Mouse Na+: HCO3- cotransporter isoform NBC-3 (kNBC-3): cloning, expression, and renal distribution

BACKGROUND

Na+:HCO3- cotransporters mediate the transport of HCO3- into or out of the cell. We recently reported the partial cloning and characterization of a new human Na+:HCO3- cotransporter (referred to as NBC-3 or kNBC-3). The purpose of the present studies was to clone the mouse kNBC-3 and to examine its properties and expression in the kidney.

METHODS

Using primers from human kNBC-3 cDNA and 5' and 3' rapid amplification cDNA end polymerase chain reaction (RACE PCR), the mouse kNBC-3 full-length cDNA was cloned from inner medullary collecting duct (mIMCD-3) cells. The tissue distribution and functional properties of NBC-3 was determined using established methods.

RESULTS

The coding region of the mouse kNBC-3 has 1089 amino acids and shows 73 and 56% identity to human NBC-2 and NBC-1, respectively. The renal distribution of kNBC-3 demonstrated a unique expression pattern: Whereas kNBC-1 is predominantly expressed in the cortex and is absent in the inner medulla, kNBC-3 shows an intense expression level in the inner medulla and is absent in the cortex. Expression studies in oocytes indicated that NBC-3 mediates Na-dependent HCO3- cotransport. Electrophysiological experiments demonstrated that unlike kNBC-1, which is electrogenic, kNBC-3 is electroneutral.

CONCLUSIONS

Based on its distribution and electroneutrality, we propose that kNBC-3 mediates the transport of HCO3- into the cells.

Abc protein transport of MRI contrast agents in canalicular rat liver plasma vesicles and yeast vacuoles

The mechanism of excretion into bile of hepatospecific magnetic resonance imaging (MRI) contrast media employed labeled Gd-reagents EOB.DTPA, BOPTA, B 20790 (iopanoate-linked), and B 21690 (glycocholate-linked) for measurement in rat liver canalicular plasma membrane vesicles and yeast vacuoles. The presence of ATP gave threefold greater transport of B 20790 and B 21690 than of EOB.DTPA and BOPTA. In yeast vacuoles the ATP stimulatory effect was eightfold with B 20790 and fivefold greater for B 21690, whereas in YCF1- or YLLO115w-deleted yeast cells the transport was significantly reduced and absent from double mutants, YCF1 and YLLO15w. The transport was similar in wild-type and deletant cells for B 21690; taurocholate gave 85% inhibition. These data suggest that bilary secretion of structurally related MRI agents depend on molecular structure. The findings are suggestive as of possible value for clinical diagnosis of inherited hyperbilirubinemias and other liver disorders.

Pendrin: an apical Cl-/OH-/HCO3- exchanger in the kidney cortex

The identities of the apical Cl-/base exchangers in kidney proximal tubule and cortical collecting duct (CCD) cells remain unknown. Pendrin (PDS), which is expressed at high levels in the thyroid and its mutation causes Pendred's syndrome, is shown to be an anion exchanger. We investigated the renal distribution of PDS and its function. Our results demonstrate that pendrin mRNA expression in the rat kidney is abundant and limited to the cortex. Proximal tubule suspensions isolated from kidney cortex were highly enriched in pendrin mRNA. Immunoblot analysis studies localized pendrin to cortical brush-border membranes. Nephron segment RT-PCR localized pendrin mRNA to proximal tubule and CCD. Expression studies in HEK-293 cells demonstrated that pendrin functions in the Cl-/OH-, Cl-/HCO3-, and Cl-/formate exchange modes. The conclusion is that pendrin is an apical Cl-/base exchanger in the kidney proximal tubule and CCD and mediates Cl-/OH-, Cl-/HCO3-, and Cl-/formate exchange.

Role of basolateral carbonic anhydrase in proximal tubular fluid and bicarbonate absorption

Membrane-bound carbonic anhydrase (CA) is critical to renal acidification. The role of CA activity on the basolateral membrane of the proximal tubule has not been defined clearly. To investigate this issue in microperfused rabbit proximal straight tubules in vitro, we measured fluid and HCO(3)(-) absorption and cell pH before and after the extracellular CA inhibitor p-fluorobenzyl-aminobenzolamide was applied in the bath to inhibit only basolateral CA. This inhibitor was 1% as permeant as acetazolamide. Neutral dextran (2 g/dl, molecular mass 70,000) was used as a colloid to support fluid absorption because albumin could affect CO(2) diffusion and rheogenic HCO(3)(-) efflux. Indeed, dextran in the bath stimulated fluid absorption by 55% over albumin. Basolateral CA inhibition reduced fluid absorption ( approximately 30%) and markedly decreased HCO(3)(-) absorption ( approximately 60%), both reversible when CA was added to the bathing solution. In the presence of luminal CA inhibition, which reduced fluid ( approximately 16%) and HCO(3)(-) ( approximately 66%) absorption, inhibition of basolateral CA further decreased the absorption of fluid (to 74% of baseline) and HCO(3)(-) (to 22% of baseline). CA inhibition also alkalinized cell pH by approximately 0.2 units, suggesting the presence of an alkaline disequilibrium pH in the interspace, which would secondarily block HCO(3)(-) exit from the cell and thereby decrease luminal proton secretion (HCO(3)(-) absorption). These data clearly indicate that basolateral CA has an important role in mediating fluid and especially HCO(3)(-) absorption in the proximal straight tubule.

Effects of cumene hydroperoxide on cellular cation composition in frog kidney proximal tubular cells

Effects of cumene hydroperoxide were studied on the peritubular membrane potential and cellular cation composition in frog kidney proximal tubular cells. After perfusion of isolated frog kidneys for 30 min with 1.3x10(-4) mol l(-1) cumene hydroperoxide Ringer solution, the peritubular membrane potential gradually declined. The ouabain-like effects were demonstrated on cell Na and K activities after 1 h of perfusion with cumene hydroperoxide. The peritubular apparent transference number for potassium was decreased. Intracellular pH was not altered in the presence of cumene hydroperoxide. Intracellular free Ca(2+) concentration increased slowly and moderately. The concentration of the malondialdehyde in the kidney homogenates, measured as an index of lipid peroxidation, was increased. A previously observable effect of cumene hydroperoxide on the peritubular membrane potential was prevented by oxygen radical scavengers.

The products of YCF1 and YLL015w (BPT1) cooperate for the ATP-dependent vacuolar transport of unconjugated bilirubin in Saccharomyces cerevisiae

Since bilirubin-like pigments are present in the environment as degradation products of heme-containing proteins, yeast could have developed a detoxifying system to transport these compounds into their vacuoles. Vacuoles from Saccharomyces cerevisiae showed an ATP-dependent, saturative transport of unconjugated bilirubin (UCB) that was reduced by 60% and 40% in YCF1 and YLL015w-deleted cells, respectively; the double deletant showed no UCB uptake. Conversely, the transport of bile acids (taurocholate) was comparable in wild and deleted stains. These data identify YCF1 and YLL015w, named BPT1 (Bile Pigment Transporter), as the genes responsible for ATP-dependent UCB transport in yeast. Since YCF1 and YLL015w are rather homologous with multidrug resistant proteins (MRPs), they also suggest the involvement of this class of transporters in the ATP-dependent transport of unconjugated bilirubin.

The products of YCF1 and YLL015w (BPT1) cooperate for the ATP-dependent vacuolar transport of unconjugated bilirubin in Saccharomyces cerevisiae

Since bilirubin-like pigments are present in the environment as degradation products of heme-containing proteins, yeast could have developed a detoxifying system to transport these compounds into their vacuoles. Vacuoles from Saccharomyces cerevisiae showed an ATP-dependent, saturative transport of unconjugated bilirubin (UCB) that was reduced by 60% and 40% in YCF1 and YLL015w-deleted cells, respectively; the double deletant showed no UCB uptake. Conversely, the transport of bile acids (taurocholate) was comparable in wild and deleted stains. These data identify YCF1 and YLL015w, named BPT1 (Bile Pigment Transporter), as the genes responsible for ATP-dependent UCB transport in yeast. Since YCF1 and YLL015w are rather homologous with multidrug resistant proteins (MRPs), they also suggest the involvement of this class of transporters in the ATP-dependent transport of unconjugated bilirubin.

Early effects of rush immunotherapy with Dermatophagoides pteronyssinus in asthmatics

In this study we assessed the effects of Dermatophagoides pteronyssinus (Dpt) rush immunotherapy in comparison with placebo treatment in our asthmatic patients. Fourteen highly Dpt-susceptible adults were randomized in two groups (immunotherapy and placebo) and treated in single-blind manner. Patients were selected according to the recommendation of the immunotherapy position paper (1993). To minimize side effects we modified the protocol by adjusting allergen doses for each patient separately. Immunologic (total and Dpt-specific serum IgE and IgG antibodies/EIA, Pharmacia) and clinical parameters (spirometry, medication score and skin testing) were recorded before treatment, after 2 weeks, at the second month and after 4 months of immunotherapy onset. None of the patients had life-threatening side effects in the course of the treatment. The results obtained demonstrated significant influence of immunotherapy on Dpt-specific serum IgG synthesis (Kruskal Wallis test, p < 0.05) and on the late phase skin reaction with Dpt (U-test, p < 0.05) at the end of the second month of immunotherapy onset. In the immunotherapy group, we also registered a negative correlation between concentrations of Dpt-specific serum IgE and IgG antibodies (p = -0.83; p < 0.05), at the end of the second month. In addition, diversities among patients, expressed by immunologic parameters, were related to the amount of delivered allergen. There were no significant differences between groups concerning medication score from opposite to better FEV, PEFR and dPEFR results (Kruskal Wallis, p < 0.05) in the placebo group. In conclusion, Dpt-specific serum IgG concentration, immunologic score and late phase skin reactivity to allergen appeared to be the valid parameters of rush-immunotherapy achievement, while delivered allergen dose also seemed to be an influencing factor.

[Arthroscopy after repeated spraining of the ankle.]

The authors analyze a group of 43 patients operated at the casualty department of the Ministry of Defence Hospital in Bratislava during the past two years where arthroscopy of the ankle was made. They identify a group of patients who were indicated for arthroscopic surgery after repeated distorsions of the ankle. They compare in these patients the preoperative finding on the native X-ray with the CT finding and finding on operation. Based on postoperative results they try to find an answer to the problem whether arthroscopy of the ankle is indicated after repeated distorsions. Key words: arthroscopy of the ankle joint, distorsion of the ankle joint, intraarticular "mouse".

[Treatment and reconstruction of bullet wounds of the calf]

The paper describes complete management of bullet wounds of the crus. After thorough primary surgical treatment and fixation of the fracture the authors recommend the method of muscle flap of the soleus muscle and anterior tibial muscle and the method of musculocutaneous flap of the gastrocnemius muscle to achieve reconstitution of soft tissues. In fractures with tissue loss, after healing of the reconstituted areas of soft tissues, they advocate plastics with sponge and corticosponge grafts. Of the 12 bullet wounds of the extremities treated 3 were injuries of the crus.

[Treatment and reconstruction of open fractures of the forearm with tissue loss]

The importance of the forearm calls for attempts to preserve it and to restore its function even in severe injuries with extensive tissue loss. At early and comprehensive first surgical treatment providing sufficient perfusion good results can be achieved also in cases that do not appear promising at first sight. After treating the soft tissues fixation of the fracture is to be ensured externally and reconstitution surgery is to proceed according to the management plan. Of the reported series of 11 severe compound fractures the functional result of management was not favorable only in a single case.

[Reconstruction of open fractures of the leg associated with extensive loss of skin and soft tissues using muscular and cutaneo-muscular flaps]

The author draws attention to the serious character of open fractures of the leg with extensive losses of the skin and soft tissues. He emphasizes the need of high standard primary surgical treatment and osteosynthesis of the fracture by external fixation device according to a therapeutic plan. To ensure an undisturbed consolidation of the fracture in indicated cases, the author implements the reconstruction of acute skin losses and soft tissue losses by a muscular flap of the m. soleus or m. tibialis ant. and by dermatomuscular flaps of the gastrocnemius muscle. Of 42 casualties with open fractures of the leg with losses of soft tissues the author made in 9 reconstructions of acute losses by muscular and dermatomuscular flaps. The author presents the documentation of two patients with open fractures of the leg who were treated by muscular and dermatomuscular flaps.