Characterization of the proteins of the intestinal Na(+)-K(+)-2Cl- cotransporter

Acclimation of brackish water pearl spot (Etroplus suratensis) to various salinities: relative changes in abundance of branchial Na(+)/K (+)-ATPase and Na (+)/K (+)/2Cl (-) co-transporter in relation to osmoregulatory parameters

The present study was conducted to elucidate the osmoregulatory ability of the fish pearl spot (Etroplus suratensis) to know the scope of this species for aquaculture under various salinities. Juvenile pearl spot were divided into three groups and acclimated to freshwater (FW), brackish water (BW) or seawater (SW) for 15 days. The fish exhibited effective salinity tolerance under osmotic challenges. Although the plasma osmolality and Na(+), K(+) and Cl(-) levels increased with the increasing salinities, the parameters remained within the physiological range. The muscle water contents were constant among FW-, BW- and SW-acclimated fish. Two Na+/K+-ATPase α-isoforms (NKA α) were expressed in gills during acclimation in FW, BW and SW. Abundance of one isoform was up-regulated in response to seawater acclimation, suggesting its role in ion secretion similar to NKA α1b, while expression of another isoform was simultaneously up-regulated in response to both FW and SW acclimation, suggesting the presence of isoforms switching phenomenon during acclimation to different salinities. Nevertheless, NKA enzyme activities in the gills of the SW and FW individuals were higher (p < 0.05) than in BW counterparts. Immunohistochemistry revealed that Na(+)/K(+)-ATPase immunoreactive (NKA-IR) cells were mainly distributed in the interlamellar region of the gill filaments in FW groups and in the apical portion of the filaments in BW and SW groups. The number of NKA-IR cells in the gills of the FW-acclimated fish was almost similar to that of SW individuals, which exceeded that of the BW individuals. The NKA-IR cells of BW and SW were bigger in size than their FW counterparts. Besides, the relative abundance of branchial Na(+)/K(+)/2Cl(-) co-transporter showed stronger evidence in favor of involvement of this protein in hypo-osmoregulation, requiring ion secretion by the chloride cells. To the best of our knowledge, this is the first study reporting the wide salinity tolerance of E. suratensis involving differential activation of ion transporters and thereby suggesting its potential as candidate for fish farming under different external salinities.

Molecular physiology of SPAK and OSR1: two Ste20-related protein kinases regulating ion transport

SPAK (Ste20-related proline alanine rich kinase) and OSR1 (oxidative stress responsive kinase) are members of the germinal center kinase VI subfamily of the mammalian Ste20 (Sterile20)-related protein kinase family. Although there are 30 enzymes in this protein kinase family, their conservation across the fungi, plant, and animal kingdom confirms their evolutionary importance. Already, a large volume of work has accumulated on the tissue distribution, binding partners, signaling cascades, and physiological roles of mammalian SPAK and OSR1 in multiple organ systems. After reviewing this basic information, we will examine newer studies that demonstrate the pathophysiological consequences to SPAK and/or OSR1 disruption, discuss the development and analysis of genetically engineered mouse models, and address the possible role these serine/threonine kinases might have in cancer proliferation and migration.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

A brief history of the study of fish osmoregulation: the central role of the Mt. Desert Island Biological Laboratory

The Mt. Desert Island Biological Laboratory (MDIBL) has played a central role in the study of fish osmoregulation for the past 80 years. In particular, scientists at the MDIBL have made significant discoveries in the basic pattern of fish osmoregulation, the function of aglomerular kidneys and proximal tubular secretion, the roles of NaCl cotransporters in intestinal uptake and gill and rectal gland secretion, the role of the shark rectal gland in osmoregulation, the mechanisms of salt secretion by the teleost fish gill epithelium, and the evolution of the ionic uptake mechanisms in fish gills. This short review presents the history of these discoveries and their relationships to the study of epithelial transport in general.

NKCC1 (SLC12a2) induces a secondary axis in Xenopus laevis embryos independently of its co-transporter function

NKCC1 is a broadly expressed Na(+)-K(+)-Cl(-) co-transporter involved in regulation of ion flux across the cell membrane and in regulating cell volume. Whilst much is known about the co-transporter activity of NKCC1 and its regulation by protein kinases and phosphatases, little is known about the activities of NKCC1 that are co-transporter independent. In this report we show that over-expression of NKCC1 in embryos of Xenopus laevis induces secondary axes, independently of its co-transporter activity. In addition, over-expression of NKCC1 results in the formation of neural tissue in ectodermal explants. We also show that NKCC1 is expressed broadly but non-uniformly in embryos of Xenopus laevis and Xenopus tropicalis, with prominent expression in the notochord, nervous system and stomach. These results provide insights into an additional, previously unreported activity of NKCCl.

The role of volume-sensitive ion transport systems in regulation of epithelial transport

This review focuses on using the knowledge on volume-sensitive transport systems in Ehrlich ascites tumour cells and NIH-3T3 cells to elucidate osmotic regulation of salt transport in epithelia. Using the intestine of the European eel (Anguilla anguilla) (an absorptive epithelium of the type described in the renal cortex thick ascending limb (cTAL)) we have focused on the role of swelling-activated K+- and anion-conductive pathways in response to hypotonicity, and on the role of the apical (luminal) Na+-K+-2Cl- cotransporter (NKCC2) in the response to hypertonicity. The shrinkage-induced activation of NKCC2 involves an interaction between the cytoskeleton and protein phosphorylation events via PKC and myosin light chain kinase (MLCK). Killifish (Fundulus heteroclitus) opercular epithelium is a Cl(-)-secreting epithelium of the type described in exocrine glands, having a CFTR channel on the apical side and the Na+/K+ ATPase, NKCC1 and a K+ channel on the basolateral side. Osmotic control of Cl- secretion across the operculum epithelium includes: (i) hyperosmotic shrinkage activation of NKCC1 via PKC, MLCK, p38, OSR1 and SPAK; (ii) deactivation of NKCC by hypotonic cell swelling and a protein phosphatase, and (iii) a protein tyrosine kinase acting on the focal adhesion kinase (FAK) to set levels of NKCC activity.

The Na+/K+/2Cl- cotransporter in the sea bass Dicentrarchus labrax during ontogeny: involvement in osmoregulation

This study combines a cellular and molecular analysis of the Na(+)/K(+)/2Cl(-) cotransporter (NKCC) to determine the osmoregulatory role of this protein in different tissues during the ontogeny of the sea bass. We have characterized the complete sequence of the NKCC1 isoform isolated from the sea bass gills and have identified, by immunofluorescence, NKCC1, and other isoforms, within the epithelium of the major osmoregulatory organs. Different (absorptive and secretory) functions have been attributed to this protein according to the tissue and salinity. The effects of short- (1-4 days), medium- (7-21 days) and long (6 months)-term freshwater (FW) adaptations were investigated, in comparison with seawater (SW)-maintained sea bass. In adult sea bass after long-term adaptation to FW and SW, the gills had the highest expression of NKCC mRNA compared with the median/posterior kidney and to the posterior intestine. Expression of NKCC mRNA in the kidney was 95% (SW) and 63% (FW) lower, and in the intestine 98% (SW) and 77% (FW) lower. Compared to SW-maintained sea bass, long-term FW adaptation induced a significant 5.6-fold decrease in the branchial NKCC gene expression whereas the intestinal and renal expressions did not vary significantly. The cells of the intestine and collecting ducts as well as a part of the epithelium lining the urinary bladder expressed NKCC apically. Within the gill chloride cells, NKCC was found basolaterally in SW-acclimated fish; some apically stained cells were detected after 7 days of FW exposure and their relative number increased progressively following FW acclimation. The appearance of FW-type chloride cells induces a functional shift of the gills from a secretory to an absorptive epithelium, which was only completed after long-term exposure to FW. Short- and medium-term exposure to FW induced a progressive decrease in total NKCC content and an increase in functionally different branchial chloride cells. During development, the cotransporter was already expressed in tegumentary ionocytes and along the digestive tract of late embryos. NKCC was recorded in the branchial chamber and along the renal collecting ducts in prelarvae and also in the dorsal part of the urinary bladder in larvae. The expression of NKCC along the osmoregulatory epithelial cells and the presence of Na(+)/K(+)-ATPase within these cells contribute to the increase of the osmoregulatory capacity during sea bass ontogeny.

The Na+-K+-2Cl- cotransporter and the osmotic stress response in a model salt transport epithelium

Epithelia are physiologically exposed to osmotic stress resulting in alteration of cell volume in several aspects of their functioning; therefore, the activation of 'emergency' systems of rapid cell volume regulation is fundamental in their physiology. In this review, the physiological response to osmotic stress, particularly hypertonic stress, was described in a salt-transporting epithelium, the intestine of the euryhaline teleost European eel. This epithelium is physiologically exposed to changes in extracellular osmolarity and represents a good physiological model for functional studies on cellular volume regulation, permitting the study of volume regulated ion transport mechanisms in a native tissue. An absorptive form of the cotransporter, homologue of the renal NKCC2, localized on the apical membrane, was found in the intestine of the euryhaline teleost European eel. This cotransporter accounts for the luminal uptake of Cl-; it operates in series with a basolateral Cl- conductance and presumably a basolateral electroneutral KCl cotransport and in parallel with a luminal K+ conductance. The ion transport model described for eel intestine, based on the operation of an absorptive luminal Na+-K+-2Cl-, is basically the same as the model that has been proposed for the thick ascending limb (cTAL) of the mammalian renal cortex. This paper focuses on the role of Na+-K+-2Cl- cotransport in the responses to hypertonic stress in the eel intestine and the role of cytoskeleton (either actin-based or tubulin based) is discussed.

SPAK and OSR1, key kinases involved in the regulation of chloride transport

Reversible phosphorylation by protein kinases is probably one of the most important examples of post-translational modification of ion transport proteins. Ste20-related proline alanine-rich kinase (SPAK) and oxidative stress response kinase (OSR1) are two serine/threonine kinases belonging to the germinal centre-like kinase subfamily VI. Genetic analysis suggests that OSR1 evolved first, with SPAK arising following a gene duplication in vertebrate evolution. SPAK and OSR1 are two recently discovered kinases which have been linked to several key cellular processes, including cell differentiation, cell transformation and proliferation, cytoskeleton rearrangement, and most recently, regulation of ion transporters. Na-K-2Cl cotransporter activity is regulated by phosphorylation. Pharmacological evidence has identified several kinases and phosphatases which alter cotransporter function, however, no direct linkage between these enzymes and the cotransporter has been demonstrated. This article will review some of the physical and physiological properties of SPAK and OSR1, and present new evidence of a direct interaction between the Na-K-Cl cotransporter and the stress kinases.

A model of pulmonary adenocarcinoma in transgenic mice expressing the simian virus 40 T antigen driven by the rat Calbindin-D9K (CaBP9K) promoter

Lung cancer is the most frequent cause of cancer deaths. Its origin and development remain poorly understood, partly because of the lack of pertinent animal models. This study produced transgenic mice expressing the simian virus (SV) 40 T antigen (Tag) driven by a 1011 base-pair DNA fragment of the rat Calbindin-D9K (CaBP9K) promoter. All transgenic animals developed multifocal pulmonary tumours with pathological and ultrastructural features consistent with adenocarcinomas. Using immunohistochemistry, northern blot or western blot, tumours were found to express the transcription factor TTF-1, as well as specific markers of the peripheral airway Clara cells (CC10) and alveolar type II cells (surfactant proteins A, B, C, and D). This model, with its similarities to human adenocarcinoma, should be useful not only for addressing the mechanisms underlying the development and progression of lung cancer, but also for testing new therapeutic approaches.

Gill Na+-K+-2Cl−cotransporter abundance and location in Atlantic salmon: effects of seawater and smolting

Na+-K+-2Cl−cotransporter abundance and location was examined in the gills of Atlantic salmon ( Salmo salar) during seawater acclimation and smolting. Western blots revealed three bands centered at 285, 160, and 120 kDa. The Na+-K+-2Cl−cotransporter was colocalized with Na+-K+-ATPase to chloride cells on both the primary filament and secondary lamellae. Parr acclimated to 30 parts per thousand seawater had increased gill Na+-K+-2Cl− cotransporter abundance, large and numerous Na+-K+-2Cl− cotransporter immunoreactive chloride cells on the primary filament, and reduced numbers on the secondary lamellae. Gill Na+-K+-2Cl− cotransporter levels were low in presmolts (February) and increased 3.3-fold in smolts (May), coincident with elevated seawater tolerance. Cotransporter levels decreased below presmolt values in postsmolts in freshwater (June). The size and number of immunoreactive chloride cells on the primary filament increased threefold during smolting and decreased in postsmolts. Gill Na+-K+-ATPase activity and Na+-K+-2Cl− cotransporter abundance increased in parallel during both seawater acclimation and smolting. These data indicate a direct role of the Na+-K+-2Cl− cotransporter in salt secretion by gill chloride cells of teleost fish.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Dietary manipulation of delta-6-desaturase modifies phospholipid arachidonic acid levels and the urinary excretion of calcium and oxalate in the rat: insight in calcium lithogenesis

An anomalous n-6 polyunsaturated fatty acid composition in plasma and erythrocyte membrane phospholipids, namely increased levels of arachidonic acid (AA), has been reported in calcium nephrolithiasis and has been proposed to play an important role in its pathogenesis. To confirm this, in rats we modified phospholipid AA levels by dietary manipulation of the delta-6-desaturase, the rate-limiting enzyme of the fatty acid biosynthetic pathway, and evaluated the effect on cellular and renal functions predisposing to lithogenesis. Increased AA levels led to conditions at risk for nephrolithiasis: higher oxalate flux and lower sodium cotransport in erythrocytes and a rise in urinary prostaglandin E2, calcium, sodium, and oxalate levels; reduced AA levels reversed these changes. In vitro, in human erythrocytes the incorporation of exogenous AA into membranes increased band 3 protein phosphorylation directly activating the Ser/Thr protein kinase CK1 and induced a parallel raise in band 3-mediated oxalate transport. These findings demonstrate the pivotal role of phospholipid AA in modulating erythrocyte and renal transport of calcium and oxalate.

CFTR upregulates the expression of the basolateral Na(+)-K(+)-2Cl(-) cotransporter in cultured pancreatic duct cells

The purpose of the current experiments was 1) to assess basolateral Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) expression and 2) to ascertain the role of cystic fibrosis transmembrane conductance regulator (CFTR) in the regulation of this transporter in a prototypical pancreatic duct epithelial cell line. Previously validated human pancreatic duct cell lines (CFPAC-1), which exhibit physiological features prototypical of cystic fibrosis, and normal pancreatic duct epithelia (stable recombinant CFTR-bearing CFPAC-1 cells, termed CFPAC-WT) were grown to confluence before molecular and functional studies. High-stringency Northern blot hybridization, utilizing specific cDNA probes, confirmed that NKCC1 was expressed in both cell lines and its mRNA levels were twofold higher in CFPAC-WT cells than in CFPAC-1 cells (P < 0.01, n = 3). Na(+)-K(+)-2Cl(-) cotransporter activity, assayed as the bumetanide-sensitive, Na(+)- and Cl(-)-dependent NH(+)(4) entry into the cell (with NH(+)(4) acting as a substitute for K(+)), increased by approximately 115% in CFPAC-WT cells compared with CFPAC-1 cells (P < 0.01, n = 6). Reducing the intracellular Cl(-) by incubating the cells in a Cl(-)-free medium increased Na(+)-K(+)-2Cl(-) cotransporter activity by twofold (P < 0.01, n = 4) only in CFPAC-WT cells. We concluded that NKCC1 is expressed in pancreatic duct cells and mediates the entry of Cl(-). NKCC1 activity is enhanced in the presence of an inward Cl(-) gradient. The results further indicate that the presence of functional CFTR enhances the expression of NKCC1. We speculate that CFTR regulates this process in a Cl(-)-dependent manner.

Mechanisms of carbachol-induced alterations in K+ transport across the rat colon

The effect of carbachol, an agonist of the Ca2+ pathway, on K+ transport in rat proximal and distal colon was studied by measuring unidirectional fluxes, uptake, and efflux of Rb+, a marker for K+, in mucosa-submucosa preparations. Unidirectional ion flux measurements revealed that carbachol stimulated K+ secretion in the proximal colon by a marked increase in the serosa-to-mucosa flux (J(Rb)sm) and a more moderate rise in the mucosa-to-serosa flux (J(Rb)ms). In the distal colon carbachol had no effect on J(Rb)ms but J(Rb)sm was reduced after a transient increase finally resulting in an inhibition of K+ secretion. Carbachol caused a stimulation of mucosal Rb+ uptake in the distal colon, which was diminished in the presence of inhibitors of the apical H+-K+-ATPase, vanadate and ouabain. In contrast, in the proximal colon the serosal Rb+ uptake was enhanced by carbachol, an effect, which could be prevented by bumetanide, an inhibitor of the basolateral Na+-K+-2Cl(-)-cotransporter. Efflux experiments revealed that carbachol caused a transient increase of apical and basolateral Rb+ permeability in both colonic segments. In the distal colon, stimulated K+ efflux to the serosal side was reduced by quinine, efflux to the mucosal side was blocked by tetraethylammonium. In the proximal colon, carbachol-activated apical and basolateral K+ efflux were inhibited by Ba2+. In conclusion, these data suggest that in the distal colon carbachol stimulates the H+-K+-ATPase and the basolateral K+ efflux through quinine-sensitive K+ channels, whereas in the proximal colon carbachol induces K+ secretion due to a stimulation of the basolateral Na+-K+-2Cl(-)-cotransporter and an increased efflux to the luminal side via Ba2+-sensitive apical K+ channels.

Cl- transport in an immortalized human epithelial cell line (NCM460) derived from the normal transverse colon

Cells of a newly described, immortalized, epithelial, human transverse colonic cell line, NCM460, reach approximately 90% confluence on plastic and develop transepithelial resistances of 120-250 Omega . cm2 on porous substrates. Its utility as a model for the transverse human colon was validated by comparing second messenger-mediated Cl- transport, using the fluorescent probe 6-methoxy-quinolyl acetoethyl ester, in NCM460 cells and colonocytes isolated from human transverse crypts. Basal Cl- influx was increased (P < 0.01) by PGE1 (1 microM), forskolin (1 microM), 8-bromoadenosine 3'5'-cyclic monophosphate (100 microM), heat-stable Escherichia coli enterotoxin (STa; 1 microM), 8-bromoguanosine 3'5'-cyclic monophosphate (100 microM), histamine (1 microM), and phorbol 12,13-dibutyrate (1 microM) in both cell types. The Cl- channel blocker diphenylamine 2-carboxylic acid (50 microM) and the Na+-K+-2Cl- cotransport inhibitor furosemide (1 microM), but not the K+ channel blocker Ba2+ (3 mM), inhibited these Cl- permeabilities. These cells possess transcripts for cystic fibrosis transmembrane conductance regulator, Na+-K+-2Cl- cotransporter, STa receptor, and intestine-specific cGMP-dependent protein kinase II. Thus cAMP-, cGMP-, and Ca2+-dependent secretagogues act on NCM460 and primary colonocytes to stimulate Cl- transport. This validates the utility of NCM460 as a model for transverse colonic crypts and is the first demonstration of a colonic cell line whose origin is known.

Stimulation of Na+-K+-2Cl- cotransporter in neuronal cells by excitatory neurotransmitter glutamate

Na+-K+-2Cl- cotransporters are important in renal salt reabsorption and in salt secretion by epithelia. They are also essential in maintenance and regulation of ion gradients and cell volume in both epithelial and nonepithelial cells. Expression of Na+-K+-2Cl- cotransporters in brain tissues is high; however, little is known about their function and regulation in neurons. In this study, we examined regulation of the Na+-K+-2Cl- cotransporter by the excitatory neurotransmitter glutamate. The cotransporter activity in human neuroblastoma SH-SY5Y cells was assessed by bumetanide-sensitive K+ influx, and protein expression was evaluated by Western blot analysis. Glutamate was found to induce a dose- and time-dependent stimulation of Na+-K+-2Cl- cotransporter activity in SH-SY5Y cells. Moreover, both the glutamate ionotropic receptor agonist N-methyl-D-aspartic acid (NMDA) and the metabotropic receptor agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) significantly stimulated the cotransport activity in these cells. NMDA-mediated stimulation of the Na+-K+-2Cl- cotransporter was abolished by the selective NMDA-receptor antagonist (+)-MK-801 hydrogen maleate. trans-ACPD-mediated effect on the cotransporter was blocked by the metabotropic receptor antagonist (+)-alpha-methyl-(4-carboxyphenyl)glycine. The results demonstrate that Na+-K+-2Cl- cotransporters in neurons are regulated by activation of both ionotropic and metabotropic glutamate receptors.

Impaired surfactant synthesis in fetal type II lung cells from gsd/gsd rats

The importance of intracellular glycogen for surfactant synthesis was investigated in fetal type II lung cells isolated from rats with a glycogen storage disorder, designated gsd/gsd. Compared to cells from a control Wistar strain, cultured gsd/gsd pneumocytes were glycogenrich and contained fewer and smaller lamellar inclusions. Freshly isolated cells from day 19-21 fetuses of control rats demonstrated the expected gestational rise and fall of cellular glycogen seen in intact fetal lungs. At day 20, when tissue glycogen peaks, cellular glycogen content was 48 and 70 nmol glucose/micrograms DNA in isolated type II cells of control and gsd/gsd lungs, respectively. In control cells, while active glycogen phosphorylase changed from 35 to 65% of total during 24 h of culture, glycogen fell 85%. In contrast, gsd/gsd cell phosphorylase was not activated, phosphorylase kinase activity was nondetectable, and glycogen per cell remained unchanged. [3H]Choline incorporation into total PC and disaturated PC (DSPC) was 50 and 62% lower, respectively, in gsd/gsd type II cells compared to controls in the absence of exogenous substrate. Cellular content of the surfactant-associated protein SP-A was similar in control and gsd/gsd cells at day 20, and increased 3- to 4-fold during a subsequent 24-h interval of tissue culture. These results suggest that PC synthesis is dramatically impaired in type II cells in which glycogen cannot be mobilized, but SP-A is synthesized at normal rates. This work characterizes the isolated gsd/gsd fetal type II cell and supports its use in future studies to determine the importance and relative utilization of specific nonglycogen substrates.

Molecular characterization of the epithelial Na-K-Cl cotransporter isoforms

Recent advances in the molecular characterization of specific isoforms of the Na-K-Cl cotransporter have allowed rapid progress in the study of the structure, function, and regulation of these members of a family of Cl-dependent cation cotransporters. Two distinct isoforms have been identified, one from Cl(-)-secretory epithelia and another found specifically in the diluting segment of the vertebrate kidney, a Cl(-)-absorptive epithelium. The discovery of three alternatively spliced variants of the absorptive isoform, which differ only by 31 amino acids and which appear to be differentially distributed within the mammalian thick ascending limb of the loop of Henle, highlight this spliced region as an important functional component of the protein.