86 μmol/L, n_H = 0.71 ± 0.09 (n = 8). The peak value of I_CRAC in the I-V relationship was decreased by berberine 30 μmol/L at potential negative to -80 mV (n = 8, P < 0.05 vs control). But the reverse potential of I_CRAC occurred at voltage 0 mV in all cells.

CONCLUSION: Berberine has inhibitory effects on potassium and calcium currents in isolated rat hepatocytes, which may be involved in hepatoprotection.

Chromosome 21 KIR channels in brain development

Two KIR (K+ Inwardly Rectifying) channel genes have been identified on chromosome 21, in a region associated with important phenotypic features of trisomy 21, including mental retardation: KIR3.2 (GIRK2) and KIR4.2. We analysed the expression of these channel genes in developing human and mouse brains to determine the possible role of the corresponding channels in brain development and function. KIR3.2, which has been extensively studied in the mouse, was found to be expressed in the human cerebellum during development. The KIR4.2 channel is expressed later in development in both mice and humans. We compared the expression of these channels in terms of RNA and protein levels and discussed the potential synergy and consequences of the overexpression of these channels in Down's syndrome brain development.

Effects of palmatine on potassium and calcium currents in isolated rat hepatocytes

AIM: To study the effects of palmatine, a known inhibitor on delayed rectifier potassium current and L-type calcium current (I_Ca,L) in guinea pig ventricular myocytes, on the potassium and calcium currents in isolated rat hepatocytes.

METHODS: Tight-seal whole-cell patch-clamp techniques were performed to investigate the effects of palmatine on the delayed outward potassium currents (I_K), inward rectifier potassium current (I_K1) and Ca²⁺ release-activated Ca²⁺ current (I_CRAC) in enzymatically isolated rat hepatocytes.

RESULTS: Palmatine 0.3-100 μM reduced I_K in a concentration-dependent manner with EC₅₀ of 41.62 ± 10.11 μM and nH, 0.48 ± 0.07 (n = 8). The effect of the drug was poorly reversible after washout. When the bath solution was changed to tetraethylammonium (TEA) 8 mM, I_K was inhibited. Palmatine 10 μM and 100 μM shifted the I-V curves of I_K downward, and the block of I_K was voltage-independent. Palmatine 0.3-100 μM also inhibited I_CRAC in a concentration-dependent manner. The fitting parameters were as follows: EC₅₀ = 51.19 ± 15.18 mM, and n_H = 0.46 ± 0.07 (n = 8). The peak value of I_CRAC in the I-V relationship was decreased by palmatine 10 μM and 100 μM. But the reverse potential of I_CRAC occurred at Voltage = 0 mV in all cells. Palmatine 0.3-100 μM failed to have any significant effect on either inward or outward components of I_K1 at any membrane potential examined.

CONCLUSION: The inhibitory effects on I_K and I_CRAC could be one of the mechanisms that palmatine exerts protective effect on hepatocytes.