Alteration of luminal membrane structure by antidiuretic hormone

New insights into the dynamic regulation of water and acid-base balance by renal epithelial cells

Maintaining tight control over body fluid and acid-base homeostasis is essential for human health and is a major function of the kidney. The collecting duct is a mosaic of two cell populations that are highly specialized to perform these two distinct processes. The antidiuretic hormone vasopressin (VP) and its receptor, the V2R, play a central role in regulating the urinary concentrating mechanism by stimulating accumulation of the aquaporin 2 (AQP2) water channel in the apical membrane of collecting duct principal cells. This increases epithelial water permeability and allows osmotic water reabsorption to occur. An understanding of the basic cell biology/physiology of AQP2 regulation and trafficking has informed the development of new potential treatments for diseases such as nephrogenic diabetes insipidus, in which the VP/V2R/AQP2 signaling axis is defective. Tubule acidification due to the activation of intercalated cells is also critical to organ function, and defects lead to several pathological conditions in humans. Therefore, it is important to understand how these "professional" proton-secreting cells respond to environmental and cellular cues. Using epididymal proton-secreting cells as a model system, we identified the soluble adenylate cyclase (sAC) as a sensor that detects luminal bicarbonate and activates the vacuolar proton-pumping ATPase (V-ATPase) via cAMP to regulate tubular pH. Renal intercalated cells also express sAC and respond to cAMP by increasing proton secretion, supporting the hypothesis that sAC could function as a luminal sensor in renal tubules to regulate acid-base balance. This review summarizes recent advances in our understanding of these fundamental processes.

Structural correlates of the transepithelial water transport

Transepithelial permeability is one of the fundamental problems in cell biology. Epithelial cell layers protect the organism from its environment and form a selective barrier to the exchange of molecules between the lumen of an organ and an underlying tissue. This chapter discusses some problems and analyzes the participation of intercellular junctions in the paracellular transport of water, migration of intramembrane particles in the apical membrane during its permeability changes for isotonic fluid in cells of leaky epithelia, insertion of water channels into the apical membrane and their cytoplasmic sources in cells of tight epithelia under ADH (antidiuretic hormone)-induced water flows, the osmoregulating function of giant vacuoles in the transcellular fluxes of hypotonic fluid across tight epithelia, and the role of actin filaments and microtubules in the transcellular transport of water across epithelia.

Hypercalcemia and electrolyte disturbances in malignancy

Hypercalcemia and electrolyte abnormalities are common problems in patients with malignancy. In this article we discuss the pathophysiology, clinical features, and management of hypercalcemia, which is the most common metabolic abnormality. We also analyze the electrolyte disturbances that occur in association with malignancy, including hyponatremia, hypokalemia, hypomagnesemia, hypophosphatemia, and hyperkalemia. Recognition and treatment of these disturbances are important parts of the management of patients with malignant disease.

Renal aquaporins

Aquaporins (AQPs) are a newly recognized family of transmembrane proteins that function as molecular water channels. At least four aquaporins are expressed in the kidney where they mediate rapid water transport across water-permeable epithelia and play critical roles in urinary concentrating and diluting processes. AQP1 is constitutively expressed at extremely high levels in the proximal tubule and descending limb of Henle's loop. AQP2, -3 and -4 are expressed predominantly in the collecting duct system. AQP2 is the predominant water channel in the apical plasma membrane and AQP3 and -4 are found in the basolateral plasma membrane. Short-term regulation of collecting duct water permeability by vasopressin is largely a consequence of regulated trafficking of AQP2-containing vesicles to and from the apical plasma membrane.

Morphological alterations and cytoskeletal reorganization in opossum kidney (OK) cells during osmotic swelling and volume regulation

Cells from a variety of tissues regulate their volume when exposed to anisotonic conditions. After exposure of cells to hypotonic conditions, the rapid phase of cell swelling is followed by a slower phase of cell shrinkage towards the initial volume. The present study investigates morphological alterations of adherent and fully spread cells after exposure to hypotonic conditions and the reorganization of cytoskeletal components such as F-actin, actin-binding proteins, microtubules and intermediate-sized filaments. We used cells of a continuous epithelial cell line from the opossum kidney (OK cells), which were exposed to hypotonic conditions for a period of 60 min at 25 degrees C. The osmolarity was reduced by 40% from 320 mosmol/l (isotonic conditions) to 192 mosmol/l (hypotonic conditions). The initial swelling after exposure of OK cells to hypotonic conditions caused enhanced ruffling membrane activity, formation of lamellipodia and an extended space between adjacent cells which was caused by a more rounded cell shape. Moreover, the height of cells located in the centre of cell clusters increased by 32 +/- 8% (mean value +/- SEM) as checked by morphometric analysis of the vertical distance between the apical and basolateral F-actin domain. Although the fluorescence intensity and organization of F-actin in a horizontal direction remained unaltered during cell swelling, we observed a loss of periodicity and irregular distribution of myosin aggregates and a partial rearrangement of vimentin filaments in the form of short fragments. In all experiments the organization of microtubles was observed to be unaltered.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The effect of vasopressin on the cytoskeleton of the epithelial cell

Vasopressin (AVP) promotes the fusion of vesicles containing water channels with the apical membrane of receptor cells in the amphibian bladder and mammalian kidney. Fusion is accompanied by depolymerization of the actin cytoskeleton. In this review, we present the evidence for actin depolymerization by AVP in the whole cell, and the application of confocal microscopy and immunogold electron microscopy in localizing depolymerization to the apical region of the receptor cell.

Apical endosomes isolated from kidney collecting duct principal cells lack subunits of the proton pumping ATPase

Endocytic vesicles that are involved in the vasopressin-stimulated recycling of water channels to and from the apical membrane of kidney collecting duct principal cells were isolated from rat renal papilla by differential and Percoll density gradient centrifugation. Fluorescence quenching measurements showed that the isolated vesicles maintained a high, HgCl2-sensitive water permeability, consistent with the presence of vasopressin-sensitive water channels. They did not, however, exhibit ATP-dependent luminal acidification, nor any N-ethylmaleimide-sensitive ATPase activity, properties that are characteristic of most acidic endosomal compartments. Western blotting with specific antibodies showed that the 31- and 70-kD cytoplasmically oriented subunits of the vacuolar proton pump were not detectable in these apical endosomes from the papilla, whereas they were present in endosomes prepared in parallel from the cortex. In contrast, the 56-kD subunit of the proton pump was abundant in papillary endosomes, and was localized at the apical pole of principal cells by immunocytochemistry. Finally, an antibody that recognizes the 16-kD transmembrane subunit of oat tonoplast ATPase cross-reacted with a distinct 16-kD band in cortical endosomes, but no 16-kD band was detectable in endosomes from the papilla. This antibody also recognized a 16-kD band in affinity-purified H+ ATPase preparations from bovine kidney medulla. Therefore, early endosomes derived from the apical plasma membrane of collecting duct principal cells fail to acidify because they lack functionally important subunits of a vacuolar-type proton pumping ATPase, including the 16-kD transmembrane domain that serves as the proton-conducting channel, and the 70-kD cytoplasmic subunit that contains the ATPase catalytic site. This specialized, non-acidic early endosomal compartment appears to be involved primarily in the hormonally induced recycling of water channels to and from the apical plasma membrane of vasopressin-sensitive cells in the kidney collecting duct.

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.

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)

Surface hydrophobicity and water transport of the toad urinary bladder: effects of vasopressin

The present study investigated whether the hydrophobic properties (wettability) of the luminal surface of the toad urinary bladder might play a role in modulating water transport across this epithelium. In the absence of vasopressin (ADH), water transport across the tissue was low, while luminal surface hydrophobicity (water contact angle) was relatively high. Following stimulation by ADH, water transport increased and surface hydrophobicity decreased. The addition of indomethacin to inhibit ADH-induced prostaglandin synthesis did not reduce these actions of ADH. In an attempt to alter water transport in this tissue, a liposomal suspension of surface-active phospholipids was administered to the luminal surface. This addition had no detectable influence on the low basal rates of water transport, but blocked the ADH-induced stimulation of water transport. We suggest that surface-active phospholipids on the toad bladder luminal membrane may contribute to the hydrophobic characteristics of this tissue. ADH may act to decrease surface hydrophobicity, facilitating the movement of water molecules across an otherwise impermeable epithelium. This surface alteration may be associated with the appearance of water channels in the apical membrane.

Effect of pH on vasopressin-induced water permeability in collecting ducts of isolated rat papillae

1. The effects of basolateral and luminal pH on diffusional water permeability of microperfused collecting ducts of isolated rat papillae were examined in the presence and absence of vasopressin at two concentrations. 2. In the absence of vasopressin, collecting duct diffusional water permeabilities did not differ when the pH of the luminal fluid was varied. Similarly, in the absence of vasopressin, collecting duct diffusional water permeabilities did not differ when the bath pH was varied. 3. In the presence of 50 microU/ml vasopressin, increases in diffusional water permeability of collecting ducts perfused with solutions at pH 5.0, 7.4 or 9.0 did not differ significantly. Similarly, increases in diffusional water permeability induced by 200 microU/ml vasopressin were not different when collecting ducts were perfused with solutions at pH 5.0, 7.4 or 9.0. 4. The presence of vasopressin (50 microU/ml) in the bathing medium at pH 6.4, 7.4 and 8.4 induced increments in diffusional water permeability of 0.40 +/- 0.21 (n = 14, P greater than 0.05), 1.56 +/- 0.27 (n = 27, P less than 0.001) and 1.67 +/- 0.24 (n = 12, P less than 0.001) microns/s, respectively. The increment in water permeability at pH 6.4 was significantly less than that at pH 7.4 (P less than 0.001). 5. The presence of vasopressin (200 microU/ml) in the bathing medium at pH 6.4, 7.4 and 8.4 induced increments in diffusional water permeability of 2.16 +/- 0.54 (n = 9, P less than 0.01), 2.55 +/- 0.51 (n = 17, P less than 0.001) and 0.98 +/- 0.34 (n = 11, P less than 0.05) microns/s respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Junctional complexes and cell polarity in the urinary tubule

In this review, we demonstrate how differentiated membrane domains can be detected in epithelial cells using conventional light and electron microscopy, freeze-fracture electron microscopy and the immuno- and cytochemical detection of membrane components. Using specific examples from the kidney, we show how the polarized insertion of these components into either apical or basolateral plasma membrane regions on either side of the tight junction barrier is related to specific functions of principal and intercalated cells in the collecting duct. In addition, distinct basal and lateral membrane domains have been revealed in some cells that are maintained in the absence of a tight junctional barrier in the plane of the membrane. This suggests that other factors, possibly related to cytoskeletal elements, may be involved in the functional segregation of these membrane areas. We propose that epithelial cell plasma membranes should be subdivided into apical, lateral and basal regions, and that the term "basolateral" may be an oversimplification.

Electrophysiological analysis of sodium-transport in the colon of the frog (Rana esculenta). Modulation of apical membrane properties by antidiuretic hormone

Sodium transport and apical bioelectrical membrane properties were investigated in frog colonic epithelium in the absence and presence of the antidiuretic hormone arginine-vasotocin (AVT). Apical Na-permeability and intracellular Na-activity were evaluated by analysis of current-voltage relationships in the serosally K-depolarized tissue. Tissue- and apical membrane capacitance were measured by voltages step analysis. The frog colon was found to be a tight epithelium with a transepithelial resistance of 2.63 +/- 0.25 k omega.muF (n = 17). 85-90% of short circuit current (11.2 +/- 1.1 microA.microF.l-1; n = 17) was related to electrogenic Na-transport from mucosa to serosa. Graded doses of amiloride (less than 50 mumol.l-1) induced Michaelis-Menten-type inhibition kinetics. Serosal addition of 10(-6) mol.l-1 AVT induced a significant increase in sodium current (25%), apical sodium permeability (19%) and tissue capacitance (4.3%) whereas intracellular Na-activity remained unchanged. There was a good correlation between increased Na-current and apical Na-permeability. No correlation was found between Na-current and membrane capacitance. Our results demonstrate that in contrast to other species the amphibian colon shows a natriferic reaction to AVT. We suggest that the regulation of Na-transport in frog colon is similar to that in the toad urinary bladder. It is caused by an activation of preexisting apical Na-channels and not by fusion of subapical cytoplasmic vesicles with the apical membrane.

Endosomes from kidney collecting tubule cells contain the vasopressin-sensitive water channel

The mechanism by which vasopressin rapidly and dramatically increases the water permeability of target epithelial cell membranes is thought to involve a cycle of exo- and endocytosis during which vesicles carrying 'water channels' are successively inserted into, and removed from the apical plasma membrane of epithelial cells. Clusters of intramembranous particles, visible by freeze-fracture electron microscopy and presumed to represent water channels, appear on apical membranes in parallel with increased transepithelial water flow. In the collecting duct, these clusters are located in clathrin-coated pits which are subsequently internalized. There has been no direct evidence, however, that subcellular membranes in vasopressin-sensitive epithelia contain functional water channels. In this report, we have used fluorophores that are sensitive to volume and do not pass through membranes to label and to measure directly the osmotic water permeability of endocytosed vesicles isolated from renal papilla. We present direct evidence that vasopressin induces the appearance of a population of endocytic vesicles whose limiting membranes contain water channels.

Cell membrane water permeability of rabbit cortical collecting duct

The water permeability (Posm) of the cell membranes of isolated perfused rabbit cortical collecting ducts was measured by quantitative light microscopy. Water permeability of the basolateral membrane, corrected for surface area, was 66 microns X sec-1 for principal cells and 62.3 microns X sec-1 for intercalated cells. Apical membrane Posm values corrected for surface area, were 19.2 and 25 microns X sec-1 for principal and intercalated cells, respectively, in the absence of antidiuretic hormone (ADH). Principal and intercalated cells both responded to ADH by increasing Posm of their apical membranes to 92.2 and 86.2 microns X sec-1, respectively. The ratio of the total basolateral cell membrane osmotic water permeability to that of the apical cell membrane was approximately 27:1 in the absence of ADH and approximately 7:1 in the presence of the hormone for both cell types. This asymmetry in water permeability is most likely due to the fact that basolateral membrane surface area is at least 7 to 8 times greater than that of the apical membrane. Both cell types exhibited volume regulatory decrease when exposed to dilute serosal bathing solutions. Upon exposure to a hyperosmotic serosal bath (390 mosM), principal cells did not volume regulate while two physiologically distinct groups of intercalated cells were observed. One group of intercalated cells failed to volume regulate; the second group showed almost complete volume regulatory increase behavior.

Morphometric analysis of epithelial cells of frog urinary bladder. I. Effect of antidiuretic hormone, calcium ionophore (A23187) and PGE2

Changes in epithelial cell morphology, especially at the apical plasma membrane, are frequently cited as initial evidence for antidiuretic hormone (ADH)-induced increase in membrane permeability. The effects of ADH and agents that alter and modify calcium and prostaglandin concentrations on the morphology and cytology of the epithelial cells of frog (Rana pipiens) urinary bladder are presented using the techniques of transmission and scanning electron microscopy. It was found that, like ADH, calcium ionophore, A23187, produce intense microvilli formation, microfilament mobilization and an increase in the density of granules and membrane associated vesicles, suggesting a prominent role of calcium in these processes. Moreover, our results suggest that these membrane and cytosolic transformations may be mediated in part through prostaglandin formation, as exogenous PGE2 mimicked these effects, and indomethacin, a prostaglandin synthesis inhibitor, attenuated ionophore's effect on luminal cytomorphology. However, unlike ADH, prostaglandins and ionophore inhibit hormonal-induced increase in transepithelial water flow. These results suggest that other components more distal to the luminal membrane, perhaps the basolateral membrane, may be rate-limiting for transepithelial water flow and possibly are regulated by either changes in calcium concentrations or prostaglandins.

Parathyroid hormone-induced changes of the brush border topography and cytoskeleton in cultured renal proximal tubular cells

In order to examine the possibility of parathyroid hormone-mediated ultrastructural rearrangements in target epithelium, isolated canine renal proximal tubular cells were grown on a collagen-coated semipermeable membrane in a defined medium. Scanning and transmission electron microscopy of these monolayers revealed abundant microvilli. Exposure of the proximal tubular cells to parathyroid hormone resulted in a biphasic changes involving: dramatic shortening and rarefaction of microvilli within 1 min; and recovery of microvillar topography after 5 min. A similar shortening of microvilli was observed following exposure to ionomycin, whereas incubation with cyclic AMP resulted in an elongation of microvilli. Parathyroid hormone stimulated cyclic AMP production and increased cytoplasmic free calcium concentration in cultured proximal tubular cells. Pretreatment of cells with a calmodulin inhibitor abolished the effect of parathyroid hormone on brush border topography. Shortening of microvilli was associated with a disappearance of microvillar core filaments. Staining of F-actin with fluoresceinphalloidin showed that parathyroid hormone resulted in fragmentation of stress fibers. It is concluded that parathyroid hormone-induced cell activation involves cytoplasmic-free calcium, calmodulin, and the cytoskeleton.

Hydraulic water permeability and transepithelial voltage in the isolated perfused rabbit cortical collecting tubule following acute unilateral ureteral obstruction

Ureteral obstruction affects the kidney's ability to conserve water and sodium. Using the isolated perfused tubule technique, we studied cortical collecting tubules (CCT) taken from rabbits subjected to a sham operation or to 4 h of unilateral ureteral obstruction (UUO). Tubules were perfused in the presence of an osmotic gradient directed to promote water movement from lumen to bath, and volume flux (Jv), hydraulic water permeability (Lp), and transepithelial voltage (V1) were determined. In tubules from sham-operated and UUO animals, basal (before exposure to vasopressin) J, and Lp were not different from zero. After addition of 200 microU . ml-1 of arginine vasopressin (aVP) to the bath, Jv and Lp increased to 1.64 +/- 0.23 nl . mm-1 . min-1 and 127.9 +/- 19.8 cm . s-1 . atm-1 x 10(7), respectively, in tubules from sham-operated animals, but not only 0.27 +/- 0.09 nl . mm-1 . min-1 an 18.8 +/- 6.2 cm . s-1 . atm-1 . 10(7) in tubules from UUO animals. Pretreatment with desoxycorticosterone acetate (DOCA) or indomethacin in vivo did not prevent the blunted vasopressin response seen in tubules taken from UUO animals. The Jv and Lp responses to the cyclic AMP (cAMP) analogue, 8-Br-cAMP, were also diminished in tubules taken from UUO animals compared with shams. V1, measured during the basal period, was diminished in tubules from UUO kidneys (-5.0 +/- 2.1 mV) compared with shams (-21.9 +/- 4.1 mV), and pretreatment with DOCA did no prevent the effects of UUO on V1. In contrast, tubules taken from animals that received indomethacin prior to UUO developed voltages not different from voltages in tubules taken from sham-operated animals (-17.3 +/- 1.7 mV). We conclude that, although CCT from UUO animals can maintain osmotic gradients, their ability to respond to vasopressin by increasing Lp is impaired by an intrinsic defect located at a step beyond the generation of cAMP, and that prostaglandin inhibition or DOCA pretreatment do not reverse the decreased responsiveness of Lp to aVP. UUO also diminished V1, and this abnormality was prevented by previous treatment with indomethacin, suggesting that prostaglandins may mediate the effect of UUO on V1.