A calcium-permeable channel in the apical membrane of primary cultures of the rabbit distal bright convoluted tubule

Modeling Distal Convoluted Tubule (Patho)Physiology: An Overview of Past Developments and an Outlook Toward the Future

The kidneys are essential for maintaining electrolyte homeostasis. Blood electrolyte composition is controlled by active reabsorption and secretion processes in dedicated segments of the kidney tubule. Specifically, the distal convoluted tubule (DCT) and connecting tubule are important for regulating the final excretion of sodium, magnesium, and calcium. Studies unravelling the specific function of these segments have greatly improved our understanding of DCT (patho)physiology. Over the years, experimental models used to study the DCT have changed and the field has advanced from early dissection studies with rats and rabbits to the use of various transgenic mouse models. Developments in dissection techniques and cell culture methods have resulted in immortalized mouse DCT cell lines and made it possible to specifically obtain DCT fragments for ex vivo studies. However, we still do not fully understand the complex (patho)physiology of this segment and there is need for advanced human DCT models. Recently, kidney organoids and tubuloids have emerged as new complex cell models that provide excellent opportunities for physiological studies, disease modeling, drug discovery, and even personalized medicine in the future. This review presents an overview of cell models used to study the DCT and provides an outlook on kidney organoids and tubuloids as model for DCT (patho)physiology. Impact statement This study provides a detailed overview of past and future developments on cell models used to study kidney (patho)physiology and specifically the distal convoluted tubule (DCT) segment. Hereby, we highlight the need for an advanced human cell model of this segment and summarize recent advances in the field of kidney organoids and tubuloids with a focus on DCT properties. The findings reported in this review are significant for future developments toward an advanced human model of the DCT that will help to increase our understanding of DCT (patho)physiology.

New evidence of a dihydropyridine-activated cationic channel in the MDCK cell line

Newborn rat distal cells express an apical Ca2+ channel activated by dihydropyridine drugs. Similarly, in Madin-Darby canine kidney (MDCK) cells, nifedipine increased Ca2+i in a concentration-dependent manner (IC50=4 μM) in fura-2-loaded cells. Response to nifedipine was abolished by EGTA, suggesting that it depends on extracellular calcium. Ca2+ channel antagonist isradipine and agonist BayK8644 increased Ca2+i indicating that this effect is related to the dihydropyridine group. Diltiazem (20 μM) and gadolinium (200 μM) decreased the nifedipine effect (62 and 43%, respectively). Lanthanum (100 μM) did not change the response. Valinomycin clamping of the membrane potential did not modify nifedipine-induced increment, indicating that it was unrelated to potassium fluxes. We performed whole cell clamp experiments in MDCK cells maintained at -50 mV with perfusion solution containing 10 mM CaCl2. Nifedipine (20 μM) induced an increase in current (1.2±0.3 nA), which was partially inhibited by Gd3+. No significant current was induced by nifedipine in the presence of 0.5 mM EGTA. To determine the effects of nifedipine on the membrane potential, we performed oxonol fluorescence experiments. The addition of nifedipine or Bay K8644 induced depolarization, highly dependent on external sodium. Nifedipine (20 μM) induced depolarization of 6.9±0.8 mV (n=21). EC50 to nifedipine was in the 10 μM range. We conclude that MDCK cells exhibit a dihydropyridine-activated cationic channel.

Characterization of mechanisms for Ca2+ and HCO3(-)/CO3(2-) acquisition for shell formation in embryos of the freshwater common pond snail Lymnaea stagnalis

The freshwater common pond snail Lymnaea stagnalis produces embryos that complete direct development, hatching as shell-bearing individuals within 10 days despite relatively low ambient calcium and carbonate availability. This development is impaired by removal of ambient total calcium but not by removal of bicarbonate and/or carbonate. In this study we utilized pharmacological agents to target possible acquisition pathways for both Ca(2+) and accumulation of carbonate in post-metamorphic, shell-laying embryos. Using whole egg mass flux measurements and ion-specific microelectrode analytical techniques, we have demonstrated that carbonic anhydrase-catalyzed hydration of CO(2) is central in the acquisition of both shell-forming ions because it provides the hydrogen ions for an electrogenic vacuolar-type H(+)-ATPase that fuels the uptake of Ca(2+) via voltage-dependent Ca(2+) channels and possibly an electrogenic Ca(2+)/1H(+) exchanger. Additionally, CO(2) hydration provides an endogenous source of HCO(3)(-). Thus, hydration of endogenous CO(2) forms HCO(3)(-) for calcification while hydrogen ions are excreted, contributing to continued Ca(2+) uptake, as well as creating favorable alkaline internal conditions for calcification. The connections between Ca(2+) and HCO(3)(-) acquisition mechanisms that we describe here provide new insight into this efficient, embryonic calcification in freshwater.

Variable effects of the mitoK(ATP) channel modulators diazoxide and 5-HD in ATP-depleted renal epithelial cells

The role of mitochondrial K(ATP) (mitoK(ATP)) channels in renal ischemia-reperfusion injury is controversial with studies showing both protective and deleterious effects. In this study, we compared the effects of the putative mitoK(ATP) opener, diazoxide, and the mitoK(ATP) blocker, 5-hydroxydecanoate (5-HD) on cytotoxicity and apoptosis in tubular epithelial cells derived from rat (NRK-52E) and pig (LLC-PK1) following in vitro ischemic injury. Following ATP depletion-recovery, there was a significant increase in cytotoxicity in both NRK cells and LLC-PK1 cells although NRK cells were more sensitive to the injury. Diazoxide treatment attenuated cytotoxicity in both cell types and 5-HD treatment-increased cytotoxicity in the sensitive NRK cells in a superoxide-dependant manner. The protective effect of diazoxide was also reversed in the presence of 5-HD in ATP-depleted NRK cells. The ATP depletion-mediated increase in superoxide was enhanced by both diazoxide and 5-HD with the effect being more pronounced in the cells undergoing 5-HD treatment. Further, ATP depletion-induced activation of caspase-3 was decreased by diazoxide in NRK cells. In order to determine the signaling pathways involved in apoptosis, we examined the activation of Erk and JNK in ATP-depleted NRK cells. Diazoxide-activated Erk in ATP-depleted cells, but did not have any effect on JNK activation. In contrast, 5-HD did not impact Erk levels but increased JNK activation even under controlled conditions. Further, the use of a JNK inhibitor with 5-HD reversed the deleterious effects of 5-HD. This study demonstrates that in cells that are sensitive to ATP depletion-recovery, mitoK(ATP) channels protect against ATP depletion-mediated cytotoxicity and apoptosis through Erk- and JNK-dependant mechanisms.

Lanthanum Effect on the Dynamics of Tight Junction Opening and Closing

We present a comparative study in frog urinary bladders (FUB) and A6 cell monolayers (A6CM) on the effect of La3+ on tight junction (TJ) dynamics. These tissues react similarly to changes of basolateral Ca2+ (Ca(2+)bl), while responding differently to the action of La3+(bl). In FUB, La(3+)bl shows a Ca(2+)-antagonistic effect that promotes TJ opening in the presence of a normal Ca(2+)bl concentration. In A6CM, in contrast, La(3+)bl always shows a clear Ca(2+)-agonistic effect. The fact that a concentration of La(3+)bl one fifth of the normal Ca(2+)bl leads in FUB to TJ opening and in A6CM to a complete recovery of the TJ seal indicates a high affinity of La3+ for the Ca(2+)-binding sites in both tissues. In FUB, apical La3+ (La(3+)ap) exhibits, differently from its basolateral effect, an evident Ca(2+)-agonistic effect, suggesting a dual effect of La3+, depending on which side of the bladder La3+ is applied. In A6CM La(3+)ap has a Ca(2+)-agonistic effect similar to La(3+)bl. The effects of La(3+)bl in FUB and in A6CM are consistent, according to our previous publications, with La3+ acting antagonistically or agonistically, respectively, on the Ca2+ binding sites of zonula adhaerens. Despite the fact that the effect of La(3+)ap is clear in both tissues, its site of action is yet to be determined. Protonation of the Ca(2+)-binding sites causes a decrease of its agonistic effect on A6CM, consistent with a negatively charged binding site. In A6CM La3+ apparently replaces Ca2+, mimicking the effect of Ca2+ triggering the cascade of events leading to TJ closure. In FUB, La3+ interacts with the binding sites, dislodging Ca2+, with a high affinity, but this interaction is inadequate to initiate or sustain the process of junction closing. Possibly, the difference between the two preparations resides in subtle conformation differences of the outer segment of E-cadherin molecules.

Characterization of three types of calcium channel in the luminal membrane of the distal nephron

We previously reported a dual kinetics of Ca2+transport by the distal tubule luminal membrane of the kidney, suggesting the presence of several types of channels. To better characterize these channels, we examined the effects of specific inhibitors (i.e., diltiazem, an L-type channel; ω-conotoxin MVIIC, a P/Q-type channel; and mibefradil, a T-type channel antagonist) on 0.1 and 0.5 mM Ca2+uptake by rabbit nephron luminal membranes. None of these inhibitors influenced Ca2+uptake by the proximal tubule membranes. In contrast, in the absence of sodium (Na+), the three channel antagonists decreased Ca2+transport by the distal membranes, and their action depended on the substrate concentrations: 50 µM diltiazem decreased 0.1 mM Ca2+uptake from 0.65 ± 0.07 to 0.48 ± 0.06 pmol·µg–1·10 s–1(P < 0.05) without influencing 0.5 mM Ca2+transport, whereas 100 nM ω-conotoxin MVIIC decreased 0.5 mM Ca2+uptake from 1.02 ± 0.05 to 0.90 ± 0.05 pmol·µg–1·10 s–1(P < 0.02) and 1 µM mibefradil decreased it from 1.13 ± 0.09 to 0.94 ± 0.09 pmol·µg–1·10 s–1(P < 0.05); the latter two inhibitors left 0.1 mM Ca2+transport unchanged. Diltiazem decreased the Vmaxof the high-affinity channels, whereas ω-conotoxin MVIIC and mibefradil influenced exclusively the Vmaxof the low-affinity channels. These results not only confirm that the distal luminal membrane is the site of Ca2+channels, but they suggest that these channels belong to the L, P/Q, and T types.Key words: renal calcium transport, calcium channels, diltiazem, mibefradil, ω-conotoxin.

Plasma membrane Ca2+-ATPase and NCX1 Na+/Ca2+ exchanger expression in distal convoluted tubule cells

The plasma membrane Ca2+-ATPase (PMCA) and the NCX1 Na+/Ca2+ exchanger regulate intracellular Ca2+ concentrations and mediate Ca2+ efflux in absorptive epithelial cells. We characterized the PMCA isoforms and subtypes expressed in mouse distal convoluted tubule (mDCT) cells and Na+/Ca2+ exchanger protein expression in mDCT cells. In lysates of mDCT cells, immunoprecipitation and Western blot analysis, performed with a monoclonal antibody to PMCA, revealed a 140-kDa protein consistent with PMCA. Laser-scanning confocal fluorescence microscopy indicated that PMCA and NCX1 expression is restricted to basolateral membranes only in confluent mDCT cells, because subconfluent cultures predominately express intracellular localizations. PMCA isoform-specific PCR primers generated appropriately sized products only for PMCA1 and PMCA4 from DCT cells but PMCA1-4 from whole mouse kidney. Assessment of splice site C within the calmodulin-binding domain demonstrated the presence of PMCA1b and PMCA4b mRNAs in mDCT cells. Northern blot analysis of mDCT cell RNA revealed transcripts of 7.5 and 5.5 kb for PMCA1 and 8.5 and 7.5 kb for PMCA4. We conclude that DCT cells express PMCA transcripts encoding PMCA1b and PMCA4b. Basolateral localization of the Na+/Ca2+ exchanger and MCAs support the idea that multiple PMCA isoforms, in concert with the Na+/Ca2+ exchanger, mediate basal or hormone-stimulated Ca2+ efflux by distal tubules.

Role of aspartate residues in Ca(2+) affinity and permeation of the distal ECaC1

The Ca(2+) affinity and permeation of the epithelial Ca(2+) channel (ECaC1) were investigated after expression in Xenopus oocytes. ECaC1 displayed anomalous mole-fraction effects. Extracellular Ca(2+) and Mg(2+) reversibly inhibited ECaC1 whole cell Li(+) currents: IC(50) = 2.2 +/- 0.4 microM (n = 9) and 235 +/- 35 microM (n = 10), respectively. These values compare well with the Ca(2+) affinity of the L-type voltage-gated Ca(2+) (Ca(V)1.2) channel measured under the same conditions, suggesting that high-affinity Ca(2+) binding is a well-conserved feature of epithelial and voltage-gated Ca(2+) channels. Neutralization of D550 and E535 in the pore region had no significant effect on Ca(2+) and Mg(2+) affinities. In contrast, neutralization of D542 significantly decreased Ca(2+) affinity (IC(50) = 1.1 +/- 0.2 mM, n = 6) and Mg(2+) affinity (IC(50) > 25 +/- 3 mM, n = 4). Despite a 1,000-fold decrease in Ca(2+) affinity in D542N, Ca(2+) permeation properties and the Ca(2+)-to-Ba(2+) conductance ratio remained comparable to values for wild-type ECaC1. Together, our observations suggest that D542 plays a critical role in Ca(2+) affinity but not in Ca(2+) permeation in ECaC1.

Effect of angiotensin II on calcium reabsorption by the luminal membranes of the nephron

In the rat and the rabbit, a number of studies have reported the effects of angiotensin II (ANG II) on Na(+) reabsorption by the proximal (PT) and distal (DT) convoluted tubules of the kidney. The aim of the present study was to examine the effect of ANG II on Ca(2+) uptake by the luminal membranes of the PT and DT of the rabbit. Incubation of PT and DT with 10(-12) M ANG II enhanced the initial Ca(2+) uptake in the two segments. Dose-response experiments revealed, for Ca(2+) as well as for Na(+) transport, a biphasic action with a maximal effect at 10(-12) M. Ca(2+) transport by the DT luminal membrane presents a dual kinetic. ANG II action influenced the high-affinity Ca(2+) channel, increasing maximal velocity from 0.72 +/- 0.03 to 0.90 +/- 0.05 pmol x microg(-1) x 10 s(-1) (P < 0.05, n = 3) and leaving the Michaelis-Menten constant unchanged. The effect of ANG II was abolished by losartan, suggesting that the hormone is acting through AT1 receptors. In the PT, calphostin C inhibited the effect of the hormone. It is therefore probable that protein kinase C is involved as a messenger. In the DT, however, neither Rp cAMP, calphostin C, nor econazole (a phospholipase A inhibitor) influenced the hormone action. Therefore, the mechanisms involved in the hormone action remain undetermined. Finally, we questioned whether ANG II acts in the same DT segment as does parathyroid hormone on Ca(2+) transport. The two hormones increased Ca(2+) transport, but their actions were not additive, suggesting that they both influence the same channels in the same segment of the distal nephron, i.e., the segment responsible for the high-affinity calcium channel.

Altered expression of renal aquaporins and Na+ transporters in rats treated with L-type calcium blocker

Nifedipine, a calcium antagonist, has diuretic and natriuretic properties. However, the molecular mechanisms by which these effects are produced are poorly understood. We examined kidney abundance of aquaporins (AQP1, AQP2, and AQP3) and major sodium transporters [type 3 Na/H exchanger (NHE-3); type 2 Na-Pi cotransporter (NaPi-2); Na-K-ATPase; type 1 bumetanide-sensitive cotransporter (BSC-1); and thiazide-sensitive Na-Cl cotransporter (TSC)] as well as inner medullary abundance of AQP2, phosphorylated-AQP2 (p-AQP2), AQP3, and calcium-sensing receptor (CaR). Rats treated with nifedipine orally (700 mg/kg) for 19 days had a significant increase in urine output, whereas urinary osmolality and solute-free water reabsorption were markedly reduced. Consistent with this, immunoblotting revealed a significant decrease in the abundance of whole kidney AQP2 (47 ± 7% of control rats, P< 0.05) and in inner medullary AQP2 (60 ± 7%) as well as in p-AQP2 abundance (17 ± 6%) in nifedipine-treated rats. In contrast, whole kidney AQP3 abundance was significantly increased (219 ± 28%). Of potential importance in modulating AQP2 levels, the abundance of CaR in the inner medulla was significantly increased (295 ± 25%) in nifedipine-treated rats. Nifedipine treatment was also associated with increased urinary sodium excretion. Consistent with this, semiquantitative immunoblotting revealed significant reductions in the abundance of proximal tubule Na+ transporters: NHE-3 (3 ± 1%), NaPi-2 (53 ± 12%), and Na-K-ATPase (74 ± 5%). In contrast, the abundance of the distal tubule Na-Cl cotransporter (TSC) was markedly increased (240 ± 29%), whereas BSC-1 in the thick ascending limb was not altered. In conclusion, 1) increased urine output and reduced urinary concentration in nifedipine-treated-rats may, in part, be due to downregulation of AQP2 and p-AQP2 levels; 2) CaR might be involved in the regulation of water reabsorption in the inner medulla collecting duct; 3) reduced expression of proximal tubule Na+ transporters (NHE-3, NaPi-2, and Na, K-ATPase) may be involved in the increased urinary sodium excretion; and 4) increase in TSC expression may occur as a compensatory mechanism.

Nifedipine-activated Ca(2+) permeability in newborn rat cortical collecting duct cells in primary culture

To characterize Ca(2+) transport in newborn rat cortical collecting duct (CCD) cells, we used nifedipine, which in adult rat distal tubules inhibits the intracellular Ca(2+) concentration ([Ca(2+)](i)) increase in response to hormonal activation. We found that the dihydropyridine (DHP) nifedipine (20 microM) produced an increase in [Ca(2+)](i) from 87.6 +/- 3.3 nM to 389.9 +/- 29.0 nM in 65% of the cells. Similar effects of other DHP (BAY K 8644, isradipine) were also observed. Conversely, DHPs did not induce any increase in [Ca(2+)](i) in cells obtained from proximal convoluted tubule. In CCD cells, neither verapamil nor diltiazem induced any rise in [Ca(2+)](i). Experiments in the presence of EGTA showed that external Ca(2+) was required for the nifedipine effect, while lanthanum (20 microM), gadolinium (100 microM), and diltiazem (20 microM) inhibited the effect. Experiments done in the presence of valinomycin resulted in the same nifedipine effect, showing that K(+) channels were not involved in the nifedipine-induced [Ca(2+)](i) rise. H(2)O(2) also triggered [Ca(2+)](i) rise. However, nifedipine-induced [Ca(2+)](i) increase was not affected by protamine. In conclusion, the present results indicate that 1) primary cultures of cells from terminal nephron of newborn rats are a useful tool for investigating Ca(2+) transport mechanisms during growth, and 2) newborn rat CCD cells in primary culture exhibit a new apical nifedipine-activated Ca(2+) channel of capacitive type (either transient receptor potential or leak channel).

Human calcium transport protein CaT1

Transcellular calcium transport occurs in many epithelial tissues including intestine, kidney, and placenta. We identified the human ortholog (hCaT1) of a recently cloned rat calcium transport protein, CaT1, that mediates intestinal calcium uptake. hCaT1 messenger RNA is present in the gastrointestinal tract, including esophagus, stomach, duodenum, jejunum, ileum, and colon. High levels of hCaT1 transcripts are also present in pancreas, placenta, prostate, and salivary gland, while moderate levels are present in liver, kidney, and testis. hCaT1 mRNA is also expressed in the colorectal cancer cell line, SW480, and the chronic myelogenous leukemia cell line, K-562. The hCaT1 gene was assigned to the long arm of chromosome 7, bands q33-34, by fluorescence in situ hybridization. When expressed in Xenopus laevis oocytes, hCaT1 promotes saturable Ca(2+) uptake with a Michaelis constant of 0.25 mM. Our studies suggest a role for hCaT1 in cellular calcium uptake in a variety of tissues, including the transcellular calcium transport pathway in intestine.

A rat kidney-specific calcium transporter in the distal nephron

Active absorption of calcium from the intestine and reabsorption of calcium from the kidney are major determinants of whole body calcium homeostasis. Two recently cloned proteins, CaT1 and ECaC, have been postulated to mediate apical calcium uptake by rat intestine and rabbit kidney, respectively. By screening a rat kidney cortex library with a CaT1 probe, we isolated a cDNA encoding a protein (CaT2) with 84.2 and 73.4% amino acid identities to ECaC and CaT1, respectively. Unlike ECaC, CaT2 is kidney-specific in the rat and was not detected in intestine, brain, adrenal gland, heart, skeletal muscle, liver, lung, spleen, thymus, and testis by Northern analysis or reverse transcription polymerase chain reaction. The expression pattern of CaT2 in kidney was similar to that of calbindin D(28K) and the sodium calcium exchanger 1, NCX1, by in situ hybridization of adjacent sections. Furthermore, the mRNAs for CaT2 and calbindin D(28K) were colocalized in the same cells. CaT2 mediated saturable calcium uptake with a Michaelis constant (K(m)) of 0.66 mm when expressed in Xenopus laevis oocytes. Under voltage clamp condition, CaT2 promoted inward currents in X. laevis oocytes upon external application of Ca(2+). Sr(2+) and Ba(2+) but not Mg(2+) also evoked inward currents in CaT2-expressing oocytes. Similar to the alkaline earth metal ions, application of Cd(2+) elicited inward current in CaT2-expressing oocytes with a K(m) of 1.3 mm. Cd(2+), however, also potently inhibited CaT2-mediated Ca(2+) uptake with an IC(50) of 5.4 micrometer. Ca(2+) evoked currents were reduced at low pH and increased at high pH and were only slightly affected by the L-type voltage-dependent calcium channel antagonists, nifedipine, verapamil, diltiazem, and the agonist, Bay K 8644, even at relatively high concentrations. In conclusion, CaT2 may participate in calcium entry into the cells of the distal convoluted tubule and connecting segment of the nephron, where active reabsorption of calcium takes place via the transcellular route. The high sensitivity of CaT2 to Cd(2+) also provides a potential explanation for Cd(2+)-induced hypercalciuria and resultant renal stone formation.

Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy

The distal tubule of the mammalian kidney, defined as the region between the macula densa and the collecting duct, is morphologically and functionally heterogeneous. This heterogeneity has stymied attempts to define functional properties of individual cell types and has led to controversy concerning mechanisms and regulation of ion transport. Recently, molecular techniques have been used to identify and localize ion transport pathways along the distal tubule and to identify human diseases that result from abnormal distal tubule function. Results of these studies have clarified the roles of individual distal cell types. They suggest that the basic molecular architecture of the distal nephron is surprisingly similar in mammalian species investigated to date. The results have also reemphasized the role played by the distal tubule in regulating urinary potassium excretion. They have clarified how both peptide and steroid hormones, including aldosterone and estrogen, regulate ion transport by distal convoluted tubule cells. Furthermore, they highlight the central role that the distal tubule plays in systemic calcium homeostasis. Disorders of distal nephron function, such as Gitelman's syndrome, nephrolithiasis, and adaptation to diuretic drug administration, emphasize the importance of this relatively short nephron segment to human physiology. This review integrates molecular and functional results to provide a contemporary picture of distal tubule function in mammals.

Effect of calbindin D 28K on sodium transport by the luminal membrane of the rabbit nephron

We previously reported that in the rabbit, the vitamin D-dependent calcium binding protein 28K (CaBP 28K) increases calcium (Ca2+) transport in the distal tubule by opening a high affinity Ca2+ channel in the luminal membrane. Since Na+ and Ca2+ transports are interdependent in this membrane, we questioned whether the calbindin has any influence on Na+ transport. Luminal membranes from rabbit proximal and distal tubules were purified and 22Na uptake by the membrane vesicles was measured using the rapid filtration technique. The vesicles were loaded with 280 mM mannitol and 20 mM Tris-Hepes pH 7.4, with either 3 microM CaBP or the carrier. Incubation medium contained 1 mM 22NaCl, 278 mM mannitol, and 20 mM Tris-Hepes pH 7.4. The presence of 3 microM CaBP 28K in the distal luminal membrane vesicles increased the 0.5 mM Ca2+ uptake from 0.91 +/- 0.21 to 1.84 +/- 0.33 pmol/microg/10 s (P < 0.01) and decreased 1 mM Na+ uptake from 0.62 +/- 0.15 to 0.27 +/- 0.08 pmol/microg/10 s (P < 0.05). A similar decrease of Na+ uptake was observed in proximal luminal membrane experiments. The effect on Na+ uptake by the distal membrane was dose-dependent with a IC50 of 4.5 microM. Addition of 2 mM Ca2+ to the incubation medium decreased 1 mM Na + uptake from 0.62 +/- 0.15 to 0.49 +/- 0.12 pmol/microg/10 s (P < 0.05), but did not influence the effect of CaBP 28K on Na+ uptake. Experiments performed in the presence and absence of ethyl isopropyl amiloride (EIPA) suggest that the effect of calbindin involves the Na+/H+ exchanger activity.

Characterization of a membrane calcium pathway induced by rotavirus infection in cultured cells

Some viruses induce changes in membrane permeability during infection. We have shown previously that the porcine strain of rotavirus, OSU, induced an increase in the permeability to Na+, K+, and Ca2+ during replication in MA104 cells. In this work, we have characterized the divalent cation entry pathway by measuring intracellular Ca2+ in fura-2-loaded MA104 and HT29 cells in suspension. The permeability to Ca2+ and other cations was evaluated by the change of the intracellular concentration following an extracellular cation pulse. Rotavirus infection induced an increase in permeability to Ca2+, Ba2+, Sr2+, Mn2+, and Co2+. The rate of cation entry decreased over time as the intracellular concentration increased during the first 20 s. This indicates that regulatory mechanisms, including channel inactivation, are triggered. La3+ did not enter the cell and blocked the entry of the divalent cations in a dose-dependent manner. Metoxyverapamil (D600), a blocker of L-type voltage-gated channels, partially inhibited the entry of Ca2+ in virus-infected MA104 and HT29 cells. The results suggest that rotavirus infection of cultured cells activates a cation channel rather than nonspecific permeation through the plasma membrane. This activation involves the synthesis of viral proteins through mechanisms yet unknown. The increase in intracellular Ca2+ induced by the activation of this channel may be related to the increase in cytoplasmic and endoplasmic reticulum Ca2+ pools required for virus maturation and cell death.

Codependence of renal calcium and sodium transport

Calcium and sodium absorption by the kidney normally proceed in parallel. However, a number of physiological, pharmacological, pathological, and genetic conditions dissociate this relation. In each instance, the dissociation can be traced to the distal convoluted tubule, where calcium and sodium transport are inversely related. Based on the identification of the relevant sodium transporters in these cells and on analysis of the mechanism of calcium transport, an explanation for this inverse relation can be developed. Apical membrane calcium entry is mediated by voltage-sensitive calcium channels that are activated upon membrane hyperpolarization. Basolateral calcium efflux is effected primarily by Na+/Ca2+ exchange. According to the model, inhibition of sodium entry through either the Na-Cl cotransporter or the Na+ channel hyperpolarizes the cell, as does parathyroid hormone, thereby activating the calcium entry channel and increasing the driving force for diffusional entry. Membrane hyperpolarization also increases the driving force of calcium efflux through the Na+/Ca2+ exchanger. Thus sodium-dependent changes of calcium transport are indirect and occur secondarily through effects on membrane voltage.

Ca2+ transport by the luminal membrane of the distal nephron: action and interaction of protein kinases A and C

We previously reported that parathyroid hormone and calcitonin increase Ca2+ uptake by purified distal luminal membranes. This effect is mimicked by high concentrations of cAMP. However, both hormones stimulate adenylate cyclase and phospholipase C. The purpose of the present study was to investigate the role of the phospholipase C pathway in the hormone action, and the interrelationship between the two messengers. Distal tubules from rabbit kidneys were incubated with dibutyryl cAMP (dbcAMP) or PMA, or both, and Ca2+ uptake by purified luminal membranes was measured by the rapid filtration technique. Incubation of the distal tubules with 1 mM dbcAMP significantly increased Ca2+ transport by the luminal membranes. A dose-response curve showed a half-maximal stimulation with 0.82 mM dbcAMP. In contrast, treatment of the tubules with 10 nM, 100 nM or 1 microM PMA did not influence Ca2+ uptake by these membranes. However, the addition of 100 nM PMA to low concentrations of dbcAMP strongly increased this uptake. The presence of cAMP or protein kinase C inhibitors prevented the effects of either a high concentration of dbcAMP alone or a low concentration of dbcAMP combined with 100 nM PMA. Our laboratory has already reported that Ca2+ uptake by the distal luminal membranes displays two-component kinetics. dbcAMP increased the Vmax of the low-affinity component, whereas a combination of the two messengers stimulated the Vmax of both the low- and high-affinity components. From these results, we conclude that: (1) in the distal tubule cells, activation of both protein kinases A and C is necessary for the stimulation of Ca2+ transport by the luminal membrane; (2) the combined effect of protein kinases A and C involves both components of the Ca2+-transport kinetics.

Calcium site specificity. Early Ca2+-related tight junction events

The molecular mechanisms by which Ca2+ and metal ions interact with the binding sites that modulate the tight junctions (TJs) have not been fully described. Metal ions were used as probes of these sites in the frog urinary bladder. Basolateral Ca2+ withdrawal induces the opening of the TJs, a process that is abruptly terminated when Ca2+ is readmitted, and is followed by a complete recovery of the TJ seal. Mg2+ and Ba2+ were incapable of keeping the TJ sealed or of inducing TJ recovery. In addition, Mg2+ causes a reversible concentration-dependent inhibition of the Ca2+-induced TJ recovery. The effects of extracellular Ca2+ manipulation on the TJs apparently is not mediated by changes of cytosolic Ca2+ concentration. The transition elements, Mn2+ and Cd2+, act as Ca2+ agonists. In the absence of Ca2+, they prevent TJ opening and almost immediately halt the process of TJ opening caused by Ca2+ withdrawal. In addition, Mn2+ promotes an almost complete recovery of the TJ seal. Cd2+, in spite of stabilizing the TJs in the closed state and halting TJ opening, does not promote TJ recovery, an effect that apparently results from a superimposed toxic effect that is markedly attenuated by the presence of Ca2+. The interruption of TJ opening caused by Ca2+, Cd2+, or Mn2+, and the stability they confer to the closed TJs, might result from the interaction of these ions with E-cadherin. Addition of La3+ (2 microM) to the basolateral Ca2+-containing solution causes an increase of TJ permeability that fully reverses when La3+ is removed. This effect of La3+, observed in the presence of Ca2+ (1 mM), indicates a high La3+ affinity for the Ca2+-binding sites. This ability of La3+ to open TJs in the presence of Ca2+ is a relevant aspect that must be considered when using La3+ in the evaluation of TJ permeability of epithelial and endothelial membranes, particularly when used during in vivo perfusion or in the absence of fixatives.

Cl- and K+ conductances activated by cell swelling in primary cultures of rabbit distal bright convoluted tubules

Ionic currents induced by cell swelling were characterized in primary cultures of rabbit distal bright convoluted tubule (DCTb) by the whole cell patch-clamp technique. Cl- currents were produced spontaneously by whole cell recording with an isotonic pipette solution or by exposure to a hypotonic stress. Initial Cl- currents exhibited outwardly rectifying current-voltage relationship, whereas steady-state currents showed strong decay with depolarizing pulses. The ion selectivity sequence was I- = Br- > Cl- >> glutamate. Currents were inhibited by 0.1 mM 5-nitro-2-(3-phenylpropylamino) benzoic acid and 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and strongly blocked by 1 mM diphenylamine-2-carboxylate. Currents were insensitive to intracellular Ca2+ but required the presence of extracellular Ca2+. They were not activated in cells pretreated with 200 nM staurosporine, 50 microM LaCl3, 10 microM nifedipine, 100 microM verapamil, 5 microM tamoxifen, and 50 microM dideoxyforskolin. Staurosporine, tamoxifen, verapamil, or the absence of external Ca2+ was without effect on the fully developed Cl- currents. Osmotic shock also activated K+ currents in Cl- free conditions. These currents were time independent, activated at depolarized potentials, and inhibited by 5 mM BaCl2. The activation of Cl- and K+ currents by an osmotic shock may be implicated in regulatory volume decrease in DCTb cells.

Effect of calcitonin on calcium transport by the luminal and basolateral membranes of the rabbit nephron

In the rabbit, calcitonin has been shown to enhance calcium (Ca2+) reabsorption in the early distal tubule. The aim of the present study was to investigate the mechanism of this action, using isolated luminal and basolateral membranes of distal tubules. The tubule suspensions were preincubated in the presence or absence of 10(-7) M calcitonin. The luminal or basolateral membranes were subsequently purified and 45Ca transport through the vesicles was measured using the rapid filtration technique. Results were compared with those obtained from proximal tubule membranes. In the proximal tubules, calcitonin had no effect on Ca2+ uptake by luminal membranes. In the distal tubules, the presence of Na+ in the incubation medium strongly decreased the uptake of Ca2+ by luminal membranes. Preincubation of distal tubules with calcitonin partially restored this uptake. We previously reported a dual kinetics of Ca2+ uptake by the distal luminal membranes. Calcitonin enhanced Ca2+ transport by the low affinity component, increasing the Vmax and leaving the K(m) unchanged. Renal calcitonin receptors usually couple to both adenylate cyclase and phospholipase C. To determine through which messenger(s) calcitonin enhances Ca2+ transport by the distal tubules, we first confirmed that the hormone stimulates cAMP and IP3 release. Incubation of the distal tubules with 10(-7) M calcitonin significantly increased both messengers. In contrast, calcitonin did not influence the IP3 nor the cAMP content of proximal tubules. Therefore, we studied the actions of cAMP and phorbol 12-myristate 13 acetate (PMA) on Ca2+ transport by the distal luminal membranes. Incubation of distal tubule suspensions with dibutyryl cAMP significantly increased Ca2+ uptake by the luminal membranes. However, incubation of these tubules with various concentrations of PMA (10 nM, 100 nM and 1 microM) had no effect on this uptake. Calcitonin also influenced Ca2+ transport by the distal basolateral membrane. Incubation of distal tubule suspensions with 10(-7) M calcitonin activated the Na+/Ca2+ exchanger activity, almost doubling the Na+ dependent Ca2+ uptake. Here again this action was mimicked by cAMP. We conclude that calcitonin increases Ca2+ transport by the distal tubule through two mechanisms: the opening of low affinity Ca2+ channels in the luminal membrane and the stimulation of the Na+/Ca2+ exchanger in the basolateral membrane, both actions depending on the activation of adenylate cyclase.

Deficient cation channel regulation in neurons from mice with targeted disruption of the extracellular Ca2+-sensing receptor gene

This study presents evidence that a receptor sensitive to the concentration of extracellular Ca2+ (Ca[2+]o) (CaR) is functionally coupled to ion channels involved in modulation of neuronal excitability. This receptor is expressed in hippocampus and other brain regions, suggesting that it could mediate some of the well-recognized but poorly understood direct actions of extracellular Ca2+ (Ca[2+]o) on neuronal function. The effects of polycationic CaR agonists on the activity of a nonselective cation channel (NCC) in cultured hippocampal neurons from wild-type mice and from mice homozygous for targeted disruption of the CaR gene (CaR -/-) were compared in this study. The CaR agonists, neomycin (100 microM), spermine (300 microM), and elevation of Ca(2+)o from 0.75 to 3 mM, significantly increased the probability of channel opening (Po) in wild-type neurons. None of these agents, however, produced any effect on Po in neurons from mice lacking the CaR. The same NCC, however, could be activated by thapsigargin in neurons from both wild-type mice and CaR-deficient mice, most likely through an associated increase in the cytosolic free calcium concentration (Ca[i]). Thus the CaR regulates the activity of Ca2+-permeable NCC in hippocampal neurons and could potentially modulate key neuronal functions, including neurotransmission and neuronal excitability, via membrane depolarization.

Evidence from incorporation experiments for an anionic channel of small conductance at the apical membrane of the rabbit distal tubule

Many of the hormone-regulated ion transport processes in distal nephron involve transcellular pathways which require a passive entry of ions at the apical membrane of the distal tubule cells. To investigate molecular mechanisms underlying the ionic permeability of the distal tubule apical membrane, a study was undertaken in which vesicles prepared from apical membranes from isolated rabbit distal tubules were fused onto a planar lipid bilayer. These experiments led to the identification of several ionic channels including a Cl(-)-permeable channel of 14 pS with a Na+ over Cl- permeability ratio, PNa/PCl < 0.09. The open channel probability (Po) showed a weak voltage dependency with Po increasing slightly at negative potential values (intracellular (trans) relative to extracellular (cis) for right-side-out vesicles). Channel activity was inhibited by NPPB at high concentrations (> 100 microM) and by DIDS (300 microM). A small inhibitory effect was also observed in the presence of DPC at concentrations ranging from 200 microM to 500 microM. The presence of SO4(2-) (32 mmol/l) in the trans solution caused a complete inhibition of channel activity, but no modification of channel behaviour was observed with the non-selective channel blocking agent gadolinium (Gd3+) at 100 microM. Finally, addition of the catalytic subunit of protein kinase A into the trans chamber (60 U/ml to 80 U/ml) led to an increase in channel activity characterized by a greater number of active channels coupled to an increase of the individual channel open probability. The action of the protein kinase A could be cancelled by the addition of a non specific protein phosphatase, such as alkaline phosphatase. Our results suggest that the apical membrane of the rabbit distal tubule contains a Cl- permeable channel of small conductance the activity of which may be modulated by hormones linked to the adenylate cyclase pathway.

Stimulation of calcium transport by amiloride in mouse distal convoluted tubule cells

This study examined the mechanism by which amiloride dissociates Na and Ca transport in distal convoluted tubules. Control rates of Na uptake averaged 288 nmol/(min mg protein) and were inhibited 39% by microM amiloride. Amiloride had no effect on Cl uptake. Resting membrane voltage, measured with the voltage-sensitive dye DiOC6 (3), averaged -70 mV. Amiloride hyperpolarized cells in a reversible manner by 18 mV. Control rates of Ca uptake averaged 2.8 nmol/(min mg protein) and increased by 39% in the presence of amiloride. Alterations of intracellular Ca activity were measured in single cells loaded with Fura2-AM. Control intracellular Ca activity averaged 100 nM. Amiloride increased intracellular Ca activity in a concentration-dependent manner to a maximum of 330 nM at microM amiloride. Amiloride analogues ethylisopropyl amiloride (EIPA) and dimethylbenzamil (DMB), which preferentially block Na/H and Na/Ca exchange, respectively, had no effect on Na or Ca influx or on intracellular Ca activity. The dihydropyridine Ca channel blocker nifedipine inhibited amiloride-stimulated Ca uptake and the rise of intracellular Ca activity but had no effect on membrane voltage. It is concluded that amiloride blocks Na entry mediated by Na channels. Inhibition of Na entry results in membrane hyperpolarization, which activates Ca entry by dihydropyridine-sensitive Ca channels.

The renal cGMP-gated cation channel: its molecular structure and physiological role

Cyclic nucleotide-gated cation channels, which are permeable to monovalent and divalent cations, are expressed in a number of tissues. cDNAs encoding cGMP-gated cation channel subunits have been cloned in retinal rods, cones, olfactory neuroepithelium, pineal gland, aorta, testis, heart, and most recently kidney. Patch clamp studies have identified and characterized cGMP-gated cation channels in the cortical collecting duct (CCD) and inner medullary collecting duct (IMCD). cGMP-gated cation channels in kidney share many biophysical and molecular properties with the retinal rod cGMP-gated channel. However, unlike the retinal rod channel, the cGMP-gated cation channel in kidney is inhibited by cGMP and stimulated by increased calcium levels. In the IMCD the cGMP-gated cation channel mediates electrogenic sodium absorption which is inhibited by ANP via cGMP. Recently, cGMP-gated cation channel poly(A+) RNA has been identified in other nephron segments by RT-PCR and in situ PCR hybridization. Furthermore, cGMP-gated cation channel protein has also been identified in all nephron segments by Western blot analysis. These observations suggest that cGMP-gated cation channels, or closely related gene products, may play an important physiological role in all nephron segments. Hormones that increase intracellular cGMP may regulate sodium, and perhaps calcium, uptake in nephron segments proximal to the IMCD. Increases in cell sodium and calcium may regulate other transport and signaling pathways.

A ubiquitous non-selective cation channel in the mouse renal tubule with variable sensitivity to calcium

Basolateral membranes of microdissected collagenase-treated fragments of renal tubules from the mouse were examined using the cell-attached and the cell-free variants of the patch-clamp technique. With a K(+)-rich solution in the pipette, a highly active, inwardly rectifying K+ channel was observed on intact cells of the cortical collecting tubule (CCT). The mean inward and outward conductances were 38.5 +/- 3.1 pS and 17.3 +/- 1.8 pS, respectively (n = 4). In contrast, cell-attached patches were usually inactive when a Na(+)-rich solution filled the patch pipette. However, another type of channel with a conductance of 20-30 pS exhibited a sparse activity in 4/20 CCT. In excised, inside-out patches, the most frequent channel in CCT had an ohmic unit conductance of 27.1 +/- 1.2 pS (n = 17), excluded anions (PCl/PNa = 0.09), discriminated little between NH4+, K+ and Na+ (PNH4/PNa = 1.5; PK/PNa = 0.9), and was much less permeable to Ca2+ and Ba2+ than to Na+ (PCa/PNa = 0.09; PBa/PNa approximately 0). The cation channel was moderately voltage-dependent, showing a decreased open probability (Po) at negative voltages. It was activated by internal calcium (threshold: 1 mumol/l-0.1 mmol/l calcium), and inhibited by the adenine nucleotides ATP, ADP and AMP with half-maximal inhibition of Po at 1.2 mumol/l AMP. As in other cell models, 3',5'-dichlorodiphenylamine-2-carboxylic acid blocked channel activity when added to the internal surface of the membrane patch. Extending our study to other parts of the renal tubule, we found that the basolateral membranes of the proximal (pars recta), distal convoluted, connecting and outer medullary collecting tubules, the thin descending limb and the medullary thick ascending limb all contained a similar Ca- and ATP-sensitive cation channel. The calcium sensitivity varied from one part to another.

Activation of calcium influx by ATP and store depletion in primary cultures of renal proximal cells

Cytoplasmic calcium changes and calcium influx evoked by adenosine triphosphate (ATP) were investigated in primary cultures of rabbit proximal convoluted tubule cells. Extracellular ATP (50 microM) induced a biphasic increase of [Ca2+]i measured with the calcium probe fura-2. In the early phase, the mobilization of intracellular pools resulted in a transient increase of [Ca2+]i from 106 +/- 11 nM (n = 36) to 1059 +/- 115% (n = 29) of the resting level within 10 s. In the presence of external calcium, [Ca2+]i then decreased within 3 min to a sustained level (398 +/- 38%, n = 8). Measurements of fura-2 quenching by external manganese revealed that this phase was the result of an increased Ca2+ uptake, blocked by lanthanum (10 microM) and verapamil (100 microM) but not by the nifedipin (25 microM). Internal calcium store depletion by ATP induced an increased calcium influx through lanthanum- and verapamil-sensitive, nifedipin-insensitive calcium channels, located on the apical membrane of the cells. As indicated by 86Rb+ efflux measurements, ATP activated a potassium efflux that was blocked by barium and Leiurus quinquestriatus hebraeus (LQH) venom (containing charybdotoxin) indicating the involvement of Ca(2+)-sensitive K+ channels. Moreover, in the presence of the LQH venom, the internal calcium stores were not replenished after being depleted by ATP. Our results indicate that an ATP-evoked hyperpolarization of the plasma membrane leads to increased Ca2+ influx, which facilitates the replenishment of the internal stores.