[MR imaging for the evaluation of accompanying injuries in cases of distal forearm fractures in children and adolescents]. / MRT zur Beurteilung von Begleitverletzungen distaler Unterarmbrüche im Wachstumsalter.

INTRODUCTION: The study was made to evaluate the role of MR imaging in pediatric distal forearm fractures by comparison with the findings of plain radiographs and MRI. MATERIAL AND METHOD: 38 patients (27 boys and 11 girls, mean age of 12 years, range 7 to 15 years) with radiographically open distal radius and ulna growth plates requiring first aid for a fracture of the distal third of the forearm, were included in this study. The fractures were diagnosed on plain radiographs and conservative treatment was performed. In 35 patients MR imaging was performed within 3 weeks after the accident and in 3 patients MRI was performed after 6 to 9 weeks because of persistent wrist pain. RESULTS: Fifteen Salter/Harris II injuries of the radius and 1 of the ulna, 1 torus fracture of the radius and 2 of the ulna, 12 greenstick fractures of the radius and 3 of the ulna, 10 complete displaced radius fractures and 15 ulnar styloid fractures were found on plain radiographs. Twelve patients had evidence of associated triangular fibrocartilage complex (TFCC) lesions in MRI, there was no statistical correlation between TFCC lesions and fracture types, fracture dislocations or patients age (p > 0.5). One patient had an avulsion of the radioscaphocapitate ligament from the radius accompanying a greenstick fracture of the distal radius. 19 bone bruises and two radiographically occult fractures were identified. In 2 patients, a bone marrow oedema was seen in the radial epiphysis immediately adjacent to the germinal zone of the growth plate. In these patients premature physeal arrest occurred. CONCLUSION: MRI plays an important role in the evaluation of acute pediatric wrist injuries. It allows a better evaluation of osseous lesions than plain radiographs. In our study, a tear of the triangular fibrocartilage complex accompanied distal radius fractures in 32 % of patients. Simultaneous rupture of the TFCC insertion in the fovea ulnaris and ulnar styloid fracture lead to destabilisation of the distal radioulnar joint (DRUJ).

[Long-term results following ORIF of dorsal dislocated distal intraarticular radius fractures]. / Langzeitergebnisse operativ versorgter distal intraartikulärer Speichenfrakturen.

PURPOSE: To evaluate the sequelae of distal intraarticular radius fracture with regard to the development of arthritis and clinical symptoms. PATIENTS AND METHOD: In a retrospective follow-up examination, 72 patients with a distal intraarticular radius fracture could be included for clinical and radiological investigation 9 years following the trauma. All fractures were treated by ORIF and cortico-cancellous bone grafting. RESULTS: Radiological evaluation revealed 5.1 degrees palmar tilt, 19.1 degrees radial tilt and the ulnar variance amounted to -0.5 mm. The articular cavity depth in the sagittal plane measured with 4.6 mm, 1.2 mm more than the non-involved side. Articular step-off was noticed in 6 patients. According to the Knirk and Jupiter classification system, two patients healed without arthritis, 43 patients presented arthritis stage 1, and 27 stage 2. Evaluation of the data showed a significant correlation between arthritis and articular cavity depth. But arthritis had neither influence on the DASH, nor the pain level. On the other hand, arthritis led to decreased sagittal wrist motion. CONCLUSION: ORIF of distal intraarticular radius fractures led to predictable results concerning restoration of length and form of the distal radius. Arthritis had a minor influence on the clinical end result.

Effects of calcium channel blockers on NIH 3T3 fibroblasts expressing the Ha-ras oncogene.

NIH 3T3 fibroblasts expressing the ras oncogene (+ras cells) respond to bradykinin, bombesin or serum with sustained oscillations of cell membrane potential reflecting oscillations of intracellular calcium activity and subsequent activation of calcium-sensitive K+ channels. In contrast, identical cells not expressing the oncogene (-ras cells) respond to bradykinin with a single, transient hyperpolarization of the cell membrane. Furthermore, +ras cells are characterized by a serum-independent proliferation, an increase in cell volume and a marked reorganization of the cytoskeleton. It has been shown previously that the calcium channel blocker nifedipine, but not verapamil and diltiazem, inhibits oscillations of cell membrane potential as well as proliferation. In this study, we have examined the effect of several calcium channel blockers (bepridil, nifedipine, verapamil, diltiazem) on the proliferation, volume and cytoskeletal reorganization of +ras cells. Bepridil (10 mumol/l), which is also shown here to inhibit oscillations of cell membrane potential, and nifedipine (10 mumol/l) caused a decrease in cell number, whereas verapamil and diltiazem (10 mumol/l each) resulted in growth rates which did not differ from untreated +ras cells. The increase in cell volume as observed in untreated +ras cells was also observed for cells treated with verapamil and diltiazem, whereas cell volumes of +ras cells treated with bepridil and nifedipine were markedly reduced and similar to the values obtained for -ras cells. In addition, bepridil and nifedipine markedly inhibited cytoskeletal rearrangement, i.e depolymerization of actin-containing stress fibers. This inhibitory effect was not observed for verapamil and diltiazem.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the human gene coding for the swelling-dependent chloride channel ICln at position 11q13.5-14.1 (CLNS1A) and further characterization of the chromosome 6 (CLNS1B) localization.

Expression cloning revealed a chloride channel (ICln) that we found to be fundamental for the regulatory volume decrease in a variety of cells. The chromosomal localization of the human ICln-gene showed two loci, one at chromosome 11 in position q13.5-q14.1, termed CLNS1A, and a second one at chromosome 6 at position p12.1-q13, termed CLNS1B. In this study, we offer a detailed characterization of the CLNS1A gene and provide the exact position (6p12) and sequence data of CLNS1B, an intronless gene 91.3% homologous to the coding region of CLNS1A.

The gastric H,K-ATPase blocker lansoprazole is an inhibitor of chloride channels.

1. It was postulated that swelling dependent chloride channels are involved in the proton secretion of parietal cells. Since omeprazole, lansoprazole and its acid activated sulphenamide form AG2000 are structurally related to phenol derivatives known to block swelling dependent chloride channels, we set out to test, whether these substances--which are known to block the H,K-ATPase--could also lead to an inhibition of swelling-dependent chloride channels. Swelling-dependent chloride channels--characterized in many different cell types--show highly conserved biophysical and pharmacological features, therefore we investigated the effect of omeprazole, lansoprazole and its acid activated sulphenamide form AG2000 on swelling-dependent chloride channels elicited in fibroblasts, after the reduction of the extracellular osmolarity. 2. Omeprazole, lansoprazole and its acid activated sulphenamide form AG2000 are able to block swelling-dependent chloride channels (IClswell). 3. Lansoprazole and its protonated metabolite AG2000 act on at least two different sites of the IClswell protein: on an extracellular site which seems to be in a functional proximity to the nucleotide binding site, and on an intracellular site which allows the formation of disulfide-bridges. 4. The inhibition of the proton pump and the simultaneous blocking of chloride channels by omeprazole, lansoprazole and its acid activated sulphenamide form AG2000, as described here could be an effective mode to restrict proton secretion in parietal cells.

Blockade of swelling-induced chloride channels by phenol derivatives.

1. In NIH3T3 fibroblasts, the chloride channel involved in regulatory volume decrease (RVD) was identified as ICln, a protein isolated from a cDNA library derived from Madin Darby canine Kidney (MDCK) cells. ICln expressed in Xenopus laevis oocytes gives rise to an outwardly rectifying chloride current, sensitive to the extracellular addition of nucleotides and the known chloride channel blockers, DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and NPPB (5-nitro-2-(3-phenylpropylamino)-benzoic acid). We set out to study whether substances structurally similar to NPPB are able to interfere with RVD. 2. RVD in NIH3T3 fibroblasts and MDCK cells is temperature-dependent. 3. RVD, the swelling-dependent chloride current and the depolarization seen after reducing extracellular osmolarity can be blocked by gossypol and NDGA (nordihydroguaiaretic acid), both structurally related to NPPB. 4. The cyclic AMP-dependent chloride current elicited in CaCo cells is less sensitive to the two substances tested while the calcium-activated chloride current in fibroblasts is insensitive. 5. The binding site for the two phenol derivatives onto ICln seems to be distinct but closely related to the nucleotide binding site identified as G x G x G, a glycine repeat located at the predicted outer mouth of the ICln channel protein.

[Clinical long-term outcome after Kapandji-Sauvé procedure]. / Klinische Langzeitergebnisse der Kapandji-Sauvé-Operation1.

PURPOSE: The present study was designed to evaluate long-term outcome of upper extremities and subjective self-assessment of patient disability after a Kapandji-Sauvé procedure by means of the DASH score. PATIENTS AND METHOD: Between 1986 and 1996, a modified Kapandji-Sauvé procedure was performed in 117 patients with painfully limited forearm rotation and arthrosis of the distal radioulnar joint (DRUJ). Of the 117 patients, 73 women and 32 men, whose ages at operation ranged from 22 to 74 years (average, 58 years) were retrospectively reviewed clinically and radiologically eight years (range, five to twelve years) after the operation. The DASH questionnaire was used in 43 patients. RESULTS: The mean DASH score was 28 points (range, 0 to 53 points). The mean score in part A was 1.9 points, in part B 1.8 points. Worst outcomes were noted for activities requiring the exertion of force. Pain was reduced in 97 % of the patients. Forearm rotation and grip strength improved in all patients. CONCLUSION: Our clinical findings suggest that the Kapandji-Sauvé procedure is indicated in symptomatic, non-reconstructable disorders of the DRU-joint with or without ulnocarpal impaction syndrome. The DASH questionnaire provides a general view of functional outcome after the Kapandji-Sauvé procedure, though rotation is absolutely necessary to evaluate the success of the operation.

Proximal humeral fractures: what is semi-rigid? Biomechanical properties of semi-rigid implants, a biomechanical cadaver based evaluation.

INTRODUCTION: Proximal humerus fractures remain challenging especially in the elderly. Biomechanical data put semi-rigid implants in favour of osteopenic or osteoporotic situation. Little surgical side damage is associated with a minimal invasive approach of these implants. The aim of this study was to evaluate the mechanical properties of three such implants. MATERIAL AND METHODS: Fresh frozen cadaver specimens were mounted as proposed by the distributors. Three different implants were used: LCP-PH (locking compression plate proximal humerus, Synthes, Austria), HB (humerus block, Synthes, Austria), and IMC (intramedullary claw, ITS, Austria). Subcapital fracture was simulated by resecting a 5 mm gap. All specimens were comparable in "B" (one), "M" (ineral) and "D" (ensity). Four load cases were tested: varus bending, medial shearing and axial torque. A cyclic test (1,000 cycles) was performed in the first load case (varus stress) for all three implants. RESULTS: The LCP-PH was the most rigid in all three load cases, always followed by the HB. The IMC was the most elastic device with almost immeasurable values in axial torque. In the cyclic setting, the load reduction of the HB followed by the LCP-PH was significantly better than that for the IMC. CONCLUSION: The differences in stiffness are varying tremendously. The IMC is the implant with the lowest stiffness in all load cases and the highest load reduction. New "semi-rigids" claim good clinical performance, yet prospective clinical studies have to prove this. It is unlikely that the IMC can maintain fracture reduction in fracture situations of complex nature (no ligamentotaxis).

Fixation of nondisplaced scaphoid fractures: making treatment cost effective. Prospective controlled trial.

INTRODUCTION: Nondisplaced scaphoid waist fractures treated with prolonged plaster immobilisation often lead in transient joint stiffness and to a delay in return to sport and work activity. The long time off work increases the work off compensation costs. Internal fixation of scaphoid fractures has resulted in a shorter time to union and to return to work and sports. This prospective study compares cast immobilisation with screw fixation and the direct cost with indirect cost of conservative and minimally invasive treatment of undisplaced scaphoid fractures. MATERIALS AND METHODS: Forty-seven patients with an acute nondisplaced waist fracture of the scaphoid were allocated into either cast immobilisation or internal screw fixation for this study. Cost data concerning the groups of nonoperated and operated patients were analysed. Range of wrist motion, grip strength, DASH-score, time to fracture union, return to work time and the needed physiotherapy at the final follow-up at 6 months were evaluated. RESULTS: Twenty-one patients were included in the group of screw fixation and 23 patients were included in the group of cast immobilisation. At final follow-up there was no significant difference in the range of motion of the wrist or in grip strength. The operatively treated group had a better mean DASH-score than the conservative group. Fracture union was seen in the screw fixation group at a mean of 43 days and in the cast immobilisation group at a mean of 74 days (P < 0.5). The average time of return to work was 8 days for patients who had an internal screw fixation, while those treated with a cast returned to work at a mean of 55 days (P < 0.5). In total the internal fixation of undisplaced scaphoid fractures is less expensive than conservative treatment. CONCLUSION: Internal screw fixation of nondisplaced scaphoid fractures had a shorter time to bony union and the patients returned earlier to work compared with cast immobilisation. Although it is assumed that operative treatment is more expensive, in this study the cost was not found to be higher.

Bifocal disruption of the knee extensor apparatus ("floating patella") in a 13-year-old patient.

We present the unusual case of a traumatic "floating patella" in a 13-year-old healthy boy. No predisposing factors were diagnosed. The obvious bony avulsion was treated by immediate open reduction and screwing, whereas the quadriceps tendon rupture was missed diagnostically in the first step. During the course of the treatment, the extension lag lead to the delayed diagnosis of the concomitant rupture of the quadriceps tendon. After the surgical treatment of the latter, the patient healed with full function of the extensor apparatus and full ROM. Bifocal injury of the knee extensor apparatus is therefore possible in young adolescents without presenting the predisposing factors and should thus be considered.

Antisense oligonucleotides suppress cell-volume-induced activation of chloride channels.

Cell volume regulation is an essential feature of most cells. After swelling in hypotonic media, the simultaneous activation of potassium and chloride channels is believed to be the initial, time-determining step in cell volume regulation. The activation of both pathways is functionally linked and enables the cells to lose ions and water, subsequently leading to cell shrinkage and readjustment of the initial volume. NIH 3T3 fibroblasts efficiently regulate their volume after swelling and bear chloride channels that are activated by decreasing extracellular osmolarity. The chloride current elicited in these cells after swelling is reminiscent of the current found in oocytes expressing an outwardly rectifying chloride current termed ICln. Introduction of antisense oligodeoxynucleotides complementary to the first 30 nucleotides of the coding region of the ICln channel into NIH 3T3 fibroblasts suppresses the activation of the swelling-induced chloride current. The experiments directly demonstrate an unambiguous link between a volume-activated chloride current and a cloned protein involved in chloride transport.

Fluorescence-optical measurements of chloride movements in cells using the membrane-permeable dye diH-MEQ.

Fluorescence-optical measurements of the intracellular chloride concentration facilitate identification of chloride movements across the cell membrane of living cells. The two main dyes used for this purpose are 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 6-methoxy-quinolyl acetoethyl ester (MQAE). The use of both substances is impaired by their poor membrane permeability and therefore limited loading of the cells to be studied. Here we report the use of 6-methoxy-N-ethylquinolinium iodide (MEQ), a chloride-sensitive dye for which a membrane-permeable form is easily prepared. This makes the loading procedure as easy as with the acetoxymethyl (AM) forms of other dyes for sensing intracellular ions. In addition, the original method, which described absolute concentration measurements of chloride in the cytosol, was modified in so far as only relative measurements were made. This avoids the known limitations of single wavelength excitation and emission dyes with respect to exact concentration measurements. Moreover, to enhance the signal-to-noise ratio the driving force for chloride was considerably increased by changing the original direction of the anion flux in the cells under investigation. We verified the method by using fibroblasts and activating ICln, a putative chloride channel cloned from epithelial cells and of paramount importance in the regulatory volume decrease in these cells. In the presence of SCN- the MEQ quench measured in NIH 3T3 fibroblasts is dramatically enhanced in hypotonically challenged cells compared with cells under isotonic conditions. Antisense oligodeoxynucleotides sensing ICln considerably impeded the swelling-induced chloride current (ICl) in NIH 3T3 fibroblasts. Accordingly, the chloride movement measured by the SCN- quench of the MEQ signal was significantly reduced. Similar results can be obtained in the presence of 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) or 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), two known blockers of chloride transport in the plasma membrane of a variety of cells. In conclusion, fluroscence-optical measurements using MEQ as the chloride-sensitive dye provide a reliable and easy-to-use method for measuring changes of the chloride flux across the cell membrane of living cells.

Na(+)/H(+)exchangers: linking osmotic dysequilibrium to modified cell function.

The Na(+)/H(+) exchangers (NHEs) are among the major ion transporters involved in cell volume regulation. NHE activation leads to a cellular influx of Na(+) ions and extrusion of H(+) ions, which are readily replenished from intracellular buffers. This will result in a net import of Na(+). In many systems NHE operates in parallel to Cl(-)/ HCO3(-) exchange, resulting in cellular uptake of NaCl. The influx of osmotically obliged water will consequently lead to cell swelling. This makes NHEs suitable to serve as powerful mechanisms for increasing cell volume (CV). The low volume threshold for NHE activation enables the cells to respond to very minute reductions of the CV. By the coupling to the export of H(+) ions cell volume regulatory NHE activation may lead to changes in intracellular pH. On the other hand NHEs are activated by a broad variety of ligands and by intracellular acidosis, which, in turn, may consequently lead to cell swelling. In addition, NHEs are linked to other intracellular proteins and structures, like e.g. the cytoskeleton, which themelves are involved in the regulation of numerous cellular processes. Therefore NHEs link CV regulation to a diversity of cellular functions, both in physiological and pathophysiological conditions. Six isoforms of the Na(+)/H(+) exchanger, termed NHE1--6, have been cloned so far. NHE 1--5 are located in the plasma membrane, whereas NHE6 is sorted to the mitochondrial membrane. NHE1 and NHE6 are the ubiquitously expressed isoforms. The expression of the isoforms NHE2 to NHE5 is restricted to specific tissues and the pattern of their expression, as well as their subcellular localization indicate that they fulfill specialized functions. Cell shrinkage induced activation has been shown for NHE1,2 and 4. In contrast, NHE3 is inhibited by cell shrinkage. In many cells several isoforms are present and assigned to specific membrane domains where they may serve a functional crosstalk between the different ion transporters.

Determination of protein-protein interactions of ICIn by the yeast two-hybrid system.

ICln is a ubiquitously expressed eukaryotic protein. Expression of the protein in Xenopus laevis oocytes, the knocking-down of the protein in fibroblasts, or the reconstitution of the protein in lipid bilayer led to the assumption that this protein is an ionic channel or a significant part thereof. However, other possible roles for ICln in potential regulatory mechanisms have been postulated, as diverse as regulator of cell morphology by interacting with the Skb1 protein and/or interaction with core spliceosomal proteins. Here we show that ICln is able to interact with SnRNP core proteins SmD1, SmD2, SmD3, SmX5 and SmB/B'.

Antiviral drugs from the nucleoside analog family block volume-activated chloride channels.

BACKGROUND: The antiviral drugs AZT and acyclovir are generally used in the treatment of infections with human immunodeficiency virus (HIV) and herpes simplex virus (HSV). These substances are known to impede virus replication by premature nucleic acid chain termination. It is not yet clear, however, if this is the sole mechanism responsible for the antiviral and/or the numerous side effects observed in patients treated with these agents. We investigated the swelling-induced chloride current in fibroblasts, which we demonstrated is closely related or identical to a cloned epithelial chloride channel, ICln: This chloride channel can be blocked by nucleotides. MATERIALS AND METHODS: Electrophysiological, fluorescence optical, and volume measurements were made to determine the effect of nucleoside analogs on the swelling-dependent chloride current (ICl) in NIH 3T3 fibroblasts and in human T cell lymphoma (H9) cells and the cAMP-dependent chloride current in CaCo cells. RESULTS: AZT and acyclovir block the swelling-dependent chloride current and the chloride flux in fibroblasts, and the regulatory volume decrease (RVD) and ICl in H9 cells. This immediate effect can be substantially reduced by the simultaneous incubation of the cells with thymidine-5'-diphosphate (TDP) or uridine, both of which are by themselves unable to affect ICl. CONCLUSIONS: We show here a novel molecular mechanism by which antiviral drugs of the nucleoside analog family could lead to impairments of the kidney, bone marrow, gastrointestinal, and neuronal functions, and how these side effects could possibly be restricted by the presence of TDP or uridine.

ICln: a chloride channel paramount for cell volume regulation.

Cell volume regulation is a ubiquitous cell regulatory mechanism based on meticulously controlled ion transport mechanisms. Keeping the absolute volume constant seems to be of the highest priority for most cells and is achieved at the expense of altered intracellular ion concentrations. We have been able to demonstrate that ICln, a chloride channel cloned from epithelial cells, is paramount for the ability of swollen cells to regulate their volume back to that under resting conditions. A unique feature of ICln is the distinct sensitivity of these channels for nucleotides and nucleoside analogues added to the extracellular fluid. In addition, cromolyn sodium and nedocromil sodium, drugs used by patients with asthma, are able to impede the function of these channels.

Chromosomal localization of the genes (CLNS1A and CLNS1B) coding for the swelling-dependent chloride channel ICln.

ICln is a cloned chloride channel paramount for regulatory volume decrease. Two different loci that carry the coding region for ICln were identified in the human genome. By PCR strategies an intronless copy of the gene was located on chromosome 6 at position 6p12.1-6q13 (CLNS1B). By fluorescence in situ hybridization a copy carrying introns with a putative length of 19 kb was located at chromosome 11 on position 11q13.5-q14.1 (CLNS1A). The characterization and chromosomal localization of the ICln gene offer the opportunity to study the regulatory sites of this gene in greater detail and could be helpful in establishing linkages between ICln and potential human diseases.

The promoter for constitutive expression of the human ICln gene CLNS1A.

The ICln protein is expressed ubiquitously in mammals. Experiments designed to knock down the ICln protein in NIH 3T3 fibroblasts as well as in epithelial cells led to the conclusion that this protein is crucially involved in volume regulation after cytoplasmic swelling. Reconstitution of the ICln protein in lipid bilayers revealed the ion channel nature of ICln. Here we describe a new human promoter sequence, composed of 89 nucleotides, which is responsible for a highly constitutive expression of the ICln protein. The promoter sequence lacks a TATA box, and the transcription can be effected at multiple sites. In addition to the starting sites, upstream sequence elements are mandatory for an efficient transcription of the ICln gene (CLNS1A). These new nucleotide elements were defined by site-directed mutagenesis.

Functional reconstitution of ICln in lipid bilayers.

Reconstitution of purified ICln in lipid bilayer leads to functional ion channels showing varying rectification. The reconstituted single channels have a conductance of approximately equal to 3 pS and their open probability is sensitive to nucleoside analogues. Mutation of a putative nucleotide binding site identified at the predicted extracellular mouth of the ICln channel protein leads to the reduction of the nucleoside-analogue sensitivity. Reconstituted ICln channels can be permeated both by cations and anions. The relative permeability of cations over anions depends on the presence of calcium. In the presence of calcium reconstituted ICln channels are more permeable to bromide than chloride, and more permeable to potassium than sodium. Similarly in NIH3T3 fibroblasts, the relative permeability of cations over anions of swelling-dependent chloride channels depends on extracellular calcium. Site-directed mutagenesis revealed the calcium-binding site responsible for the shift of the selectivity from cations towards anions of reconstituted ICln channels. Additional indirect structural information has been obtained by mutating a histidine in the predicted pore region of ICln. This histidine seems to have access to the ion-conducting tunnel of the pore. Our experiments show that ICln can act as an ionic channel, which does not exclude additional functions of the protein in regulatory mechanisms of the cell. Since knocking down the ICln protein in fibroblasts and epithelial cells leads to an impaired regulatory volume decrease (RVD) after cytoplasmic swelling and reconstituted ICln channels show several biophysical features of ion channels activated after swelling, ICln is a molecular candidate for these channels.

Structure and function of the ion channel ICln.

Normal function of organs and cells is tightly linked to the cytoarchitecture. Control of the cell volume is therefore vital for the organism. A widely established strategy of cells to counteract swelling is the activation of chloride and potassium channels, which leads to a net efflux of salt followed by water - a process termed regulatory volume decrease. Since there is evidence for swelling-dependent chloride channels (IClswell) being activated also during pathological processes, the identification of the molecular entity underlying IClswell is of utmost importance. Several proteins are discussed as the channel forming IClswell, i.e. phospholemman, p-glycoprotein, CLC-3 and ICln. In this review we would like to focus on the properties of ICln, a protein cloned from a Madin Darby canine kidney (MDCK) cell library whose expression in Xenopus laevis oocytes resulted in a nucleotide sensitive outwardly rectifying chloride current closely resembling the biophysical properties of IClswell.