Ion channels and transporters represent two major types of pathways of transmembrane transport for ions. Distinct from ion channels which conduct passive ionic diffusion, ion transporters mediate active transport of ions. In the perspective of biochemistry, ion transporters are enzymes that catalyze the movement of ions across the plasma membrane. In the present review, we selected the Na(+)/HCO3(-) cotransporter (NBC) as an example to analyze the key biochemical and biophysical properties of ion transporters, including stoichiometry, turnover number and transport capacity. Moreover, we provided an analysis of the electrophysiological principles of NBC based on the laws of thermodynamics. Based on the thermodynamical analysis, we showed how the stoichiometry of an NBC determines the direction of its ion transport. Finally, we reviewed the methodology for experimental determination of the stoichiometry of NBC, as well as the physiological significance of the stoichiometry of NBCs in specific tissues.
Electrophysiological Phenomena ; Ion Transport ; Sodium-Bicarbonate Symporters

Na⁺/HCO₃⁻ cotransporter NBCe1 is an electrogenic member of the solute carrier 4 (SLC4) family and plays important roles in intracellular pH regulation as well as transepithelial HCO₃⁻ movement. The physiological and pathological significance of NBCe1 has been well established by genetic studies with humans as well as knock-out study with mouse. NBCe1 is expressed in diverse tissues in mammals. The transporter plays an essential role in the maintenance of acid-base homeostasis in our body, being responsible for more ~80% of HCO₃⁻ reabsorption in the proximal renal tubule. In humans, a number of SLC4A4 mutations have been associated with proximal renal tubule acidosis that is often accompanied with short stature, ocular abnormalities (including cataract, glaucoma, and band keratopathy), migraine, and/or defects in dental enamel development. In the present article, we review the molecular physiology, the structure/function relationship, the mechanisms underlying the functional regulation of NBCe1, as well as the physiological and pathological roles of the transporter.
Acid-Base Equilibrium ; Acidosis, Renal Tubular ; genetics ; Animals ; Humans ; Mice ; Mutation ; Sodium-Bicarbonate Symporters ; genetics ; physiology

PiT2 is an inorganic phosphate (Pi) transporter whose mutations are linked to primary familial brain calcification (PFBC). PiT2 mainly consists of two ProDom (PD) domains and a large intracellular loop region (loop7). The PD domains are crucial for the Pi transport, but the role of PiT2-loop7 remains unclear. In PFBC patients, mutations in PiT2-loop7 are mainly nonsense or frameshift mutations that probably cause PFBC due to C-PD1131 deletion. To date, six missense mutations have been identified in PiT2-loop7; however, the mechanisms by which these mutations cause PFBC are poorly understood. Here, we found that the p.T390A and p.S434W mutations in PiT2-loop7 decreased the Pi transport activity and cell surface levels of PiT2. Furthermore, we showed that these two mutations attenuated its membrane localization by affecting adenosine monophosphate-activated protein kinase (AMPK)- or protein kinase B (AKT)-mediated PiT2 phosphorylation. In contrast, the p.S121C and p.S601W mutations in the PD domains did not affect PiT2 phosphorylation but rather impaired its substrate-binding abilities. These results suggested that missense mutations in PiT2-loop7 can cause Pi dyshomeostasis by affecting the phosphorylation-regulated cell-surface localization of PiT2. This study helps understand the pathogenesis of PFBC caused by PiT2-loop7 missense mutations and indicates that increasing the phosphorylation levels of PiT2-loop7 could be a promising strategy for developing PFBC therapies.
Humans ; Cell Membrane ; Mutation, Missense ; Phosphates/metabolism* ; Sodium-Phosphate Cotransporter Proteins, Type III/genetics*

In conventional pharmacological research in the field of mental disorders, pharmacological effect and dose have been estimated by ethological approach and in vitro data of affinity to the site of action. In addition, the frequency of administration has been estimated from drug kinetics in blood. However, there is a problem regarding an objective index of drug effects in the living body. Furthermore, the possibility that the concentration of drug in blood does not necessarily reflect the drug kinetics in target organs has been pointed out. Positron emission tomography (PET) techniques have made progress for more than 20 years, and made it possible to measure the distribution and kinetics of small molecule components in living brain. In this article, we focused on rational drug dosing using receptor occupancy and proof-of-concept of drugs in the drug development process using PET.
Brain ; Central Nervous System ; Drug Evaluation ; Electrons ; Kinetics ; Mental Disorders ; Norepinephrine Plasma Membrane Transport Proteins ; Positron-Emission Tomography ; Receptors, Dopamine D2 ; Serotonin Plasma Membrane Transport Proteins

OBJECTIVE: Bupropion is an antidepressant with proven efficacy for smoking cessation, however the response rate is some limited. With this background, the authors investigated the difference of early bupropion response in smoking cessation according to individual genetic polymorphism and temperamental characteristics. METHOD: Subjects were 113 Korean male volunteers who were nicotine dependent and wanted to quit smoking. Authors compared 6 candidate genes (DRD2, DRD4, dopamine transporter, norepinephrine transporter, serotonin transporter, COMT), and Temperament Character Inventory (TCI) between response group and non-response group after 3 weeks bupropion treatment. RESULT: Among 6 candidate genes, DRD2 homozygotes (A2/A2+A1/A1), COMT H/H genotype and H allele carriers showed high rate of smoking cessation by bupropion. NET-8 GG genotype and G allele carriers showed low rate of smoking cessation by bupropion. Persistence score in TCI was significant between two groups. CONCLUSION: DRD2, COMT, NET-8 genetic polymorphisms and some temperamental characteristics could predict success of smoking cessation by early treatment response of bupropion.
Alleles ; Bupropion* ; Dopamine Plasma Membrane Transport Proteins ; Genotype ; Homozygote ; Humans ; Male ; Nicotine ; Norepinephrine Plasma Membrane Transport Proteins ; Polymorphism, Genetic* ; Serotonin Plasma Membrane Transport Proteins ; Smoke* ; Smoking Cessation* ; Smoking* ; Temperament* ; Volunteers

The electrogenic sodium/bicarbonate cotransporter 1 (NBCe1) on the basolateral side of the renal proximal tubule plays a pivotal role in systemic acid-base homeostasis. Mutations in the gene encoding NBCe1 cause severe proximal renal tubular acidosis accompanied by other extrarenal symptoms. The proximal tubule reabsorbs most of the sodium filtered in the glomerulus, contributing to the regulation of plasma volume and blood pressure. NBCe1 and other sodium transporters in the proximal tubule are regulated by hormones, such as angiotensin II and insulin. Angiotensin II is probably the most important stimulator of sodium reabsorption. Proximal tubule AT(1A) receptor is crucial for the systemic pressor effect of angiotensin II. In rodents and rabbits, the effect on proximal tubule NBCe1 is biphasic; at low concentration, angiotensin II stimulates NBCe1 via PKC/cAMP/ERK, whereas at high concentration, it inhibits NBCe1 via NO/cGMP/cGKII. In contrast, in human proximal tubule, angiotensin II has a dose-dependent monophasic stimulatory effect via NO/cGMP/ERK. Insulin stimulates the proximal tubule sodium transport, which is IRS2-dependent. We found that in insulin resistance and overt diabetic nephropathy, stimulatory effect of insulin on proximal tubule transport was preserved. Our results suggest that the preserved stimulation of the proximal tubule enhances sodium reabsorption, contributing to the pathogenesis of hypertension with metabolic syndrome. We describe recent findings regarding the role of proximal tubule transport in the regulation of blood pressure, focusing on the effects of angiotensin II and insulin.
Acidosis, Renal Tubular ; Angiotensin II ; Blood Pressure* ; Diabetic Nephropathies ; Homeostasis ; Humans ; Hypertension ; Insulin ; Insulin Resistance ; Kidney Tubules, Proximal ; Plasma Volume ; Rabbits ; Rodentia ; Sodium ; Sodium-Bicarbonate Symporters

OBJECTIVES: The studies on the genetic risk factors of the children of alcoholics (COAs) are still in an early stage. The A 1 allele of the dopamine receptor 2 gene (DRD2) may be associated with the negative affect and positive alcohol expectancy of the COAs. In addition, several researchers reported that COAs might be associated with the GABAA receptor beta subunit gene (GABRB3) and serotonin transporter gene (5-HTTLPR). In this study, we investigated the association of polymorphism of the DRD2, Dopamine D4 receptor gene (DRD4), GABRB3, 5-HTTLPR with COAs to examine the genetic risk factors of COAs. METHODS: Twenty-two COAs and 23 control children (children of non-Alcoholics ; Non-COAs) were included for the genetic study. All COAs aged 6 to 18 were recruited and selected from families of alcoholic patients in alcohol clinics of three university and mental hospital. Alcoholism of parents was classified as type I (non-antisocial, late onset) and type II (antisocial, early onset) by Cloninger's classification. The genotyping of the DRD2, DRD4, GABRB3, 5-HTTLPR was carried out. Chi-square method was used for evaluating the associations between genetic polymorphism and the COAs. RESULTS: The frequency of A1+ allele of DRD2 in COAs were significantly higher than Non-COAs (Chi-square=4.45, df=1, p=0.035). Significant association between the genotype of DRD4 and COAs was found (Chi-square=8.32, df=1, p=0.004). G1- alleles of GABRB3 in COAs were significantly higher than in Non-COAs (Chi-square=6.622, df=1, p=0.022). We found no association of the polymorphic alleles of 5-HTTLPR with the COAs (Chi-square=0.021, df=1, p=0.884). There were significant associations between the type of parental alcoholism and depression of COAs. CONCLUSION: We found that the children of alcoholics had significantly increased genetic risk of alcohol drinking expectancy. This study provides some preliminary information on the risk and protective factors associated with the COAs, which can be used as a foundation for prevention and intervention of future psychopathology.
Alcohol Drinking ; Alcoholics* ; Alcoholism ; Alleles ; Child* ; Classification ; Depression ; Dopamine* ; Genotype ; Hospitals, Psychiatric ; Humans ; Korea* ; Parents ; Polymorphism, Genetic ; Psychopathology ; Receptors, Dopamine ; Receptors, Dopamine D4 ; Receptors, GABA-A* ; Risk Factors ; Serotonin Plasma Membrane Transport Proteins* ; Serotonin*

The potassium depletion has remarkable and opposite effect on kidney and body growth and has affected the expression of the several ion transporters. Previously, Ahn et al. have reported that HK alpha 1 and 2 subunit gene were upregulated in the hypokalemic rat kidney. To clone the unreported genes expressed in potassium deficiency, differential display PCR-based cloning strategy was used in normal and potassium-depleted rat kidney and a novel gene was isolated. Sequence analysis with blast search program identified a cDNA clone encoding an isoform of kidney sodium bicarbonate cotransporter-1. The tissue and cellular expression pattern of this gene were investigated with Northern analyses and in situ hybridization histochemistry (ISH) in normal and hypokalemic rats. This novel transcript was highly expressed in kidney and brain and at lower levels in distal colon, urinary bladder, and heart but not in salivary gland, stomach, liver, and lung in normal rat. In potassium-depleted rat, this transcript was upregulated in kidney, brain, and distal colon. By ISH, cellular distribution of this gene was highly expressed in S3 segment of proximal tubule, distal convoluted tubule, and cortical collecting duct of kidney and lower third of intestinal glands of distal colon but at lower levels in cortical and medullary thick ascending limb and medullary collecting duct of kidney and middle third of intestinal glands of distal colon. From these results, this candidate gene may play an important role in HCO3-transport by these organs during potassium depletion.
Animals ; Brain ; Clone Cells* ; Cloning, Organism* ; Colon ; DNA, Complementary ; Extremities ; Heart ; Hypokalemia ; In Situ Hybridization ; Intestinal Mucosa ; Ion Transport ; Kidney ; Liver ; Lung ; Potassium ; Potassium Deficiency ; Rats* ; Salivary Glands ; Sequence Analysis ; Sodium Bicarbonate ; Sodium-Bicarbonate Symporters* ; Stomach ; Urinary Bladder

OBJECTIVE: Serotonin transporter (5-HTT) is a key synaptic regulator of serotonergic neurotransmission and a major site of action of most antidepressants. The functional polymorphism of 5-HTT gene is reported to be associated with antidepressant responsiveness. Norepinephrine transporter (NET) and dopamine transporter (DAT) are also the targets for antidepressant drugs, and these biogenic amine transporters share a similar structure and mode of action as 5-HTT. We investigated the association between genetic polymorphisms of biogenic amine transporters and antidepressant response. METHODS: We genotyped 203 patients with major depressive disorder and 147 normal controls, using polymerase chain reaction (PCR) of genomic DNA with primers flanking the second intron and promoter regions of 5-HTT gene, and the 3' untranslated region of DAT. NET-1 (Thr99Ile) and NET-8 (1287 G/A) polymorphism were characterized by amplification and restriction fragment length polymorphisms (RFLP) analysis. RESULTS: VNTR polymorphism in the 3' untranslated region of DAT (p=0.020) was associated with a diagnosis of depression, but was influenced by age effect. We found that NET-8 polymorphism (p=0.015) in NET gene had significant associations with antidepressant response, as did the allelic variations of the promoter (p<0.0001) and intron2 (p=0.023) region in 5-HTT gene. The choice of drug had no effect on drug responsiveness. CONCLUSIONS: These results suggest that allelic variations of 5-HTT and NET genes affect the antidepressant responsiveness.
3' Untranslated Regions ; Antidepressive Agents ; Biogenic Amines* ; Depression ; Depressive Disorder, Major ; Diagnosis ; DNA ; Dopamine Plasma Membrane Transport Proteins ; Humans ; Introns ; Norepinephrine Plasma Membrane Transport Proteins ; Polymerase Chain Reaction ; Polymorphism, Genetic* ; Polymorphism, Restriction Fragment Length ; Promoter Regions, Genetic ; Serotonin Plasma Membrane Transport Proteins ; Synaptic Transmission

PURPOSE: The present study was aimed to determine whether there exist an altered regulation of tubular transporters and nitric oxide system in the kidneys in maleic acid-nduced metabolic acidosis. METHODS: Male Sprague-awley rats were treated with maleic acid (2 mmol/kg, every 24 hours, intraperitoneally) for 2 days. Control rats were injected with saline. At 24 hours following the second injection, rats were killed by decapitation. Plasma HCO3-and anion gap were measured. The protein expression of type 3 Na+/H+ exchanger (NHE3), type 1 Na+:HCO3- cotransporter (NBC1), and aquaporin (AQP)-1 in the cortex of the kidneys was determined by Western blot analysis. In addition, the expression of isoforms of nitric oxide synthase (NOS) was determined. Contents of nitric oxide metabolites (nitrite/ nitrate, NOx) were also measured in urine by colorimetric assay. RESULTS: Plasma concentrations of HCO3- were significantly decreased following the treatment of maleic acid, while plasma anion gap was did not differ between the experimental and the control groups. In the experimental group, the protein expression of NHE3 was significantly increased in the cortex of the kidney although the expression of NBC1 was not altered significantly. The expression of inducible NOS (iNOS), endothelial NOS (eNOS), and neuronal NOS (nNOS) was significantly increased in the cortex of the kidney. Accordingly, urine NOx contents were increased in the experimental group. In contrast, the expression of AQP1 was not altered. CONCLUSION: These results indicated that upregulation of NHE3 and nitric oxide system may play a role in regulation of acid-ase balance.
Acid-Base Equilibrium ; Acidosis* ; Animals ; Blotting, Western ; Decapitation ; Humans ; Kidney* ; Male ; Neurons ; Nitric Oxide Synthase ; Nitric Oxide* ; Plasma ; Protein Isoforms ; Rats* ; Sodium-Bicarbonate Symporters ; Sodium-Hydrogen Antiporter ; Up-Regulation