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

The effect of metal ions and antidiuretic hormone on oxygen consumption in toad bladder

1. The sodium-dependent oxygen consumption of pieces of toad bladder (Bufo marinus) has been investigated using an oxygen electrode.2. The effect of polyvalent cations (Ca(2+), Sr(2+), Mg(2+), Eu(3+), La(3+) and Mn(2+)) on sodium-dependent oxygen consumption has been measured. All cations inhibited oxygen consumption, the order of effectiveness being Ca(2+) > Sr(2+) > Mg(2+) > Mn(2+) > Eu(3+) > La(3+).3. Treatment of bladder pieces with antidiuretic hormone (50 m-u./ml.) decreased the effectiveness of Ca(2+) and Sr(2+) as inhibitors of sodium-dependent oxygen consumption. Mn(2+), Eu(2+) and La(2+) were more effective after hormonal treatment, while the effectiveness of Mg(2+) was unaltered.4. The results have been interpreted in terms of a model in which sodium entry to the transporting mechanisms of the epithelium is controlled by Ca(2+), and in which antidiuretic hormone alters Ca(2+) binding and so affects sodium transport.

The kinetics of sodium transport in the toad bladder. I. Determination of the transport pool

A compartmental model of toad bladder sodium content has been developed, whereby it is possible to measure the four unidirectional fluxes across the opposite faces of the transport compartment, as well as the amount of sodium in the compartment. (24)Na is added to the mucosal medium of a short-circuited bladder mounted between halves of a chamber in which the fluid is stirred by rotating impellers. After a steady state is reached, nonradioactive medium is flushed through both sides of the chamber, collected, and counted. The data from each chamber are fitted to sums of exponentials and interpreted in terms of conventional compartmental analysis. Three exponentials are required, with half-times of 0.2, 2.2, and 14.0 min. It is shown that the first of these represents chamber washout, the second the transport pool, and the third a tissue compartment which is not involved in active sodium transport and which does not communicate with the transport pool. The second compartment contains 10.5 microEq of sodium per 100 mg dry weight, an amount equal to approximately 30% of total tissue sodium. The results also indicate, as expected from electrophysiological data, that the mucosal-facing side of the transport compartment is over 10 times as permeable to sodium as the serosal, or pump, side.

The kinetics of sodium transport in the toad bladder. II. Dual effects of vasopressin

In the accompanying paper, a compartmental model for the toad bladder sodium transport system was developed. In the present paper, the model is tested by determining the effects of antidiuretic hormone on the pools and fluxes. It is shown that this hormone affects only that sodium pool previously designated as the transport pool, and that the effects are on two separate sites. In the first place, the hormone stimulates entry at the mucosal side of the transport compartment, and by this means brings about an increase in the amount of sodium contained in the compartment. Second, the hormone has a distinct stimulatory effect on the rate coefficient for efflux across the serosal boundary, the pump rate coefficient. Evidence is presented that under control conditions, the pump rate coefficient is a decreasing function of the pool size, a characteristic feature of a saturating system. Therefore, the effect of vasopressin in increasing both the pool size and the pump rate coefficient must be construed as a direct effect on the pump, and not one which is secondary to the increase in the pool size. Furthermore, it is shown that the effect of the hormone on the sodium pump is not dependent on the presence of sodium in the serosal medium.

The effect of antidiuretic hormone on Na movement across frog skin

1. The effect of antidiuretic hormone (ADH) on the movement and distribution of Na was studied. This was done using three different approaches: (a) the measurement of Na and (22)Na in slices of epithelium of skins which were exposed to Ringer of varied composition containing (22)Na, (b) the measurement of the influx of Na from the outer to the inner bathing solution with (22)Na added to the outside, and (c) the use of a recently introduced technique which permits the direct evaluation of the flux from the outer solution --> epithelium, (J(OT)), i.e. the flux across the barrier which is generally regarded as the site of ADH activity.2. ADH increased the influx from the outer to the inner bathing solution of Na (50%) not only when the concentration of Na on the outside was 115 mM (i.e. higher than in the epithelium) but even when the concentration was 1 mM (67%).3. When the skin was bathed with 1mM-Na Ringer on the outside, ADH increased the unidirectional Na flux J(OT) by 56% (Rana pipiens) and 71% (Leptodactylus ocellatus). When the concentration was 115 mM a small increase (17%) was observed in paired skins of R. pipiens. Under this condition no change was observed in L. ocellatus.4. The amount of epithelial sodium which is labelled by (22)Na added to the outside was taken to reflect the amount of Na involved in Na transport across the epithelium. Depending on whether the concentration of Na on the outside was high (115 mM) or low (1 mM), ADH produced an increase, or a decrease, of both the total Na content and the amount of (22)Na exchanged.5. When the concentration of Na on the outside was low, ADH increased the total influx and J(OT) in spite of the fact that it lowers the total Na content and does not affect the exchangeable pool of Na. This observation is inconsistent with the view that the effect of ADH is due to the fact that the increased permeability of the outer barrier allows more Na into the cell, and that the resulting increase of Na concentration in the cytoplasm accelerates the Na pumps at the inner side of the cells.6. It is concluded that ADH speeds up Na movements at the outward facing barrier, and that this exchange which facilitates the penetration of Na into a transporting compartment produces also a gain or a loss of Na in compartments not directly involved in Na transport across the epithelium. One compartment which is not involved in Na transport might be the cytoplasm of the epithelial cells.

Sodium transport across the isolated epithelium of the frog skin

1. A method to separate the epithelium from the underlying layers of the frog skin is described. The method is based on the combined use of collagenase and hydrostatic pressures.2. The potential difference and the short-circuit current values of isolated epithelia and whole skins are similar. Na net flux and short-circuit current are equivalent.3. The time course of changes in potential following rapid changes in composition of the bathing solutions shows that the barrier to K diffusion at the internal surface of the isolated epithelium is larger than the barrier to Na diffusion at the external surface.4. In the isolated epithelium there are 133 m-mole K(+) and 24.7 m-mole Na/l. cellular water. The amount of extracellular water was considered to be equal to the inulin space.5. Arginine vasopressin (0.1 u./ml.) markedly increased short-circuit current and potential difference in isolated epithelia. The amount of Na in the epithelium that equilibrated with Na in the external solution was not increased by the hormone.6. Ouabain (10(-4)M) reduced short circuit current and potential difference to values close to zero. The ouabain treated epithelia contained an increased amount of Na originating in the internal solution. On the other hand the amount of Na that originated from the external solution was not increased.7. The amount of epithelial Na that equilibrated with Na in the external solution was 0.009 mu-equiv/cm(2). This figure is about ten times smaller than the values found in whole skins.

Direct measurement of uptake of sodium at the outer surface of the frog skin

A combination of the methods described by Schultz et al. (6) and by Ussing and Zerahn (9) was used to measure directly the unidirectional uptake of sodium from the outside solution into the frog skin, under short-circuit conditions. The sodium uptake was determined at six sodium concentrations ranging from 3.4 to 114 mM. NaCl was replaced by choline chloride in the solutions bathing both sides of the skin. Sodium uptake is not a linear function of sodium concentration but appears to be composed of two components, a saturating one and one that varies linearly with concentration. The sodium uptake is inhibited by the addition of lithium to the outside solution. The effect appears to be primarily on the saturating component and has the characteristics of competitive inhibition. In addition, lithium uptake by the skin is inhibited by sodium. The effects of lithium cannot be ascribed to changes in electrical potential difference. Measurements with microelectrodes indicate that under short-circuit condition there is no change in the intracellular potential when lithium chloride is added to the outside solution.

Changes in sodium pool and kinetics of sodium transport in frog skin produced by amiloride

1. Amiloride produces a decrease in size of the active sodium transport pool of isolated frog skin.2. Rate coefficients for sodium movement into and out of the transporting cells across the outside membrane are decreased by amiloride. The rate coefficient for sodium extrusion across the inside membrane is not significantly affected.3. In the presence and in the absence of amiloride, the relation of sodium transport to outside sodium concentration exhibits similar saturation kinetics but amiloride reduces sodium transport rate at every sodium concentration of the outside solution.4. Labelling of skin with (14)C-amiloride from the outside solution is significantly greater than labelling with (14)C-inulin.5. The results of these studies suggest that amiloride reacts with sites on the outside membrane of the transporting cells as a result of which the rate of sodium movement across this membrane is diminished.

From frog skin to sheep rumen: a survey of transport of salts and water across multicellular structures

All higher animals, whether they live in water or on dry land, are faced with the necessity of regulating rather closely their intake and excretion of salts and water in order to maintain the constancy of their internal ionic environment. The kidney is in general the most important organ of the body as far as the excretion of sodium, potassium, chloride and water is concerned, but there are other tissues which also play a part in controlling the ionic balance between the internal and external environments, such as the intestinal mucosa, the skin and urinary bladder in amphibia, the gill epithelium in fishes, the salt gland in marine birds, and the epithelium of the rumen in ruminants. In addition to excretory and absorptive organs of this type, there are others which are secretory and whose function involves the production of fluids differing in ionic composition from the blood plasma. Examples include the glands which secrete saliva and sweat, the oxyntic acid-producing cells of the gastric mucosa, and the epithelium of the stria vascularis which generates the potassium-rich endolymph of the mammalian cochlea. The purpose of this article is to consider briefly what is known about the active transport of salts and water across some typical multicellular secretory tissues, and to attempt in the process to discern what properties they have in common and in what respects they are specialized.

Ion transport and the development of hydrogen ion secretion in the stomach of the metamorphosing bullfrog tadpole

Isolated bullfrog tadpole stomachs secrete H(+) by stage XXIV of metamorphosis, when tail reabsorption is nearly complete. At this stage the PD shows characteristic responses identical to those of the adult. The appearance of HCl secretion correlates well with other studies showing the morphogenesis of oxyntic cells. Prior to the development of H(+) secretion tadpole stomachs maintain a PD similar in polarity and magnitude to that of the adult; i.e., secretory (S) side negative with respect to the nutrient (N) side. The interdependence with aerobic metabolism appeared to increase progressively through metamorphosis; however, glycolytic inhibitors always abolished the PD. Isotopic flux analysis showed that the transepithelial movement of Na(+) was consistent with passive diffusion, whereas an active transport of Cl(-) from N to S was clearly indicated. Variations in [Na(+)], [K(+)], and [Cl(-)] in the bathing solutions induced changes consistent with the following functional description of the pre-H(+)-secreting tadpole stomach. (a) The S side is relatively permeable to Cl(-), but not to Na(+) or K(+). (b) An equilibrium potential for K(+) and Cl(-) exists at the N interface. (c) Ouabain abolishes the selective K(+) permeablity at the N interface and reduces the total PD. (d) Effects of Na(+) replacement by choline in the N solution become manifest only below 10-20 mM. It is concluded that prior to development of H(+) secretion, the tadpole gastric PD is generated by a Cl(-) pump from N to S and a Na(+) pump operating from the cell interior toward the N side.

Transcellular transport of isosmotic volumes by the rabbit gall-bladder in vitro

1. Fluid transport rate and oxygen consumption (Q(O2)) were studied in rabbit gall-bladder preparations in vitro exposed on both sides to identical Ringer solutions with NaCl concentrations (and osmolarities) varying from 70 to 140 m-equiv Na(+)/l.).2. The time sequence of acute effects on transport rate resulting from sudden changes in the NaCl concentration of the bathing solutions indicated that, (a) as a primary effect, fluid volume transfer rate remained unaffected whereas Na transport rate changed abruptly in direct proportion to the Na concentration of the bathing media; (b) a secondary, delayed and partly reversible depression of fluid transfer rate following elevation of the NaCl concentration was observed only when the rate of transport was relatively high initially.3. A fixed, and highly significant, linear relationship between changes in transport-linked oxygen consumption (DeltaQ(O2)) and measured net fluid volume transport (DeltaT(vol)) was found independent of the NaCl concentration of the bathing media, dQ(O2)/dT(vol) being 0.22 +/- 11% and 0.25 +/- 8% in bladders incubated in solutions containing 140 and 70 m-equiv Na(+)/l. respectively.4. Oxygen consumption per equiv of Na(+) (calculated) transported varied in inverse proportion to the Na concentration of the bathing media, dQ(O2)/dT(Na) being 0.0016 +/- 11% and 0.0036 +/- 8% in ;140 R' and ;70 R' solutions, respectively.5. Removal of K from the bathing solutions was followed by a gradual and partly reversible depression of fluid transport rate to a minimum level (about 100 x 10(-4) mul H(2)O. min(-1).mg(-1)) independent of the initial transport rate.6. It is concluded that the range of absorption rates of isosmotic fluid from the gall-bladder lumen represents a range of energy requiring capacities for transfer of fluid volume units; the data suggest that the intracellular (cytoplasmic) ion composition, depending on the presence of external K, as well as hormonal action may influence the capacity of the transcellular fluid transport mechanism.7. A model (a ;mechanical volume pump') for transcellular transfer of fluid volume units, allowing for flexible specificity with regard to the actively transported solutes, and requiring the presence of Na(+) and Cl(-), is proposed.

Effect of a saline environment on sodium transport by the toad colon

1. Colons isolated from saline-adapted or aldosterone-injected toads maintained transmural potential differences with the serosal side positive to the mucosa. The short-circuit currents of colons taken from aldosterone-injected toads could be expressed quantitatively by the net flux of sodium measured isotopically. This did not apply to saline-adapted colons where the net sodium flux was from serosa to mucosa.2. The short-circuit currents of colons taken from aldosterone-injected animals increased as the sodium concentration was raised from 10 to 50 mM, then decreased as the sodium concentration was further increased to 115 mM. Adaptation to saline changed this relationship, the short-circuit current becoming directly dependent on the sodium concentration.3. Faecal sodium was higher than serum sodium in saline-adapted toads. There was little or no change in the level of serum sodium or potassium. The urine of saline-adapted toads also contained high concentrations of sodium.4. The total and ouabain-sensitive ATPase activities of mucosal scrapings taken from saline-adapted colons were about half those found in the aldosterone-injected animal. Ten per cent of the total ATPase activity could be inhibited by ouabain.5. Microsomal fractions of mucosal scrapings taken from saline-adapted toads contained 4 times less Na(+) + K(+)-activated ATPase than did corresponding fractions from aldosterone-injected animals. High concentrations of sodium inhibited the microsomal ATPase activity irrespective of the previous conditions of adaptation.6. Regulation of sodium movements across the toad colon appears to be a complex process with the mucosal cells changing their properties so that they either absorb or secrete sodium ions depending on the physiological state of the animal.

The effect of sodium concentration on the content and distribution of sodium in the frog skin

1. The content and distribution of sodium in the epithelium of the frog skin (Leptodactylus ocellatus L.) have been studied.2. The inulin space, the (22)Na exchange, and the amounts of water and sodium were measured in samples of connective tissue. The results indicate that the necessary assumptions generally made to calculate the sodium and water contents of the epithelial cells as the difference between the total content in the tissue and the amounts contained in the inulin space are not valid in the frog skin.3. The mean concentration of sodium in the epithelium has been obtained from direct measurements of sodium and water in samples of epithelium. To measure the water content of the epithelium a new technique has been developed. When the skin is bathed with Ringer solution containing 115 mM-Na on both sides, the mean concentration of sodium in the epithelium is 79 mM. When the concentration of sodium in the Ringer is 1 mM the mean concentration in the epithelium is 25 mM. When the skin is bathed with Ringer with 1 mM-Na on the outside and 115 mM-Na on the inside-a situation which resembles the natural condition in the skin-the mean concentration of sodium in the epithelium is 52 mM.4. The compartmentalization of Na was studied by comparing the sodium content and the degree of exchange with (22)Na in the bathing solutions. In these experiments the skins were exposed to Ringer solutions with different concentrations of sodium, and (22)Na on one or both sides.5. The results indicate that the epithelium has a compartment of sodium which is not exchangeable in 40-80 min and whose size is not appreciably changed by a threefold change in the Na content in the epithelium and a hundredfold change in the concentration of the bathing solution.6. Sodium exchangeable in 40-80 min seems to be contained in two different compartments: (a) a large one that contains fixed sodium is mainly connected to the inside, and does not appear to participate directly in sodium transport across the frog skin; (b) a small one, that is bounded on the inside by a Na-impermeable barrier, and that seems to comprise the sodium involved in active transport. When the skin is bathed with Ringer solutions with 115 mM-Na on the inside and 1 mM-Na on the outside, the transporting compartment contains some 13% of the total sodium in the epithelium.7. The results are interpreted on the basis of a model recently proposed by Cereijido & Rotunno (1968). The major feature of this model is that the sodium transporting compartment is confined to the plasma membrane of the epithelial cells.

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.

Amiloride: a potent inhibitor of sodium transport across the toad bladder

1 Amiloride inhibits Na transport and short-circuit current (SCC) across the toad bladder. It is 1000 times more active at the mucosal than serosal surface. The lowest effective concentration was 10(-7)M.2. The inhibition was non-competitive with the sodium on the mucosal side of the bladder.3. Vasopressin, cyclic adenosine monophosphate (AMP) and aldosterone increased Na transport and SCC across the bladder and these effects were inhibited by amiloride.4. The antagonism of amiloride for vasopressin was non-competitive.5. Amphotericin B also increases Na transport across the bladder but its action was not changed by amiloride.6. Amiloride was without effects on SCC and diffusion potentials in bladders metabolically inhibited with CN(-) and iodoacetic acid (IAA).7. Neither plasma albumin, Ca(2+) nor adenosine triphosphate (ATP) altered the effects of amiloride.8. The only structural analogue of amiloride found to reduce SCC similarly was guanidine which was 1000 times less active. Pyrazine and a substituted pyrazine analogue were without effect. Neither guanidine nor the substituted pyrazine compound were competitive with amiloride.9. Amiloride had no effect on the osmotic permeability of the toad bladder either in the presence or absence of vasopressin.10. Na transport across the toad colon was also reduced by 10(-5)M amiloride at the mucosal surface.11. The possible mechanism of action of amiloride is discussed.