Modulation of the epithelial Ca2+ channel ECaC by extracellular pH

Structural mechanism of TRPV5 inhibition by econazole

The calcium-selective TRPV5 channel activated by phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] is involved in calcium homeostasis. Recently, cryoelectron microscopy (cryo-EM) provided molecular details of TRPV5 modulation by exogenous and endogenous molecules. However, the details of TRPV5 inhibition by the antifungal agent econazole (ECN) remain elusive due to the low resolution of the currently available structure. In this study, we employ cryo-EM to comprehensively examine how the ECN inhibits TRPV5. By combining our structural findings with site-directed mutagenesis, calcium measurements, electrophysiology, and molecular dynamics simulations, we determined that residues F472 and L475 on the S4 helix, along with residue W495 on the S5 helix, collectively constitute the ECN-binding site. Additionally, the structure of TRPV5 in the presence of ECN and PI(4,5)P2, which does not show the bound activator, reveals a potential inhibition mechanism in which ECN competes with PI(4,5)P2, preventing the latter from binding, and ultimately pore closure.

Biochemistry and pathophysiology of the Transient Potential Receptor Vanilloid 6 (TRPV6) calcium channel

TRPV6 is a Transient Receptor Potential Vanilloid (TRPV) cation channel with high selectivity for Ca²⁺ ions. First identified in 1999 in a search for the gene which mediates intestinal Ca²⁺ absorption, its far more extensive repertoire as a guardian of intracellular Ca²⁺ has since become apparent. Studies on TRPV6-deficient mice demonstrated additional important roles in placental Ca²⁺ transport, fetal bone development and male fertility. The first reports of inherited deficiency in newborn babies appeared in 2018, revealing its physiological importance in humans. There is currently strong evidence that TRPV6 also contributes to the pathogenesis of some common cancers. The recently reported association of TRPV6 deficiency with non-alcoholic chronic pancreatitis suggests a role in normal pancreatic function. Over time and with greater awareness of TRPV6, other disease-associations are likely to emerge. Powerful analytical tools have provided invaluable insights into the structure and operation of TRPV6. Its roles in Ca²⁺ signaling and carcinogenesis, and the use of channel inhibitors in cancer treatment are being intensively investigated. This review first briefly describes the biochemistry and physiology of the channel, and analytical methods used to investigate these. The focus subsequently shifts to the clinical disorders associated with abnormal expression and the underlying pathophysiology. The aims of this review are to increase awareness of this channel, and to draw together findings from a wide range of sources which may help to formulate new ideas for further studies.

The role of environmental calcium in the extreme acid tolerance of northern banjo frog (Limnodynastes terraereginae) larvae

Many aquatically respiring animals acutely exposed to low pH waters suffer inhibition of ion uptake, and loss of branchial (gill) epithelial integrity, culminating in a fatal loss of body Na+. Environmental calcium levels ([Ca2+]e) are pivotal in maintaining branchial junction integrity, with supplemental Ca2+ reversing the negative effects of low pH in some animals. Tolerance of some naturally acidic environments by aquatic animals is further complicated by low [Ca2+]e, yet many of these environments are surprisingly biodiverse. How animals overcome the damaging actions of low pH and low environmental Ca2+ remains unknown. We examined the effects of [Ca2+]e on the response to low pH in larvae of the highly acid tolerant frog Limnodynastes terraereginae. Acute exposure to low pH water in the presence of low (5 µmol L-1) [Ca2+]e increased net Na+ efflux. Provision of additional [Ca2+]e reduced net Na+ efflux, but the effect was saturable. Acclimation to both low and high (250 µmol L-1) [Ca2+]e improved the resistance of larvae to Na+ efflux at low pH. Exposure to the Ca2+ channel inhibitor ruthenium red resulted in an abrupt loss of tolerance in low pH acclimated larvae. Acclimation to acidic water increased branchial gene expression of the intracellular Ca2+ transport protein calbindin, consistent with a role for increased transcellular Ca2+ trafficking in the tolerance of acidic water. This study supports a role for [Ca2+]e in promoting branchial integrity and highlights a potential mechanism via the maintenance of transcellular Ca2+ uptake in the acid tolerance of L. terraereginae larvae.

Increased calcium uptake and improved trabecular bone properties in intestinal alkaline phosphatase knockout mice

Previous studies have demonstrated a negative correlation between intestinal alkaline phosphatase (IAP) activity and calcium (Ca) absorption in the gut, as IAP acts as a protective mechanism inhibiting high Ca entry into enterocytes, preventing Ca overload. Here we evaluated Ca absorption and bone properties in knockout mice (KO) completely devoid of duodenal IAP (Akp3 ^-/- mice). Female C57BL/6 control mice (WT, n = 7) and KO mice (n = 10) were used to determine Ca absorption in vivo and by in situ isolated duodenal loops followed by histomorphometric analysis of duodenal villi and crypts. Bone mineral density, morphometry, histomorphometry and trabecular connectivity and biomechanical properties were measured on bones. We observed mild atrophy of the villi with lower absorption surface and a significantly higher Ca uptake in KO mice. While no changes were seen in cortical bone, we found better trabecular connectivity and biomechanical properties in the femurs of KO mice compared to WT mice. Our data indicate that IAP KO mice display higher intestinal Ca uptake, which over time appears to correlate with a positive effect on the biomechanical properties of trabecular bone.

Coupling of TRPV6 and TMEM16A in epithelial principal cells of the rat epididymis

Principal cells regulate the ionic environment of the epididymal lumen via unknown mechanisms. Gao et al. use electrophysiological and pharmacological tools to characterize rat principal cells and reveal a TRPV6-mediated calcium conductance and TMEM16A-mediated calcium-activated chloride conductance.

The epididymis establishes a congenial environment for sperm maturation and protection. Its fluid is acidic, and the calcium concentration is low and declines along the length of the epididymal tubule. However, our knowledge of ionic currents and mechanisms of calcium homeostasis in rat epididymal epithelial cells remains enigmatic. In this study, to better understand calcium regulation in the epididymis, we use the patch-clamp method to record from single rat cauda epididymal principal cells. We detect a constitutively active Ca²⁺ current with characteristics that match the epithelial calcium channel TRPV6. Electrophysiological and pharmacological data also reveal a constitutively active calcium-activated chloride conductance (CaCC). Removal of extracellular calcium attenuates not only the TRPV6-like conductance, but also the CaCC. Lanthanide block is time dependent such that the TRPV6-like component is inhibited first, followed by the CaCC. The putative CaCC blocker niflumic acid partially inhibits whole-cell currents, whereas La³⁺ almost abolishes whole-cell currents in principal cells. Membrane potential measurements reveal an interplay between La³⁺-sensitive ion channels and those that are sensitive to the specific TMEM16A inhibitor tannic acid. In vivo perfusion of the cauda epididymal tubule shows a substantial rate of Ca²⁺ reabsorption from the luminal side, which is dose-dependently suppressed by ruthenium red, a putative blocker of epithelial Ca²⁺ channels and CaCC. Finally, we discover messenger RNA for both TRPV6 and TMEM16A in the rat epididymis and show that their proteins colocalize in the apical membrane of principal cells. Collectively, these data provide evidence for a coupling mechanism between TRPV6 and TMEM16A in principal cells that may play an important role in the regulation of calcium homeostasis in the epididymis.

TRPV5: a Ca(2+) channel for the fine-tuning of Ca(2+) reabsorption

TRPV5 is one of the two channels in the TRPV family that exhibit high selectivity to Ca(2+) ions. TRPV5 mediates Ca(2+) influx into cells as the first step to transport Ca(2+) across epithelia. The specialized distribution in the distal tubule of the kidney positions TRPV5 as a key player in Ca(2+) reabsorption. The responsiveness in expression and/or activity of TRPV5 to hormones such as 1,25-dihydroxyvitamin D3, parathyroid hormone, estrogen, and testosterone makes TRPV5 suitable for its role in the fine-tuning of Ca(2+) reabsorption. This role is further optimized by the modulation of TRPV5 trafficking and activity via its binding partners; co-expressed proteins; tubular factors such as calbindin-D28k, calmodulin, klotho, uromodulin, and plasmin; extracellular and intracellular factors such as proton, Mg(2+), Ca(2+), and phosphatidylinositol-4,5-bisphosphate; and fluid flow. These regulations allow TRPV5 to adjust its overall activity in response to the body's demand for Ca(2+) and to prevent kidney stone formation. A point mutation in mouse Trpv5 gene leads to hypercalciuria similar to Trpv5 knockout mice, suggesting a possible role of TRPV5 in hypercalciuric disorders in humans. In addition, the single nucleotide polymorphisms in Trpv5 gene prevalently present in African descents may contribute to the efficient renal Ca(2+) reabsorption among African descendants. TRPV5 represents a potential therapeutic target for disorders with altered Ca(2+) homeostasis.

TRP channels

TRP channels constitute a large superfamily of cation channel forming proteins, all related to the gene product of the transient receptor potential (trp) locus in Drosophila. In mammals, 28 different TRP channel genes have been identified, which exhibit a large variety of functional properties and play diverse cellular and physiological roles. In this article, we provide a brief and systematic summary of expression, function, and (patho)physiological role of the mammalian TRP channels.

Calcium reabsorption in the distal tubule: regulation by sodium, pH, and flow

We developed a mathematical model of Ca(2+) transport along the late distal convoluted tubule (DCT2) and the connecting tubule (CNT) to investigate the mechanisms that regulate Ca(2+) reabsorption in the DCT2-CNT. The model accounts for apical Ca(2+) influx across transient receptor potential vanilloid 5 (TRPV5) channels and basolateral Ca(2+) efflux via plasma membrane Ca(2+)-ATPase pumps and type 1 Na(+)/Ca(2+) exchangers (NCX1). Model simulations reproduce experimentally observed variations in Ca(2+) uptake as a function of extracellular pH, Na(+), and Mg(2+) concentration. Our results indicate that amiloride enhances Ca(2+) reabsorption in the DCT2-CNT predominantly by increasing the driving force across NCX1, thereby stimulating Ca(2+) efflux. They also suggest that because aldosterone upregulates both apical and basolateral Na(+) transport pathways, it has a lesser impact on Ca(2+) reabsorption than amiloride. Conversely, the model predicts that full NCX1 inhibition and parathyroidectomy each augment the Ca(2+) load delivered to the collecting duct severalfold. In addition, our results suggest that regulation of TRPV5 activity by luminal pH has a small impact, per se, on transepithelial Ca(2+) fluxes; the reduction in Ca(2+) reabsorption induced by metabolic acidosis likely stems from decreases in TRPV5 expression. In contrast, elevations in luminal Ca(2+) are predicted to significantly decrease TRPV5 activity via the Ca(2+)-sensing receptor. Nevertheless, following the administration of furosemide, the calcium-sensing receptor-mediated increase in Ca(2+) reabsorption in the DCT2-CNT is calculated to be insufficient to prevent hypercalciuria. Altogether, our model predicts complex interactions between calcium and sodium reabsorption in the DCT2-CNT.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Hearing loss associated with enlargement of the vestibular aqueduct: mechanistic insights from clinical phenotypes, genotypes, and mouse models

Enlargement of the vestibular aqueduct (EVA) is one of the most common inner ear malformations associated with sensorineural hearing loss in children. The delayed onset and progressive nature of this phenotype offer a window of opportunity to prevent or retard progression of hearing loss. EVA is not the direct cause of hearing loss in these patients, but rather is a radiologic marker for some underlying pathogenetic defect. Mutations of the SLC26A4 gene are a common cause of EVA. Studies of an Slc26a4 knockout mouse demonstrate that acidification and enlargement of the scala media are early events in the pathogenesis of deafness. The enlargement is driven by fluid secretion in the vestibular labyrinth and a failure of fluid absorption in the embryonic endolymphatic sac. Elucidating the mechanism of hearing loss may offer clues to potential therapeutic strategies.

Luminal calcium concentration controls intestinal calcium absorption by modification of intestinal alkaline phosphatase activity

Intestinal alkaline phosphatase (IAP) is a brush-border phosphomonoesterase. Its location suggests an involvement in the uptake of nutrients, but its role has not yet been defined. IAP expression parallels that of other proteins involved in Ca absorption under vitamin D stimulation. Experiments carried out in vitro with purified IAP have demonstrated an interaction between Ca and IAP. The gut is prepared to face different levels of Ca intake over time, but high Ca intake in a situation of a low-Ca diet over time would cause excessive entry of Ca into the enterocytes. The presence of a mechanism to block Ca entry and to avoid possible adverse effects is thus predictable. Thus, in the present study, Sprague-Dawley rats were fed with different amounts of Ca in the diet (0.2, 1 and 2 g%), and the percentage of Ca absorption (%Ca) in the presence and absence of L-phenylalanine (Phe) was calculated. The presence of Phe caused a significant increase in %Ca (52.3 (SEM 6.5) % in the presence of Phe v. 31.1 (sem 8.9) % in the absence of Phe, regardless of the amount of Ca intake; paired t test, P = 0.02). When data were analysed with respect to Ca intake, a significant difference was found only in the group with low Ca intake (paired t test, P = 0.03). Additionally, IAP activity increased significantly (ANOVA, P < 0.05) as Ca concentrations increased in the duodenal lumen. The present study provides in vivo evidence that luminal Ca concentration increases the activity of IAP and simultaneously decreases %Ca, acting as a minute-to-minute regulatory mechanism of Ca entry.

The calcium-permeable non-selective cation channel TRPM2 is modulated by cellular acidification

TRPM2 is a calcium-permeable non-selective cation channel expressed in the plasma membrane and in lysosomes that is critically involved in aggravating reactive oxygen species (ROS)-induced inflammatory processes and has been implicated in cell death. TRPM2 is gated by ADP-ribose (ADPR) and modulated by physiological processes that produce peroxide, cyclic ADP-ribose (cADPR), nicotinamide adenine dinucleotide phosphate (NAADP) and Ca(2+). We investigated the role of extra- and intracellular acidification on heterologously expressed TRPM2 in HEK293 cells. Our results show that TRPM2 is inhibited by external acidification with an IC(50) of pH 6.5 and is completely suppressed by internal pH of 6. Current inhibition requires channel opening and is strongly voltage dependent, being most effective at negative potentials. In addition, increased cytosolic pH buffering capacity or elevated [Ca(2+)](i) reduces the rate of current inactivation elicited by extracellular acidification, and Na(+) and Ca(2+) influence the efficacy of proton-induced inactivation. Together, these results suggest that external protons permeate TRPM2 channels to gain access to an intracellular site that regulates channel activity. Consistent with this notion, single-channel measurements in HEK293 cells reveal that internal protons induce channel closure without affecting single-channel conductance, whereas external protons affect channel open probability as well as single-channel conductance of native TRPM2 in neutrophils. We conclude that protons compete with Na(+) and Ca(2+) for channel permeation and channel closure results from a competitive antagonism of protons at an intracellular Ca(2+) binding site.

Chloride, gluconate, sulfate, and short-chain fatty acids affect calcium flux rates across the sheep forestomach epithelium

In ruminants, more than 50% of overall gastrointestinal Ca absorption can occur preintestinally, and the anions of orally applied Ca salts are thought to play an important role in stimulating ruminal Ca absorption. This assumption is based mainly on ion-exchange studies that have used gluconate as the control anion, which may bind Ca2+ ions and interfere with treatment effects. In the present study, we investigated the distinct effects of different anions on Ca absorption across the sheep rumen and on the concentration of free Ca2+ ions ([Ca2+]ion). We showed that gluconate, sulfate, and short-chain fatty acids (SCFA) remarkably reduced [Ca2+]ion in buffer solutions. Nevertheless, increasing the Cl or SCFA concentration by 60 mM stimulated net ruminal Ca absorption 5- to 7-fold, but these effects could be antagonized by gluconate. Therefore, ion-exchange experiments must be (re)evaluated very carefully, because changes in [Ca2+]ion in the presence of gluconate, sulfate, or SCFA not only might entail an underestimation of Ca flux rates, but also might have effects on other cellular pathways that are Ca2+ dependent. Concerning the optimal Ca supply for dairy cows, the present study suggests that CaCl2 formulations and Ca salts of the SCFA stimulate Ca absorption across the rumen wall and are beneficial in preventing or correcting a Ca deficiency.

Mechanism of urinary calcium regulation by urinary magnesium and pH

Urinary magnesium and pH are known to modulate urinary calcium excretion, but the mechanisms underlying these relationships are unknown. In this study, the data from 17 clinical trials in which urinary magnesium and pH were pharmacologically manipulated were analyzed, and it was found that the change in urinary calcium excretion is directly proportional to the change in magnesium excretion and inversely proportional to the change in urine pH; a regression equation was generated to relate these variables (R(2) = 0.58). For further exploration of these relationships, intravenous calcium chloride, magnesium chloride, or vehicle was administered to rats. Magnesium infusion significantly increased urinary calcium excretion (normalized to urinary creatinine), but calcium infusion did not affect magnesium excretion. Parathyroidectomy did not prevent this magnesium-induced hypercalciuria. The effect of magnesium loading on calciuria was still observed after treatment with furosemide, which disrupts calcium and magnesium absorption in the thick ascending limb, suggesting that the effect may be mediated by the distal nephron. The calcium channel TRPV5, normally present in the distal tubule, was expressed in Xenopus oocytes. Calcium uptake by TRPV5 was directly inhibited by magnesium and low pH. In summary, these data are compatible with the hypothesis that urinary magnesium directly inhibits renal calcium absorption, which can be negated by high luminal pH, and that this regulation likely takes place in the distal tubule.

Calciotropic and Magnesiotropic TRP Channels

Significant progress has been made into our understanding of the molecular mechanisms responsible for Ca2+ and Mg2+ homeostasis. Members of the transient receptor potential channel (TRP) superfamily proved essential to the maintenance of divalent cation levels by regulating their absorption from renal and intestinal lumina. This review highlights the molecular and functional aspects of these new calciotropic and magnesiotropic TRPs in health and disease.

Regulation of the epithelial calcium channel TRPV5 by extracellular factors

PURPOSE OF REVIEW

Recent studies have greatly increased our knowledge concerning the regulation of renal calcium handling. This review focuses on newly identified calciotropic factors present in the pro-urine and the mechanisms by which they control the transient receptor potential channel vanilloid subtype 5 (TRPV5) which forms the gatekeeper of active renal calcium reabsorption.

RECENT FINDINGS

The antiaging hormone klotho regulates TRPV5 activity via a novel mechanism modifying its glycosylation status, thereby entrapping the channel at the cell surface. Functional characterization of tissue kallikrein knockout mice revealed that these animals exhibit a pronounced hypercalciuria, comparable to the calcium leak observed in TRPV5 knockout mice. Recently, it has been demonstrated that tissue kallikrein stimulates active calcium reabsorption via the bradykinin receptor type 2 pathway involving protein kinase C-dependent activation of TRPV5. Finally, the extracellular pH appears to act as a dynamic switch controlling cell surface expression of TRPV5.

SUMMARY

Unraveling the molecular mechanisms of TRPV5 channel regulation by the antiaging hormone klotho, tissue kallikrein and extracellular pH demonstrated the existence of novel regulatory mechanisms of active calcium reabsorption acting from the tubular lumen.

Molecular determinants of Mg2+ and Ca2+ permeability and pH sensitivity in TRPM6 and TRPM7

The channel kinases TRPM6 and TRPM7 have recently been discovered to play important roles in Mg2+ and Ca2+ homeostasis, which is critical to both human health and cell viability. However, the molecular basis underlying these channels' unique Mg2+ and Ca2+ permeability and pH sensitivity remains unknown. Here we have created a series of amino acid substitutions in the putative pore of TRPM7 to evaluate the origin of the permeability of the channel and its regulation by pH. Two mutants of TRPM7, E1047Q and E1052Q, produced dramatic changes in channel properties. The I-V relations of E1052Q and E1047Q were significantly different from WT TRPM7, with the inward currents of 8- and 12-fold larger than TRPM7, respectively. The binding affinity of Ca2+ and Mg2+ was decreased by 50- to 140-fold in E1052Q and E1047Q, respectively. Ca2+ and Mg2+ currents in E1052Q were 70% smaller than those of TRPM7. Strikingly, E1047Q largely abolished Ca2+ and Mg2+ permeation, rendering TRPM7 a monovalent selective channel. In addition, the ability of protons to potentiate inward currents was lost in E1047Q, indicating that E1047 is critical to Ca2+ and Mg2+ permeability of TRPM7, and its pH sensitivity. Mutation of the corresponding residues in the pore of TRPM6, E1024Q and E1029Q, produced nearly identical changes to the channel properties of TRPM6. Our results indicate that these two glutamates are key determinants of both channels' divalent selectivity and pH sensitivity. These findings reveal the molecular mechanisms underpinning physiological/pathological functions of TRPM6 and TRPM7, and will extend our understanding of the pore structures of TRPM channels.

Loss of cochlear HCO3- secretion causes deafness via endolymphatic acidification and inhibition of Ca2+ reabsorption in a Pendred syndrome mouse model

Pendred syndrome, characterized by childhood deafness and postpuberty goiter, is caused by mutations of SLC26A4, which codes for the anion exchanger pendrin. The goal of the present study was to determine how loss of pendrin leads to hair cell degeneration and deafness. We evaluated pendrin function by ratiometric microfluorometry, hearing by auditory brain stem recordings, and expression of K(+) and Ca(2+) channels by confocal immunohistochemistry. Cochlear pH and Ca(2+) concentrations and endocochlear potential (EP) were measured with double-barreled ion-selective microelectrodes. Pendrin in the cochlea was characterized as a formate-permeable and DIDS-sensitive anion exchanger that is likely to mediate HCO(3)(-) secretion into endolymph. Hence endolymph in Slc26a4(+/-) mice was more alkaline than perilymph, and the loss of pendrin in Slc26a4(-/-) mice led to an acidification of endolymph. The stria vascularis of Slc26a4(-/-) mice expressed the K(+) channel Kcnj10 and generated a small endocochlear potential before the normal onset of hearing at postnatal day 12. This small potential and the expression of Kcnj10 were lost during further development, and Slc26a4(-/-) mice did not acquire hearing. Endolymphatic acidification may be responsible for inhibition of Ca(2+) reabsorption from endolymph via the acid-sensitive epithelial Ca(2+) channels Trpv5 and Trpv6. Hence the endolymphatic Ca(2+) concentration was found elevated in Slc26a4(-/-) mice. This elevation may inhibit sensory transduction necessary for hearing and promote the degeneration of the sensory hair cells. Degeneration of the hair cells closes a window of opportunity to restore the normal development of hearing in Slc26a4(-/-) mice and possibly human patients suffering from Pendred syndrome.

TRPV5, the gateway to Ca2+ homeostasis

Ca2+ homeostasis in the body is tightly controlled, and is a balance between absorption in the intestine, excretion via the urine, and exchange from bone. Recently, the epithelial Ca2+ channel (TRPV5) has been identified as the gene responsible for the Ca2+ influx in epithelial cells of the renal distal convoluted tubule. TRPV5 is unique within the family of transient receptor potential (TRP) channels due to its high Ca2+ selectivity. Ca2+ flux through TRPV5 is controlled in three ways. First, TRPV5 gene expression is regulated by calciotropic hormones such as vitamin D3 and parathyroid hormone. Second, Ca2+ transport through TRPV5 is controlled by modulating channel activity. Intracellular Ca2+, for example, regulates channel activity by feedback inhibition. Third, TRPV5 is controlled by mobilization of the channel through trafficking toward the plasma membrane. The newly identified anti-aging hormone Klotho regulates TRPV5 by cleaving off sugar residues from the extracellular domain of the protein, resulting in a prolonged expression of TRPV5 at the plasma membrane. Inactivation of TRPV5 in mice leads to severe hypercalciuria, which is compensated by increased intestinal Ca2+ absorption due to augmented vitamin D3 levels. Furthermore, TRPV5 deficiency in mice is associated with polyuria, urine acidification, and reduced bone thickness. Some pharmaceutical compounds, such as the immunosuppressant FK506, affect the Ca2+ balance by modulating TRPV5 gene expression. This underlines the importance of elucidating the role of TRPV5 in Ca(2+)-related disorders, thereby enhancing the possibilities for pharmacological intervention. This chapter describes a unique TRP channel and highlights its regulation and function in renal Ca2+ reabsorption and overall Ca2+ homeostasis.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

Extracellular pH dynamically controls cell surface delivery of functional TRPV5 channels

Extracellular pH has long been known to affect the rate and magnitude of ion transport processes among others via regulation of ion channel activity. The Ca(2+)-selective transient receptor potential vanilloid 5 (TRPV5) channel constitutes the apical entry gate in Ca(2+)-transporting cells, contributing significantly to the overall Ca(2+) balance. Here, we demonstrate that extracellular pH determines the cell surface expression of TRPV5 via a unique mechanism. By a comprehensive approach using total internal reflection fluorescence microscopy, cell surface protein labeling, electrophysiology, (45)Ca(2+) uptake assays, and functional channel recovery after chemobleaching, this study shows that upon extracellular alkalinization, a pool of TRPV5-containing vesicles is rapidly recruited to the cell surface without collapsing into the plasma membrane. These vesicles contain functional TRPV5 channels since extracellular alkalinization is accompanied by increased TRPV5 activity. Conversely, upon subsequent extracellular acidification, vesicles are retrieved from the plasma membrane, simultaneously resulting in decreased TRPV5 activity. Thus, TRPV5 accesses the extracellular compartment via transient openings of vesicles, suggesting that rapid responses of constitutive active TRP channels to physiological stimuli rely on vesicular "kiss and linger" interactions with the plasma membrane.

Effect of chloride on pH microclimate and electrogenic Na+ absorption across the rumen epithelium of goat and sheep

Active Na+ absorption across rumen epithelium comprises Na+/H+ exchange and a nonselective cation conductance (NSCC). Luminal chloride is able to stimulate Na+ absorption, which has been attributed to an interaction between Cl-/HCO3- and Na+/H+ exchangers. However, isolated rumen epithelial cells also express a Cl- conductance. We investigated whether Cl- has an additional effect on electrogenic Na+ absorption via NSCC. NSCC was estimated from short-circuit current (Isc) across epithelia of goat and sheep rumen in Ussing chambers. Epithelial surface pH (pHs) was measured with 5-N-hexadecanoyl-aminofluorescence. Membrane potentials were measured with microelelectrodes. Luminal, but not serosal, Cl- stimulated the Ca2+ and Mg2+ sensitive Isc. This effect was independent of the replacing anion (gluconate or acetate) and of the presence of bicarbonate. The mean pHs of rumen epithelium amounted to 7.47 +/- 0.03 in a low-Cl- solution. It was increased by 0.21 pH units when luminal Cl- was increased from 10 to 68 mM. Increasing mucosal pH from 7.5 to 8.0 also increased the Ca2+ and Mg2+ sensitive Isc and transepithelial conductance and reduced the fractional resistance of the apical membrane. Luminal Cl- depolarized the apical membrane of rumen epithelium. 5-Nitro-2-(3-phenylpropylamino)-benzoate reduced the divalent cation sensitive Isc, but only in low-Cl- solutions. The results show that luminal Cl- can increase the microclimate pH via apical Cl-/HCO3- or Cl-/OH- exchangers. Electrogenic Na+ absorption via NSCC increases with pH, explaining part of the Cl- effects on Na+ absorption. The data further show that the Cl- conductance of rumen epithelium must be located at the basolateral membrane.

Regulation of TRPV5 and TRPV6 by associated proteins

The epithelial Ca2+ channels TRPV5 and TRPV6 are the most Ca2+-selective members of the TRP channel superfamily. These channels are the prime target for hormonal control of the active Ca2+ flux from the urine space or intestinal lumen to the blood compartment. Insight into their regulation is, therefore, pivotal in our understanding of the (patho)physiology of Ca2+ homeostasis. The recent elucidation of TRPV5/6-associated proteins has provided new insight into the molecular mechanisms underlying the regulation of these channels. In this review, we describe the various means of TRPV5/6 regulation, the role of channel-associated proteins herein, and the relationship between both processes.

Permeation and selectivity of TRP channels

Ion channels are pore-forming transmembrane proteins that allow ions to permeate biological membranes. Pore structure plays a crucial role in determining the ion permeation and selectivity properties of particular channels. In the past few decades, efforts have been undertaken to identify key elements of the pore regions of different classes of ion channels. In this review, we summarize current knowledge about permeation and selectivity of channel proteins from the transient receptor potential (TRP) superfamily. Whereas all TRP channels are permeable for cations, only two TRP channels are impermeable for Ca2+ (TRPM4, TRPM5), and two others are highly Ca2+ permeable (TRPV5, TRPV6). Despite the great advances in the TRP channel field during the past decade, only a limited number of reports have dealt with functional characterization of pore properties, biophysical aspects of cation permeation, or description of pore structures of TRP channels. This review gives an overview of available experimental and theoretical data and discusses the functional impact of pore-structure modifications on TRP channel properties.

Acid-base status determines the renal expression of Ca2+ and Mg2+ transport proteins

Chronic metabolic acidosis results in renal Ca2+ and Mg2+ wasting, whereas chronic metabolic alkalosis is known to exert the reverse effects. It was hypothesized that these adaptations are mediated at least in part by the renal Ca2+ and Mg2+ transport proteins. The aim of this study, therefore, was to determine the effect of systemic acid-base status on renal expression of the epithelial Ca2+ channel TRPV5, the Ca2+-binding protein calbindin-D28K, and the epithelial Mg2+ channel TRPM6 in relation to Ca2+ and Mg2+ excretion. Chronic metabolic acidosis that was induced by NH4Cl loading or administration of the carbonic anhydrase inhibitor acetazolamide for 6 d enhanced calciuresis accompanied by decreased renal TRPV5 and calbindin-D28K mRNA and protein abundance in wild-type mice. In contrast, metabolic acidosis did not affect Ca2+ excretion in TRPV5 knockout (TRPV5-/-) mice, in which active Ca2+ reabsorption is effectively abolished. This demonstrates that downregulation of renal Ca2+ transport proteins is responsible for the hypercalciuria. Conversely, chronic metabolic alkalosis that was induced by NaHCO3 administration for 6 d increased the expression of Ca2+ transport proteins accompanied by diminished urine Ca2+ excretion in wild-type mice. However, this Ca2+-sparing action persisted in TRPV5-/- mice, suggesting that additional mechanisms apart from upregulation of active Ca2+ transport contribute to the hypocalciuria. Furthermore, chronic metabolic acidosis decreased renal TRPM6 expression, increased Mg2+ excretion, and decreased serum Mg2+ concentration, whereas chronic metabolic alkalosis resulted in the exact opposite effects. In conclusion, these data suggest that regulation of Ca2+ and Mg2+ transport proteins contributes importantly to the effects of acid-base status on renal divalent handling.

TRPV5 and TRPV6 in Ca(2+) (re)absorption: regulating Ca(2+) entry at the gate

Many physiological functions rely on the exact maintenance of body Ca(2+) balance. Therefore, the extracellular Ca(2+) concentration is tightly regulated by the concerted actions of intestinal Ca(2+) absorption, exchange of Ca(2+) to and from bone, and renal Ca(2+) reabsorption. Renal distal convoluted and connecting tubular cells as well as duodenal epithelial cells are unique in their ability to mediate transcellular (re)absorption of Ca(2+) at large and highly variable rates. Two members of the transient receptor potential (TRP) superfamily, TRP vanilloid (TRPV)5 and TRPV6, are specialized epithelial Ca(2+) channels responsible for the critical Ca(2+) entry step in transcellular Ca(2+) (re)absorption in intestine and kidney, respectively. Because transcellular Ca(2+) transport is fine-tuned to the body's specific requirements, regulation of the transmembrane Ca(2+) flux through TRPV5/6 is of particular importance and has, therefore, to be conspicuously controlled. We present an overview of the current knowledge and recent advances concerning the coordinated regulation of Ca(2+) influx through the epithelial Ca(2+) channels TRPV5 and TRPV6 in transcellular Ca(2+) (re)absorption.

TRPV5 and TRPV6 in Ca(2+) (re)absorption: regulating Ca(2+) entry at the gate

Many physiological functions rely on the exact maintenance of body Ca(2+) balance. Therefore, the extracellular Ca(2+) concentration is tightly regulated by the concerted actions of intestinal Ca(2+) absorption, exchange of Ca(2+) to and from bone, and renal Ca(2+) reabsorption. Renal distal convoluted and connecting tubular cells as well as duodenal epithelial cells are unique in their ability to mediate transcellular (re)absorption of Ca(2+) at large and highly variable rates. Two members of the transient receptor potential (TRP) superfamily, TRP vanilloid (TRPV)5 and TRPV6, are specialized epithelial Ca(2+) channels responsible for the critical Ca(2+) entry step in transcellular Ca(2+) (re)absorption in intestine and kidney, respectively. Because transcellular Ca(2+) transport is fine-tuned to the body's specific requirements, regulation of the transmembrane Ca(2+) flux through TRPV5/6 is of particular importance and has, therefore, to be conspicuously controlled. We present an overview of the current knowledge and recent advances concerning the coordinated regulation of Ca(2+) influx through the epithelial Ca(2+) channels TRPV5 and TRPV6 in transcellular Ca(2+) (re)absorption.

Invertebrate TRP proteins as functional models for mammalian channels

Transient receptor potential (TRP) channels constitute a large and diverse family of channel proteins that are expressed in many tissues and cell types in both vertebrates and invertebrates. While the biophysical features of many of the mammalian TRP channels have been described, relatively little is known about their biological roles. Invertebrate TRPs offer valuable genetic handles for characterizing the functions of these cation channels in vivo. Importantly, studies in model organisms can help to identify fundamental mechanisms involved in normal cellular functions and human disease. In this review, we give an overview of the different TRP channels known in the two most utilized invertebrate models, the nematode Caenorhabditis elegans and the fruit-fly Drosophila melanogaster, and discuss briefly the heuristic impact of these invertebrate channels with respect to TRP function in mammals.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

Recurrent calcium nephrolithiasis associated with primary aldosteronism

Typical manifestations of hyperaldosteronism include salt retention, hypokalemia, and metabolic alkalosis. However, a consequence infrequently recognized and described is hypocitraturia. In combination with hypercalciuria, aldosterone-induced hypocitraturia can trigger calcium nephrolithiasis. The authors report a case of an individual with primary hyperaldosteronism from an adrenal adenoma that resulted in hypocitraturia. The patient had severe recurrent renal calcium calculi that corrected with adrenalectomy. The clinical physiology of renal calcium and citrate handling in hyperaldosteronism is reviewed.

Regulation of calcium signaling by polycystin-2

Autosomal dominant PKD (ADPKD) is a common lethal genetic disorder characterized by progressive development of fluid-filled cysts in the kidney and other target organs. ADPKD is caused by mutations in the PKD1 and PKD2 genes, encoding the transmembrane proteins polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Although the function and putative interacting ligands of PC1 are largely unknown, recent evidence indicates that PC2 behaves as a TRP-type Ca2+-permeable nonselective cation channel. The PC2 channel is implicated in the transient increase in cytosolic Ca2+in renal epithelial cells and may be linked to the activation of subsequent signaling pathways. Recent studies also indicate that PC1 functionally interacts with PC2 such that the PC1-PC2 channel complex is an obligatory novel signaling pathway implicated in the transduction of environmental signals into cellular events. The present review purposely avoids issues of regulation of PC2 expression and trafficking and focuses instead on the evidence for the TRP-type cation channel function of PC2. How its role as a cation channel may unmask mechanisms that trigger Ca2+transport and regulation is the focus of attention. PC2 channel function may be essential in renal cell function and kidney development. Nonrenal-targeted expression of PC2 and related proteins, including the cardiovascular system, also suggests previously unforeseeable roles in signal transduction.

Extracellular Zinc and ATP Restore Chloride Secretion across Cystic Fibrosis Airway Epithelia by Triggering Calcium Entry

Cystic fibrosis (CF) is caused by defective cyclic AMP-dependent cystic fibrosis transmembrane conductance regulator Cl(-) channels. Thus, CF epithelia fail to transport Cl(-) and water. A postulated therapeutic avenue in CF is activation of alternative Ca(2+)-dependent Cl(-) channels. We hypothesized that stimulation of Ca(2+) entry from the extracellular space could trigger a sustained Ca(2+) signal to activate Ca(2+)-dependent Cl(-) channels. Cytosolic [Ca(2+)](i) was measured in non-polarized human CF (IB3-1) and non-CF (16HBE14o(-)) airway epithelial cells. Primary human CF and non-CF airway epithelial monolayers as well as Calu-3 monolayers were used to assess anion secretion. In vivo nasal potential difference measurements were performed in non-CF and two different CF mouse (DeltaF508 homozygous and bitransgenic gut-corrected but lung-null) models. Zinc and ATP induced a sustained, reversible, and reproducible increase in cytosolic Ca(2+) in CF and non-CF cells with chemistry and pharmacology most consistent with activation of P2X purinergic receptor channels. P2X purinergic receptor channel-mediated Ca(2+) entry stimulated sustained Cl(-) and HCO(3)(-) secretion in CF and non-CF epithelial monolayers. In non-CF mice, zinc and ATP induced a significant Cl(-) secretory response similar to the effects of agonists that increase intracellular cAMP levels. More importantly, in both CF mouse models, Cl(-) permeability of nasal epithelia was restored in a sustained manner by zinc and ATP. These effects were reversible and reacquirable upon removal and readdition of agonists. Our data suggest that activation of P2X calcium entry channels may have profound therapeutic benefit for CF that is independent of cystic fibrosis transmembrane conductance regulator genotype.

TRPV4 calcium entry channel: a paradigm for gating diversity

The vanilloid receptor-1 (VR1, now TRPV1) was the founding member of a subgroup of cation channels within the TRP family. The TRPV subgroup contains six mammalian members, which all function as Ca2+ entry channels gated by a variety of physical and chemical stimuli. TRPV4, which displays 45% sequence identity with TRPV1, is characterized by a surprising gating promiscuity: it is activated by hypotonic cell swelling, heat, synthetic 4alpha-phorbols, and several endogenous substances including arachidonic acid (AA), the endocannabinoids anandamide and 2-AG, and cytochrome P-450 metabolites of AA, such as epoxyeicosatrienoic acids. This review summarizes data on TRPV4 as a paradigm of gating diversity in this subfamily of Ca2+ entry channels.

Mechanism and Molecular Determinant for Regulation of Rabbit Transient Receptor Potential Type 5 (TRPV5) Channel by Extracellular pH

The transient receptor potential type 5 (TRPV5) channel is present in kidney and intestine and important for transepithelial (re)absorption of calcium in these tissues. We report that in whole-cell patch clamp recording extracellular acidification inhibited rabbit TRPV5 with apparent pKa approximately 6.55. The two extracellular loops between the fifth and sixth transmembrane segments of TRPV5 presumably form part of the outer opening of the pore and likely are important in binding and regulation by external protons. We found that mutation of glutamate 522 to glutamine (E522Q) decreased the sensitivity of the channel to extracellular acidification. Mutations of other titratable amino acids within the two extracellular loops to non-titratable amino acids had no effect on pH sensitivity. Substitutions of aspartate or other titratable amino acids for glutamate 522 conferred an increase in pH sensitivity. The pH sensitivity mediated by glutamate 522 was independent of extracellular or intracellular Mg2+. Single channel analysis revealed that extracellular acidification reduced single channel conductance as well as open probability of the wild type channel. In contrast to wild type channel, extracellular acidification did not reduce open probability for E522Q mutant. Methanethiosulfonate reagents inhibited the activity of glutamine 522 to cysteine mutant channel with a reaction rate constant approaching that with free thiols in solution, suggesting that glutamate 522 is located on the surface of the channel. These data suggest that glutamate 522 of the rabbit TRPV5 is a "pH sensor," and extracellular protons inhibit TRPV5 likely by altering conformation of the channel protein.

The epithelial calcium channels, TRPV5 & TRPV6: from identification towards regulation

The epithelial calcium channels, TRPV5 and TRPV6, have been extensively studied in epithelial tissues controlling the Ca(2+) homeostasis and exhibit a range of distinctive properties that distinguish them from other TRP channels. This review focuses on the tissue distribution, the functional properties, the architecture and the regulation of the expression and activity of the TRPV5 and TRPV6 channel.

Sustained calcium entry through P2X nucleotide receptor channels in human airway epithelial cells

Purinergic receptor stimulation has potential therapeutic effects for cystic fibrosis (CF). Thus, we explored roles for P2Y and P2X receptors in stably increasing [Ca(2+)](i) in human CF (IB3-1) and non-CF (16HBE14o(-)) airway epithelial cells. Cytosolic Ca(2+) was measured by fluorospectrometry using the fluorescent dye Fura-2/AM. Expression of P2X receptor (P2XR) subtypes was assessed by immunoblotting and biotinylation. In IB3-1 cells, ATP and other P2Y agonists caused only a transient increase in [Ca(2+)](i) derived from intracellular stores in a Na(+)-rich environment. In contrast, ATP induced an increase in [Ca(2+)](i) that had transient and sustained components in a Na(+)-free medium; the sustained plateau was potentiated by zinc or increasing extracellular pH. Benzoyl-benzoyl-ATP, a P2XR-selective agonist, increased [Ca(2+)](i) only in Na(+)-free medium, suggesting competition between Na(+) and Ca(2+) through P2XRs. Biochemical evidence showed that the P2X(4) receptor is the major subtype shared by these airway epithelial cells. A role for store-operated Ca(2+) channels, voltage-dependent Ca(2+) channels, or Na(+)/Ca(2+) exchanger in the ATP-induced sustained Ca(2+) signal was ruled out. In conclusion, these data show that epithelial P2X(4) receptors serve as ATP-gated calcium entry channels that induce a sustained increase in [Ca(2+)](i). In airway epithelia, a P2XR-mediated Ca(2+) signal may have therapeutic benefit for CF.

Molecular mechanism of active Ca2+ reabsorption in the distal nephron

The identification of the epithelial Ca(2+) channel (ECaC) complements the group of Ca(2+) transport proteins including calbindin-D28K, Na(+)/Ca(2+) exchanger and plasma membrane Ca(2+)-ATPase, which are co-expressed in 1,25(OH)2D3- responsive nephron segments. ECaC constitutes the rate-limiting apical entry step in the process of active transcellular Ca(2+) transport and belongs to a superfamily of Ca(2+) channels that includes the vanilloid receptor and transient receptor potential channels. This new Ca(2+) channel consists of six transmembrane-spanning domains, including a pore-forming hydrophobic stretch between domain 5 and 6. The C- and N-terminal tails contain several conserved regulatory sites, implying that the channel function is modulated by regulatory proteins. The distinctive functional properties of ECaC include a constitutively activated Ca(2+) permeability, a high selectivity for Ca(2+), hyperpolarization-stimulated and Ca(2+)-dependent feedback regulation of channel activity, and 1,25(OH)2D3-induced gene activation. This review covers the distinctive properties of this new highly Ca(2+)-selective channel and highlights the implications for active transcellular Ca(2+) reabsorption in health and disease.

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.

Function and expression of the epithelial Ca(2+) channel family: comparison of mammalian ECaC1 and 2

1. The epithelial Ca(2+) channel (ECaC) family represents a unique group of Ca(2+)-selective channels that share limited homology to the ligand-gated capsaicin receptors, the osmolarity-sensitive channel OTRPC4, as well as the transient receptor potential family. Southern blot analysis demonstrated that this family is restricted to two members, ECaC1 and ECaC2 (also named CaT1). 2. RT-PCR analysis demonstrated that the two channels are co-expressed in calbindin-D-containing epithelia, including small intestine, pancreas and placenta, whereas kidney and brain only express ECaC1 and stomach solely ECaC2. 3. From an electrophysiological point of view, ECaC1 and ECaC2 are highly similar channels. Differences concern divalent cation permeability, the kinetics of Ca(2+)-dependent inactivation and recovery from inactivation. 4. Ruthenium red is a potent blocker of ECaC activity. Interestingly, ECaC2 has a 100-fold lower affinity for ruthenium red (IC(50) 9 +/- 1 microM) than ECaC1 (IC(50) 121 +/- 13 nM). 5. ECaCs are modulated by intracellular Mg(2+) and ATP. ECaC1 and ECaC2 activity rapidly decay in the absence of intracellular ATP. This effect is further accelerated at higher intracellular Mg(2+) concentrations. 6. In conclusion, ECaC1 and ECaC2 are homologous channels, with an almost identical pore region. They can be discriminated by their sensitivity for ruthenium red and show differences in Ca(2+)-dependent regulation.