Tubular and cellular localization of the cardiac L-type calcium channel in rat kidney

Calcium channel blocker in patients with chronic kidney disease

BACKGROUND

Chronic kidney disease (CKD) is involved in a progressive deterioration in renal function over the years and is now a global public health problem. Currently, reducing the number of patients progressing to end-stage renal failure is urgently necessary. Hypertension and CKD interact with each other, and good control of blood pressure (BP) can improve CKD patients' prognosis. With the current global trend for more strict BP control, the importance of BP management and the need for medication to achieve this strict goal are increasing. Calcium channel blockers (CCBs), which target voltage-dependent calcium channels, are frequently used in combination with renin-angiotensin-aldosterone system inhibitors for CKD patients because of their strong BP-lowering properties and relatively few adverse side effects. Calcium channels have several subtypes, including L, N, T, P/Q, and R, and three types of CCBs, L-type CCBs, L-/T-type CCBs, and L-/N-type CCBs, that are available. Nowadays, the new functions and effects of the CCBs are being elucidated.

CONCLUSION

We should use different types of CCBs properly depending on their pharmacological effects, such as the strength of antihypertensive effects and the organ protection effects, taking into account the pathophysiology of the patients. In this article, the role and the use of CCBs in CKD patients are reviewed.

Iron uptake by ZIP8 and ZIP14 in human proximal tubular epithelial cells

In patients with iron overload disorders, increasing number of reports of renal dysfunction and renal iron deposition support an association between increased iron exposure and renal injury. In systemic iron overload, elevated circulating levels of transferrin-bound (TBI) and non-transferrin-bound iron (NTBI) are filtered to the renal proximal tubules, where they may cause injury. However, the mechanisms of tubular iron handling remain elusive. To unravel molecular renal proximal tubular NTBI and TBI handling, human conditionally immortalized proximal tubular epithelial cells (ciPTECs) were incubated with ⁵⁵Fe as NTBI and fluorescently labeled holo-transferrin as TBI. Ferrous iron importers ZIP8 and ZIP14 were localized in the ciPTEC plasma membrane. Whereas silencing of either ZIP8 or ZIP14 alone did not affect ⁵⁵Fe uptake, combined silencing significantly reduced ⁵⁵Fe uptake compared to control (p < 0.05). Furthermore, transferrin receptor 1 (TfR1) and ZIP14, but not ZIP8, colocalized with early endosome antigen 1 (EEA1). TfR1 and ZIP14 also colocalized with uptake of fluorescently labeled transferrin. Furthermore, ZIP14 silencing decreased ⁵⁵Fe uptake after ⁵⁵Fe-Transferrin exposure (p < 0.05), suggesting ZIP14 could be involved in early endosomal transport of TBI-derived iron into the cytosol. Our data suggest that human proximal tubular epithelial cells take up TBI and NTBI, where ZIP8 and ZIP14 are both involved in NTBI uptake, but ZIP14, not ZIP8, mediates TBI-derived iron uptake. This knowledge provides more insights in the mechanisms of renal iron handling and suggests that ZIP8 and ZIP14 could be potential targets for limiting renal iron reabsorption and enhancing urinary iron excretion in systemic iron overload disorders.

The Wnt11 Signaling Pathway in Potential Cellular EMT and Osteochondral Differentiation Progression in Nephrolithiasis Formation

The molecular events leading to nephrolithiasis are extremely complex. Previous studies demonstrated that calcium and transforming growth factor-β1 (TGF-β1) may participate in the pathogenesis of stone formation, but the explicit mechanism has not been defined. Using a self-created genetic hypercalciuric stone-forming (GHS) rat model, we observed that the increased level of serous/uric TGF-β1 and elevated intracellular calcium in primary renal tubular epithelial cells (PRECs) was associated with nephrolithiasis progression in vivo. In the setting of high calcium plus high TGF-β1 in vitro, PRECs showed great potential epithelial to mesenchymal transition (EMT) progression and osteochondral differentiation properties, representing the multifarious increased mesenchymal and osteochondral phenotypes (Zeb1, Snail1, Col2A1, OPN, Sox9, Runx2) and decreased epithelial phenotypes (E-cadherin, CK19) bythe detection of mRNAs and corresponding proteins. Moreover, TGF-β-dependent Wnt11 knockdown and L-type Ca²⁺ channel blocker could greatly reverse EMT progression and osteochondral differentiation in PRECs. TGF-β1 alone could effectively promote EMT, but it had no effect on osteochondral differentiation in NRK cells (Rat kidney epithelial cell line). Stimulation with Ca²⁺ alone did not accelerate differentiation of NRK. Co-incubation of extracellular Ca²⁺ and TGF-β1 synergistically promotes EMT and osteochondral differentiation in NRK control cells. Our data supplied a novel view that the pathogenesis of calcium stone development may be associated with synergic effects of TGF-β1 and Ca²⁺, which promote EMT and osteochondral differentiation via Wnt11 and the L-type calcium channel.

Forskolin Regulates L-Type Calcium Channel through Interaction between Actinin 4 and β3 Subunit in Osteoblasts

Voltage-dependent L-type calcium channels that permit cellular calcium influx are essential in calcium-mediated modulation of cellular signaling. Although the regulation of voltage-dependent L-type calcium channels is linked to many factors including cAMP-dependent protein kinase A (PKA) activity and actin cytoskeleton, little is known about the detailed mechanisms underlying the regulation in osteoblasts. Our present study investigated the modulation of L-type calcium channel activities through the effects of forskolin on actin reorganization and on its functional interaction with actin binding protein actinin 4. The results showed that forskolin did not significantly affect the trafficking of pore forming α_1c subunit and its interaction with actin binding protein actinin 4, whereas it significantly increased the expression of β₃ subunit and its interaction with actinin 4 in osteoblast cells as assessed by co-immunoprecipitation, pull-down assay, and immunostaining. Further mapping showed that the ABD and EF domains of actinin 4 were interaction sites. This interaction is independent of PKA phosphorylation. Knockdown of actinin 4 significantly decreased the activities of L-type calcium channels. Our study revealed a new aspect of the mechanisms by which the forskolin activation of adenylyl cyclase - cAMP cascade regulates the L-type calcium channel in osteoblast cells, besides the PKA mediated phosphorylation of the channel subunits. These data provide insight into the important role of interconnection among adenylyl cyclase, cAMP, PKA, the actin cytoskeleton, and the channel proteins in the regulation of voltage-dependent L-type calcium channels in osteoblast cells.

L-type calcium channel modulates cystic kidney phenotype

In polycystic kidney disease (PKD), abnormal proliferation and genomic instability of renal epithelia have been associated with cyst formation and kidney enlargement. We recently showed that L-type calcium channel (CaV1.2) is localized to primary cilia of epithelial cells. Previous studies have also shown that low intracellular calcium level was associated with the hyperproliferation phenotype in the epithelial cells. However, the relationship between calcium channel and cystic kidney phenotype is largely unknown. In this study, we generated cells with somatic deficient Pkd1 or Pkd2 to examine ciliary CaV1.2 function via lentiviral knockdown or pharmacological verapamil inhibition. Although inhibition of CaV1.2 expression or function did not change division and growth patterns in wild-type epithelium, it led to hyperproliferation and polyploidy in mutant cells. Lack of CaV1.2 in Pkd mutant cells also decreased the intracellular calcium level. This contributed to a decrease in CaM kinase activity, which played a significant role in regulating Akt and Erk signaling pathways. Consistent with our in vitro results, CaV1.2 knockdown in zebrafish and Pkd1 heterozygous mice facilitated the formation of kidney cysts. Larger cysts were developed faster in Pkd1 heterozygous mice with CaV1.2 knockdown. Overall, our findings emphasized the importance of CaV1.2 expression in kidneys with somatic Pkd mutation. We further suggest that CaV1.2 could serve as a modifier gene to cystic kidney phenotype.

Cilioplasm is a cellular compartment for calcium signaling in response to mechanical and chemical stimuli

Primary cilia with a diameter of ~200 nm have been implicated in development and disease. Calcium signaling within a primary cilium has never been directly visualized and has therefore remained a speculation. Fluid-shear stress and dopamine receptor type-5 (DR5) agonist are among the few stimuli that require cilia for intracellular calcium signal transduction. However, it is not known if these stimuli initiate calcium signaling within the cilium or if the calcium signal originates in the cytoplasm. Using an integrated single-cell imaging technique, we demonstrate for the first time that calcium signaling triggered by fluid-shear stress initiates in the primary cilium and can be distinguished from the subsequent cytosolic calcium response through the ryanodine receptor. Importantly, this flow-induced calcium signaling depends on the ciliary polycystin-2 calcium channel. While DR5-specific agonist induces calcium signaling mainly in the cilioplasm via ciliary CaV1.2, thrombin specifically induces cytosolic calcium signaling through the IP3 receptor. Furthermore, a non-specific calcium ionophore triggers both ciliary and cytosolic calcium responses. We suggest that cilia not only act as sensory organelles but also function as calcium signaling compartments. Cilium-dependent signaling can spread to the cytoplasm or be contained within the cilioplasm. Our study thus provides the first model to understand signaling within the cilioplasm of a living cell.

Functional and pharmacological consequences of the distribution of voltage-gated calcium channels in the renal blood vessels

Calcium channel blockers are widely used to treat hypertension because they inhibit voltage-gated calcium channels that mediate transmembrane calcium influx in, for example, vascular smooth muscle and cardiomyocytes. The calcium channel family consists of several subfamilies, of which the L-type is usually associated with vascular contractility. However, the L-, T- and P-/Q-types of calcium channels are present in the renal vasculature and are differentially involved in controlling vascular contractility, thereby contributing to regulation of kidney function and blood pressure. In the preglomerular vascular bed, all the three channel families are present. However, the T-type channel is the only channel in cortical efferent arterioles which is in contrast to the juxtamedullary efferent arteriole, and that leads to diverse functional effects of L- and T-type channel inhibition. Furthermore, by different mechanisms, T-type channels may contribute to both constriction and dilation of the arterioles. Finally, P-/Q-type channels are involved in the regulation of human intrarenal arterial contractility. The calcium blockers used in the clinic affect not only L-type but also P-/Q- and T-type channels. Therefore, the distinct effect obtained by inhibiting a given subtype or set of channels under experimental settings should be considered when choosing a calcium blocker for treatment. T-type channels seem to be crucial for regulating the GFR and the filtration fraction. Use of blockers is expected to lead to preferential efferent vasodilation, reduction of glomerular pressure and proteinuria. Therefore, renovascular T-type channels might provide novel therapeutic targets, and may have superior renoprotective effects compared to conventional calcium blockers.

Involvement of N-type Ca(2+) channels in the fibrotic process of the kidney in rats

N-type Ca(2+) channels are densely distributed in sympathetic nerves that innervate renal tubules. However, the role of N-type Ca(2+) channels in renal fibrosis remains unknown. To address this issue, we examined the difference between the effects of amlodipine (an L-type Ca(2+) channel blocker) and cilnidipine (a dual L/N-type Ca(2+) channel blocker) on fibrotic changes using a rat unilateral ureteral obstruction (UUO) model. The expression of both L-type and N-type Ca(2+) channels was significantly upregulated in UUO kidneys compared with that in contralateral kidneys. There were no significant differences in mean blood pressure among the rats tested. Both amlodipine and cilnidipine significantly attenuated fibrotic changes in UUO kidneys. The antifibrotic effect of cilnidipine was more potent than that of amlodipine. Amlodipine as well as cilnidipine reduced type III collagen deposition, α-smooth muscle actin (α-SMA) expression, and interstitial cell proliferation. In addition, cilnidipine significantly reduced deposition of type I collagen and macrophage infiltration in UUO kidneys. With the use of in vivo bromodeoxyuridine labeling, label-retaining cells (LRCs) were identified as a population of tubular cells that participate in epithelial-mesenchymal transition after UUO. Some LRCs migrated into the interstitium, expressed α-SMA and vimentin, and produced several extracellular matrixes in UUO kidneys. The number of interstitial LRCs was significantly decreased by cilnidipine but not amlodipine. These data suggest that N-type Ca(2+) channels contribute to multiple steps of renal fibrosis, and its blockade may thus be a useful therapeutic approach for prevention of renal fibrosis.

Calcium signaling in renal tubular cells

The kidney handles calcium by filtration and reabsorption. About 60% of the plasma calcium is filterable, and 99% is reabsorbed in the tubule. In the proximal tubule, the reabsorption is passive and paracellular, but in the distal tubule is active and transcellular. Thus, renal tubular cells are exposed to very high concentrations of calcium in both, the extracellular and the intracellular compartments. Extracellular calcium signaling is transmitted by the calcium sensing receptor, located both in the luminal and basolateral sides of tubular cells. This receptor is able to control levels of extracellular calcium and acts in consequence to maintain calcium homeostasis. Furthermore, renal tubular cells possess several calcium channels that regulate some of the cell functions. Among those, voltage gated calcium channels, transient receptor potential channels and N-methyl-D-aspartate receptor channels have been reported to control several functions. Those functions include survival, apoptosis, differentiation, epithelial-mesenchymal transition, and active vitamin D and renin synthesis.

17β-Estradiol induces nongenomic effects in renal intercalated cells through G protein-coupled estrogen receptor 1

Steroid hormones such as 17β-estradiol (E2) are known to modulate ion transporter expression in the kidney through classic intracellular receptors. Steroid hormones are also known to cause rapid nongenomic responses in a variety of nonrenal tissues. However, little is known about renal short-term effects of steroid hormones. Here, we studied the acute actions of E2 on intracellular Ca(2+) signaling in isolated distal convoluted tubules (DCT2), connecting tubules (CNT), and initial cortical collecting ducts (iCCD) by fluo 4 fluorometry. Physiological concentrations of E2 induced transient increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) in a subpopulation of cells. The [Ca(2+)](i) increases required extracellular Ca(2+) and were inhibited by Gd(3+). Strikingly, the classic E2 receptor antagonist ICI 182,780 also increased [Ca(2+)](i), which is inconsistent with the activation of classic E2 receptors. G protein-coupled estrogen receptor 1 (GPER1 or GPR30) was detected in microdissected DCT2/CNT/iCCD by RT-PCR. Stimulation with the specific GPER1 agonist G-1 induced similar [Ca(2+)](i) increases as E2, and in tubules from GPER1 knockout mice, E2, G-1, and ICI 182,780 failed to induce [Ca(2+)](i) elevations. The intercalated cells showed both E2-induced concanamycin-sensitive H(+)-ATPase activity by BCECF fluorometry and the E2-mediated [Ca(2+)](i) increment. We propose that E2 via GPER1 evokes [Ca(2+)](i) transients and increases H(+)-ATPase activity in intercalated cells in mouse DCT2/CNT/iCCD.

L-type calcium channel activity in osteoblast cells is regulated by the actin cytoskeleton independent of protein trafficking

Voltage-dependent L-type calcium channels (VDCC) play important roles in many cellular processes. The interaction of the actin cytoskeleton with the channel in nonexcitable cells is less well understood. We performed whole-cell patch-clamp surface biotinylation and calcium imaging on different osteoblast cells to determine channel kinetics, amplitude, surface abundance, and intracellular calcium, respectively. Patch-clamp studies showed that actin polymerization by phalloidin increased the peak current density of I (Ca), whereas actin depolymerization by cytochalasin D (CD) significantly decreased the current amplitude. This result is consistent with calcium imaging, which showed that CD significantly decreased Bay K8644-induced intracellular calcium increase. Surface biotinylation studies showed that CD is not able to affect the surface expression of the pore-forming subunit α(1C). Interestingly, application of CD caused a significantly negative shift in the steady-state inactivation kinetics of I (Ca). There were decreases in the voltage at half-maximal inactivation that changed in a dose-dependent manner. CD also reduced the effect of activated vitamin D(3) (1α,25-D3) on VDCC and intracellular calcium. We conclude that in osteoblasts the actin cytoskeleton affects α(1C) by altering the channel kinetic properties, instead of changing the surface expression, and it is able to regulate 1α,25-D3 signaling through VDCC. Our study provides a new insight into calcium regulation in osteoblasts, which are essential in many physiological functions of this cell.

A nicardipine-sensitive Ca2+ entry contributes to the hypotonicity-induced increase in [Ca2+]i of principal cells in rat cortical collecting duct

We examined the mechanisms involved in the [Ca(2+)](i) response to the extracellular hypotonicity in the principal cells of freshly isolated rat cortical collecting duct (CCD), using Fura-2/AM fluorescence imaging. Reduction of extracellular osmolality from 305 (control) to 195 mosmol/kgH(2)O (hypotonic) evoked transient increase in [Ca(2+)](i) of principal cells of rat CCDs. The [Ca(2+)](i) increase was markedly attenuated by the removal of extracellular Ca(2+)(.) The application of a P(2) purinoceptor antagonist, suramin failed to inhibit the hypotonicity-induced [Ca(2+)](i) increase. The [Ca(2+)](i) increase in response to extracellular hypotonicity was not influenced by application of Gd(3+) and ruthenium red. On the other hand, a voltage-gated Ca(2+) channel inhibitor, nicardipine, significantly reduced the peak amplitude of [Ca(2+)](i) increase in the principal cells. In order to assess Ca(2+) entry during the hypotonic stimulation, we measured the quenching of Fura-2 fluorescence intensity by Mn(2+). The hypotonic stimulation enhanced quenching of Fura-2 fluorescence by Mn(2+), indicating that a Ca(2+)-permeable pathway was activated by the hypotonicity. The hypotonicity-mediated enhancement of Mn(2+) quenching was significantly inhibited by nicardipine. These results strongly suggested that a nicardipine-sensitive Ca(2+) entry pathway would contribute to the mechanisms underlying the hypotonicity-induced [Ca(2+)](i) elevation of principal cells in rat CCD.

Cilnidipine suppresses podocyte injury and proteinuria in metabolic syndrome rats: possible involvement of N-type calcium channel in podocyte

OBJECTIVES

Clinical studies have indicated the beneficial effect of an L/N-type calcium channel blocker (CCB), cilnidipine, on the progression of proteinuria in hypertensive patients compared with an L-type CCB, amlodipine. In the present study, we examined the effects of cilnidipine and amlodipine on the renal injury in spontaneously hypertensive rat/ND mcr-cp (SHR/ND) and their underlying mechanism.

METHODS AND RESULTS

SHR/ND were treated with vehicle (nU10), cilnidipine [33 mg/kg per day, orally (p.o.); nU11] or amlodipine (20 mg/kg per day, p.o.; nU9) for 20 weeks. SHR/ND developed proteinuria in an age-dependent manner. Cilnidipine suppressed the proteinuria greater than amlodipine did. The immunohistochemical analysis showed that N-type calcium channel and Wilm's tumor factor, a marker of podocyte, were co-expressed. SHR/ND had significantly greater desmin staining, an indicator of podocyte injury, with lower podocin and nephrin expression in the glomeruli than Wistar-Kyoto rat or SHR. Cilnidipine significantly prevented the increase in desmin staining and restored the glomerular podocin and nephrin expression compared with amlodipine. Cilnidipine also prevented the increase in renal angiotensin II content, the expression and membrane translocation of NADPH oxidase subunits and dihydroethidium staining in SHR/ND. In contrast, amlodipine failed to change these renal parameters.

CONCLUSION

These data suggest that cilnidipine suppressed the development of proteinuria greater than amlodipine possibly through inhibiting N-type calcium channel-dependent podocyte injury in SHR/ND.

A voltage-dependent Ca2+ influx pathway regulates the Ca2+-dependent Cl(-) conductance of renal IMCD-3 cells

We have previously shown that the membrane conductance of mIMCD-3 cells at a holding potential of 0 mV is dominated by a Ca2+-dependent Cl(-) current (I(CLCA)). Here we report that I(CLCA) activity is also voltage dependent and that this dependence on voltage is linked to the opening of a novel Al3+-sensitive, voltage-dependent, Ca2+ influx pathway. Using whole-cell patch-clamp recordings at a physiological holding potential (-60 mV), ICLCA was found to be inactive and resting currents were predominantly K+ selective. However, membrane depolarization to 0 mV resulted in a slow, sigmoidal, activation of ICLCA (T(0.5) approximately 500 s), while repolarization in turn resulted in a monoexponential decay in I(CLCA) (T (0.5) approximately 100 s). The activation of I(CLCA) by depolarization was reduced by lowering extracellular Ca2+ and completely inhibited by buffering cytosolic Ca2+ with EGTA, suggesting a role for Ca2+ influx in the activation of I(CLCA). However, raising bulk cytosolic Ca2+ at -60 mV did not produce sustained I(CLCA) activity. Therefore I(CLCA) is dependent on both an increase in intracellular Ca2+ and depolarization to be active. We further show that membrane depolarization is coupled to opening of a Ca2+ influx pathway that displays equal permeability to Ca2+ and Ba2+ ions and that is blocked by extracellular Al3+ and La3+. Furthermore, Al3+ completely and reversibly inhibited depolarization-induced activation of ICLCA, thereby directly linking Ca2+ influx to activation of I(CLCA). We speculate that during sustained membrane depolarization, calcium influx activates ICLCA which functions to modulate NaCl transport across the apical membrane of IMCD cells.

Transcriptional adaptation to Clcn5 knockout in proximal tubules of mouse kidney

Dent disease has multiple defects attributed to proximal tubule malfunction including low-molecular-weight proteinuria, aminoaciduria, phosphaturia, and glycosuria. To understand the changes in kidney function of the Clc5 chloride/proton exchanger gene knockout mouse model of Dent disease, we examined gene expression profiles from proximal S1 and S2 tubules of mouse kidneys. We found many changes in gene expression not known previously to be altered in this disease. Genes involved in lipid metabolism, organ development, and organismal physiological processes had the greatest number of significantly changed transcripts. In addition, genes of catalytic activity and transporter activity also had a great number of changed transcripts. Overall, 720 genes are expressed differentially in the proximal tubules of the Dent Clcn5 knockout mouse model compared with those of control wild-type mice. The fingerprint of these gene changes may help us to understand the phenotype of Dent disease.

Calcium channel inhibition accelerates polycystic kidney disease progression in the Cy/+ rat

In polycystic kidney disease, abnormal epithelial cell proliferation is the main factor leading to cyst formation and kidney enlargement. Cyclic AMP (cAMP) is mitogenic in cystic but antimitogenic in normal human kidney cells, which is due to reduced steady-state intracellular calcium levels in cystic compared to the normal cells. Inhibition of intracellular calcium entry with channel blockers, such as verapamil, induced cAMP-dependent cell proliferation in normal renal cells. To determine if calcium channel blockers have a similar effect on cell proliferation in vivo, Cy/+ rats, a model of dominant polycystic kidney disease, were treated with verapamil. Kidney weight and cyst index were elevated in verapamil-treated Cy/+ rats. This was associated with increased cell proliferation and apoptosis, elevated expression, and phosphorylation of B-Raf with stimulation of the mitogen-activated protein kinase MEK/ERK (mitogen-activated protein kinase kinase/extracellular-regulated kinase) pathway. Verapamil had no effect on kidney morphology or B-Raf stimulation in wild-type rats. We conclude that treatment of Cy/+ rats with calcium channel blockers increases activity of the B-Raf/MEK/ERK pathway accelerating cyst growth in the presence of endogenous cAMP, thus exacerbating renal cystic disease.

Ca2+ channel subtypes and pharmacology in the kidney

A large body of evidence has accrued indicating that voltage-gated Ca(2+) channel subtypes, including L-, T-, N-, and P/Q-type, are present within renal vascular and tubular tissues, and the blockade of these Ca(2+) channels produces diverse actions on renal microcirculation. Because nifedipine acts exclusively on L-type Ca(2+) channels, the observation that nifedipine predominantly dilates afferent arterioles implicates intrarenal heterogeneity in the distribution of L-type Ca(2+) channels and suggests that it potentially causes glomerular hypertension. In contrast, recently developed Ca(2+) channel blockers (CCBs), including mibefradil and efonidipine, exert blocking action on L-type and T-type Ca(2+) channels and elicit vasodilation of afferent and efferent arterioles, which suggests the presence of T-type Ca(2+) channels in both arterioles and the distinct impact on intraglomerular pressure. Recently, aldosterone has been established as an aggravating factor in kidney disease, and T-type Ca(2+) channels mediate aldosterone release as well as its effect on renal efferent arteriolar tone. Furthermore, T-type CCBs are reported to exert inhibitory action on inflammatory process and renin secretion. Similarly, N-type Ca(2+) channels are present in nerve terminals, and the inhibition of neurotransmitter release by N-type CCBs (eg, cilnidipine) elicits dilation of afferent and efferent arterioles and reduces glomerular pressure. Collectively, the kidney is endowed with a variety of Ca(2+) channel subtypes, and the inhibition of these channels by their specific CCBs leads to variable impact on renal microcirculation. Furthermore, multifaceted activity of CCBs on T- and N-type Ca(2+) channels may offer additive benefits through nonhemodynamic mechanisms in the progression of chronic kidney disease.

Characterization of endogenous betaine gamma-amino-n-butyric acid cotransporter glycoform and its hyperosmotic regulation in MDCK cells

Increase in mRNA expression and transport activity of the betaine gamma-amino-n-butyric acid cotransporter (BGAT) in response to hyperosmolality has been previously shown in MDCK cells. However, the hyperosmolality-induced response of endogenous BGAT protein expression was not investigated in detail. We show two forms of endogenous BGAT immunoreactivity that are expressed in MDCK II cells. Both are sensitive to Peptide N-Glycosidase F (PNGase F), suggesting that they are N-glycosylated proteins. One band, about 75 kDa, is resistant to Endo H, while the other 55 kDa band is sensitive to it, suggesting that they are fully N-glycosylated mature form in the post-Golgi compartment and core-glycosylated immature form in the endoplasmic reticulum (ER), respectively. When treated with hyperosmolality, they are significantly increased. But the rate of BGAT processing, as assessed by the ratio of mature to immature form, is not increased, suggesting that hyperosmolality does not facilitate the export of BGAT from the ER to the secretory pathway. Surface biotinylation and confocal microscopy show that hyperosmolality significantly increases the amount of the mature form of BGAT on the basolateral membrane with a very small fraction on the apical membrane. We conclude that BGAT is an N-glycosylated protein with two glycoforms and endogenous BGAT synthesis rather than processing is involved in the adaptation to the hyperosmotic stress.

Calcium restriction allows cAMP activation of the B-Raf/ERK pathway, switching cells to a cAMP-dependent growth-stimulated phenotype

cAMP can be either mitogenic or anti-mitogenic, depending on the cell type. We demonstrated previously that cAMP inhibited the proliferation of normal renal epithelial cells and stimulated the proliferation of cells derived from the cysts of polycystic kidney disease (PKD) patients. The protein products of the genes causing PKD, polycystin-1 and polycystin-2, are thought to regulate intracellular calcium levels, suggesting that abnormal polycystin function may affect calcium signaling and thus cause a switch to the cAMP growth-stimulated phenotype. To test this hypothesis, we disrupted intracellular calcium mobilization by treating immortalized mouse M-1 collecting duct cells and primary cultures of human kidney epithelial cells with calcium channel blockers and by lowering extracellular calcium with EGTA. Calcium restriction for 3-5 h converted both cell types from a normal cAMP growth-inhibited phenotype to an abnormal cAMP growth-stimulated phenotype, characteristic of PKD. In M-1 cells, we showed that calcium restriction was associated with an elevation in B-Raf protein levels and cAMP-stimulated, Ras-dependent activation of B-Raf and ERK. Moreover, the activity of Akt, a negative regulator of B-Raf, was decreased by calcium restriction. Inhibition of Akt or phosphatidylinositol 3-kinase also allowed cAMP-dependent activation of B-Raf and ERK in normal calcium. These results suggest that calcium restriction causes an inhibition of the phosphatidylinositol 3-kinase/Akt pathway, which relieves the inhibition of B-Raf to allow the cAMP growth-stimulated phenotypic switch. Finally, M-1 cells stably overexpressing an inducible polycystin-1 C-terminal cytosolic tail construct were shown to exhibit a cAMP growth-stimulated phenotype involving B-Raf and ERK activation, which was reversed by the calcium ionophore A23187. We conclude that disruption of calcium mobilization in cells that are normally growth-inhibited by cAMP can derepress the B-Raf/ERK pathway, thus converting these cells to a phenotype that is growth-stimulated by cAMP.

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.

Na,K-ATPase as a signal transducer

Recent studies have indicated that Na,K-ATPase may, in addition to being the key regulator of intracellular Na(+) and K(+) concentration, act as a signal transducer. Despite extensive research, the biological role for ouabain, a natural ligand of Na,K-ATPase, is not well understood. We have reported that exposure of rat proximal tubular cells (RPTC) to doses of ouabain that inhibit the Na,K-ATPase activity by less than 50% (10 nM - 500 micro M), will induce intracellular [Ca(2+)](i) oscillations and that this calcium signal leads to activation of the transcription factor NF-kappaB. The ouabain-induced calcium oscillations were blocked by an inhibitor of the IP(3) receptors but not by phospholipase C inhibitors nor by cellular depletion of IP(3), suggesting that the calcium signal is not due to phospholipase C-mediated IP(3) release. Fluorescence resonance energy transfer (FRET) studies suggested a close proximity between the Na,K-ATPase and IP(3) receptor. Our findings demonstrate a novel principle for calcium signaling via Na,K-ATPase.

Sodium and calcium transport pathways along the mammalian distal nephron: from rabbit to human

The final adjustment of renal sodium and calcium excretion is achieved by the distal nephron, in which transepithelial ion transport is under control of various hormones, tubular fluid composition, and flow rate. Acquired or inherited diseases leading to deranged renal sodium and calcium balance have been linked to dysfunction of the distal nephron. Diuretic drugs elicit their effects on sodium balance by specifically inhibiting sodium transport proteins in the apical plasma membrane of distal nephron segments. The identification of the major apical sodium transport proteins allows study of their precise distribution pattern along the distal nephron and helps address their cellular and molecular regulation under various physiological and pathophysiological settings. This review focuses on the topological arrangement of sodium and calcium transport proteins along the cortical distal nephron and on some aspects of their functional regulation. The availability of data on the distribution of transporters in various species points to the strengths, as well as to the limitations, of animal models for the extrapolation to humans.