المؤلفون: Van Aubel RA; Department of Pharmacology and Toxicology, Faculty of Medical Sciences, University Medical Centre Nijmegen, 6500 HB Nijmegen, The Netherlands., Peters JG, Masereeuw R, Van Os CH, Russel FG

المصدر: American journal of physiology. Renal physiology [Am J Physiol Renal Physiol] 2000 Oct; Vol. 279 (4), pp. F713-7.

نوع المنشور: Journal Article

بيانات الدورية: Publisher: American Physiological Society Country of Publication: United States NLM ID: 100901990 Publication Model: Print Cited Medium: Print ISSN: 1931-857X (Print) Linking ISSN: 15221466 NLM ISO Abbreviation: Am J Physiol Renal Physiol Subsets: MEDLINE

مواضيع طبية MeSH: Membrane Transport Proteins* , Multidrug Resistance-Associated Proteins*, ATP Binding Cassette Transporter, Subfamily B/*physiology , Adenosine Triphosphate/*physiology , Kidney/*metabolism , p-Aminohippuric Acid/*metabolism, Animals ; Anions/metabolism ; Biological Transport ; Carrier Proteins/metabolism ; Cell Line ; Kinetics ; Multidrug Resistance-Associated Protein 2 ; Rabbits ; Recombinant Proteins/metabolism ; Spodoptera/cytology ; Time Factors

مستخلص: p-Aminohippurate (PAH) is widely used as a model substrate to characterize organic anion transport in kidney proximal tubules. The carrier responsible for uptake of PAH across the basolateral membrane has been cloned and well characterized, whereas transporters mediating PAH excretion across the brush-border (apical) membrane are yet unknown. In this study we investigated whether PAH is a substrate for the apical multidrug resistance protein 2 (Mrp2). Overexpression of recombinant rabbit Mrp2 in Sf9 cells significantly increased ATP-dependent [(14)C]PAH uptake into isolated membrane vesicles compared with endogenous ATP-dependent uptake. The Michaelis-Menten constant and maximal velocity for Mrp2-mediated ATP-dependent [(14)C]PAH transport were 1.9 +/- 0.8 mM and 187 +/- 29 pmol. mg(-1). min(-1), respectively. On the basis of the inhibitory profile, the endogenous ATP-dependent PAH transporter does not appear to be an ortholog of Mrp2. Together, our results show that Mrp2 is a low-affinity ATP-dependent PAH transporter, indicating that Mrp2 might contribute to urinary PAH excretion.

المؤلفون: van Baal J; Department of Cell Physiology, University of Nijmegen, 6500 HB Nijmegen, The Netherlands., Hoenderop JG, Groenendijk M, van Os CH, Bindels RJ, Willems PH

المصدر: The American journal of physiology [Am J Physiol] 1999 Dec; Vol. 277 (6), pp. F899-906.

نوع المنشور: Journal Article

بيانات الدورية: Publisher: American Physiological Society Country of Publication: United States NLM ID: 0370511 Publication Model: Print Cited Medium: Print ISSN: 0002-9513 (Print) Linking ISSN: 00029513 NLM ISO Abbreviation: Am J Physiol Subsets: MEDLINE

مواضيع طبية MeSH: Adenosine Triphosphate/*physiology , Arginine Vasopressin/*pharmacology , Calcium/*metabolism , Cyclic AMP/*physiology , Deamino Arginine Vasopressin/*pharmacology , Kidney Cortex/*physiology , Kidney Tubules/*physiology , Parathyroid Hormone/*pharmacology , Protein Kinase C/*metabolism, 8-Bromo Cyclic Adenosine Monophosphate/pharmacology ; Adenosine/analogs & derivatives ; Adenosine/pharmacology ; Adenosine Triphosphate/pharmacology ; Alkaloids ; Animals ; Benzophenanthridines ; Cells, Cultured ; Dinoprostone/pharmacology ; Enzyme Inhibitors/pharmacology ; Indomethacin/pharmacology ; Kidney Cortex/drug effects ; Kidney Tubules/drug effects ; Kidney Tubules, Collecting/drug effects ; Kidney Tubules, Collecting/physiology ; Models, Biological ; Phenanthridines/pharmacology ; Purinergic P1 Receptor Antagonists ; Rabbits ; Tetradecanoylphorbol Acetate/pharmacology

مستخلص: Exogenous ATP markedly reduced 1-desamino-8-D-arginine vasopressin (dDAVP)-stimulated Ca2+ transport and cAMP accumulation in primary cultures of rabbit connecting tubule and cortical collecting duct cells. Similarly, ATP inhibited the stimulatory effect of 8-bromo-cAMP. At first sight, this is in agreement with the "classic" concept that dDAVP exerts its stimulatory effect via cAMP. However, dDAVP-stimulated Ca2+ transport was markedly reduced by the protein kinase C (PKC) inhibitor chelerythrine, reported previously to inhibit the cAMP-independent pathway responsible for parathyroid hormone-, [Arg8]vasopressin-, PGE2-, and adenosine-stimulated Ca2+ transport. Chelerythrine also inhibited the increase in Ca2+ transport evoked by the cAMP-independent A1 receptor agonist N6-cyclopentyladenosine (CPA). Downregulation of phorbol ester-sensitive PKC isoforms by chronic phorbol ester treatment has been shown before to be without effect on hormone-stimulated Ca2+ transport, indicating that the chelerythrine-inhibitable pathway consists of a phorbol ester-insensitive PKC isoform. Here, this maneuver did not affect ATP inhibition of dDAVP-stimulated Ca2+ transport and cAMP formation, while abolishing ATP inhibition of CPA-stimulated Ca2+ transport. These findings show that ATP acts via 1) a phorbol ester-sensitive PKC isoform to inhibit hormonal stimulation of Ca2+ transport at the level of the chelerythrine-inhibitable pathway involving a phorbol ester-insensitive PKC isoform and 2) a phorbol ester-insensitive mechanism to inhibit V2 receptor-mediated concomitant activation of this pathway and adenylyl cyclase.

المؤلفون: van Aubel RA; Department of Pharmacology, University of Nijmegen, 6500 HB Nijmegen, The Netherlands., van Kuijck MA, Koenderink JB, Deen PM, van Os CH, Russel FG

المصدر: Molecular pharmacology [Mol Pharmacol] 1998 Jun; Vol. 53 (6), pp. 1062-7.

نوع المنشور: Journal Article; Research Support, Non-U.S. Gov't

بيانات الدورية: Publisher: American Society for Pharmacology and Experimental Therapeutics Country of Publication: United States NLM ID: 0035623 Publication Model: Print Cited Medium: Print ISSN: 0026-895X (Print) Linking ISSN: 0026895X NLM ISO Abbreviation: Mol Pharmacol Subsets: MEDLINE

مواضيع طبية MeSH: Drug Resistance, Multiple*, Adenosine Triphosphate/*pharmacology , Carrier Proteins/*physiology, Animals ; Anion Transport Proteins ; Biological Transport ; Carrier Proteins/antagonists & inhibitors ; Carrier Proteins/biosynthesis ; Rabbits ; Rats ; Recombinant Proteins/biosynthesis ; Spodoptera

مستخلص: The multidrug resistance-associated protein Mrp2 is expressed in liver, kidney, and small intestine and mediates ATP-dependent transport of conjugated organic anions across the apical membrane of epithelial cells. We recently cloned a rabbit cDNA encoding a protein that on basis of highest amino acid homology and tissue distribution was considered to be the rabbit homolog of rat Mrp2. To investigate whether rabbit Mrp2 mediates ATP-dependent transport similar to rat Mrp2, we expressed rabbit Mrp2 in Spodoptera frugiperda (Sf9) cells using recombinant baculovirus. Mrp2 was expressed as an underglycosylated protein in Sf9 cells and to a higher level compared with rabbit liver and renal proximal tubules. Both 17beta-estradiol-17-beta-D-glucuronide ([3H]E217betaG, 50 nM) and [3H]leukotriene C4 (3 nM) were taken up by Sf9-Mrp2 membrane vesicles in an ATP-dependent fashion. Uptake of [3H]E217betaG was dependent on the osmolarity of the medium and saturable for ATP (Km = 623 microM). Leukotriene C4, MK571, phenolphthalein glucuronide, and fluorescein-methotrexate were good inhibitors of [3H]E217betaG transport. The inhibitory potency of cyclosporin A and methotrexate was moderate, whereas fluorescein, alpha-naphthyl-beta-D-glucuronide, and p-nitrophenyl-beta-D-glucuronide did not inhibit transport. In conclusion, we show direct ATP-dependent transport by recombinant rabbit Mrp2 and provide new data on Mrp2 inhibitor specificity.

المؤلفون: van Kuijck MA; Department of Cell Physiology, University of Nijmegen, The Netherlands., van Aubel RA, Busch AE, Lang F, Russel FG, Bindels RJ, van Os CH, Deen PM

المصدر: Proceedings of the National Academy of Sciences of the United States of America [Proc Natl Acad Sci U S A] 1996 May 28; Vol. 93 (11), pp. 5401-6.

نوع المنشور: Journal Article; Research Support, Non-U.S. Gov't

بيانات الدورية: Publisher: National Academy of Sciences Country of Publication: United States NLM ID: 7505876 Publication Model: Print Cited Medium: Print ISSN: 0027-8424 (Print) Linking ISSN: 00278424 NLM ISO Abbreviation: Proc Natl Acad Sci U S A Subsets: MEDLINE

مواضيع طبية MeSH: ATP-Binding Cassette Transporters/*physiology , Adenosine Triphosphate/*metabolism , Chloride Channels/*physiology , Kidney/*physiology, 1-Methyl-3-isobutylxanthine/pharmacology ; ATP Binding Cassette Transporter, Subfamily B, Member 1/chemistry ; ATP-Binding Cassette Transporters/biosynthesis ; ATP-Binding Cassette Transporters/chemistry ; Amino Acid Sequence ; Animals ; Blotting, Northern ; Cell Membrane/physiology ; Cloning, Molecular ; Colforsin/pharmacology ; Epithelium/physiology ; Female ; Humans ; Intestine, Small/physiology ; Liver/physiology ; Membrane Potentials ; Models, Structural ; Molecular Sequence Data ; Niflumic Acid/pharmacology ; Oocytes/drug effects ; Oocytes/physiology ; Organ Specificity ; Patch-Clamp Techniques ; Phylogeny ; Polymerase Chain Reaction ; Protein Structure, Secondary ; Rabbits ; Recombinant Proteins/biosynthesis ; Recombinant Proteins/metabolism ; Sequence Homology, Amino Acid ; Xenopus laevis

مستخلص: Cystic fibrosis transmembrane conductance regulator (CFTR) is an ATP-regulated, cAMP-activated chloride channel located in the apical membrane of many epithelial secretory cells. Here we report cloning of a cAMP-activated epithelial basolateral chloride conductance regulator (EBCR) that appears to be a basolateral CFTR counterpart. This novel chloride channel or regulator shows 49% identity with multidrug resistance-associated protein (MRP) and 29% identity with CFTR. On expression in Xenopus oocytes, EBCR confers a cAMP-activated chloride conductance that is inhibited by the chloride channel blockers niflumic acid, 5-nitro-2-(3-phenylpropylamine)benzoic acid, and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Northern blot analysis reveals high expression in small intestine, kidney, and liver. In kidney, immunohistochemistry shows a conspicuous basolateral localization mainly in the thick ascending limb of Henle's loop, distal convoluted tubules and to a lesser extent connecting tubules. These data suggest that in the kidney EBCR is involved in hormone-regulated chloride reabsorption.

المؤلفون: Flik G; Department of Animal Physiology, Faculty of Science, University of Nijmegen, The Netherlands., Schoenmakers TJ, Groot JA, van Os CH, Wendelaar Bonga SE

المصدر: The Journal of membrane biology [J Membr Biol] 1990 Jan; Vol. 113 (1), pp. 13-22.

نوع المنشور: Journal Article; Research Support, Non-U.S. Gov't

بيانات الدورية: Publisher: Springer Country of Publication: United States NLM ID: 0211301 Publication Model: Print Cited Medium: Print ISSN: 0022-2631 (Print) Linking ISSN: 00222631 NLM ISO Abbreviation: J Membr Biol Subsets: MEDLINE

مواضيع طبية MeSH: Intestinal Absorption*, Adenosine Triphosphate/*pharmacology , Calcium/*metabolism , Fishes/*metabolism , Sodium/*pharmacology, Adenosine Triphosphatases/metabolism ; Adenosine Triphosphate/metabolism ; Animals ; Bepridil/pharmacology ; Biological Transport, Active/drug effects ; Calcium/pharmacology ; Carrier Proteins/metabolism ; Kinetics ; Male ; Ouabain/pharmacology ; Sodium/metabolism ; Sodium-Calcium Exchanger

مستخلص: Measurements of unidirectional calcium fluxes in stripped intestinal epithelium of the tilapia, Oreochromis mossambicus, in the presence of ouabain or in the absence of sodium indicated that calcium absorption via the fish intestine is sodium dependent. Active Ca2+ transport mechanisms in the enterocyte plasma membrane were analyzed. The maximum capacity of the ATP-dependent Ca2+ pump (Vm: 0.63 nmol.min-1.mg-1, Km:27 nM Ca2+) is calculated to be 2.17 nmol.min-1.mg-1, correcting for 29% inside-out oriented vesicles in the membrane preparation. The maximum capacity of the Na+/Ca2+ exchanger with high affinity for Ca2+ (Vm:7.2 nmol.min-1.mg-1, Km:181 nM Ca2+) is calculated to be 13.6 nmol.min-1.mg-1, correcting for 53% resealed vesicles and assuming symmetrical behavior of the Na+/Ca2+ exchanger. The high affinity for Ca2+ and the sixfold higher capacity of the exchanger compared to the ATPase suggest strongly that the Na+/Ca2+ exchanger will contribute substantially to Ca2+ extrusion in the fish enterocyte. Further evidence for an important contribution of Na+/Ca2+ exchange to Ca2+ extrusion was obtained from studies in which the simultaneous operation of ATP- and Na(+)-gradient-driven Ca2+ pumps in inside-out vesicles was evaluated. The fish enterocyte appears to present a model for a Ca2+ transporting cell, in which Na+/Ca2+ exchange activity with high affinity for Ca2+ extrudes Ca2+ from the cell.

المؤلفون: van Corven EJ, Verbost PM, de Jong MD, van Os CH

المصدر: Cell calcium [Cell Calcium] 1987 Jun; Vol. 8 (3), pp. 197-206.

نوع المنشور: Journal Article; Research Support, Non-U.S. Gov't

بيانات الدورية: Publisher: Elsevier Country of Publication: Netherlands NLM ID: 8006226 Publication Model: Print Cited Medium: Print ISSN: 0143-4160 (Print) Linking ISSN: 01434160 NLM ISO Abbreviation: Cell Calcium Subsets: MEDLINE

مواضيع طبية MeSH: Adenosine Triphosphate/*physiology , Calcium/*metabolism , Inositol Phosphates/*pharmacology , Intestinal Absorption/*drug effects , Sugar Phosphates/*pharmacology, Animals ; In Vitro Techniques ; Inositol 1,4,5-Trisphosphate ; Intestine, Small/metabolism ; Kinetics ; Male ; Mitochondria/drug effects ; Mitochondria/metabolism ; Rats ; Saponins/pharmacology ; Vanadates ; Vanadium/pharmacology

مستخلص: Isolated rat enterocytes were permeabilized by saponin treatment. 45Ca2+ was accumulated by these cells when provided with ATP in a medium containing Ca2+ ligands. The use of oxalate, vanadate and mitochondrial inhibitors indicated that both non-mitochondrial and mitochondrial pools are involved. Kinetic analysis of non-mitochondrial Ca2+ uptake revealed a Km of 0.1 microM Ca2+ and a Vmax of 0.4 nmol Ca2+/mg protein X min for this Ca2+-pumping ATPase activity. Mitochondria started to take up Ca2+ between 0.2 and 0.3 microM free Ca2+ reaching maximal rates around 2 microM. At 1 microM free Ca2+ mitochondria accumulated 20 times more Ca2+ than the non-mitochondrial pool. Inositol 1,4,5-trisphosphate released 40% of the Ca2+ content of the non-mitochondrial pool. Half-maximal release was observed at 0.5 and 1.5 microM IP3 in duodenal and ileal cells respectively. These findings support the possibility that the phosphatidyl inositide metabolism plays a role in regulation of electrolyte transport in enterocytes.

المؤلفون: van Corven EJ, de Jong MD, van Os CH

المصدر: Cell calcium [Cell Calcium] 1986 Apr; Vol. 7 (2), pp. 89-99.

نوع المنشور: Comparative Study; Journal Article; Research Support, Non-U.S. Gov't

بيانات الدورية: Publisher: Elsevier Country of Publication: Netherlands NLM ID: 8006226 Publication Model: Print Cited Medium: Print ISSN: 0143-4160 (Print) Linking ISSN: 01434160 NLM ISO Abbreviation: Cell Calcium Subsets: MEDLINE

مواضيع طبية MeSH: Adenosine Triphosphate/*metabolism , Calcium/*metabolism , Intestine, Small/*metabolism, Animals ; Biological Transport, Active/drug effects ; Buffers ; Calcium-Transporting ATPases/metabolism ; Cell Membrane/metabolism ; Cell Separation/methods ; Citrates ; In Vitro Techniques ; Intestine, Small/cytology ; Male ; Rats ; Rats, Inbred Strains ; Trypsin/pharmacology ; Vibration

مستخلص: Epithelial villus cells from rat small intestine were isolated either in citrate buffers or by vibration in EDTA containing solutions. Basolateral plasmamembrane vesicles (BLMV) were isolated and active Ca2+-transport studied. The rate of ATP-dependent Ca2+-transport in BLMV was 11.2, 1.2 and 0.8 nmol Ca2+/min.mg protein in duodenum, mid-jejunum and terminal ileum when enterocytes had been isolated in citrate buffers. These transport rates were 10.5, 4.8 and 5.8 respectively when cells were isolated by vibration. The specific activities of various marker enzymes were not influenced by the cell isolation procedure. The enrichment factors for (Na+-K+)-ATPase and the latency of this enzyme activity, the mannitol spaces and the half-times for mannitol equilibration among the three BLMV preparations were independent of the cell isolation method. It was demonstrated that active Ca2+-transport in BLMV from jejunum and ileum could be destroyed when cells isolated by vibration were incubated with low proteolytic activity (1 microgram/ml trypsin). Therefore, Ca2+-ATPase in jejunum and ileum is far more susceptible to extracellular proteolytic activity than Ca2+-ATPase more proximally located. Addition of various protease inhibitors to the citrate buffer only partly prevented the selective damage of Ca2+-transport in basolateral membranes.

المؤلفون: van Corven EJ, van Os CH

المصدر: Progress in clinical and biological research [Prog Clin Biol Res] 1984; Vol. 168, pp. 295-9.

نوع المنشور: Journal Article; Research Support, Non-U.S. Gov't

بيانات الدورية: Publisher: Wiley-Liss Country of Publication: United States NLM ID: 7605701 Publication Model: Print Cited Medium: Print ISSN: 0361-7742 (Print) Linking ISSN: 03617742 NLM ISO Abbreviation: Prog Clin Biol Res Subsets: MEDLINE

مواضيع طبية MeSH: Adenosine Triphosphate/*metabolism , Calcium/*metabolism , Duodenum/*metabolism, Alkaline Phosphatase/metabolism ; Animals ; Biological Transport, Active ; Cell Membrane/metabolism ; Duodenum/enzymology ; In Vitro Techniques ; Microvilli/enzymology ; Microvilli/metabolism ; Rats ; Sodium-Potassium-Exchanging ATPase/metabolism

المؤلفون: Ghijsen WE, De Jong MD, Van Os CH

المصدر: Biochimica et biophysica acta [Biochim Biophys Acta] 1982 Jul 28; Vol. 689 (2), pp. 327-36.

نوع المنشور: Journal Article; Research Support, Non-U.S. Gov't

بيانات الدورية: Publisher: Elsevier Pub. Co Country of Publication: Netherlands NLM ID: 0217513 Publication Model: Print Cited Medium: Print ISSN: 0006-3002 (Print) Linking ISSN: 00063002 NLM ISO Abbreviation: Biochim Biophys Acta Subsets: MEDLINE

مواضيع طبية MeSH: Adenosine Triphosphate/*metabolism , Calcium/*metabolism , Calcium-Transporting ATPases/*metabolism , Cell Membrane/*metabolism , Duodenum/*metabolism, Animals ; Biological Transport, Active/drug effects ; Calcimycin/pharmacology ; Cell Membrane/drug effects ; Kinetics ; Male ; Oligomycins/pharmacology ; Rats ; Rats, Inbred Strains ; Temperature ; Theophylline/pharmacology ; Vanadates ; Vanadium/pharmacology

مستخلص: Isolated basolateral plasma membrane vesicles from rat duodenum epithelial cells exhibit ATP-dependent calcium-accumulation and Ca2+ -dependent ATPase activity. Calcium accumulation stimulated by ATP is prevented by the calcium ionophore A23187, inhibited 80% by 0.1 mM orthovanadate but is not effected by oligomycin. Calcium accumulation is not observed with the substrate beta-gamma-(CH2)-ATP, ADP and p-nitrophenyl phosphate. Kinetic studies reveal an apparent Km of 0.2 microM Ca2+ and a Vmax of 5.3 nmol Ca2+/min per mg protein for the ATP-dependent calcium-uptake system. Calmodulin and phenothiazines have no effect on calcium accumulation in freshly prepared membranes, but small effects are inducible after a wash with a 5 mM EGTA. The kinetic parameters of Ca2+ -ATPase are: Km = 0.25 microM Ca2+ and Vmax = 19.2 nmol Pi/min per mg protein. Three techniques, osmotic shock, treatment with Triton X-100 or the channel-forming peptide alamethicin, reveal that about 40% of the vesicles are resealed. Assuming that half of the resealed vesicles have an inside-out orientation, the Vmax of ATP-dependent calcium uptake amounts to 25 nmol Ca2+/min per mg protein and of the Ca2+ -ATPase to 23 nmol Pi/min per mg protein. The close correlation between kinetic parameters of Ca2+ -ATPase and ATP-dependent calcium-transport strongly suggests that both systems are expressions of a Ca2+ -pump located in duodenal basolateral plasma membranes.

المؤلفون: van Heeswijk MP, Geertsen JA, van Os CH

المصدر: The Journal of membrane biology [J Membr Biol] 1984; Vol. 79 (1), pp. 19-31.

نوع المنشور: Journal Article; Research Support, Non-U.S. Gov't

بيانات الدورية: Publisher: Springer Country of Publication: United States NLM ID: 0211301 Publication Model: Print Cited Medium: Print ISSN: 0022-2631 (Print) Linking ISSN: 00222631 NLM ISO Abbreviation: J Membr Biol Subsets: MEDLINE

مواضيع طبية MeSH: Adenosine Triphosphate/*metabolism , Calcium/*metabolism , Kidney Cortex/*metabolism , Sodium/*metabolism, Animals ; Biological Transport ; Cell Membrane/metabolism ; In Vitro Techniques ; Kidney Cortex/ultrastructure ; Kinetics ; Male ; Rats ; Rats, Inbred Strains

مستخلص: Basolateral plasma membranes from rat kidney cortex have been purified 40-fold by a combination of differential centrifugation, centrifugation in a discontinuous sucrose gradient followed by centrifugation in 8% percoll. The ratio of leaky membrane vesicles (L) versus right-side-out (RO) and inside-out (IO) resealed vesicles appeared to be L:RO:IO = 4:3:1. High-affinity Ca2+-ATPase, ATP-dependent Ca2+ transport and Na+/Ca2+ exchange have been studied with special emphasis on the relative transport capacities of the two Ca2+ transport systems. The kinetic parameters of Ca2+-ATPase activity in digitonin-treated membranes are: Km = 0.11 microM Ca2+ and Vmax = 81 +/- 4 nmol Pi/min X mg protein at 37 degrees C. ATP-dependent Ca2+ transport amounts to 4.3 +/- 0.2 and 7.4 +/- 0.3 nmol Ca2+/min X mg protein at 25 and 37 degrees C, respectively, with an affinity for Ca2+ of 0.13 and 0.07 microM at 25 and 37 degrees C. After correction for the percentage of IO-resealed vesicles involved in ATP-dependent Ca2+ transport, a stoichiometry of 0.7 mol Ca2+ transported per mol ATP is found for the Ca2+-ATPase. In the presence of 75 mM Na+ in the incubation medium ATP-dependent Ca2+ uptake is inhibited 22%. When Na+ is present at 5 mM an extra Ca2+ accumulation is observed which amounts to 15% of the ATP-dependent Ca2+ transport rate. This extra Ca2+ accumulation induced by low Na+ is fully inhibited by preincubation of the vesicles with 1 mM ouabain, which indicates that (Na+-K+)-ATPase generates a Na+ gradient favorable for Ca2+ accumulation via the Na+/Ca2+ exchanger. In the absence of ATP, a Na+ gradient-dependent Ca2+ uptake is measured which rate amounts to 5% of the ATP-dependent Ca2+ transport capacity. The Na+ gradient-dependent Ca2+ uptake is abolished by the ionophore monensin but not influenced by the presence of valinomycin. The affinity of the Na+/Ca2+ exchange system for Ca2+ is between 0.1 and 0.2 microM Ca2+, in the presence as well as in the absence of ATP. This affinity is surprisingly close to the affinity measured for the ATP-dependent Ca2+ pump. Based on these observations it is concluded that in isolated basolateral membranes from rat kidney cortex the Ca2+-ATPase system exceeds the capacity of the Na+/Ca2+ exchanger four- to fivefold and it is therefore unlikely that the latter system plays a primary role in the Ca2+ homeostasis of rat kidney cortex cells.