Cellular Calcium Transport Research Articles

Vitamin D3 mediated peptides-calcium chelate self-assembly: Fabrication, stability and improvement on cellular calcium transport

A self-assembled peptides-calcium-Vitamin D3 ternary delivery system (CSPH-Ca-VD3) was prepared to investigate the promotion of cellular calcium transport. The constructed CSPH-Ca-VD3 nanocomplex exhibited a spherical structure with a size of 135.2 ± 10.2 nm. Based on the thermodynamic calculation of fluorescent spectra, hydrophobic interaction was shown as the major driving force for this nanocomplex structure. CSPH-Ca-VD3 nanocomplex possessed excellent stability during simulated gastrointestinal digestion, contributing to the prevention of acid degradation of VD3 and the enhancement of calcium solubility. Furthermore, the calcium transport efficiency in the form of CSPH-Ca-VD3 (4 mg/mL) across a Caco-2 cells monolayer was significantly increased 2.3-fold compared to that of free Ca2+, mainly attributed to the upregulation in the presence of CSPH-Ca-VD3 of TRPV6, calbindin D9k and PMCA1b expression in Caco-2 cells. The present study provided a basis for developing a novel delivery system of peptides-calcium chelate with the dual effects of VD3 protection and calcium uptake promotion.

Structure, Function, and Applications of Soybean Calcium Transporters.

Glycine max is a calcium-loving crop. The external application of calcium fertilizer is beneficial to the increase of soybean yield. Indeed, calcium is a vital nutrient in plant growth and development. As a core metal ion in signaling transduction, calcium content is maintained in dynamic balance under normal circumstances. Now, eight transporters were found to control the uptake and efflux of calcium. Though these calcium transporters have been identified through genome-wide analysis, only a few of them were functionally verified. Therefore, in this study, we summarized the current knowledge of soybean calcium transporters in structural features, expression characteristics, roles in stress response, and prospects. The above results will be helpful in understanding the function of cellular calcium transport and provide a theoretical basis for elevating soybean yield.

Hydrogen Sulfide Regulates SERCA2a Ubiquitylation via Muscle RING Finger-1 S-Sulfhydration to Affect Cardiac Contractility in db/db Mice.

Hydrogen sulfide (H2S), as a gasotransmitter, is involved in various pathophysiological processes. Diabetic cardiomyopathy (DCM) is a major complication of diabetes mellitus (DM), which leads to structural and functional abnormalities of the myocardium and eventually causes heart failure (HF). Systolic and diastolic dysfunction are fundamental features of heart failure. SERCA2a, as a key enzyme for calcium transport in the endoplasmic reticulum (ER), affects the process of myocardial relaxation and contraction. H2S can protect the cardiac function against diabetic hearts, however, its mechanisms are unclear. This study found that exogenous H2S affects cellular calcium transport by regulating the H2S/MuRF1/SERCA2a/cardiac contractile pathway. Our results showed that, compared with the db/db mice, exogenous H2S restored the protein expression levels of CSE and SERCA2a, and the activity of SERCA2a, while reducing cytosolic calcium concentrations and MuRF1 expression. We demonstrated that MuRF1 could interact with SERCA2a via co-immunoprecipitation. Using LC-MS/MS protein ubiquitylation analysis, we identified 147 proteins with increased ubiquitination levels, including SERCA2a, in the cardiac tissues of the db/db mice compared with NaHS-treated db/db mice. Our studies further revealed that NaHS administration modified MuRF1 S-sulfhydration and enhanced the activity and expression of SERCA2a. Under hyperglycemia and hyperlipidemia, overexpression of the MuRF1-Cys44 mutant plasmid reduced the S-sulfhydration level of MuRF1 and decreased the ubiquitination level of SERCA2a and the intracellular Ca2+ concentration. These findings suggested that H2S modulates SERCA2a ubiquitination through MuRF1 S-sulfhydration of Cys44 to prevent decreased myocardial contractility due to increased cytosolic calcium.

Parkinson's Disease and Impairment in Mitochondrial Metabolism: A Pathognomic Signature.

Mitochondrial bioenergetics is vital for the proper functioning of cellular compartments. Impairments in mitochondrial DNA encoding the respiratory chain complexes and other assisting proteins, accumulation of intracellular reactive oxygen species, an imbalance in cellular calcium transport, or the presence of organic pollutants, high fat-ketogenic diets or toxins, and advancing age can result in complex disorders, including cancer, metabolic disease, and neurodegenerative disorders. Such manifestations are distinctly exhibited in several age-related neurodegenerative diseases, such as in Parkinson's disease (PD). Defects in complex I along with perturbed signaling pathways is a common manifestation of PD. Impaired oxidative phosphorylation could increase the susceptibility to PD. Therefore, unraveling the mechanisms of mitochondrial complexes in clinical scenarios will assist in developing potential early biomarkers and standard tests for energy failure diagnosis and assist to pave a new path for targeted therapeutics against PD.

Molecular Mechanisms of Bortezomib Action: Novel Evidence for the miRNA-mRNA Interaction Involvement.

Bortezomib is an anti-tumor agent, which inhibits 26S proteasome degrading ubiquitinated proteins. While apoptotic transcription-associated activation in response to bortezomib has been suggested, mechanisms related to its influence on post-transcriptional gene silencing mediated regulation by non-coding RNAs remain not fully elucidated. In the present study, we examined changes in global gene and miRNA expression and analyzed the identified miRNA–mRNA interactions after bortezomib exposure in human neuroblastoma cells to define pathways affected by this agent in this type of cells. Cell viability assays were performed to assess cytotoxicity of bortezomib. Global gene and miRNA expression profiles of neuroblastoma cells after 24-h incubation with bortezomib were determined using genome-wide RNA and miRNA microarray technology. Obtained results were then confirmed by qRT-PCR and Western blot. Further bioinformatical analysis was performed to identify affected biological processes and pathways. In total, 719 genes and 28 miRNAs were downregulated, and 319 genes and 61 miRNAs were upregulated in neuroblastoma cells treated with bortezomib. Possible interactions between dysregulated miRNA/mRNA, which could be linked to bortezomib-induced neurotoxicity, affect neurogenesis, cellular calcium transport, and neuron death. Bortezomib might exert toxic effects on neuroblastoma cells and regulate miRNA–mRNA interactions influencing vital cellular functions. Further studies on the role of specific miRNA–mRNA interactions are needed to elucidate mechanisms of bortezomib action.

Multiple myeloma with primary systemic amyloidosis affecting cutaneous, gastro intestinal and cardio-vascular system

Amyloidosis refers to a group of disorders characterized by extracellular deposition of protein fibrils. Primary systemic amyloidosis is commonly due to an underlying plasma cell dyscrasia. Infiltrative amyloid cardiomyopathy is a rare cause of predominantly diastolic myocardial disease. Restrictive cardiomyopathy is the main finding in cardiac amyloidosis and results from the replacement of normal myocardial contractile elements by infiltration and interstitial deposits of amyloid, leading to alterations in cellular metabolism, calcium transport, receptor regulation, and cellular edema. Injury can also occur from circulating light chains in the absence of amyloid fibril formation. Cardiac amyloidosis should be considered in any patient presenting with congestive heart failure, preserved systolic function, and a discrepancy between a low QRS voltage on electrocardiography and an apparent left ventricular hypertrophy on sonogram. The pattern of left ventricular diastolic dysfunction changes during the course of amyloidosis and the classically described restrictive physiology occurs only in advanced stages of the disease. This is a case report where an unusual extensive cutaneous and cardiac involvement provided the sign of widespread systemic deposition of amyloid protein which eventually led to the diagnosis of multiple myeloma.

Antihypertensive and neuroprotective actions of pyridoxine and its derivatives.

Vitamin B6 plays a crucial role in the nervous system as the amino acid decarboxylases involved in the synthesis of all putative neurotransmitters requires the coenzyme pyridoxal phosphate. Vitamin B6 in its various forms has antioxidant properties. Pyridoxal phosphate has a role in regulating cellular calcium transport through both the voltage-mediated and ATP-mediated purinergic mechanisms of cellular calcium influx and, hence, has a role in the control of hypertension. Pharmacological doses of vitamin B6 appear to decrease the high blood pressure associated with both genetic and nongenetic models of hypertension. Vitamin B6 has a crucial role in the normal function of the central and peripheral nervous systems. It also protects against ischemia and glutamate-induced neurotoxicity.

Signal transducer and activator of transcription 5a inhibited by pimozide may regulate survival of goat mammary gland epithelial cells by regulating parathyroid hormone-related protein

The signal transducer and activator of transcription 5a (Stat5a) modulates genes involved in proliferation and survival and plays pivotal roles in regulating the function of the mammary gland during pregnancy, lactation, and involution. However, there is little information about the effects of Stat5a on apoptosis of goat mammary gland epithelial cells (GMECs). In addition, parathyroid hormone-related protein (PTHrP) is a key regulator in cellular calcium transport, mammary gland development and breast tumor biology. This study aimed to explore the interaction of Stat5a and PTHrP in GMEC apoptosis. Quantitative real time PCR (qRT-PCR) suggested that Stat5a was predominantly expressed in the mammary gland, lung, liver and spleen of goats. Treating the GMECs with pimozide, an inhibitor of Stat5a that decreases Stat5a tyrosine phosphorylation, increased PTHrP levels in GMECs in a dose-dependent manner and simultaneously promoted apoptosis of the GMECs. We also demonstrated that PTHrP inhibition induced GMEC apoptosis and restrained cell proliferation. In contrast, PTHrP overexpression protected GMECs from pimozide- and calcium-induced apoptosis, and promoted cell proliferation. Furthermore, pimozide and CaCl2 downregulated the antiapoptotic protein Bcl-2 mRNA expression, respectively, and these effects were protected by PTHrP overexpression. Interestingly, we also found that Stat5a suppressed the expression of matrix metalloproteinase 9 (MMP-9) which can induce goat mammary epithelial cell migration, but PTHrP increased MMP-9 mRNA level. Thus, Stat5a may regulate GMEC survival by regulating the expression of PTHrP.

Calcium regulation in aortic smooth muscle cells during the initial phase of tunicamycin-induced endo/sarcoplasmic reticulum stress

Endo/sarcoplasmic reticulum stress and the unfolded protein response have been implicated as underlying mechanisms of cell death in many pathological conditions. We have confirmed that long-term exposure to 10µM tunicamycin induced the endo/sarcoplasmic reticulum stress in cultured vascular smooth muscle cells. Since tunicamycin is reported to induce the stress response by inhibiting protein glycosylation, we attempted to investigate a causal link between accumulation of unfolded proteins and dysregulation of cellular calcium transport. However, we found that tunicamycin caused an immediate release of calcium from the endo/sarcoplasmic reticulum, which was sensitive to thapsigargin, and an influx of calcium through the plasma membrane, resulting in a significant increase in cytoplasmic calcium and depletion of endo/sarcoplasmic reticulum calcium. Furthermore, we observed that tunicamycin also induced contraction in intact vascular smooth muscle. By applying established procedures and antagonists, we established that tunicamycin did not directly activate physiological calcium channels, such as store-operated channels, voltage gated calcium channels, ryanodine receptors or inositol trisphosphate receptors. Instead, we found that its effects on cellular calcium fluxes closely resembled those of the known calcium ionophore, ionomycin. We have concluded that tunicamycin directly permeabilizes the plasma membrane and endo/sarcoplasmic reticulum to calcium, and is, therefore, inappropriate for studying the relationship between accumulation of unfolded proteins and endo/sarcoplasmic reticulum calcium dysregulation during the endo/sarcoplasmic reticulum stress response. In contrast, we also report that two other well-known endo/sarcoplasmic reticulum stress inducers, brefeldin A and dithiothreitol, did not exhibit similar increases in calcium permeability.

Role of local vitamin D signaling and cellular calcium transport system in bone homeostasis

Mouse genetic studies have demonstrated that the 1,25-dihydroxyvitamin D [1,25(OH)2D] endocrine system is required for calcium (Ca(2+)) and bone homeostasis. These studies reported severe hypocalcemia and impaired bone mineralization associated with rickets in mutant mice. Specific phenotypes of these mice with an engineered deletion of 1,25(OH)2D cell signaling resemble the features observed in humans with the same congenital disease or severe 1,25(OH)2D deficiency. Decreased active intestinal Ca(2+) absorption because of reduced expression of epithelial Ca(2+) channels is a crucial mechanism that contributes to the major phenotypes observed in the mutant mice. The importance of intestinal Ca(2+) absorption supported by 1,25(OH)2D-mediated transport was further emphasized by the observation that Ca(2+) supplementation rescues hypocalcemia and restores bone mineralization in both patients and mice lacking 1,25(OH)2D signaling. This observation questions the direct role of 1,25(OH)2D signaling in bone tissue. Studies regarding tissue-specific manipulation of 1,25(OH)2D function have provided a consensus on this issue by demonstrating a direct action of 1,25(OH)2D on cells in bone tissue through bone metabolism and mineral homeostasis. In addition, movement of Ca(2+) from the bone as a result of osteoclastic bone resorption also provides a large Ca(2+) supply in Ca(2+) homeostasis; however, the system controlling Ca(2+) homeostasis in osteoclasts has not been fully identified. Transient receptor potential vanilloid (TRPV) 4 mediates Ca(2+) influx during the late stage of osteoclast differentiation, thereby regulating the Ca(2+) signaling essential for cellular events during osteoclast differentiation; however, the system-modifying effect of TRPV4 activity should be determined. Furthermore, it remains unknown how local Ca(2+) metabolism participates in systemic Ca(2+) homeostasis through bone remodeling. New insights are therefore required to understand this issue.

Phosphoproteomic analysis of differential expression of AGS cellular proteins in response to Helicobanter pyiori infection

Objective To research the differential expression of trace phosphorylated proteins in human gastric adenocarcinoma epithelial (AGS) cells infected by Helicobacter pylori. Methods H. pylori 26695 strain infected AGS cells 4 h and AGS cells was cultivated for 4 h as a comparison. The proteins of AGS and comparison AGS cells were extracted. Their phosphorylated proteins were enriched by metal ion af-finity adsorption enrichment techniques. After desalinated and purified the phosphorylated proteins samples were separated by 2-dimensional polyacrylamide gel electrophoresis (2-DE) technique. Computer assisted image analysis was used to analyze the differential proteomic expression. The significantly differentially ex-pressed proteins were unambiguously assigned identities by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF/TOF). Results Fifteen kinds of proteins were down-regulated, 4 kinds of new proteins were observed, 1 kind of proteins were up-regulated, 1 kind of proteins unexpression. The 21 proteins that were significantly differentially expressed , including cellular calcium ion homeostasis, transcription, interpretation, protein folding and transport, ribosomal assembly, centrosome replication, chromosome stability, cellular structure, cellular proliferation and apoptosis. Conclusion H. priori can cause a wide range change to human gastric adenocarcinoma epithelial cell protein pheshorylation. This change character has great significance to further comprehensive understanding of the pathogenesis of H. pylori. Key words: Affinity adsorption; Phosphorylated proteins; 2-DE; Pathogenic mechanism

Invited commentary

It is a privilege to review and comment on this interesting article. The experimental study presented by Dr Munakata and coworkers demonstrates the role of phosphatidyl-inositol specific phospholipase C (PI-PLC) in the myocardium exposed to a short period of normothermic ischemia using an isolated rabbit heart model. The authors used a specific inhibitor of PI-PLC, U73122 and its inactive analogue U73343 to modify the effect of PI-PLC to activate protein kinase C (PKC). The authors also were able to show reversal of the U73122 induced post-ischemic cardiac dysfunction by direct PKC activation with phorbol myristate acetate (PMA). This suggests that the myocardial protective effect of PI-PLC after ischemia occurs through PKC activation. Many studies have shown a significant myocardial protective effect of ischemic preconditioning [1Murry C.E. Richard V.J. Reimer K.A. Jennings R.B. Ischemic preconditioning slows energy metabolism and delays ultrastructural damage during a sustained ischemic episode.Circ Res. 1990; 66: 913-931Crossref PubMed Scopus (746) Google Scholar]. However, the mechanism and its clinical significance have been discussed for more than 10 years without a definitive answer. One promising candidate for the mechanism of ischemic preconditioning is activation of PKC by PI-PLC [2De Jonge H.W. Van Heugten H.A. Lamers J.M. Signal transduction by the phosphatidylinositol cycle in myocardium.J Mol Cell Cardiol. 1995; 27: 93-106Abstract Full Text PDF PubMed Scopus (62) Google Scholar] to protect the heart against impairment in intracellular calcium flux. The authors’ observation in this article that PI-PLC inhibition impairs diastolic calcium flux without a significant decrease in cellular ATP level is new and important. Further investigation using modifications of the cellular calcium transport system, eg, sarcoplasmic reticulum inhibition and membrane L-type calcium channel inhibition would be interesting and may clarify the mechanism underlying the intrinsic, antiischemic property of myocardium [3Liu D.M. Katnik C. Stafford M. Adams D.J. P2Y purinoceptor activation mobilizes intracellular Ca2+ and induces a membrane current in rat intracardiac neurones.J Physiol. 2000; 526: 287-298Crossref PubMed Scopus (25) Google Scholar]. Although the myocardial protective effect related to PKC activation during ischemia attracts much interest, many controversies remain. These controversies may due to the difference between species, experimental models, ischemic durations, and degree of ischemic insult. In addition, differences in the role of PKC-ε and PKC-α during short periods of ischemia is not fully understood. Researchers and clinicians must resolve these problems before this mechanism can be used to improve clinical myocardial protection. Further investigation is required. I congratulate these investigators for stimulating our thinking about the mechanism of the intrinsic protective property of the myocardium against ischemic reperfusion damage.

Perinuclear, perigranular and sub-plasmalemmal mitochondria have distinct functions in the regulation of cellular calcium transport

We have identified three distinct groups of mitochondria in normal living pancreatic acinar cells, located (i) in the peripheral basolateral region close to the plasma membrane, (ii) around the nucleus and (iii) in the periphery of the granular region separating the granules from the basolateral area. Three-dimensional reconstruction of confocal slices showed that the perigranular mitochondria form a barrier surrounding the whole of the granular region. Cytosolic Ca(2+) oscillations initiated in the granular area triggered mitochondrial Ca(2+) uptake mainly in the perigranular area. The most intensive uptake occurred in the mitochondria close to the apical plasma membrane. Store-operated Ca(2+) influx through the basolateral membrane caused preferential Ca(2+) uptake into sub-plasmalemmal mitochondria. The perinuclear mitochondria were activated specifically by local uncaging of Ca(2+) in the nucleus. These mitochondria could isolate nuclear and cytosolic Ca(2+) signalling. Photobleaching experiments indicated that different groups of mitochondria were not luminally connected. The three mitochondrial groups are activated independently by specific spatiotemporal patterns of cytosolic Ca(2+) signals and can therefore participate in the local regulation of Ca(2+) homeostasis and energy supply.

Splenic uptake of Tc-99m MDP in patients with hematologic abnormalities recently treated with chemotherapy.

Two patients are described who had hematologic abnormalities (acute promyelocytic leukemia and non-Hodgkin lymphoma) recently treated with chemotherapy and unexpected increased splenic uptake of Tc-99m MDP in the absence of hypercalcemia. Although not common, case reports have described increased splenic uptake on bone scintigraphy in patients with wide-ranging hematologic abnormalities, including lymphoma and leukemia. The mechanism of this phenomenon remains uncertain. Possible mechanisms include transient hypercalcemia, altered iron metabolism, splenic infarction, or previous chemotherapy causing cell membrane disruption or altered cellular calcium transport.

Genetic regulation of placental function: a quantitative in situ hybridization study of calcium binding protein (Calbindin-D 9k) and calcium ATPase mRNAs in sheep placenta

The calcium requirement of the ovine fetus increases progressively throughout pregnancy. The 9-kDa calcium binding protein (calbindin-D 9k; 9CBP) is considered to be a reliable marker for epithelia mediating calcium transport. This quantitative in situ hybridization study shows that the levels of 9CBP mRNA show a pregnancy stage-related increase which correlates with fetal calcium demand only in maternal endometrial gland and fetal interplacentomal trophoblast epithelia. Levels of 9CBP mRNA in the placentome, which has by far the greater area of maternofetal contact, show no changes during pregnancy. mRNA for the CaATPase enzyme, a second requirement for calcium transport, is shown to be present in epithelia in interplacentomal and placentomal regions but shows no change in concentration as pregnancy progresses. Results with the 9CBP and CaATPase mRNAs confirm our recent immunocytochemical results with ruminant placenta and indicate the basis for a cellular calcium transport system analogous to that in the enterocyte. The interplacentomal trophoblast system appears to be eminently suitable for investigations of details of the cellular mechanism and control of epithelial calcium transport.

Cellular Calcium Transport in Fish: Unique and Universal Mechanisms

The tilapia, Oreochromis mossambicus, is a truly euryhaline species in that it lives, grows, and reproduces in freshwater as well as in full-strength seawater. The gills, intestine, and kidneys show ionoregulatory adaptations fundamental for the calcium balance of this fish in these vastly different ionic media. This review focuses on calcium flows in these ionoregulatory organs and the changes that occur in the Cal²⁺-transporting mechanisms in the basolateral plasma membrane compartment of the cells that make up their ion-transporting epithelia. Influx of Ca²⁺ via the gills is comparable in freshwater and seawater; also, the Ca²⁺-ATPase and Na⁺/Ca²⁺ exchanger in branchial epithelial plasma membranes have comparable activities in fish adapted to freshwater and in those adapted to seawater. Ussing chamber experiments with isolated opercular membranes (a flat epithelium with chloride cells) suggest that chloride-cell-mediated, inward Ca²⁺ transport is largely dependent on Na⁺-dependent mechanisms. The Ca²⁺-...

Cellular calcium transport in renal epithelia: measurement, mechanisms, and regulation

The kidneys play a vital role in mineral homeostasis. In this review, the handling of calcium and the methods currently applied to measuring its intracellular concentration are discussed. The bulk of calcium absorption proceeds in proximal tubules, with smaller fractions recovered by thick ascending limbs, distal convoluted tubules, and connecting tubules. Hormonally regulated transcellular calcium absorption is essentially limited to distal convoluted and connecting tubules. At physiological concentrations, parathyroid hormone, calcitonin, and vitamin D increase net calcium absorption. Calcium absorption by polarized epithelial cells is a two-step process wherein calcium enters the cell across apical plasma membranes and exits across basolateral membranes. Recent electrophysiological and pharmacological experiments demonstrate that apical entry is mediated by calcium channels, which are modestly calcium selective, sensitive to dihydropyridine-type calcium channel blockers, and exhibit a wide range of single-channel conductances. Cellular calcium efflux is mediated by Ca(2+)-ATPase and by Na+/Ca2+ exchange. Ca(2+)-ATPase activity is highest in segments that exhibit significant rates of active calcium absorption. Multiple plasma membrane Ca(2+)-ATPase isoforms have been found in the kidney. Several renal Na+/Ca2+ exchange isoforms have been identified, and their role in effecting calcium efflux is under investigation.

Increased erythrocyte calcium pump activity in a kindred with familial benign hypercalcaemia.

The hypothesis that a global defect in cellular calcium transport may be critical in the development of familial benign hypercalcaemia (FBH) was investigated. Nine hypercalcaemic patients from a kindred with FBH and nine normal subjects were evaluated. Our results indicate that calcium pump activity in the FBH kindred was significantly higher (28%, P < 0.005) when compared to normal subjects. These findings suggest that alterations in calcium pump activity in target tissues may play a role in the development of FBH.

Strontium can be used as a tracer to examine calcification in the adult barnacle (Balanus amphititre amphititre)

Calcification in barnacles occurs at the base of each shell plate; as new CaCO3is deposited at the base, the shell grows upward. Although barnacle shell growth patterns have been characterized, the cellular calcium transport mechanisms involved in biomineralization are unknown; calcium is thought to be transported through the mineralizing epithelial cells into a limited diffusional space between the cells and the shell where calcification occurs. To follow transport of calcium in the cells and newly mineralized shell, one might consider using45Ca and autoradiography.Alternatively, electron probe x-ray microanalysis (EPXMA) imaging has better spatial resolution than autoradiography, but cannot discriminate45Ca from the stable isotope. However, in studies of mammalian bone formation using EPXMA, strontium was used successfully as a tracer for calcium. To assess whether strontium could serve as a tracer for calcium in barnacles, it was first necessary to determine if strontium could be incorporated into barnacle shells.

Calcium transport in renal epithelial cells

Extracellular calcium homeostasis involves coordinated calcium absorption by the intestine, calcium resorption from bone, and calcium reabsorption by the kidney. This review addresses the mechanism and regulation of renal calcium transport. Calcium reabsorption occurs throughout the nephron. However, distal tubules are the nephron site at which calcium reabsorption is regulated by parathyroid hormone, calcitonin, and 1 alpha,25-dihydroxyvitamin D3 and where the magnitude of net reabsorption is largely determined. These and related observations underscore the view that distal tubules are highly specialized to permit fine regulation of calcium excretion in response to alterations in extracellular calcium levels. Progress in understanding the mechanism and regulation of calcium transport has emerged from application of single cell fluorescence, patch clamp, and molecular biological approaches. These techniques permit the examination of ion transport at the cellular level and its regulation at subcellular and molecular levels. This editorial review focuses on recent and emerging observations and attempts to integrate them into models of cellular calcium transport.