Water, proton, and urea transport in toad bladder endosomes that contain the vasopressin-sensitive water channel

Aquaporin channels may modulate ventilator-induced lung injury

Adult Respiratory Distress Syndrome is a disease with functional lung heterogeneity and thus a ventilator-delivered breath may over-distend non-involved areas. In rats we examined ventilator-delivered tidal volume (TV) breaths of 7 and 20 ml/kg on lung water as evidence of lung injury. We examined the role of aquaporins on ventilator-induced lung injury (VILI) by infusing HgCl(2) which inhibits aquaporins by binding cysteine. Wet to dry lung weight ratio (W/D) as evidence of lung water was 4.47+/-0.1 SEM in controls, 4.6+/-0.1 and 5.5+/-0.2 (P<0.05) in rats ventilated at 7 and 20 ml/kg, respectively. Pulmonary artery pressure (PAP) rose from 23+/-1 to 26+/-1 mmHg (P<0.05, n=7) and cardiac output fell from 104+/-2 to 67+/-3 ml/min (P<0.05) in rats ventilated at 20 ml/kg. Left ventricular end diastolic pressure (n=3) was unchanged. Evans Blue dye, an albumin marker, increased from a control 37+/-11 to 97+/-41 mg/g wet lung in TV 20 rats (P<0.05). HgCl(2) infused slowly by tail vein did not significantly raise PAP, but did increase W/D to 6+/-0.2 (P<0.05) in rats ventilated at 20 ml/kg but not at 7 ml/kg. Equimolar cysteine infusions prevented the HgCl(2) from increasing the W/D above that seen with TV 20 ml/kg. Thus ventilation with TV of 20 ml/kg produced a protein-rich lung edema. Aquaporin channels may have a protective effect in VILI.

Highly water-permeable type I alveolar epithelial cells confer high water permeability between the airspace and vasculature in rat lung

Water permeability measured between the airspace and vasculature in intact sheep and mouse lungs is high. More than 95% of the internal surface area of the lung is lined by alveolar epithelial type I cells. The purpose of this study was to test whether osmotic water permeability (Pf) in type I alveolar epithelial cells is high enough to account for the high Pf of the intact lung. Pf measured between the airspace and vasculature in the perfused fluid-filled rat lung by the pleural surface fluorescence method was high (0.019 +/- 0.004 cm/s at 12 degrees C) and weakly temperature-dependent (activation energy 3.7 kcal/mol). To resolve the contributions of type I and type II alveolar epithelial cells to lung water permeability, Pf was measured by stopped-flow light scattering in suspensions of purified type I or type II cells obtained by immunoaffinity procedures. In response to a sudden change in external solution osmolality from 300 to 600 mOsm, the volume of type I cells decreased rapidly with a half-time (t1/2) of 60-80 ms at 10 degrees C, giving a plasma membrane Pf of 0.06-0.08 cm/s. Pf in type I cells was independent of osmotic gradient size and was weakly temperature-dependent (activation energy 3.4 kcal/mol). In contrast, t1/2 for type II cells in suspension was much slower, approximately 1 s; Pf for type II cells was 0.013 cm/s. Vesicles derived from type I cells also had a very high Pf of 0.06-0.08 cm/s at 10 degrees C that was inhibited 95% by HgCl2. The Pf in type I cells is the highest measured for any mammalian cell membrane and would account for the high water permeability of the lung.

cDNA cloning of a functional water channel from toad urinary bladder epithelium

A cDNA was cloned from the epithelium of toad (Bufo marinas) urinary bladder, based on homology to the mammalian aquaporins (AQP). The cDNA [947 base pairs (bp), identified as AQP-t1] encoded a 272-amino acid protein with 76% identity to mammalian aquaporin-1 (AQP-1) and 88% identity to frog water channel FA-CHIP. AQP-t1 cDNA was nearly identical to a fragment of a nonfunctional cDNA cloned recently from toad bladder ["AQP-TB"; J. Siner, A. Paredes, C. Hosselet, T. Hammond, K. Strange, and H.W. Harris, Am. J. Physiol. 270 (Cell Physiol. 39): C372-C381, 1996], except for reading frame shifts at bp 253, 264, and 682, two single amino acid deletions, a different 3'-coding sequence downstream from bp 786, and a different 5' sequence upstream from bp 9. Water permeability (Pf) in Xenopus laevis oocytes expressing AQP-t1 cRNA was strongly increased from (0.83 +/- 0.06) x 10(-3) cm/s (water-injected control) to (17 +/- 4) x 10(-3) cm/s, with 80% inhibition by 0.3 mM HgCl2; glycerol and urea permeabilities were not increased. Northern blot analysis showed a single AQP-t1 mRNA of 2.8 kb in eye > lung > urinary bladder > skin > stomach approximately heart, brain, and intestine. AQP-t1 mRNA expression was not changed by a 3-day dehydration of toads or an 8-h stimulation of Pf in isolated bladders by forskolin. These results indicate that the epithelium of toad urinary bladder expresses a functional homologue of AQP-1 and FA-CHIP that is probably not vasopressin regulated.

Water transport across mammalian cell membranes

This review summarizes recent progress in water-transporting mechanisms across cell membranes. Modern biophysical concepts of water transport and new measurement strategies are evaluated. A family of water-transporting proteins (water channels, aquaporins) has been identified, consisting of small hydrophobic proteins expressed widely in epithelial and nonepithelial tissues. The functional properties, genetics, and cellular distributions of these proteins are summarized. The majority of molecular-level information about water-transporting mechanisms comes from studies on CHIP28, a 28-kDa glycoprotein that forms tetramers in membranes; each monomer contains six putative helical domains surrounding a central aqueous pathway and functions independently as a water-selective channel. Only mutations in the vasopressin-sensitive water channel have been shown to cause human disease (non-X-linked congenital nephrogenic diabetes insipidus); the physiological significance of other water channels remains unproven. One mercurial-insensitive water channel has been identified, which has the unique feature of multiple overlapping transcriptional units. Systems for expression of water channel proteins are described, including Xenopus oocytes, mammalian and insect cells, and bacteria. Further work should be directed at elucidation of the role of water channels in normal physiology and disease, molecular analysis of regulatory mechanisms, and water channel structure determination at atomic resolution.

Cellular distribution of the aquaporins: a family of water channel proteins

A group of transmembrane proteins that are related to the major intrinsic protein of lens fibers (MIP26) have been named "aquaporins" to reflect their role as water channels. These proteins are located at strategic membrane sites in a variety of epithelia, most of which have well-defined physiological functions in fluid absorption or secretion. However, some aquaporins have been localized in cell types where their role is at present unknown. Most of the aquaporins are delivered to the plasma membrane in a non-regulated (constitutive) fashion, but AQP2 enters the regulated exocytotic pathway and its membrane expression is controlled by the action of the antidiuretic hormone, vasopressin. These pathways of constitutive versus regulated delivery to the plasma membrane have been reconstituted in transfected LLC-PK1 epithelial cells, indicating that the information encoded within the protein sequence is sufficient to allow sorting of newly synthesized protein into distinct intracellular vesicles. Finally, different members of the aquaporin family can be targeted to apical, basolateral or both apical and basolateral plasma membrane domains of polarized epithelial cells. This implies that signals for the polarized targeting of these proteins also is located in non-homologous regions of these similar proteins. Thus, future investigations on the aquaporin family of proteins will provide important information not only on the physiology of membrane transport processes in many cell types, but also on the targeting and trafficking signals that allow proteins to enter distinct intracellular vesicular pathways in epithelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Discovery of aquaporins: a breakthrough in research on renal water transport

Several membranes of the kidney are highly water permeable, thereby enabling this organ to retain large quantities of water. Recently, the molecular identification of water channels responsible for this high water permeability has finally been accomplished. At present, four distinct renal water channels have been identified, all members of the family of major intrinsic proteins. Aquaporin 1 (AQP1), aquaporin 2 (AQP2) and the mercury-insensitive water channel (MIWC) are water-selective channel proteins, whereas the fourth, referred to as aquaporin 3 (AQP3), permits transport of urea and glycerol as well. Furthermore, a putative renal water channel (WCH3) has been found. AQP1 is expressed in apical and basolateral membranes of proximal tubules and descending limbs of Henle, AQP2 predominantly in apical membranes of principal and inner medullary collecting duct cells and AQP3 in basolateral membranes of kidney collecting duct cells. MIWC is expressed in the inner medulla of the kidney and has been suggested to be localised in the vasa recta. The human genes encoding AQP1 and AQP2 have been cloned, permitting deduction of their amino acid sequence, prediction of their two-dimensional structure by hydropathy analysis, speculations on their way of functioning and DNA analysis in patients with diseases possibly caused by mutant aquaporins. Mutations in the AQP1 gene were recently detected in clinically normal individuals, a finding which contradicts the presumed vital importance of this protein. Mutations in the AQP2 gene were shown to cause autosomal recessive nephrogenic diabetes insipidus. The renal unresponsiveness to arginine vasopressin, which characterises this disease, is in accordance with the assumption that AQP2 is the effector protein of the renal vasopressin pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

ADH-induced recycling of fluid-phase marker from endosomes to the mucosal surface in toad bladder

In the toad urinary bladder, the reversal of antidiuretic hormone (ADH) stimulation results in the endocytosis of apical membrane water channels, along with any fluid-phase marker present in the mucosal bathing solution. We have loaded vesicles with horseradish peroxidase (HRP), then restimulated the bladders and measured the reappearance of endocytosed HRP in the mucosal bath. HRP-loaded bladders that were restimulated showed HRP release that peaked sharply within 15 min after restimulation. Varying the interval between loading and restimulation did not vary HRP release significantly. Restimulation with forskolin gave HRP release values similar to ADH. The amount of HRP released correlated with the magnitude of water permeability induced. The demonstration that fluid-phase markers can be recycled from endosomes to the apical surface in a hormone-dependent fashion indicates that endocytosed membrane, containing water channels, is able to rapidly recycle back to the surface in response to hormone restimulation. In addition, marker release declined progressively with repeated restimulation, totaling < 30% of the retrieved amount. This result indicates that a relatively large proportion of the retrieved marker reaches a nonrecycling compartment.

Isolation of highly purified, functional endosomes from toad urinary bladder

Endosomes are difficult to isolate as they share size and density properties with much more abundant cellular organelles such as mitochondria. In cultured cell lines the tandem use of charge-dependent isolation techniques and differential centrifugation is necessary to isolate endosomes. Endosomal populations of the toad urinary bladder are of special interest because they are thought to contain a water channel. Understanding of the molecular structure of the water channel has been constrained, as there is currently no practical method to isolate functional water-channel-containing vesicles. This study reports the tandem use of charge-dependent techniques and centrifugation to isolate populations of endosomes from the toad urinary bladder. To purify water-channel-containing vesicles aqueous two-phase partition was utilized to fractionate a preparation partially purified by differential centrifugation. Populations of endosomes were analysed by small-particle flow cytometry techniques. A 5-fold enrichment in endosomes, achieved with aqueous two-phase partition, allowed us to identify two populations of endosomes of diverse size in a toad bladder endosomal fraction. Preenrichment also improved the efficiency of flow cytometry sorting, allowing isolation of the two endosomal populations in sufficient quantities for secondary analysis. A population of larger endosomes had very high water permeability, indicating the presence of water channels. The two populations had different SDS/PAGE fingerprints. Electron micrographs of the flow-sorted material shows a uniform population of membrane vesicles devoid of mitochondria and other identifiable cellular organelles. Hence, aqueous two-phase partition and flow cytometry allow identification of two populations of endosomes in the toad urinary bladder which have diverse structural and functional properties. Isolation of functional water-channel-containing vesicles allows co-localization of water-channel function with candidate water-channel proteins.

The molecular structure of the antidiuretic hormone elicited water channel

Measurements of osmotic water permeability (Pf) have shown that the plasma membranes of human red cells and certain epithelial cells possess specialized water channels. Although these water channels have been characterized extensively using biophysical techniques, the proteins that compose these unique channels have only recently been identified. Antidiuretic hormone (ADH) stimulation rapidly increases collecting duct epithelial cell Pf by fusion of water channel-containing vesicles (WCV) with their apical membranes. The proteins of WCV from toad bladder and rodent kidney have been characterized. The principal proteins in toad bladder WCV are 55,000 daltons (55 kDa) and 53 kDa and span the lipid bilayer of these vesicles. Polyclonal antisera raised against the 55-kDa and 53-kDa proteins inhibit ADH-stimulated toad bladder Pf by 80% and recognize protein bands of 46, 38 and 30 kDa in mouse kidney. Purification of WCV from rat kidney reveals enrichment of the 46-kDa protein. Recently, a 28-kDa integral membrane protein (called CHIP-28) has been isolated from human red cells. It forms functional water channels in Xenopus oocytes and when reconstituted into proteoliposomes. Large amounts of CHIP-28 protein are present in epithelial cells of the proximal tubule and descending thin limb of Henle's loop. Molecular cloning efforts are underway to elucidate the structure and function of these candidate water channel proteins.

Functional expression of cAMP-dependent and independent urea transporters in Xenopus oocytes

Facilitated transport of urea by the inner medullary collecting duct in kidney is important for the urinary concentrating mechanism. To examine the nature and tissue distribution of urea transporters, mRNA was isolated from different tissues and expressed in Xenopus oocytes. [14C]urea and [3H]methylglucose uptake were measured at 21 degrees C at 64 h after microinjection of mRNA. Relative urea uptake in oocytes injected with 50 ng of unfractionated mRNA was (n = 6-42): 1.0 (water-injected control), 1.0 +/- 0.3 (human kidney cortex), 2.9 +/- 0.5 (rat kidney papilla), 2.5 +/- 0.5 (human kidney papilla), 2.7 +/- 0.3 (rat liver), 1.1 +/- 0.3 (rat brain), 1.2 +/- 0.3 (rat muscle), and 2.6 +/- 0.3 (rabbit reticulocyte). Urea uptake was inhibited to near control values by 0.2 mM phloretin and 0.2 mM p-chloromercuribenzenesulfonate (pCMBS) in oocytes injected with mRNA from kidney medulla, liver, and reticulocyte; phloretin and pCMBS had no effect in control oocytes and oocytes injected with mRNA from kidney cortex, brain, and muscle. Urea uptake was strongly increased in oocytes injected with kidney medulla mRNA (4.4-fold over control) by a 5-min preincubation with the adenosine 3',5'-cyclic monophosphate (cAMP) agonist adenosine-3',5'-cyclic monophosphorothioate (Sp-cAMPS) or a mixture of CPT-cAMP, forskolin, and 3-isobutyl-1-methylxanthine; cAMP agonists did not affect urea uptake in oocytes expressing the reticulocyte and liver urea transporters. As an internal control, (phloretin inhibitable) glucose uptake was enhanced in all oocytes (up to 5-fold greater than control), and was not affected by pCMBS and the cAMP agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of RNA isolated from the water-shunting complex of a sap-sucking insect increases the membrane permeability for water in Xenopus oocytes

The highly specialized membranes of the filter chamber found in the digestive tract of some homopteran insects could represent a favorable material for characterizing water channels. In order to demonstrate that membrane proteins of this epithelial complex serve as water channels, we have investigated the membrane permeability for water in Xenopus oocytes injected with RNA isolated from the filter chamber. Volumes of oocytes injected with filter chamber RNA were increased by 15% following a 16-min osmotic shock, while volumes of oocytes injected with RNA from midgut not of filter chamber or with water were increased only by 8.5 and 10%, respectively. This significant difference in oocyte swelling leads us to conclude that RNA isolated from the filter chamber contains mRNA coding for water channel proteins.

Flow cytometry and sorting of amphibian bladder endocytic vesicles containing ADH-sensitive water channels

The water permeability of ADH target epithelial cells is believed to be regulated by a cycle of exo-endocytosis of vesicles containing functional water channels. These vesicles were selectively labeled in intact frog urinary bladders with an impermeant fluorescent marker, 6-carboxyfluorescein. Vesicle suspensions containing the labeled endosomes were obtained by homogenization and differential centrifugation of bladder epithelial cells. The osmotic permeability of the endocytic vesicles was measured, using a stopped-flow fluorescence technique, in the absence or in the presence of HgCl2. This permeability was found very high (500 microns/sec) and inhibited by 1 mM HgCl2 (90%), thus confirming the presence of water channels. The labeled endosomes were then separated from the other membrane vesicles by flow cytometry and sorting. Their protein content was analyzed by electrophoresis on ultrathin polyacrylamide gels. Two double bands were found at 71 and 55 kDa as well as a small band at 43 kDa. They respectively correspond to 31, 38 and 10% of the total amount of silver-stained proteins present in the sorted endosomes, while they only represent 2, 4, and less than 1% of the proteins contained in the vesicle suspension, before sorting. These highly enriched proteins (or at least one of them) are likely to be involved in the mechanism of water transport. Associated to their partial purification by differential centrifugation, the sorting of the endosomes by flow cytometry seems a good way to further characterize the water channel.

Transcellular water flow modulates water channel exocytosis and endocytosis in kidney collecting tubule

The regulation of osmotic water permeability (Pf) by vasopressin (VP) in kidney collecting tubule involves the exocytic-endocytic trafficking of vesicles containing water channels between an intracellular compartment and apical plasma membrane. To examine effects of transcellular water flow on vesicle movement, Pf was measured with 1-s time resolution in the isolated perfused rabbit cortical collecting tubule in response to addition and removal of VP (250 microU/ml) in the presence of bath greater than lumen (B greater than L), lumen greater than bath (L greater than B), and lumen = bath (L = B) osmolalities. With VP addition, Pf increased from 12 to 240-270 x 10(-4) cm/s (37 degrees C) in 10 min. At 1 min, Pf was approximately 70 x 10(-4) cm/s for B greater than L, L greater than B, and L = B conditions. At later times, Pf increased fastest for L greater than B and slowest for B greater than L osmolalities; at 5 min, Pf was 250 x 10(-4) cm/s (L greater than B) and 158 x 10(-4) cm/s (B greater than L). With VP removal, Pf returned to pre-VP levels at the fastest rate for B greater than L and the slowest rate for L greater than B osmolalities; at 30 min, Pf was 65 x 10(-4) cm/s (B greater than L) and 183 x 10(-4) cm/s (L greater than B). For a series of osmotic gradients of different magnitudes and directions, the rates of Pf increase and decrease were dependent upon the magnitude of transcellular volume flow; control studies showed that paracellular water flux, asymmetric transcellular water pathways, or changes in cell volume could not account for the data. VP-dependent endocytosis was measured by apical uptake of rhodamine-dextran; in paired studies where the same tubule was used for + and - gradients, B greater than L and L greater than B osmolalities gave 168% and 82% of uptake measured with no gradient. In contrast, endocytosis in proximal tubule was not dependent on gradient direction. These data provide evidence that transcellular volume flow modulates the vasopressin-dependent cycling of vesicles containing water channels, suggesting a novel driving mechanism to aid or oppose the targeted, hormonally directed movement of subcellular membranes.

Current understanding of the cellular biology and molecular structure of the antidiuretic hormone-stimulated water transport pathway

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Water and urea permeability properties of Xenopus oocytes: expression of mRNA from toad urinary bladder

The Xenopus oocyte was evaluated as an mRNA expression system for water and urea transporters. Osmotic water permeability (Pf) was measured from the time course of oocyte volume in response to osmotic gradients using a real-time imaging method. Diffusional water permeability (Pd) was measured by 3H2O efflux. In mature oocytes treated with collagenase to remove the follicular cell layer, Pf was 8.6 +/- 0.6 x 10(-4) (SD) cm/s (n = 32) at 25 degrees C and independent of the time after oocyte removal (0-8 days). The activation energy (Ea) for Pf was 10.2 kcal/mol (10-32 degrees C). Pf was independent of osmotic gradient size (50-200 mosmol) in swelling experiments but decreased in an unpredictable manner in shrinking experiments. Pf was not altered by removal of the vitelline membrane but was decreased by 75% when the follicular cell layer was intact. In collagenase-treated oocytes, amphotericin (0-500 micrograms/ml) increased Pf from 8 x 10(-4) to 84 x 10(-4) cm/s in a dose-dependent manner. Pd was 3.4 +/- 0.2 x 10(-4) (SE) cm/s at 25 degrees C, 1.5 +/- 0.2 x 10(-4) cm/s at 4 degrees C, and 5.1 +/- 0.5 x 10(-4) cm/s at 25 degrees C in the presence of 500 micrograms/ml amphotericin; Ea was 6.5 kcal/mol. Thus Pd, but not Pf, is unstirred layer limited.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of the formation and water permeability of endosomes from toad bladder granular cells

Osmotic water permeability (Pf) in toad bladder is regulated by the vasopressin (VP)-dependent movement of vesicles containing water channels between the cytoplasm and apical membrane of granular cells. Apical endosomes formed in the presence of serosal VP have the highest Pf of any biological or artificial membrane (Shi and Verkman. 1989. J. Gen. Physiol. 94:1101-1115). We examine here: (a) the influence of protein kinase A and C effectors on transepithelial Pf (Pfte) in intact bladders and on the number and Pf of labeled endosomes, and (b) whether endosome Pf can be modified physically or biochemically. In paired hemibladder studies, Pfte induced by maximal serosal VP (50 mU/ml, 0.03 cm/s) was not different than that induced by 8-Br-cAMP (1 mM), forskolin (50 microM), VP + 8-Br-cAMP, or VP + forskolin. Pf was measured in endosomes labeled in intact bladders with carboxyfluorescein by a stopped-flow, fluorescence-quenching assay using an isolated microsomal suspension; the number and Pf (0.08-0.11 cm/s, 18 degrees C) of labeled endosomes was not different in bladders treated with VP, forskolin, and 8-Br-cAMP. Protein kinase C activation by 1 microM mucosal phorbol myristate acetate (PMA) induced submaximal bladder Pfte (0.015 cm/s) and endosome Pf (0.022 cm/s) in the absence of VP, but had little effect on maximal Pfte and endosome Pf induced by VP. However, PMA increased by threefold the number of apical endosomes with high Pf formed in response to serosal VP. Pf of endosomes containing the VP-sensitive water channel decreased fourfold by increasing membrane fluidity with hexanol or chloroform (0-75 mM); Pf of phosphatidylcholine liposomes (0.002 cm/s) increased 2.5-fold under the same conditions. Endosome Pf was mildly pH dependent, strongly inhibited by HgCl2, but not significantly altered by GTP gamma S, Ca, ATP + protein kinase A, and phosphatase action. We conclude that: (a) water channels cycled in endocytic vesicles are functional and not subject to physiological regulation, (b) VP and forskolin do not have cAMP-independent cellular actions, (c) activation of protein kinase C stimulates trafficking of water channels, but does not increase the number of apical membrane water channels induced by maximal VP, and (d) water channel function is sensitive to membrane fluidity. By using VP and PMA together, large quantities of endosomes containing the VP-sensitive water channel are labeled with fluid-phase endocytic markers.

Development and biological applications of chloride-sensitive fluorescent indicators

Chloride movement across cell plasma and internal membranes, is of central importance for regulation of cell volume and pH, vectorial salt movement in epithelia, and, probably, intracellular traffic. Quinolinium-based chloride-sensitive fluorescent indicators provide a new approach to study chloride transport mechanisms and regulation that is complementary to 36Cl tracer methods, intracellular microelectrodes, and patch clamp. Indicator fluorescence is quenched by chloride by a collisional mechanism with Stern-Volmer constants of up to 220 M-1. Fluorescence is quenched selectively by chloride in physiological systems and responds to changes in chloride concentration in under 1 ms. The indicators are nontoxic and can be loaded into living cells for continuous measurement of intracellular chloride concentration by single-cell fluorescence microscopy. In this review, the structure-activity relationships for chloride-sensitive fluorescent indicators are described. Methodology for measurement of chloride transport in isolated vesicle and liposome systems and in intact cells is evaluated critically by use of examples from epithelial cell physiology. Future directions for synthesis of tailored chloride-sensitive indicators and new applications of indicators for studies of transport regulation and intracellular ion gradients are proposed.

Urea transport in freshly isolated and cultured cells from rat inner medullary collecting duct

Regulation of urea transport by vasopressin in inner medullary collecting duct (IMCD) cells is thought to be important for the urinary concentrating mechanism. Isolated tubule perfusion studies suggest the existence of a saturable urea carrier. We have measured 14C-urea efflux in IMCD cells which were freshly isolated and grown in primary culture. Cells were isolated from rat papilla by collagenase digestion and hypotonic shock. In suspended cells, 14C-urea efflux (Jurea) from loaded cells was exponential with time constant 59 +/- 3 sec (SEM, n = 6, 23 degrees C). Jurea had an activation energy of 4.1 kcal/mole and was inhibited 42 +/- 7% by 0.25 mM phloretin and 30-40% by the high affinity urea analogues dimethylurea and phenylurea. Jurea was increased 40-60% by addition of vasopressin (10(-8) M) or 8-bromo-cAMP (1 mM); stimulated Jurea was inhibited 55 +/- 8% by the kinase A inhibitor H-8. Phorbol esters and epidermal growth factor did not alter Jurea. IMCD cells grown in primary culture were homogeneous in appearance with greater than fivefold stimulation of cAMP by vasopressin. The exponential time constant for urea efflux was 610 +/- 20 sec (n = 3). Jurea was not altered by vasopressin, cAMP or phloretin. Another function of in vivo IMCD cells, vasopressin-dependent formation of endosomes containing water channels, was absent in the cultured cells. These results demonstrate presence of a urea transporter on suspended IMCD cells which is activated by cAMP and inhibited by phloretin and urea analogues. The urea transporter and its regulation by cAMP, and cAMP-dependent apical membrane endocytosis, are lost after growth in primary culture.