The cellular specificity of the effect of vasopressin on toad urinary bladder

Summary Phase and electron micrographs of toad bladders were obtained following dilution of bathing media in the presence and absence of vasopressin. Dilution of the mucosal medium alone resulted in no morphologic changes. Subsequent addition of vasopressin produced an increase in the cell volume of the granular cells, manifested by some or all of the following changes: increased area of granular cell profiles as observed in sections, rounding of the cell nucleus, displacement of the two components of the nuclear envelope, loss of nuclear heterochromatin, sacculation of the endoplasmic reticulum and the Golgi apparatus, and reduction in the electron density of the cell cytoplasm. No such morphologic changes were noted in the other cell types comprising the mucosal epithelium — the mitochondria-rich, the goblet, and the basal cells. On the other hand, dilution of the serosal bathing medium in the absence of vasopressin caused a marked increase in the cell volume of all these cell types. The results demonstrate that the action of vasopressin to enhance bulk water flow across toad bladder is exerted specifically on the apical surface of the granular cells. It is suggested that the hormonal effect on sodium transport may also be limited to the granular cells. The route of osmotic water flow and the possible role of the other mucosal epithelial cells is discussed.     

Effect of monensin on osmotic water flow across the toad bladder and its stimulation by vasopressin and cyclic AMP

The effects of the sodium ionophore monensin on osmotic water flow across the urinary bladder of the toad Bufo marinus were studied. Monensin alone did not alter osmotic water flow; however, the ionophore inhibited the hydrosmotic response to vasopressin and cyclic AMP in a dose-dependent manner. The inhibitory effects of monensin were apparent when the ionophore was added to th serosal bathing solution but not when it was added to the mucosal bathing solution. The inhibitory effect of serosal monensin required the presence of sodium in the serosal bathing solution but not the presence of calcium in the bathing solutions. Thus, it appears that intracellular sodium concentration is a regulator of the magnitude of the hydrosmotic response to vasopressin and cyclic AMP.     

Metabolic evidence that serosal sodium does not recycle through the active transepithelial transport pathway of toad bladder

Summary The possibility that sodium from the serosal bathing medium back-diffuses into the active sodium transport pool within the mucosal epithelial cell of the isolated toad bladder was examined by determining the effect on the metabolism of the tissue of removing sodium from the serosal medium. It was expected that if recycling of serosal sodium did occur through the active transepithelial transport pathway of the isolated toad bladder, removal of sodium from the serosal medium would reduce the rate of CO₂ production by the tissue and enhance the stoichiometric ratio of sodium ions transported across the bladder per molecule of sodium transport dependent CO₂ produced simultaneously by the bladder (J _Na/J _{CO 2}). The data revealed no significant change in this ratio (17.19 with serosal sodium and 16.13 after replacing serosal sodium with choline). Further, when transepithelial sodium transport was inhibited (a) by adding amiloride to the mucosal medium, or (b) by removing sodium from the mucosal medium, subsequent removal of sodium from the serosal medium, or (c) addition of ouabain failed to depress the basal rate of CO₂ production by the bladder [(a) rate of basal, nontransport related, CO₂ production (J _CO2 ^b ) equals 1.54±0.52 with serosal sodium and 1.54±0.37 without serosal sodium; (b)J _CO2 ^b equals 2.18±0.21 with serosal sodium and 2.09±0.21 without serosal sodium; (c) 1.14±0.26 without ouabain and 1.13±0.25 with ouabain; unite ofJ _CO2 ^b are nmoles mg d.w.^–1 min^–1]. The results support the hypothesis that little, if any, recycling of serosal sodium occurs in the toad bladder.     

The Site of the Stimulatory Action of Vasopressin on Sodium Transport in Toad Bladder

Vasopressin increases the net transport of sodium across the isolated urinary bladder of the toad by increasing the mobility of sodium ion within the tissue. This change is reflected in a decreased DC resistance of the bladder; identification of the permeability barrier which is affected localizes the site of action of vasopressin on sodium transport. Cells of the epithelial layer were impaled from the mucosal side with glass micropipettes while current pulses were passed through the bladder. The resulting voltage deflections across the bladder and between the micropipette and mucosal reference solution were proportional to the resistance across the entire bladder and across the mucosal or apical permeability barrier, respectively. The position of the exploring micropipette was not changed and vasopressin was added to the serosal medium. In 10 successful impalements, the apical permeability barrier contributed 54% of the initial total transbladder resistance, but 98% of the total resistance change following vasopressin occurred at this site. This finding provides direct evidence that vasopressin acts to increase ionic mobility selectively across the apical permeability barrier of the transporting cells of the toad bladder.     

Induction of reverse flow of Na+ through the active transport pathway in toad urinary bladder

When active Na⁺ transport across the toad urinary bladder was abolished by ouabain, a ’reversed‘ short circuit current could be induced by an Na⁺ concentration gradient. This reversed current was increased by vasopressin and inhibited by amiloride and appears to represent net Na⁺ movement ‘backwards’ through epithelial cells which normally participate in active Na⁺ transport across the bladder.     

On the effects of propionate and other short-chain fatty acids on sodium transport by the toad bladder.

1. Propionate and other unbranched short-chain fatty acids, butyrate, pentanoate, hexanoate and octanoate were found to both stimulate and inhibit active sodium transport by the toad bladder, as measured by the short-circuit current (s.c.c.). 2. Stimulation alone followed addition of low concentrations of fatty acids (0.1-1.0 mM) to either the serosal or mucosal bathing medium; stimulation was also seen after an initial period of inhibition in response to higher concentrations (approx. 5 mM) of some compounds. 3. Inhibition alone followed addition of high concentrations (5-20 mM) of these compounds. The duration and magnitude of the inhibition varied with increasing concentration and chain length of the fatty acid, and was greater following mucosal addition than serosal addition. 4. The inhibitory effect of mucosal propionate increased with decreasing pH of the mucosal bathing medium. 5. Inhibition by the fatty acids was completely reversed upon removing the compound from the bathing medium, and stimulation characteristically followed. 6. In studies designed to evaluate the role of metabolism of the fatty acids in their mucosal inhibitory effects it was found that 14-c-labelled propionate, when added to the mucosal surface of the bladder, was converted to 14-CO2, and mucosal succinate and alpha-oxoglutaric acid at 20 mM inhibited the s.c.c. slightly. However, malonate did not interfere with inhibition by mucosal propionate and two non-metabolizable acids, dimethylpropionate and benzoate, induced inhibition (and no stimulation) of the s.c.c. 7. In the presence of an inhibitory concentration of fatty acid, the ability of the bladder to respond to added pyruvate was reduced in proportion to the reduction in the level of the s.c.c., whereas the natriferic response to vasopressin was largely intact. 8. We conclude that stimulation of sodium transport by propionate and other short-chain fatty acids is due to metabolism of the compounds and provision of energy to the sodium transport mechanism. The basis of the inhibition appears complex. It may in part depend on metabolism of the fatty acids and/or uncoupling of oxidative phosphorylation, with resultant reduction in net ATP production for the sodium transport mechanism. However, the inhibition may also be caused in part by a direct effect on the mucosal entry of sodium into the transporting epithelial cells.     

Control of urea transport across toad urinary bladder by vasopressin: Effect of periodate oxidation of the mucosal cell surface

The Journal of Membrane Biology - Oxidation of toad urinary bladder epithelial cell membranes by periodate in the bathing medium altered vasopressin-stimulated transport of urea or of water and...     

Water flow in the toad urinary bladder in response to vasopressin: role of potassium

In agreement with previous reports, we found that absence of K+ from the serosal bath of the toad urinary bladder substantially impairs vasopressin and cAMP-stimulated water flow. The decreased response to vasopressin appears unrelated to prostaglandin production since inhibition of endogenous prostaglandins by pretreatment with naproxen 10(-5) M failed to prevent the effect seen with K+-free Ringer's. The resistance to vasopressin does not appear to be directly related to epithelial K+ concentrations, in that maneuvers leading to decreased intracellular K+ failed to produce a similar effect. A more likely explanation appears to be that K+-free Ringer's induces an increased cytosolic Ca++ which, in turn, decreases the hydrosmotic effects of vasopressin. Several lines of evidence argue in favor of such an explanation: (a) Increased cytosolic Ca++ had been found in other tissues with low extracellular K+; (b) The resistance to vasopressin decreases with decreased serosal Ca++; (c) The effects of K+-free Ringer's are not additive in situations believed to have increased epithelial Ca++, i.e. replacement of serosal Na+ with choline; (d) The effects of K+-free serosal bathing medium could be both prevented and/or reversed if already established by increasing serosal bath, and presumably intracellular, pH, which is believed to decrease intracellular Ca++.     

Regulation of membrane permeability by vasopressin; activation of the water permeability pathway in toad urinary bladder by N-ethyl-maleimide

1. Vasopressin induces a rapid increase in water permeability and stimulates net sodium transport in responsive epithelia through the mediation of cAMP. 2. In amphibian urinary bladder, the increase in water permeability is dependent on an intact cytoskeleton and is associated with the exocytotic insertion of tubular vesicles containing particle aggregates (the putative water channels) into the apical membrane of the granular epithelial cells. 3. In the toad bladder, mucosal addition of NEM, 0.1 mM, elicits a slow and irreversible increase in transepithelial water flow, whilst decreasing net sodium transport. 4. The hydrosmotic response to mucosal NEM is inhibited by cellular acidification, by pretreatment with cytoskeleton-disruptive drugs, and by agents that increase cytosolic calcium. 5. Mucosal NEM potentiates the hydrosmotic response to a submaximal, but not a maximal, dose of vasopressin. 6. Mucosal NEM, like vasopressin, induces both vesicle fusion and the appearance of particle aggregates at the granular cell apical surface. 7. NEM, unlike vasopressin, does not increase cellular cAMP content. 8. Mucosal NEM appears to increase transcellular water flow by activating cellular processes normally triggered by vasopressin, at a step beyond cAMP.     

Effect of sodium transport inhibition on active phosphate transport by toad bladder

Summary The relationship between active Na transport (estimated by the short-circuit (SCC)) and active inorganic phosphate (P_i) transport was studied in the toad bladder. When SCC was inhibited by amiloride, ouabaim, or removal of K from the serosal bathing solution, active P_i transport was totally inhibited. When Na was replaced isotonically by choline in either the mucosal bathing solution or both the mucosal and serosal bathing solutions, there was no measurable SCC or active P_i transport. These experiments are compatible with the hypothesis that active P_i transport occurs only in the presence of active Na transport.     

The Electrical Characteristics of Active Sodium Transport in the Toad Bladder

The mechanism responsible for active sodium transport in the urinary bladder of the toad appears to be located at the serosal boundary of the epithelial cell layer of the bladder. Studies of the potential step observed at the serosal boundary in the open-circuited state were undertaken in an attempt to define the factors responsible for its production. Glass micropipettes were used to measure the serosal potential step in bladders exposed on the serosal side to solutions of high potassium or of high potassium and low chloride concentration. Observed potentials exceed the maximum values which would have been expected if the serosal potential step were a potassium or chloride diffusion potential. Measurements of net cation flux exclude the possibility of a diffusion potential at this border due to the passive movement of any anionic species. The observed independence of transbladder potential and short-circuit current from the pH of the serosal medium over a wide range of pH makes it unlikely that the observed serosal potential step is a hydrogen ion diffusion potential. We conclude that the active sodium transport mechanism in toad bladder is "electrogenic."     

Effect of quinidine on Na,H+, and water transport by the turtle and toad bladders

Summary The effect of quinidine on Na and H⁺ transport by the turtle bladder and water transport by the toad bladder was examined. Quinidine inhibited the short-circuit current and the potential difference in a dose-dependent fashion. The effect of quinidine on the short-circuit was not dependent on extracellular calcium concentration and was not reversible with removal of the drug. Quinidine inhibited H⁺ secretion in a dose-dependent fashion. The effect of quinidine on H⁺ secretion also was not dependent on extracellular calcium concentration and was not reversible, either with removal of the drug or with stimulation of H⁺ secretion with 5% CO₂. The effect of quinidine on Na or H⁺ transport could not be elicited by an equivalent dose of tetracaine, suggesting that the inhibitory effect of quinidine is not dependent on its anesthetic properties. Quinidine also inhibited vasopressin and cyclic AMP stimulated water flow in the toad bladder. Quinidine did not alter calcium uptake by the turtle bladder but increased calcium efflux by the turtle and toad bladders. These observations suggest that a rise in cytosolic calcium is responsible for the inhibitory effect of quinidine on Na, H⁺, and water transport.     

Alterations in extracellular calcium concentration differentially influence prostaglandin and thromboxane synthesis in epithelial cells from the toad urinary bladder

The effects of alterations in extracellular calcium concentration on prostaglandin (PGE) and thromboxane (TXB₂) syntheses were studied in isolated epithelial cells from the urinary bladder of the toad, Bufo marinus. In epithelial cells prepared using collagenase, basal iPGE synthesis was greater than iTXB₂ synthesis. Increasing extracellular calcium from zero to 1 mm increased iPGE synthesis and decreased iTXB₂ synthesis equivalently such that total conversion of endogenous arachidonate to these two metabolites was unaltered. Vasopressin stimulated iPGE and iTXB₂ syntheses when the incubation buffer contained 1 mm calcium but had no effect in the presence of 0.4 μm calcium. In contrast, using an EDTA isolation method, basal iPGE and iTXB₂ syntheses were equal in the presence of zero calcium. Increasing extracellular calcium concentration to 1 mm caused a greater enhancement in iTXB₂ synthesis compared to iPGE. Increasing extracellular calcium to 2 mm was associated with a decline in iPGE and iTXB₂ syntheses back to the levels observed with no calcium added to the medium. The effect of increasing the calcium concentration was greater in phosphate than in bicarbonate buffer. In a Tris buffer the effect of altered calcium was almost completely abrogated. These studies demonstrate that the choice of buffer and alterations in extracellular calcium concentration differentially alter basal arachidonic acid metabolism to prostaglandins and thromboxane in isolated toad urinary bladder cells. The results suggest that there may exist several endogenous pools of arachidonic acid which are differentially influenced by calcium. Furthermore, the pool sensitive to vasopressin has an absolute requirement for calcium.     

Effect of monensin on osmotic water flow across the toad bladder and its stimulation by vasopressin and cyclic AMP

Summary The effects of the sodium ionophore monensin on osmotic water flow across the urinary bladder of the toadBufo marinus were studied. Monensin alone did not alter osmotic water flow; however, the ionophore inhibited the hydrosmotic response to vasopressin and cyclic AMP in a dose-dependent manner. The inhibitory effects of monensin were apparent when the ionophore was added to the serosal bathing solution but not when it was added to the mucosal bathing solution. The inhibitory effect of serosal monensin required the presence of sodium in the serosal bathing solution but not the presence of calcium in the bathing solutions. Thus, it appears that intracellular sodium concentration is a regulator of the magnitude of the hydrosmotic response to vasopressin and cyclic AMP.     

EFFECTS OF CYTOCHALASIN B ON THE RESPONSE OF TOAD URINARY BLADDER TO VASOPRESSIN

A combined physiological and morphological study of the effects of cytochalasin B (CB) on the toad urinary bladder has been carried out. CB inhibits the hydro-osmotic response to vasopressin without altering basal water permeability or diffusion, or the increase in ³H₂O diffusion observed after hormone addition. Although CB increases [²²Na]-, [³⁶Cl]-, and [¹⁴C]urea fluxes, and decreases transepithelial potential, no alteration in basal short-circuit current, the vasopressin-induced increase in this parameter, or [¹⁴C]inulin permeability occurs. In the absence of hormone, CB does not markedly alter the structure of the toad bladder. However, in the presence of vasopressin, CB induces the formation of large intracellular vacuoles. These results suggest a possible coupling of solute and water movement across the tissue.     

The coupling coefficient as an index of junctional conductance

Summary Toad bladder epithelial cells were isolated under mild conditions in a calcium-free medium; they were found to exclude trypan blue, to consume oxygen, and to respond to vasopressin with an increased rate of oxygen consumption. Since isolated toad bladder epithelial cells are mostly spherical in shape, the cell diameter can be accurately measured with an ocular micrometer of an inverted microscope. Epithelial cells swelled by 29±3% in the presence of KCN. This cyanide-induced swelling of cells was prevented by amiloride or, alternatively, by replacing NaCl by equiosmotic amounts of mannitol in the Ringer's fluid. Cells incubated in the presence of vasopressin swelled by 10±2%. Vasopressin and KCN acted synergistically in enhancing cell volume. Ouabain caused cells to swell by 9±2%, and this effect was not additive to the swelling seen with vasopressin. These observations are in accord with the theory of Leaf and his associates, that the predominant effect of vasopressin is to enhance sodium entry into the transporting epithelial cells of the toad urinary bladder.     

Interrelationships of sodium transport and carbon dioxide production by the toad bladder: response to changes in mucosal sodium concentration, to vasopressin and to availability of metabolic substrate.

Active sodium transport and CO2 production were measured simultaneously in toad bladders mounted in membrane chambers. The rate of sodium transport was varied by changing the concentration of sodium in the mucosal bath (substitution with choline), by adding vasopressin, by adding metabolic substrates and by adding malonate, and the ratio of the change of sodium transport and CO2 production was determined Mean values for deltaNa/deltaCO2 (equiv/mole) were: Na in equilibrium choline 18.3 +/- 1.1; vasopressin 15.5 +/- 2.8; and pyruvate (corrected for the increment in "nontransport" CO2) 15.4 +/- 3.5. Based on previously determined values for the respiratory quotient (R.Q.), calculated mean values for deltaNa/deltaO2 ranged between 15.5 and 18.5 equiv/mole. It appears that basal metabolism does not contribute to metabolism supporting sodium transport when the rate of sodium transport is varied. "Transport" metabolism appears much more responsive to changes in the availability of endogenous and exogenous substrates than does "nontransport" metabolism. We conclude that "transport" and "nontransport" metabolism are functionally separated in the toad bladder.     

Active sodium transport by the isolated toad bladder

Studies were made of the active ion transport by the isolated urinary bladder of the European toad, Bufo bufo, and the large American toad, Bufo marinus. The urinary bladder of the toad is a thin membrane consisting of a single layer of mucosal cells supported on a small amount of connective tissue. The bladder exhibits a characteristic transmembrane potential with the serosal surface electrically positive to the mucosal surface. Active sodium transport was demonstrated by the isolated bladder under both aerobic and anaerobic conditions. Aerobically the mean net sodium flux across the bladder wall measured with radioactive isotopes, Na²⁴ and Na²², just equalled the simultaneous short-circuit current in 42 periods each of 1 hour's duration. The electrical phenomenon exhibited by the isolated membrane was thus quantitatively accounted for solely by active transport of sodium. Anaerobically the mean net sodium flux was found to be slightly less than the short-circuit current in 21 periods of observation. The cause of this discrepancy is not known. The short-circuit current of the isolated toad bladder was regularly stimulated with pure oxytocin and vasopressin when applied to the serosal surface under aerobic and anaerobic conditions. Adrenaline failed to stimulate the short-circuit current of the toad bladder.     

Evaluation of the rate of basal oxygen consumption in the isolated frog skin and toad bladder.

In the study of active transport it is important to distinguish between oxygen consumption sustaining transepithelial transport and that responsible for other tissue functions (basal metabolism). Since amiloride blocks transepithelial active sodium transport and the associated oxygen consumption in the frog skin and toad bladder, we and others have employed this agent to evaluate the rate of basal metabolism. This technique has recently been criticized in a report that amiloride (and ouabain) increased oxygen consumption when no sodium was available for transport. We have been unable to corroborate these observations. With magnesium-Ringer as external bathing solutions, amiloride and ouabain failed to stimulate oxygen consumption. With sodium-Ringer as external bathing solution amiloride reduced oxygen consumption about 30%, to a level indistinguishable from that found on external substitution of magnesium-Ringer for sodium-Ringer. We conclude that the use of amiloride permits evaluation of the rate of basal metabolism with acceptable accuracy; a possible slight depressant effect of ouabain on basal metabolism remains to be investigated.     

Evaluation of the rate of basal oxygen consumption in the isolated frog skin and toad bladder

In the study of active transport it is important to distinguish between oxygen consumption sustaining transepithelial transport and that responsible for other tissue functions (basal metabolism). Since amiloride blocks transepithelial active sodium transport and the associated oxygen consumption in the frog skin and toad bladder, we and others have employed this agent to evaluate the rate of basal metabolism. This technique has recently been criticized in a report that amiloride (and ouabain) increased oxygen consumption when no sodium was available for transport. We have been unable to corroborate these observations.With magnesium-Ringer as external bathing solutions, amiloride and ouabain failed to stimulate oxygen consumption. With sodium-Ringer as external bathing solution amiloride reduced oxygen consumption about 30%, to a level indistinguishable from that found on external substitution of magnesium-Ringer for sodium-Ringer. We conclude that the use of amiloride permits evaluation of the rate of basal metabolism with acceptable accuracy; a possible slight depressant effect of ouabain on basal metabolism remains to be investigated.