ADH action: evidence for a membrane shuttle mechanism

Phosphorylation of aquaporin-2 does not alter the membrane water permeability of rat papillary water channel-containing vesicles

Antidiuretic hormone modulates the water permeability (Pf) of epithelial cells in the rat kidney by vesicle-mediated insertion and removal of the aquaporin-2 (AQP-2) water channel. AQP-2 possesses a single consensus cAMP-dependent protein kinase A (PKA) phosphorylation site (Ser-256) hypothesized to regulate channel Pf(Kuwahara, M., Fushimi, K., Terada, Y., Bai, L., Sasaki, S., and Marumo, F. (1995) J. Biol. Chem. 270, 10384-10387). To test whether PKA phosphorylation of AQP-2 alters channel Pf, we compared the Pf values of purified AQP-2 endosomes after incubation with either PKA or alkaline phosphatase. Studies using [gamma-32P]ATP reveal that AQP-2 endosomes contain endogenous PKA and phosphatase activities that add and remove 32P label from AQP-2. However, the Pf (0.16 +/- 0.06 cm/s) of endosomes containing phosphorylated AQP-2 (0.7 +/- 0. 3 mol of PO4/mol of protein) is not significantly different from the same AQP-2 endosomes where 95 +/- 8% of the phosphate has been removed (Pf 0.14 +/- 0.06 cm/s). These data do not support a role for PKA phosphorylation in alteration of AQP-2's Pf. Instead, AQP-2 phosphorylation by PKA may modulate AQP-2's distribution between plasma membrane and intracellular vesicle compartments.

Effect of a dynein inhibitor on vasopressin action in toad urinary bladder

1. The effect of the dynein inhibitor erythro-9-[3-(2-hydroxynonyl)] adenine (EHNA) on the osmotic water flow response to vasopressin or exogenous cAMP has been investigated in isolated toad urinary bladders. 2. Pretreatment with serosal EHNA had no effect on basal water flow, but inhibited the development and maintenance of the hydrosmotic response to vasopressin (20 mU ml-1) or 8-(4-parachlorophenylthio)-adenosine 3',5'-cyclic monophosphate (8 CPT-cAMP; 0.1 mM). 3. The inhibitory effect of EHNA on vasopressin-induced water flow was dose dependent. Inhibition occurred in the dose range in which EHNA inhibits the ATPase and motor activities of dynein in vitro. 4. EHNA also inhibited the maintenance of the high rate of water flow established by prior exposure to vasopressin. 5. The inhibitory effect of EHNA on the onset phase of the vasopressin response was attenuated after exposure of the tissue to the microtubule-disruptive drug nocodazole but was fully additive with that of cytochalasin B. 6. EHNA inhibited basal and vasopressin-stimulated transepithelial sodium transport. 7. The findings support the view that EHNA inhibits hormone-induced water flow through an action on a cytoplasmic dynein. The results are consistent with the hypothesis that dynein is involved in the microtubule-based delivery of water channel-containing vesicles to the apical membrane of the granular epithelial cells during both the onset and maintenance of the water permeability response to vasopressin.

The aquaporin family of water channels in kidney

The longstanding puzzle of membrane water permeability was advanced by the discovery of channel-forming integral protein (CHIP). This protein was shown to function as a water channel when expressed in Xenopus oocytes or when reconstituted into synthetic membranes. Site-directed mutagenesis and electron crystallography reveal tetrameric organization of CHIP, and the two halves of CHIP are tandem repeats folded into an obversely symmetric structure which resembles an hourglass. Each tetramer is comprised of functionally independent subunits. CHIP is the archetypal member of a newly-recognized family of membrane water transporters known as the "Aquaporins" (AQPs). AQP1 (CHIP) is abundant in the apical and basolateral membranes of renal proximal tubules and descending thin limbs, and is also present in a number of extra renal tissues. In the collecting duct, AQP2 is the predominant vasopressin-sensitive water channel. AQP2 is localized in the apical membrane and in intracellular vesicles which are targeted to the apical plasma membranes when stimulated by antidiuretic hormone. Humans are identified with mutations in AQP1 and AQP2 and exhibit contrasting clinical phenotypes. AQP3 resides in the basolateral membranes of collecting duct principal cells providing an exit pathway for water, and AQP4 is abundant in brain, where it apparently functions as the hypothalamic osmoreceptor responsible for secretion of antidiuretic hormone. Continued analysis of the aquaporins is providing detailed molecular insight into the fundamental physiological problems of water balance and water balance disorders.

Redistribution of aquaporin-2 water channels induced by vasopressin in rat kidney inner medullary collecting duct

Aquaporin-2 (AQP2) is the predominant vasopressin-regulated water channel of the renal collecting duct. We tested whether vasopressin induces translocation of AQP2 from intracellular vesicles into the apical plasma membrane. AQP2 was quantitated in plasma membrane and intracellular vesicle fractions prepared from the inner medulla of one kidney from each rat before or 20 min after intravenous 1-desamino-8-D-arginine vasopressin (DDAVP) treatment, using immunoblotting and densitometry. Contralateral kidneys were prepared for immunofluorescence and immunoelectron microscopy. Immunoblotting revealed that, compared with untreated controls, DDAVP treatment significantly increased the fraction of AQP2 protein associated with the plasma membrane fraction relative to intracellular vesicles. This increase averaged 2.0-fold in untreated rats and 2.9-fold in rats water loaded for 12 h. Water loading, presumably by suppressing circulating vasopressin levels, decreased the fraction of AQP2 associated with the plasma membrane by 55%, suggesting retrieval of AQP2 from the plasma membrane. In rats sequentially thirsted for 48 h to increase expression and then water loaded for 72 h to minimize plasma membrane labeling, DDAVP caused a 12-fold increase in the plasma membrane to intracellular vesicle labeling ratio. The accentuation of the DDAVP response seen after water loading is consistent with the observed increase in the fraction of AQP2 in the intracellular pool available for insertion. Immunofluorescence confirmed a marked DDAVP-induced redistribution of AQP2 from intracellular to plasma membrane domains. Furthermore, quantitative immunoelectron microscopy demonstrated a 3.4-fold increase in apical plasma membrane to intracellular vesicle labeling ratio. These results provide a direct in vivo demonstration of vasopressin-induced translocation of AQP2 into the apical plasma membrane.

Constitutive and regulated membrane expression of aquaporin 1 and aquaporin 2 water channels in stably transfected LLC-PK1 epithelial cells

The aquaporins (AQPs) are a family of homologous water-channel proteins that can be inserted into epithelial cell plasma membranes either constitutively (AQP1) or by regulated exocytosis following vasopressin stimulation (AQP2). LLC-PK1 porcine renal epithelial cells were stably transfected with cDNA encoding AQP2 (tagged with a C-terminal c-Myc epitope) or rat kidney AQP1 cDNA in an expression vector containing a cytomegalovirus promoter. Immunofluorescence staining revealed that AQP1 was mainly localized to the plasma membrane, whereas AQP2 was predominantly located on intracellular vesicles. After treatment with vasopressin or forskolin for 10 min, AQP2 was relocated to the plasma membrane, indicating that this relocation was induced by cAMP. The location of AQP1 did not change. The basal water permeability of AQP1-transfected cells was 2-fold greater than that of nontransfected cells, whereas the permeability of AQP2-transfected cells increased significantly only after vasopressin treatment. Endocytotic uptake of fluorescein isothiocyanate-coupled dextran was stimulated 6-fold by vasopressin in AQP2-transfected cells but was only slightly increased in wild-type or AQP1-transfected cells. This vasopressin-induced endocytosis was inhibited in low-K+ medium, which selectively affects clathrin-mediated endocytosis. These water channel-transfected cells represent an in vitro system that will allow the detailed dissection of mechanisms involved in the processing, targeting, and trafficking of proteins via constitutive versus regulated intracellular transport pathways.

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)

Urinary excretion of aquaporin-2 in patients with diabetes insipidus

BACKGROUND

Urine-concentrating ability is regulated by vasopressin. Recently, the specific water-channel protein of the renal collecting duct, known as aquaporin-2, was cloned. However, it is not certain whether this molecule is responsive to vasopressin.

METHODS

We measured the urinary excretion of aquaporin-2 and its response to vasopressin in 11 normal subjects and 9 patients with central or nephrogenic diabetes insipidus. The urine samples were collected during periods of dehydration and hydration and after the administration of vasopressin. Urine samples were analyzed for aquaporin-2 by the Western blot assay and immunogold labeling, and the amount of aquaporin-2 was determined by radioimmunoassay.

RESULTS

Aquaporin-2 was detectable in the urine in both soluble and membrane-bound forms. In the five normal subjects tested, the mean (+/- SE) urinary excretion of aquaporin-2 was 11.2 +/- 2.2 pmol per milligram of creatinine after a period of dehydration, and it decreased to 3.9 +/- 1.9 pmol per milligram of creatinine (P = 0.03) during the second hour after a period of hydration. In the six other normal subjects, an infusion of desmopressin (1-desamino-8-D-arginine vasopressin) increased the urinary excretion of aquaporin-2 from 0.8 +/- 0.3 to 11.2 +/- 1.6 pmol per milligram of creatinine (P < 0.001). The five patients with central diabetes insipidus also had increases in urinary excretion of aquaporin-2 in response to the administration of vasopressin, but the four patients with X-linked or non-X-linked nephrogenic diabetes insipidus did not.

CONCLUSIONS

Aquaporin-2 is detectable in the urine, and changes in the urinary excretion of this protein can be used as an index of the action of vasopressin on the kidney.

Vasopressin increases AQP-CD water channel in apical membrane of collecting duct cells in Brattleboro rats

The effect of vasopressin on subcellular localization of AQP-CD and AQP3 water channels was examined in thirsted Brattleboro rats by immunohistochemistry and immunoelectron microscopy. AQP-CD was mainly present in the cytoplasm of the collecting duct cells in association with cytoplasmic vesicles but was sparse in the apical membrane in control vehicle-injected rats. In rats given vasopressin 15 min before death, the number of immunogold particles for AQP-CD in the apical membrane increased significantly (P < 0.002) from 1.8 +/- 0.2 to 10.0 +/- 0.4/microns with a significant decrease (P < 0.05) of cytoplasmic labeling from 32.6 +/- 6.4 to 24.6 +/- 5.6/microns 2, indicating that AQP-CD is the vasopressin-regulated water channel predicted by the "shuttle" hypothesis. In contrast, AQP3 was restricted to the basolateral membrane of the collecting duct cells, and the labeling density of AQP3 was unchanged by vasopressin treatment, indicating that AQP3 is constitutively expressed and may maintain high water permeability of the basolateral membrane.

cAMP-dependent phosphorylation stimulates water permeability of aquaporin-collecting duct water channel protein expressed in Xenopus oocytes

Among water channel proteins (aquaporins), aquaporin-collecting duct (AQP-CD) is the vasopressin-regulated water channel. Vasopressin causes cAMP production in the renal collecting duct cells, and this is believed to lead to exocytic insertion of water channel into the apical membrane (shuttle hypothesis). AQP-CD contains a consensus sequence for cAMP-dependent protein kinase, residues at positions 253-256 (Arg-Arg-Gln-Ser). To determine the role of this site, Ser-256 was substituted for Ala, Leu, Thr, Asp, or Glu by site-directed mutagenesis. In Xenopus oocytes injected with wild-type or mutated AQP-CD cRNAs, osmotic water permeability (Pf) was 4.8-7.7 times higher than Pf of water-injected oocytes. Incubation with cAMP plus forskolin or direct cAMP injection into the oocytes increased Pf of wild-type, but not mutated, AQP-CD-expressing oocytes, whereas the amounts of AQP-CD expression were similar in wild and mutated types as identified by Western blot analysis. In vitro phosphorylation studies of AQP-CD proteins expressed in oocyte showed that cAMP-dependent protein kinase phosphorylated wild-type, but not mutated, AQP-CD proteins. Phosphoamino acid analysis revealed that this phosphorylation occurred at the serine residue. Moreover, phosphorylation of AQP-CD protein in intact rat kidney medulla tissues was stimulated by incubation with cAMP. Our data suggest that cAMP stimulates water permeability of AQP-CD by phosphorylation. This process may contribute to the vasopressin-regulated water permeability of collecting duct in addition to the apical insertion of AQP-CD by exocytosis.

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)

Vasopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin-CD water channels to plasma membrane

Water excretion by the kidney is regulated by the peptide hormone vasopressin. Vasopressin increases the water permeability of the renal collecting duct cells, allowing more water to be reabsorbed from collecting duct urine to blood. Despite long-standing interest in this process, the mechanism of the water permeability increase has remained undetermined. Recently, a molecular water channel (AQP-CD) has been cloned whose expression appears to be limited to the collecting duct. Previously, we immunolocalized this water channel to the apical plasma membrane (APM) and to intracellular vesicles (IVs) of collecting duct cells. Here, we test the hypothesis that vasopressin increases cellular water permeability by inducing exocytosis of AQP-CD-laden vesicles, transferring water channels from IVs to APM. Rat collecting ducts were perfused in vitro to determine water permeability and subcellular distribution of AQP-CD in the same tubules. The collecting ducts were fixed for immunoelectron microscopy before, during, and after exposure to vasopressin. Vasopressin exposure induced increases in water permeability and the absolute labeling density of AQP-CD in the APM. In parallel, the APM:IV labeling ratio increased. Furthermore, in response to vasopressin withdrawal, AQP-CD labeling density in the APM and the APM:IV labeling ratio decreased in parallel to a measured decrease in osmotic water permeability. We conclude that vasopressin increases the water permeability of collecting duct cells by inducing a reversible translocation of AQP-CD water channels from IVs to the APM.

The AQP2 water channel: effect of vasopressin treatment, microtubule disruption, and distribution in neonatal rats

Aquaporin 2 is a collecting duct water channel that is located in apical vesicles and in the apical plasma membrane of collecting duct principal cells. It shares 42% identity with the proximal tubule/thin descending limb water channel, CHIP28. The present study was aimed at addressing three questions concerning the location and behavior of the AQP2 protein under different conditions. First, does the AQP2 channel relocate to the apical membrane after vasopressin treatment? Our results show that AQP2 is diffusely distributed in cytoplasmic vesicles in collecting duct principal cells of homozygous Brattleboro rats that lack vasopressin. In rats injected with exogenous vasopressin, however, AQP2 became concentrated in the apical plasma membrane of principal cells, as determined by immunofluorescence and immunogold electron microscopy. This behavior is consistent with the idea that AQP2 is the vasopressin-sensitive water channel. Second, is the cellular location of AQP2 modified by microtubule disruption? In normal rats, AQP2 has a mainly apical and subapical location in principal cells, but in colchicine-treated rats, it is distributed on vesicles that are scattered throughout the entire cytoplasm. This is consistent with the dependence on microtubules of apical protein targeting in many cell types, and explains the inhibitory effect of microtubule disruption on the hydroosmotic response to vasopressin in sensitive epithelia, including the collecting duct. Third, is AQP2 present in neonatal rat kidneys? We show that AQP2 is abundant in principal cells from neonatal rats at all days after birth. The detection of AQP2 in early neonatal kidneys indicates that a lack of this protein is not responsible for the relatively weak urinary concentrating response to vasopressin seen in neonatal rats.

Regulation of collecting duct water channel expression by vasopressin in Brattleboro rat

AQP-CD is a vasopressin-regulated water channel expressed exclusively in the renal collecting duct. We have previously shown that AQP-CD is present in the apical plasma membrane and subapical vesicles of collecting duct cells, consistent with membrane-shuttling mechanisms that have been proposed to explain the short-term action of [Arg8] vasopressin (AVP) to regulate apical water permeability. We propose here that AVP may also have long-term actions on the collecting duct to regulate the expression of the AQP-CD water channel. We used immunoblotting, immunohistochemistry, and in vitro perfusion of renal tubules to investigate water channel regulation in collecting ducts of diabetes insipidus (Brattleboro) rats treated with a 5-day infusion of AVP or vehicle. Immunoblotting and immunohistochemistry demonstrated that collecting ducts of vehicle-infused Brattleboro rats had markedly reduced expression of AQP-CD relative to normal rats. In response to AVP infusion there was a nearly 3-fold increase in AQP-CD expression as detected by immunoblotting. Immunocytochemistry demonstrated that the increased expression was predominantly in the apical plasma membrane and subapical vesicles of collecting duct cells. Inner medullary collecting ducts of AVP-infused Brattleboro rats displayed a 3-fold increase in osmotic water permeability relative to vehicle-infused controls, in parallel with the change in AQP-CD expression. Based on these findings, we conclude that (i) long-term infusion of AVP, acting either directly or indirectly, regulates expression of the AQP-CD water channel and (ii) AQP-CD is the predominant AVP-regulated water channel.

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.

Glucose-induced changes in Na+/H+ antiport activity and gene expression in cultured vascular smooth muscle cells. Role of protein kinase C

Increased Na+/H+ antiport activity has been implicated in the pathogenesis of hypertension and vascular disease in diabetes mellitus. The independent effect of elevated extracellular glucose concentrations on Na+/H+ antiport activity in cultured rat vascular smooth muscle cells (VSMC) was thus examined. Amiloride-sensitive 22Na+ uptake by VSMC significantly increased twofold after 3 and 24 h of exposure to high glucose medium (20 mM) vs. control medium (5 mM). Direct glucose-induced Na+/H+ antiport activation was confirmed by measuring Na(+)-dependent intracellular pH recovery from intracellular acidosis. High glucose significantly increased protein kinase C (PKC) activity in VSMC and inhibition of PKC activation with H-7, staurosporine, or prior PKC downregulation prevented glucose-induced increases in Na+/H+ antiport activity in VSMC. Northern analysis of VSMC poly A+ RNA revealed that high glucose induced a threefold increase in Na+/H+ antiport (NHE-1) mRNA at 24 h. Inhibiting this increase in NHE-1 mRNA with actinomycin D prevented the sustained glucose-induced increase in Na+/H+ antiport activity. In conclusion, elevated glucose concentrations significantly influence vascular Na+/H+ antiport activity via glucose-induced PKC dependent mechanisms, thereby providing a biochemical basis for increased Na+/H+ antiport activity in the vascular tissues of patients with hypertension and diabetes mellitus.

Effect of brefeldin A on ADH-induced transport responses of toad bladder

We have used brefeldin A (BFA) to examine the role of membrane traffic in the short-circuit current (ISC) and water permeability responses of the toad urinary bladder. BFA treatment of 1 or 5 micrograms/ml had a complex effect on the response of the ISC to antidiuretic hormone (ADH) or forskolin stimulation. Although the responses to initial challenges by ADH were not impaired by BFA, subsequent ISC responses were progressively reduced. Similarly, while the response to an initial challenge by forskolin was modestly reduced by BFA, subsequent responses were markedly reduced. Inhibition of protein synthesis with cycloheximide (CHM) affected ISC responses similarly. Neither BFA nor CHM had an effect on water permeability responses. These observations show that although the membrane traffic responsible for the water permeability response is insensitive to inhibition by BFA or CHM, the stimulation of Na+ transport becomes increasingly sensitive to these inhibitors with successive challenges by ADH or forskolin. Although initial increases in Na+ transport utilize preexisting components, subsequent responses appear to require an intact system for membrane biogenesis.

Surface membrane remodeling following removal of vasopressin in toad urinary bladder

Vasopressin (ADH) increases transepithelial water flow in renal epithelia by a process that involves the insertion of water channels into the apical membrane. The objective of the present study was to examine membrane surface remodeling under conditions that promote the recovery of water channels. Hemibladders were set up as sacs with an imposed osmotic gradient. The control sacs received no hormone treatment, whereas the other sacs were stimulated with 100 mU/ml ADH for 10 or 15 min to induce exocytosis and enhanced water flow. ADH was then washed from the tissues with fresh buffer rinses to abolish the hormone actions. These tissues were then allowed to recover for 15, 30 and 60 min. During this time water channels are recovered intracellularly by a process of endocytosis. This time period was called the retrieval period. At specified time intervals, tissues were fixed and processed for SEM or embedded in epon for ultrathin sectioning for TEM studies. Control tissues, regardless of the length of time, showed little or no sign of surface remodeling that was indicative of endocytosis during pre- or post-buffer washes, whereas the ADH-treated tissues showed a time-dependent remodeling of the apical membrane during activation and following removal of the hormone during the retrieval period. At the 10 min retrieval period, greater than 47% of the granular cells showed extensive surface remodeling. By 30 and 60 min posthormone treatment during recovery, fewer than 23% of granular cells showed signs of surface membrane changes. During retrieval the apical membrane undergoes a transition with a loss of both microridges and microvilli prior to membrane restoration. These observations suggest that apical membrane remodeling is crucial for the restoration of membrane permeability following hormone activation and termination.

Cloning, characterization, and chromosomal mapping of human aquaporin of collecting duct

We recently cloned a cDNA of the collecting duct apical membrane water channel of rat kidney, which is important for the formation of concentrated urine (Fushima, K., S. Uchida, Y. Hara, Y. Hirata, F. Marumo, and S. Sasaki. 1993. Nature [Lond.]. 361:549-552). Since urine concentrating ability varies among mammalian species, we examined whether an homologous protein is present in human kidney. By screening a human kidney cDNA library, we isolated a cDNA clone, designated human aquaporin of collecting duct (hAQP-CD), that encodes a 271-amino acid protein with 91% identity to rat AQP-CD. mRNA expression of hAQP-CD was predominant in the kidney medulla compared with the cortex, immunohistochemical staining of hAQP-CD was observed only in the collecting duct cells, and the staining was dominant in the apical domain. Functional expression study in Xenopus oocytes confirmed that hAQP-CD worked as a water channel. Western blot analysis of human kidney medulla indicated that the molecular mass of hAQP-CD is 29 kD, which is the same mass expected from the amino acid sequence. Chromosomal mapping of the hAQP-CD gene assigned its location to chromosome 12q13. These results could be important for future studies of the pathophysiology of human urinary concentration mechanisms in normal and abnormal states.

Cellular and subcellular immunolocalization of vasopressin-regulated water channel in rat kidney

Vasopressin (antidiuretic hormone) regulates body water balance by controlling water permeability of the renal collecting ducts. The control mechanisms may involve alterations in the number or unit conductance of water channels in the apical plasma membrane of collecting-duct cells. How this occurs is unknown, but indirect evidence exists for the "shuttle" hypothesis, which states that vasopressin causes exocytic insertion of water channel-laden vesicles from the apical cytosol. To test key aspects of the shuttle hypothesis, we have prepared polyclonal antisera against the recently cloned collecting-duct water channel protein and used the antisera in immunolocalization studies (light and electron microscopic levels) in thin and ultrathin cryosections from rat kidney. Labeling was seen exclusively in collecting-duct principal cells and inner medullary collecting-duct cells. Apical membrane labeling was intense. There was heavy labeling of abundant small subapical vesicles and of membrane structures within multivesicular bodies. In addition, labeling of basolateral plasma membranes in inner medullary collecting ducts was present. Depriving rats of water for 24 or 48 hr markedly increased collecting-duct water-channel protein expression determined by immunoblotting and immunolabeling. These results are compatible with at least two complementary modes of water-channel regulation in collecting-duct cells: (i) control of channel distribution between the apical membrane and a reservoir in subapical vesicles (shuttle hypothesis) and (ii) regulation of the absolute level of expression of water-channel protein.

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.

Molecular aspects of water transport

Due to its fundamental importance, the movement of water across cell membranes has been an active area of research for more than 100 years. This subject is central to consideration of normal water metabolism by terrestrial animals, as well as derangements in overall water balance that are frequently encountered by nephrologists in the care of their patients. The objective of this review is to discuss the most basic aspects of cell membrane water permeability and provide a framework for these data in the context of the care of pediatric patients with renal disease. While the water permeability of most cell membranes can be accounted for by the diffusion of water across the lipid bilayer, other cells, including the red blood cell and certain epithelial cells that line the proximal and collecting tubules of the kidney and the urinary bladder of amphibians, possess specialized water channels. Water channels are composed of specialized proteins that create aqueous pores across cell membrane. Currently, there are active research efforts to isolate and characterize water channel proteins from these cell types. Data concerning the distribution, permeability and function of these various water channels will greatly enhance our knowledge of how water is transported across cell membranes.

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.

Quantitation and topography of membrane proteins in highly water-permeable vesicles from ADH-stimulated toad bladder

Antidiuretic hormone (ADH) stimulation of toad bladder granular cells rapidly increases the osmotic water permeability (Pf) of their apical membranes by insertion of highly selective water channels. Before ADH stimulation, these water channels are stored in large cytoplasmic vesicles called aggrephores. ADH causes aggrephores to fuse with the apical membrane. Termination of ADH stimulation results in prompt endocytosis of water channel-containing membranes via retrieval of these specialized regions of apical membrane. Protein components of the ADH water channel contained within these retrieved vesicles would be expected to be integral membrane protein(s) that span the vesicle's lipid bilayer to create narrow aqueous channels. Our previous work has identified proteins of 55 (actually a 55/53-kDa doublet), 17, 15, and 7 kDa as candidate ADH water channel components. We now have investigated these candidate ADH water channel proteins in purified retrieved vesicles. These vesicles do not contain a functional proton pump as assayed by Western blots of purified vesicle protein probed with anti-H(+)-ATPase antisera. Approximately 60% of vesicle protein is accounted for by three protein bands of 55, 53, and 46 kDa. Smaller contributions to vesicle protein are made by the 17- and 15-kDa proteins. Triton X-114-partitioning analysis shows that the 55, 53, 46, and 17 kDa are integral membrane proteins. Vectorial labeling analysis with two membrane-impermeant reagents shows that the 55-, 53-, and 46-kDa protein species span the lipid bilayer of these vesicles. Thus the 55-, 53-, and 46-kDa proteins possess characteristics expected for ADH water channel components. These data show that the 55- and 53- and perhaps the 46-, 17-, and 15-kDa proteins are likely components of aqueous transmembrane pores that constitute ADH water channels contained within these vesicles.

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

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Na+ channel activity in cultured renal (A6) epithelium: regulation by solution osmolarity

Solution osmolarity is known to affect Na+ transport rates across tight epithelia but this variable has been relatively ignored in studies of cultured renal epithelia. Using electrophysiological methods to study A6 epithelial monolayers, we observed a marked effect of solution tonicity on amiloride-sensitive Na+ currents (I(sc)). I(sc) for tissues bathed in symmetrical hyposmotic (170 mOsm), isosmotic (200 mOsm), and hyperosmotic (230 or 290 mOsm) NaCl Ringer's solutions averaged 25 +/- 2, 9 +/- 2, 3 +/- 0.4, and 0.6 +/- 0.5 microA/cm2, respectively. Similar results were obtained following changes in the serosal tonicity: mucosal changes did not significantly affect I(sc). The changes in I(sc) were slow and reached steady-state within 30 min. Current fluctuation analysis measurements indicated that single-channel currents and Na+ channel blocker kinetics were similar for isosmotic and hyposmotic conditions. However, the number of conducting Na+ channels was approximately threefold higher for tissues bathed in hyposmotic solutions. No channel activity was detected during hyperosmotic conditions. The results suggest that Na+ channels in A6 epithelia are highly sensitive to relatively small changes in serosal solution tonicity. Consequently, osmotic effects may partly account for the large variability in Na+ transport rates for A6 epithelia reported in the literature.

Cryofixation, ultracryomicrotomy, and X-ray microanalysis of enterocytes from chick duodenum: vitamin-D-induced formation of an apical tubulovesicular system

New methods of tissue preparation were developed to study the morphology and distribution of calcium ions in duodenal enterocytes from normal, rachitic, and vitamin D-replete (either cholecalciferol [CC] or 1,25-dihydroxycholecalciferol [1,25-DHCC] treated) chicks. Frozen hydrated sections were prepared from cryofixed tissues by ultracryomicrotomy at -125 degrees C. Sections were subsequently freeze-dried by increasing the temperature to -100 degrees C. The latter temperature was maintained throughout both the structural and elemental analyses. In cells from normal, rachitic, and vitamin D-treated [CC] animals the brush border from lanthanum-infused tissues was electron dense and calcium-lanthanum positive by x-ray analysis. In the absence of lanthanum, i.e., sucrose-infused duodena, the microvilli were still calcium positive. In the terminal web region of normal and CC-treated enterocytes, numerous, apparently interconnected, tubules and vesicles were seen. Vacuole-like structures were also seen. Such structures were especially prominent in the enterocytes from the vitamin-treated [CC] animals. Except for the vacuoles, the tubules and vesicles were electron dense in the lanthanum-infused duodena, and clear in sucrose-infused tissues. In both instances, the structures were calcium positive. Similar, but even larger structures were seen below the terminal web. Here however, the tubules and vesicles seemed to be organized into multiple complex interconnecting networks, i.e., tubulo-vesicular complexes. Both the tubules and the vesicles seemed to be interconnected via smaller channel-like entities. The extensiveness of this structure was better appreciated in the enterocytes from lanthanum-infused tissues, where it appeared similar in structure and complexity to an en face view of the sarcoplasmic reticulum of skeletal muscle. These intestinal complexes were less well developed, decreased in number, and quite often absent, in the apical cytoplasm of absorptive cells from rachitic chicks. In the enterocytes from animals treated for 24 hours with 1,25-DHCC, the same highly developed tubulo-vesicular networks were again seen in the enterocyte apical cytoplasm. They were even more developed in the 1,25-DHCC-treated animals. All structures were intensely calcium positive in enterocytes from both the lanthanum- and the sucrose-infused preparations. Numerous endocytotic (pinocytotic) vesicles were seen at the lumenal plasmalemma. Similar structures were also apparent in the terminal web region of the 1,25-DHCC-treated enterocytes. Exocytotic vesicles were seen at the apical aspect of the lateral cell membrane, below the level of the junctional complex. All components of this unique system contained high concentrations of calcium.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of cytoskeletal organization and cytosolic granule distribution by verapamil in amphibian urinary epithelia

The present study examines the role of calcium in modulating epithelial cytomorphology by using verapamil, a calcium antagonist, and considering its effects on cytosolic granule distribution and exocytosis in toad urinary bladder. The effect of verapamil on the detection and distribution of microfilaments in toad urinary bladder using immunogold labeling techniques in toad urinary bladder epithelial cells was also examined. Verapamil, which inhibits antidiuretic hormone (ADH)-mediated water flow, increased the number, size and distribution of dense calcium-containing secretory granules in bladder epithelial cells. This calcium antagonist prevented granule exocytosis, such that, six-times the number of granules were present in verapamil-treated tissues. The normal cytomorphological changes that accompany the actions of ADH were attenuated by verapamil, including ADH-induction of microvilli. ADH increased the number of actin microfilaments as determined using protein A-gold by immunolabeling, whereas, verapamil treatment was unremarkable as compared to control. The results suggest that calcium may play a prominent role in mediating granule exocytosis and membrane fusion events that normally accompany hormone action.

Endocytosis of water channels in rat kidney: cell specificity and correlation with in vivo antidiuresis

Vasopressin action in the renal collecting duct is believed to be mediated by the cycling of water channels in principal and, possibly, intercalated cells. We used 6-carboxyfluorescein (6-CF) or fluorescein-labeled dextran (FITC-dextran) to determine the location and water permeability of endocytic vesicles from papilla and inner stripe of Brattleboro rats in different states of diuresis. Fifteen minutes after FITC-dextran infusion, fluorescent vesicles were concentrated at the apical pole of principal and intercalated cells. The osmotic water permeability (Pf) of these endosomes was measured by fluorescence quenching. In papillary endosomes, Pf was high (0.04 +/- 0.004 cm/s) when rats were in physiological states of antidiuresis or after treatment with vasopressin, 1-desamino-8-D-arginine vasopressin (DDAVP), or oxytocin; endosomes isolated from these regions of untreated animals had a low Pf. The number of papillary endosomes with high Pf increased with increasing doses of DDAVP. Endosomes from the inner stripe also had a high Pf only after vasopressin treatment. Confocal microscopy of sections of papilla showed that vasopressin significantly increased endocytosis in principal cells but had no effect on intercalated cells. Our data demonstrate that the bulk of fluorescently labeled vesicles from the papilla originate from the apical membrane of principal cells and contain water channels in their limiting membrane only when the rats are in physiological states of antidiuresis. In contrast, the majority of endocytosis in intercalated cells is not involved in water channel recycling.

Endocytic vesicles from renal papilla which retrieve the vasopressin-sensitive water channel do not contain a functional H+ ATPase

The water permeability of the kidney collecting duct epithelium is regulated by vasopressin (VP)-induced recycling of water channels between an intracellular vesicular compartment and the plasma membrane of principal cells. To test whether the water channels pass through an acidic endosomal compartment during the endocytic portion of this pathway, we measured ATP-dependent acidification of FITC-dextran-labeled endosomes in isolated microsomal fractions from different regions of Brattleboro rat kidneys. Both VP-deficient controls and rat treated with exogenous VP were examined. ATP-dependent acidification was not detectable in endosomes containing water channels from distal papilla (osmotic water permeability Pf = 0.038 +/- 0.004 cm/s). In contrast, the addition of ATP resulted in a strong acidification of renal cortical endosomes (pHmin = 5.8, initial rate = 0.18-0.25 pH U/s). Acidification of cortical endosomes was reversed with nigericin and strongly inhibited by N-ethyl-maleimide. Passive proton permeability was similar and low in both cortical and papillary endosomes from rats treated or not treated with VP. The fraction of labeled endosomes present in microsomal preparations was determined by fluorescence imaging microscopy of microsomes nonspecifically bound to poly-l-lysine-coated coverslips and was 25% in cortical preparations compared to 14% (+VP) and 9% (-VP) in papillary preparations. The fraction of cortical endosomes was enriched 1.5-fold by immunoabsorption to coverslips coated with mAbs against the bovine vacuolar proton pump. In contrast, the fraction of papillary endosomes was depleted more than twofold by immunoabsorption to identical coverslips. Finally, sections of distal papilla stained with antibodies against the lysosomal glycoprotein LGP120 showed that most of the entrapped FITC-dextran did not colocalize with this lysosomal protein. These results demonstrate that vesicles which internalize water channels in kidney collecting duct principal cells lack functional proton pumps, and do not deliver the bulk of their FITC-dextran content to lysosomes. The data suggest that the principal cell contains a specialized nonacidic apical endocytic compartment which functions primarily to recycle membrane components, including water channels, to the plasma membrane.

Mechanisms and regulation of water permeability in renal epithelia

Water transport occurs in all biological membranes. A few selected membranes in the kidney, amphibian urinary bladder, and erythrocyte have very high water permeability and are thought to contain specialized water transporting units termed "water channels." The known biophysical properties of membranes containing water channels are a high osmotic water permeability coefficient (Pf), an osmotic-to-diffusional water permeability coefficient ratio (Pf/Pd) greater than unity, a low activation energy (Ea), and inhibition by mercurial compounds. The biochemical and molecular characteristics of water channel pathways are not known at present. Established and new methods to measure Pf and Pd in kidney tubules and in isolated membrane vesicles from kidney cells are reviewed and evaluated. In the mammalian proximal tubule, a high Pf results from transcellular movement of water across highly permeable apical and basolateral membranes containing water channels. It has been assumed that proximal tubule Pf is unregulated; however, recent results indicate that apical water channels are retrieved by endocytosis and that Pf is decreased fivefold with increasing transepithelial osmotic gradients. In the thin and thick ascending limbs, Pf is nearly the lowest of all biological membranes and is not subject to regulation. In contrast, collecting tubule Pf is subject to hormonal regulation by vasopressin. Vasopressin binding to receptors located at the basal membrane of principal cells initiates adenosine 3',5'-cyclic monophosphate production, which is thought ultimately to activate the exocytic insertion of intracellular vesicles containing water channels into the cell apical membrane. Vasopressin-induced endosomes from kidney collecting tubule and toad urinary bladder contain functional water channels but no proton pumps or urea transporters, supporting a membrane shuttle hypothesis that is selective for water channels. Future directions for the isolation and molecular cloning of kidney water channels are evaluated.

Composite replicas: methodologies for direct evaluation of the relationship between intramembrane and extramembrane structures

Electron microscopic studies of membrane structure have been facilitated by the recent development of the composite replica technique in which the membrane is freeze-fractured, then inverted and the surface deep-etched and replicated. Examination in stereo of this composite preparation of two replicas with interposed half-membrane and associated surface elements reveals the physical relationship between structures on the surface and within the membrane. Composite replicas of the toad urinary bladder surface demonstrated connections of filamentous glycocalyx elements to intramembrane particles (IMPs). Using a bidirectional shadowing technique, many membrane surface particles also are shown to be associated with underlying IMPs, suggesting that these membrane surface particles are projections of the IMPs above the surface of the membrane. There is evidence that elements whose attachment sites relate to the half-membrane fractured away can be displaced from the membrane surface and lost. Labelling studies using colloidal gold-labelled antibodies were carried out to assess loss of surface mesh from fractured membrane. Gold distributions and amounts were similar in labelled surface replicas, label-fracture specimens, and labelled composite replicas, yet the amount of mesh detected in the composite replicas was less than in the surface replicas. This suggests that while some unlabelled or lightly labelled surface elements can be lost from fractured membranes, ligands stabilize elements and reduce their loss apparently by cross-linking them.

Effect of mercurial compounds on net water transport and intramembrane particle aggregates in ADH-treated frog urinary bladder

It has been suggested that during the oxytocin-induced hydrosmotic response, water crosses the luminal membrane of urinary bladder epithelium cells through membrane-spanning proteins. Although specific inhibitors of osmotic water transport have not been found, certain sulfhydryl reagents such as mercurial compounds may help to identify the proteins involved in this permeation process. We tested the effects of p-chloromercuribenzene sulfonate (PCMBS) and of fluorescein-mercuric acetate (FMA) on the net water flux, the microtubule and microfilament structures of the frog urinary bladder, and the distribution of intramembrane particle aggregates in the luminal membrane. We observed that: (i) 5 mM PCMBS at pH 5 and 0.5 mM FMA at pH 8 added to the mucosal bath at the maximum of the response to oxytocin partially inhibited the net water flux. Inhibition then increased progressively when the preparation was repeatedly or continuously stimulated, until it reached a maximal inhibition at 120 min. This inhibition was not reversed even when cystein was added in the mucosal bath. PCMBS and FMA effects were also observed when cyclic AMP (3',5' cyclic adenosine monophosphate) was used to increase water permeability, (ii) PCMBS mucosal pretreatment did not modify the basal water flux but potentiated the inhibitory effect of PCMBS or FMA on the hydrosmotic response to oxytocin. (iii) Microtubule and microfilament network, visualized in target cells by immunofluorescence, was not affected by PCMBS. (iv) The maximal PCMBS or FMA inhibition was not associated with a reduction of aggregate surface area in the apical membrane. The persistence of the intramembrane particle aggregates associated with the oxytocin-induced hydrosmotic response during the net water flux inhibition by PCMBS, suggests that the PCMBS effect occurs possibly at the level of sulfhydryl groups of the water channel itself.

Pre-steady-state analysis of the turn-on and turn-off of water permeability in the kidney collecting tubule

Water transport across the mammalian collecting tubule is regulated by vasopressin-dependent water channel insertion into and retrieval from the cell apical membrane. The time course of osmotic water permeability (Pf) following addition and removal of vasopressin (VP) and 8-Br-cAMP was measured continuously by quantitative fluorescence microscopy using an impermeant fluorophore perfused in the lumen. Cortical collecting tubules were subjected to a 120 mOsm bath-to-lumen osmotic gradient at 37 degrees C with 10-15 nl/min lumen perfusion and 10-20 ml/min bath exchange rate. With addition of VP (250 microU/ml), there was a 23 +/- 3 sec (SEM, n = 16) lag in which Pf did not change, followed by a rise in Pf (initial rate 1.4 +/- 0.2 x 10(-4) cm/sec2) to a maximum of 265 +/- 10 x 10(-4) cm/sec. With addition of 8-Br-cAMP (0.01-1 mM) there was an 11 +/- 2 sec lag. For [8-Br-cAMP] = 0.01, 0.1 and 1 mM, the initial rate of Pf increase following the lag was (units 10(-4) cm/sec2): 1.1 +/- 0.1, 1.2 +/- 0.1 and 1.7 +/- 0.3. Maximum Pf was (units 10(-4) cm/sec): 64 +/- 4, 199 +/- 9 and 285 +/- 11. With removal of VP, Pf decreased to baseline (12 x 10(-4) cm/sec) with a T1/2 of 18 min; removal of 0.1 and 1 mM 8-Br-cAMP gave T1/2 of 4 and 8.5 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional colocalization of water channels and proton pumps in endosomes from kidney proximal tubule

The apical membrane of mammalian proximal tubule undergoes rapid membrane cycling by exocytosis and endocytosis. Osmotic water and ATP-driven proton transport were measured in endocytic vesicles from rabbit and rat proximal tubule apical membrane labeled in vivo with the fluid phase marker fluorescein-dextran. Osmotic water permeability (Pf) was determined from the time course of fluorescein-dextran fluorescence after exposure of endosomes to an inward osmotic gradient in a stopped-flow apparatus. Pf was 0.009 (rabbit) and 0.029 cm/s (rat) (23 degrees C) and independent of osmotic gradient size. Pf in rabbit endosomes was inhibited reversibly by HgCl2 (KI = 0.2 mM) and had an activation energy of 6.4 +/- 0.5 kcal/mol (15-35 degrees C). Endosomal proton ATPase activity was measured from the time course of internal pH, measured by fluorescein-dextran fluorescence, after the addition of external ATP. Endosomes contained an ATP-driven proton pump that was sensitive to N-ethylmaleimide and insensitive to vanadate and oligomycin. In response to saturating [ATP] the pump acidified the endosomal compartment at a rate of 0.17 (rat) and 0.029 pH unit/s (rabbit); at an external pH of 7.4, the steady-state pH was 6.4 (rat) and 6.5 (rabbit). To examine whether water channels and the proton ATPase were present in the same endosome, the time course of fluorescein-dextran fluorescence was measured in response to an osmotic gradient in the presence and absence of ATP. ATP did not alter endosome Pf, but decreased the amplitude of the fluorescence signal by 43 +/- 3% (rabbit) and 47 +/- 4% (rat).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of nocodazole on the water permeability response to vasopressin in rabbit collecting tubules perfused in vitro

1. The effect of the microtubule-disruptive agent, nocodazole (methyl [5-(2-thienylcarbonyl)-1H-benzimidazol-2-yl] carbamate), on the water permeability response to vasopressin or the synthetic cyclic AMP analogue, 8-parachlorophenylthio-cyclic AMP (8-CPT-cAMP), has been investigated in isolated cortical collecting tubules from rabbit kidneys, perfused in vitro. 2. Pre-treatment with nocodazole, 1-4 micrograms ml-1, had no significant effect on basal water permeability, but inhibited the increase in hydraulic conductivity elicited by vasopressin, 50 microU ml-1, in a dose-dependent manner. Inhibition of the response to the hormone averaged 65 +/- 6% (n = 8, P less than 0.001) at a nocodazole concentration of 4 micrograms ml-1. 3. Nocodazole, 1-4 micrograms ml-1, had no effect on the increase in lumen-negative potential difference (PD) induced by the hormone. 4. Pre-treatment with nocodazole, 4 micrograms ml-1, inhibited the development of the water permeability response to 8-CPT-cAMP, 1.8 x 10(-5) M, by 45 +/- 7% (n = 7, P less than 0.001). 5. When collecting tubules were exposed to nocodazole, 4 micrograms ml-1, after the hydrosmotic response to vasopressin had been fully established, the drug had no inhibitory effect on the maintenance of a high water permeability. 6. The results are consistent with the view that cytoplasmic microtubules play a role in the initiation of the water permeability response to vasopressin in the mammalian cortical collecting tubule at a cellular site beyond the generation of cyclic AMP.