The effect of neurohypophyseal hormones on the permeability of the toad bladder to urea

Diabetes insipidus: celebrating a century of vasopressin therapy

Diabetes mellitus, widely known to the ancients for polyuria and glycosuria, budded off diabetes insipidus (DI) about 200 years ago, based on the glucose-free polyuria that characterized a subset of patients. In the late 19th century, clinicians identified the posterior pituitary as the site of pathology, and pharmacologists found multiple bioactivities there. Early in the 20th century, the amelioration of the polyuria with extracts of the posterior pituitary inaugurated a new era in therapy and advanced the hypothesis that DI was due to a hormone deficiency. Decades later, a subset of patients with polyuria unresponsive to therapy were recognized, leading to the distinction between central DI and nephrogenic DI, an early example of a hormone-resistant condition. Recognition that the posterior pituitary had 2 hormones was followed by du Vigneaud's Nobel Prize winning isolation, sequencing, and chemical synthesis of oxytocin and vasopressin. The pure hormones accelerated the development of bioassays and immunoassays that confirmed the hormone deficiency in vasopressin-sensitive DI and abundant levels of hormone in patients with the nephrogenic disorder. With both forms of the disease, acquired and inborn defects were recognized. Emerging concepts of receptors and of genetic analysis led to the recognition of patients with mutations in the genes for 1) arginine vasopressin (AVP), 2) the AVP receptor 2 (AVPR2), and 3) the aquaporin 2 water channel (AQP2). We recount here the multiple skeins of clinical and laboratory research that intersected frequently over the centuries since the first recognition of DI.

Transport of hop aroma compounds across Caco-2 monolayers

Hop aroma compounds and digestive transformation products thereof were investigated in view of their human intestinal absorption and biotransformation processes.

Structure of the toad's urinary bladder as related to its physiology

The structure of the urinary bladder of the toad Bufo marinus was studied by light and electron microscopy. The epithelium covering the mucosal surface of the bladder is 3 to 10 microns thick and consists of squamous epithelial cells, goblet cells, and a third class of cells containing many mitochondria and possibly representing goblet cells in early stages of their secretory cycle. This epithelium is supported on a lamina propria 30 to several hundred microns thick and containing collagen fibrils, bundles of smooth muscle fibers, and blood vessels. The serosal surface of the bladder is covered by an incomplete mesothelium. The cytoplasm of the squamous epithelial cells, which greatly outnumber the other types of cells, is organized in a way characteristic of epithelial secretory cells. Mitochondria, smooth and rough surfaced endoplasmic reticulum, a Golgi apparatus, "multivesicular bodies," and isolated particles and vesicles are present. Secretion granules are found immediately under the plasma membranes of the free surfaces of the epithelial cells and are seen to fuse with these membranes and release their contents to contribute to a fibrous surface coating found only on the free mucosal surfaces of the cells. Beneath the plasma membranes on these surfaces is an additional, finely granular component. Lateral and basal plasma membranes are heavily plicated and appear ordinary in fine structure. The cells of the epithelium are tightly held together by a terminal bar apparatus and sealed together, with an intervening space of only 0.02 mmicro near the bladder lumen, in such a way as to prevent water leakage between the cells. It is demonstrated in in vitro experiments that water traversing the bladder wall passes through the cytoplasm of the epithelial cells and that a vesicle transport mechanism is not involved. In vitro experiments also show that the basal (serosal) surfaces of the epithelial cells are freely permeable to water, while the free (mucosal) surfaces are normally relatively impermeable but become permeable when the serosal surface of the bladder is treated with neurohypophyseal hormones. The permeability barrier found at the mucosal surface may be represented, structurally, either by the filamentous layer lying external to the plasma membrane, by the intracellular, granular component found just under the plasma membrane, or by both of these components of the mucosal surface complex. The polarity of the epithelial sheet is emphasized and related to the physiological role of the urinary bladder in amphibian water balance mechanisms.

Movement of sodium across the mucosal surface of the isolated toad bladder and its modification by vasopressin

Studies have been made on the isolated urinary bladder of the toad, Bufo marinus, in an attempt to evaluate gradients of chemical activity across the mucosal surfaces of the epithelial cells which would serve to maintain a net movement of sodium from the mucosal medium into the cells. The likelihood of such chemical gradients has been established by the demonstration of lower contents of sodium within the tissue, expressed as microequivalents per gram of tissue water, than of concentrations of sodium in the mucosal medium at all levels of the latter examined. The transepithelial transport of sodium and the sodium content of the tissue were found to increase rapidly with rise in concentration of sodium in the mucosal medium up to values of 30 to 60 meq per liter. Further increase in concentration of the medium above this value failed to induce further stimulation of sodium transport or increase in the sodium content of the tissue. Vasopressin increased the rate of transport of sodium at every concentration of sodium in the mucosal medium without altering this relationship. Although entry of sodium across the mucosal surface of the epithelial cells may be passive it is not by free diffusion but involves some considerable interaction with the mucosal surface of the bladder and constitutes the major determinant of the rate of transepithelial transport of sodium. Vasopressin acts to enhance this initial step in the transport of sodium.

The state of water in the isolated toad bladder in the presence and absence of vasopressin

An attempt has been made to assess the validity of applying the frictional and viscous coefficients of bulk water to the movement of water and solutes through the urinary bladder of the toad. The temperature dependence of diffusion of THO, C¹⁴-urea, C¹⁴-thiourea, and net water transfer across the bladder was determined in the presence and absence of vasopressin. The activation energy for diffusion of THO was 9.8 kcal per mole in the absence of vasopressin and 4.1 kcal per mole with the hormone present. Activation energies simultaneously determined following vasopressin for diffusion and net transfers of water were similar, and in the same range as known activation energies for diffusion and viscous flow in water. Urea had activation energies for diffusion of 4.1 and 3.9 kcal per mole in the absence and presence of vasopressin, respectively. Thiourea had a high activation energy for diffusion of 6.3 kcal per mole, which was unchanged, 6.6 kcal per mole, following hormone. These findings suggest that in its rate-limiting permeability barrier, water is present in a structured state, offering a high resistance to penetration by water. Vasopressin enlarges the aqueous channels so that the core of water they contain possesses the physical properties of ordinary bulk water. Urea penetrates the tissue via these aqueous channels while thiourea is limited by some other permeability barrier.

Studies on the movement of water through the isolated toad bladder and its modification by vasopressin

Measurements of diffusion permeability and of net transfer of water have been made across the isolated urinary bladder of the toad, Bufo marinus, and the effects thereon of mammalian neurohypophyseal hormone have been examined. In the absence of a transmembrane osmotic gradient, vasopressin increases the unidirectional flux of water from a mean of 340 to a mean of 570 µl per cm² per hour but the net water movement remains essentially zero. In the presence of an osmotic gradient but without hormone net transfer of water remains very small. On addition of hormone large net fluxes of water occur; the magnitude of which is linearly proportional to the osmotic gradient. The action of the hormone on movement of water is not dependent on the presence of sodium or on active transport of sodium. Comparison of the net transport of water and of unidirectional diffusion permeability of the membrane to water indicates that non-diffusional transport must predominate as the means by which net movement occurs in the presence of an osmotic gradient. An action of the hormone on the mucosal surface of the bladder wall is demonstrated. The effects of the hormone on water movement are most simply explained as an action to increase the permeability and porosity of the mucosal surface of the membrane.

Permeability of the isolated toad bladder to solutes and its modification by vasopressin

Measurements have been made of the permeability of the isolated urinary bladder of the toad to a number of small solute molecules, in the presence and absence of vasopressin. Vasopressin has a strikingly specific effect on increasing permeability of the bladder to a group of small, uncharged amides and alcohols while penetration by other small molecules and ions is unaffected. The movement of urea is passive, as indicated by equal flux rates in the two directions. The reflection coefficients for chloride and thiourea indicate a high degree of impermeability of the bladder to these solutes even in the presence of large net movements of water. The low concentration of thiourea in the tissue water when this compound is added to the mucosal bathing medium indicates that the major permeability barrier to thiourea is at the mucosal surface of the bladder. The findings can be accounted for by a double permeability barrier consisting of a fine selective diffusion barrier and a porous barrier in series. The former would constitute the permeability barrier to most small solutes while the latter would be the rate-limiting barrier for water and the amides. It would be the porous barrier which is affected by vasopressin. Reasons are presented which require both barriers to be contained in or near the plasma membrane at the mucosal surface of the bladder.

Pulsatile urea excretion in gulf toadfish (Opsanus beta): evidence for activation of a specific facilitated diffusion transport system

When toadfish are made ureotelic by a crowding/confinement protocol, they excrete approximately 90 % of their urea nitrogen (urea-N) production in large, irregular pulses (1-2 pulses per day) from the gill region. We investigated three hypotheses as to the mechanism of pulsatile excretion: (i) the presence of an active reabsorptive 'back-transport' mechanism that is periodically inhibited to allow urea-N excretion to occur; (ii) the periodic occurrence of a generalized, non-specific increase in gill permeability; and (iii) the presence of a specific facilitated diffusion transport system that is periodically activated. Exposure of toadfish during non-pulse periods to treatments designed to block a 'back-transport' mechanism (Na+-free sea water or the urea analogues 30 mmol l-1 thiourea or 30 mmol l-1 acetamide in the external water) did not stimulate a leakage of urea-N, thereby opposing the first hypothesis. The second hypothesis was opposed by several results. Neither injection of the potent branchial vasodilator L-isoprenaline (10(-5) mol l-1) nor infusion of NH4Cl, the latter at levels known to stimulate urea-N efflux in perfused gills, had any effect on urea-N excretion. Furthermore, during natural pulse events, when the normally very low gill permeability to urea (3x10(-7) cm s-1) increased at least 35-fold, there was no accompanying increase in permeability to either 3H2O (1.5x10(-5) cm s-1) or the paracellular marker [14C]PEG-4000 (10(-8) cm s-1). However [14C]thiourea permeability (1.5x10(-7) cm s-1) increased approximately fivefold, in support of the third hypothesis. Furthermore, when 30 mmol l-1 urea was placed in the external water, a concentration (60 000 micromol-N l-1) approximately three times that of blood (20 000 micromol-N l-1), each efflux pulse event (measured with [14C]urea) was accompanied by a net uptake, such that blood urea-N levels rose rather than fell. A proportional 1:1 relationship between influx per unit external concentration and efflux per unit internal (i.e. plasma) concentration indicated a fully bidirectional transport system. The simultaneous presence of 60 mmol l-1 thiourea in the external water inhibited the influx component by 73 %, further supporting this conclusion. These data, together with recent molecular, morphological and endocrinological evidence, strongly suggest that pulsatile urea-N excretion is caused by the periodic activation of a facilitated urea transporter in the gills, similar to the vasopressin-regulated urea transporter in the mammalian kidney.

Urea and amphibian water economy

Accumulation of urea in the body fluids enables some amphibians to tolerate high ambient salinities (Bufo viridis, Xenopus laevis, Rana cancrivora, Ambystoma tigrinum, Batrachoseps spp.) or to estivate in soil with low water potentials (Scaphiopus spp.). These species are assumed not only to accumulate urea produced in the normal metabolism, but to synthesize urea in response to water shortage. Re-examination of the data did not support the view of an osmoregulatory urea synthesis. Increased urea synthesis on exposure to high salinities in X. laevis, R. cancrivora and Batrachoseps spp. seemed to reflect reactions to an adverse environment. It is suggested that in amphibians, solute concentration in the plasma and rate of excretion of urea are coordinated so that at a certain plasma concentration, urea is excreted at the same rate at which it is produced. The higher the level of urea in the body fluids at balance between production and excretion, the higher the tolerance of the species of low external water potentials. The mechanisms that integrate the relationship between plasma solute concentration and handling of urea by the kidneys are not known.

DIFFUSION OF WEAK ACIDS ACROSS THE TOAD BLADDER. INFLUENCE OF PH ON NON-IONIC PERMEABILITY COEFFICIENTS

Studies have been carried out, using the toad bladder, to determine the influence of pH on the permeability coefficients (K_trans) of the non-ionic species of (a) a series of aliphatic acids ranging from propionic to octanoic and (b) the aromatic acids, benzoic and acetylsalicylic. The data demonstrate that as the acidity of the mucosal bathing solution is increased by changing pH from 6 to 4, the fluxes of propionic, butyric, and acetylsalicylic acids increase in direct proportion to the increase in the calculated non-ionic concentration; the permeability coefficients, therefore, remain constant. However, the fluxes of the six, seven, and eight carbon aliphatic acids and benzoic acid rise only slightly despite an almost tenfold increase in non-ionic concentration, the K_trans falling from approximately 20,000 x 10^-7 cm sec.^-1 at pH 6 to approximately 2500 x 10^-1 cm sec.^-1 at pH 4. It has been tentatively proposed that the common characteristic of the compounds exhibiting this anomalous behavior is their non-polarity and high degree of lipid solubility. Possible explanations for the differences observed between the more lipid-soluble and less lipid-soluble compounds have been considered.

THE EFFECT OF MUCOSAL AND SEROSAL SOLUTION CATIONS ON BIOELECTRIC PROPERTIES OF THE ISOLATED TOAD BLADDER

The spontaneous transtissue potential and the DC conductance of the isolated toad bladder were measured when the tissue was exposed to sulfate Ringer's solutions of modified ionic composition. Na(+) was replaced to varying extents by (C(2)H(5))(3)NH(+), (C(2)H(5))(4)N(+), Li(+), Cs(+), K(+), or Rb(+). Reversible and irreversible changes were observed. The reversible changes were consistent with equations derived from the Nernst-Planck diffusion equation, and gave the following functional description of the bladder: (a) the potential measurements were compatible with two membranes in series; (b) the mucosal surface was more permeable to Na(+) than to other monovalent cations; (c) the serosal surface was permeable to both K(+) and Na(+) but preferentially to K(+); (d) the rate of Na(+) diffusion across the mucosal membrane appeared to approach a maximum but two alternative interpretations are discussed; (e) the conductance data were consistent with the assumption of a constant concentration gradient for the penetrating ions within the membrane (Henderson's assumption) provided suitable hypotheses are made concerning the Na(+) distribution between the membrane surfaces and the bulk phases of the adjacent solutions; (f) the conductance and spontaneous potential data suggested that the mucosal membranes of a small fraction of the epithelial cells were more permeable than the mucosal membranes of the majority of these cells. The irreversible changes were almost entirely associated with cation substitution in the serosal solution. However, Li(+) produced an irreversible fall in voltage when added to either side of the tissue.

Apical membrane limits urea permeation across the rat inner medullary collecting duct

Urea diffuses across the terminal inner medullary collecting duct (IMCD) via a facilitated transport pathway. To examine the mechanism of transcellular urea transport, membrane-apparent urea (Purea) and osmotic water (Pf) permeabilities of IMCD cells were measured by quantitative light microscopy in isolated IMCD-2 tubules perfused in the absence of vasopressin. Basolateral membrane Pf, determined by addition of raffinose to the bath, was 69 microns/s. Basolateral membrane Purea, determined by substituting urea for raffinose without change in osmolality, was 14 X 10(-5) cm/s. Bath phloretin inhibited basolateral Purea by 85% without a significant effect on Pf. The basolateral reflection coefficient for urea, determined by addition of urea in the presence of phloretin, was 1.0. These results indicate that urea crosses the basolateral membrane by diffusion, and not by solvent drag. In perfused tubules, the rate of cell swelling following substitution of urea for mannitol was significantly greater with bath than lumen changes. After correcting for membrane surface area, the basolateral membrane was twofold more permeable than the apical membrane.

CONCLUSIONS

(a) in the absence of vasopressin, urea permeation across the IMCD cell is limited by the apical membrane; (b) the basolateral membrane contains a phloretin-sensitive urea transporter; (c) transepithelial urea transport occurs by movement of urea through the IMCD cell.

Oxygen free radicals and corneal endothelium

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Effect of inflammation on the corneal endothelial pump and barrier

Corneal thickness is a reflection of endothelial barrier and pump functions. The corneal edema that occurs during intraocular inflammation is a consequence of the breakdown of one or both of these parameters. Results of this study demonstrate that, during intraocular inflammation induced by an intravitreal injection of bovine serum albumin (BSA), the permeability of rabbit corneal endothelia to inulin was increased. By comparison, treatment with oral aspirin and/or subconjunctival triamcinolone acetonide prevented the endothelial barrier breakdown induced by the BSA. Concomitant with the loss of the barrier function, endothelial ouabain binding decreased in the BSA injected eye, indicating a reduction in endothelial Na/K ATPase pump site density. A subconjunctival injection of triamcinolone prevented this decrease in pump sites. The increase in endothelial permeability and the decrease in pump site density correlated with an increase in corneal thickness. It can be concluded that the intraocular inflammation induced by BSA effects corneal edema by both an increase in endothelial permeability and a decrease in Na/K ATPase pump site density. Subconjunctival triamcinolone is effective in preventing this response.

Polycations reduce vasopressin-induced water flow by endocytic removal of water channels

Several polycations added to the luminal solution were found to inhibit the vasopressin (ADH)-induced water flow in toad urinary bladder but not the ADH-induced increase in sodium transport or in urea permeability. Ultrastructural studies were conducted to evaluate the uptake of cationized ferritin. It was found that endocytosis of cationized ferritin by luminal cells was strikingly enhanced on exposure to ADH; this increased endocytosis was concomitant with inhibition of transepithelial ADH-induced water flow. Various maneuvers preventing endocytosis were also found to counteract the polycation-induced inhibition of the ADH effect. It is suggested that polycations are endocytosed in vesicles whose walls contain the water channels but not the urea or sodium channels.

Downregulation of vasopressin receptors in toad bladder

Binding of tritium-labeled vasopressin [( 3H]AVP) to a broken epithelial cell preparation of the toad's urinary bladder has been related to hormonal action on water and urea transport across the intact tissue. Hormone binding to receptor sites and permeability changes were initiated at the same concentration of hormone (0.4 nM). Half-maximal urea and water permeability responses were observed with 3.1 and 5.6 nM AVP, respectively, although half-maximal receptor saturation required considerably higher concentrations of hormone (less than 500 nM). Because maximal permeability responses were obtained with occupation of approximately 200 fmol/mg protein receptor sites and the total receptor density was in excess of 2,000 fmol/mg protein, there is apparently a receptor reserve in this tissue. The antidiuretic hormone employed by the toad is vasotocin (AVT). This compound was 60-fold more effective than AVP in displacing [3H]AVP from receptor sites. Preincubation of bladders with AVT resulted in downregulation of receptor sites. Although the magnitude of receptor loss was equivalent in the presence or absence of a transmembrane osmotic pressure gradient, the capacity of AVT to induce permeability changes was more markedly reduced in the presence of an osmotic gradient. This observation suggests that the negative-feedback signal initiated by water flow through the hormone target cell diminishes sensitivity to hormone by a mechanism other than by a reduction in the number of surface receptors or by a decrease in their affinity for the hormone.

Autoradiographic studies of solute transport across the toad bladder

The autoradiography of diffusible, hydrophilic solutes presents special problems in localization of the labeled solute under study. We present studies of the movement of 14C-labeled urea, and 14C- and 3H-labeled sucrose across the isolated urinary bladder of the toad, a vasopressin-sensitive epithelium, using a technique that avoids exposure to water throughout all processing steps and minimizes error caused by isotope scatter. We have shown a significant increase in 14C urea entry into epithelial cells following vasopressin, and a significant decrease following phloretin, an agent that selectively blocks vasopressin-stimulated urea transport. The autoradiographic technique confirms the luminal site of action of phloretin. Studies of 14C and 3H sucrose labeling show that this molecule is virtually excluded from the cell. The current method of grain counting is capable of yielding reliable information in studies of epithelial transport.

Effects of bilirubin on transepithelial transport of sodium, water, and urea

Urinary concentrating defects and renal salt wasting have been described in the hyperbilirubinemic Gunn strain strain of rat. Homozygous animals demonstrate significant reductions in renal medullary urea and sodium ion concentrations. These observations are consistent with possible bilirubin associated disorders in the transepithelial transport of water and solute. To test this hypothesis, measurements of active sodium transport and passive water and urea fluxes were made in hemibladders isolated from the Dominican toad, Bufo marinus. Tissues were exposed to amphibian bicarbonate Ringer's solution containing 0.1 mM bilirubin with 0.05% bovine serum albumin (BSA) or BSA alone. Vasopressin-stimulated sodium transport, as reflected by short circuit current (SCC), was inhibited by 18 +/- 6% in the presence of bilirubin (N = 10; P less than 0.02). Cyclic AMP (p-Cl-phenylthio cAMP 10(-5) M) stimulated SCC was inhibited to a similar degree in the presence of bilirubin. The inhibition was noted only when bilirubin was in the serosal bath, and it could be abolished with BSA 0.5%. Bilirubin had no effect on the increase in SCC induced by higher concentrations of cyclic AMP (10(-4) M), aldosterone, or amphotericin B. Furthermore, bilirubin had no effect on the hydro-osmotic response to vasopressin and vasopressin-induced changes in urea permeability. These findings show that short-term exposure to bilirubin exerts a tissue-specific effect on the vasopressin-stimulated active transport of sodium but has no effect on the vasopressin-induced fluxes of water and urea.

Effect of HEPES buffer on corneal storage in MK medium

Rabbit corneas were stored for 7 days in either MK medium containing gentamicin or modified MK medium containing HEPES buffer, gentamicin and phenol red. Corneas stored for 7 days in modified MK medium were thicker than corneas stored in MK medium. Corneal endothelial permeability to inulin and dextran was similar following 7 days of storage in either solution. Transmission electron microscopy of corneal endothelial cells stored in either solution showed intact cell membranes and organelles. In vitro perfusion of rabbit corneas in the specular microscope with Krebs Ringer bicarbonate containing HEPES buffer swelled at 17 +/- 1 micron/h, whereas those perfused with Krebs Ringer bicarbonate alone swelled at 7 +/- micron/h. Perfusion with Krebs Ringer bicarbonate containing phenol red did not result in an increased corneal swelling rate. The work indicates that HEPES buffer has an adverse effect on corneal endothelial pumping function, and this results in corneal swelling during storage as well as during perfusion in the specular microscope. The adverse effect appears to be, at least in part, transient: however, the ultimate, long term effect of HEPES buffer on corneas stored prior to penetrating keratoplasty is not known and deserves continued investigation.

Radial keratotomy and corneal permeability in Owl Monkey

Radial keratotomy was performed on Owl Monkey corneas using 8 incisions with sparing of a 3.5mm central pupillary area. Cornea endothelial membrane permeabilities were determined at 2 days, and 4 weeks, following the procedure using simultaneous flux determinations of 3H-labeled inulin and 14C-labelled dextran. Inulin permeability was increased 27% two days following the procedure, and had returned to levels comparable to the unoperated eye 4 weeks following the procedure. Dextran permeability was unaltered at both 2 days, and 4 weeks, following the procedure. This study has shown that radial keratotomy causes a transient reduction in endothelial barrier function with the production of physiologically significant 'holes' in the membrane in the immediate post-operative period. The relationship of this physiological alteration to ultimate endothelial cell function is, at the present time, unknown.

Serosal and mucosal facilitated transport of urea in urinary bladder of of Bufo bufo: evidence for an alleged water uptake

In Bufo bufo urinary bladder an urea facilitated transport has been localised on the luminal membrane. The transport fulfils the criteria for such a mechanism, i.e. is saturable and is inhibited by phloretin, a specific inhibitor for urea transport. Similarly to that of Bufo marinus and Rana esculenta the luminal membrane of Bufo bufo urinary bladder shows an ADH stimulated facilitated transport. Experiments wtih Amphotericin B, serosal phloretin (with and without ADH), have demonstrated the presence of a facilitated urea transport localised on basolateral membrane. Urea uptake on the isolated epithelial cells of Bufo bufo urinary bladder shows a characteristic feature, different from molecules passively transported such as glycerol yet inhibited by phloretin. Allegedly with urea, water flows in to the cells by a dragging or osmotic effect.

Phloretin sensitive active urea absorption in frog skin

This report presents evidence for urea active absorption by isolated skin of Rana esculenta. One of the supporting factors of such evidence is that at a low concentration the urea influx is five times greater than the outflux, in the absence of a chemical gradient. The transport shows a saturation kinetics with an apparent Km = 1.33 mM and is inhibited by un uncoupling agent (FCCP). 5 x 10(-4) M Phloretin, added to the external side, markedly inhibits inward urea transport, whereas it is ineffective when added to the serosal fluid. This provides evidence for a phloretin-sensitive mechanism located at the external side of the epithelium. Phloretin stimulates the sodium active transport; the possible coupling of urea and sodium movement is analysed.

The nature of urea transport across the luminal membrane of Bufo bufo urinary bladder

By using the washing-out technique, counterflow acceleration for urea was demonstrated on the luminal membrane of Bufo bufo urinary bladder, in the absence of ADH. This phenomenon completely disappears in the presence of phloretin 10-4 M on the luminal side and is consistent with the presence of a mobile carrier mechanism for urea transport across the luminal membrane, in basal conditions. In the presence of ADH, counterflow acceleration is completely absent. This result is in agreement with the presence of urea selective channels, induced by ADH, as proposed by Levine & Worthington (1976).

Modulation of vasopressin action by reducing agents in Bufo marinus

The effects of the reducing agent dithiothreitol (DTT) on vasopressin (AVP)-stimulated osmotic water flow and adenylate cyclase activity were studied in the urinary bladder of Bufo marinus. DTT produced concentration-dependent inhibition of the hydroosmotic water permeability response to 10 mU/ml AVP and 10 mM theophylline but did not inhibit the response to 10 mM adenosine 3',5'-cyclic monophosphate (cAMP). The inhibitory effects of DTT on AVP responsiveness were partially reversed by washing in DTT-free Ringer solution or by addition of oxidizing agents such as dehydroascorbic acid (DHA) or H2O2. The inhibitory effects of DTT were completely reversed by washing in DTT-free Ringer plus addition of DHA. In addition, the inhibitory effects of DTT on AVP-induced osmotic water flow were partially reversed by the GTP analogue 5'-guanylyl imidodiphosphate [Gpp(NH)p]. DTT also inhibited the adenylate cyclase response to AVP but did not alter the response to AVP plus Gpp(NH)p or the response to NaF. These observations suggest that the inhibitory effect of thiol compounds on AVP responsiveness may be modulated through alterations of a redox system distal to the hormone receptor but proximal to the catalytic subunit of adenylate cyclase. Inasmuch as Gpp(NH)p partially reversed the inhibitory effects of DTT on AVP-stimulated osmotic water permeability and prevented the inhibitory effect of DTT on AVP-stimulated adenylate cyclase, an effect on either GTPase or binding of GTP to the regulatory protein of adenylate cyclase is suggested by these observations.

Role of the endogenous kallikrein-kinin system in modulating vasopressin-stimulated water flow and urea permeability in the toad urinary bladder

This study investigates the endogenous kallikrein-kinin system's role as a modulator of vasopressin action in the toad urinary bladder. Kalli-krein inhibition by aprotinin, which results in decreased kinin production, significantly increased both vasopressin and 8-Br-cyclic (c) AMP-stimulated water flow. Kinin potentiation by the kininase II inhibitor captopril (SQ 14225) significantly decreased vasopressin and 8-Br-cAMP-stimulated water flow. In contrast to water flow, vasopressin-stimulated urea permeability was decreased by aprotinin and increased by captopril. We conclude that the endogenous kallikrein-kinin system represents a significant modulator of vasopressin action and it permits separate control of vasopressin-stimulated water flow and solute transport.

Inhibition of ADH-enhanced transepithelial urea and water movement by prostaglandins

The effects of PGF2 alpha and PGE2 on transepithelial urea flux and osmotic water flow were evaluated in total bladders. Mucosal to serosal urea flux and osmotic water flow were not changed from basal values by the addition of either prostaglandin to the serosal bath. However, treatment with either PGF2 inhibited both urea flux and osmotic water flow in response to ADH stimulation in a concentration-dependent manner. The hydrosmotic response to ADH was more sensitive to prostaglandin inhibition than was urea flux. The inhibitory effect of the prostaglandins on ADH-enhanced urea flux was not dependent upon inhibition of the hydrosmotic response, since both PGF2 alpha and PGE2 decreased urea flux in the absence of a transepithelial osmotic gradient. Prostaglandin E2 was a more potent inhibitor than PGF2 alpha of both ADH-enhanced urea flux and osmotic water flow. The PGF2 alpha antagonism of osmotic water flow was apparently competitive, while antagonism of urea flux was apparently noncompetitive. The results are consistent with the hypothesis of the existence of a "spare" population of prostaglandin receptors that modulate water flow, but the absence of a "spare" prostaglandin receptor population with respect to the modulation of urea flux.

Colchicine effect on the permeability of the whole epithelium and of isolated cells of frog skin

The effect of 2 X 10(-5) M colchicine on epithelial cells isolated from frog skins was investigated. Three hours of treatment with colchicine did not change either Na+ and K+ content of isolated cells or nonelectrolyte permeability. When ADH (50 mU/ml) was added, thiourea uptake values became greater than without the hormone; the same values were found in the cells previously treated with colchicine. Na+ transepithelial transport, measured by means of short-circuit current, was inhibited by the antimitotic agent both under control conditions and after ADH stimulation. These results support the view that colchicine does not directly affect ADH action on membrane permeability, but influences some mechanism that controls ADH action on transepithelial transport. Intercellular junctions appear to be the location of such a mechanism.

pH-Dependence of water and solute transport in toad urinary bladder

Stimulation of urea and water transport by vasopressin (ADH) appears to occur via independent pathways. We examined the effects of altering serosal or mucosal bath pH on transport of water, urea, and sodium. Compared to bladders with a serosal bath pH of 7.4 to 8.0, reducing the serosal bath pH to 6.8 led to a 60% fall in ADH-stimulated osmotic water flow, without decreasing the permeability of urea. Raising the serosal pH to 9.5 had the opposite effect: urea permeability was inhibited by 40% without altering water flow. Exogenous cyclic AMP-stimulated water and urea permeabilities were not dissociated, but were changed in the same direction by alterations in serosal pH: serosal acidification enhanced the effect of exogenous cyclic AMP on both urea and water, whereas the cyclic AMP effect on both was diminished by serosal alkalinization. This was especially marked for urea, suggesting that an alteration in the urea response to cyclic AMP may be particularly important in defining vasopressin-stimulated urea permeability as the serosal bath pH is altered. Mucosal acidification increased short circuit current but decreased both the urea and water response to ADH and 8-bromo-cyclic AMP. The response to cyclic AMP was less consistent. Mucosal alkalinization did not cause significant changes in either basal or stimulated transport. The data demonstrate distinct and separable effects of bath pH alterations on each of the transport systems examined.

Evidence for involvement of microtubules in the action of vasopressin in toad urinary bladder. I. Functional studies on the effects of antimitotic agents on the response to vasopressin

The antimitotic agents colchicine, podophyllotoxin, and vinblastine inhibit the action of vasopressin and cyclic AMP on osmotic water movement in the toad urinary bladder. The alkaloids have no effect on either basal or vasopressin-stimulated sodium transport or urea flux across the tissue. Inhibition of vasopressin-induced water movement is half-maximal at the following alkaloid concentrations: colchicine, 1.8 X 10(-6) M; podophyllotoxin, 5 X 10(-7)M; and vinblastine, 1 X 10(-7)M. The characteristics of the specificity, time-dependence and temperature-dependence of the inhibitory effect of colchicine are similar to the characteristics of the interaction of this drug with tubulin in vitro, and they differ from those of its effect on nucleoside transport. Inhibition of the vasopressin response by colchicine, podophyllotoxin, and vinblastine is not readily reversed. The findings support the view that the inhibition of vasopressin-induced water movement by the antimitotic agents is due to the interaction of these agents with tubulin and consequent interference with microtubule integrity and function. Taken together with the results of biochemical and morphological studies, the findings provide evidence that cytoplasmic microtubules play a critical role in the action of vasopressin on transcellular water movement in the toad bladder.

Urea uptake and translocation in toad urinary bladder: the effect of antidiuretic hormone

The uptake of C14-urea into everted and noneverted bladder sacs was compared, over short time periods (up to 2 min), with the transepithelial urea fluxes. This method allowed the study of the time course of urea uptake and distribution, while previously this problem was only studied in steady-state conditions. When mucosal uptake was studied no accumulation of C14-urea inside the tissue was observed, indicating that the mucosal border could be the limiting step. Comparative studies of urea and inulin uptake from the serosal side showed that urea equilibrated with the water epithelial cells in less than 30 sec. This accumulation suggested again that the mucosal border is an effective barrier for urea translocation. The kinetics of the increase in urea permeability induced by antidiuretic hormone was also studied and it was similar (T1/2:4.3 min) to the kinetics of the increase in water permeability induced by the hormone (T1/2:5.6 min). A strong parallelism was also observed between the time course of the increases in water and urea permeabilities induced by medium hypertonicity (T1/2 25 and 26 min, respectively). The values obtained for the permeability coefficient ktrans), either at rest or under ADH were similar to those previously reported employing steady-state techniques (28+/-8 and 432+/-25 cm-sec-1-10(-7), respectively).