Cyanide inhibition of chloride conductance across toad skin

The effect of cyanide (CN(-)) on voltage-activated or cAMP-induced passive chloride conductance (G(Cl)) was analyzed in isolated toad skin. Comparatively low concentrations of CN(-) inhibited G(Cl) almost completely and fully reversibly, regardless of whether it was applied from the mucosal or serosal side. The IC(50) was 180 +/- 12 microm for voltage-activated G(Cl) and 305 +/- 30 microm for the cAMP-inducted conductance. At [CN] <100 microm, the initial inhibition frequently declined partly in the continuous presence of CN(-). Inhibition was independent of the presence of Ca(2+). Inhibition was stronger at more alkaline pH, which suggests that dissociated CN(-) is the effective inhibitor. The onset of the inhibition of voltage-activated or cAMP-induced G(Cl) by CN(-) occurred with half-times of 34 +/- 10 sec, whereas reversibility upon washout was twice as fast (18 +/- 7 sec). If [CN(-)] <200 microm was applied under inactivating conditions (serosa -30 mV), the reduction of G(Cl) was stronger upon subsequent voltage-activation than under steady-state activated conditions. This effect was essentially complete less than 30 sec after apical addition of CN(-), but G(t) recovered thereafter partially in the continuous presence of CN(-). Dinitrophenol inhibited G(Cl) similarly, while omission of oxygen did not affect it. These observations, as well as the time course of inhibition and the full reversibility, suggest that interference of CN(-) with oxidative phosphorylation and subsequent metabolic depletion is not the reason for the inhibition of G(Cl). We propose that the inhibition is directly on G(Cl), presumably by competition with Cl(-) at a rate-limiting site in the pathway. Location and molecular nature of this site remain to be identified.

Mitochondria-rich cells in anuran amphibia: chloride conductance and regional distribution over the body surface

The distribution and density (D(mrc)) of mitochondria-rich cells (MR cells) in skin epithelium, were determined over the whole body surface in nine species of anuran Amphibia that live in a variety of habitats. It was found that the more terrestrial species (beginning with Hyla arborea) have a higher density of MR cells in their pelvic region. In the skin of aquatic (Xenopus laevis) or fossorial (Pelobates syriacus) species, D(mrc) is evenly distributed over the whole body surface. In dorsal skin pieces of H. arborea that lack detectable MR cells, transepithelial voltage activation did not induce Cl(-) conductance as it did in ventral pieces. Skins from Bufo viridis and X. laevis, both have MR cells in their skin, differ markedly in their biophysical properties: a Cl(-) specific current conductance is predominant in the skin epithelium of B. viridis, and is absent in X. laevis. In the latter, anionic conductance is due to glandular secretion. The biophysical properties cannot therefore be related solely to the presence or density of MR cells. Mitochondria-rich cells are sites of Cl(-) conductance across the skin of those amphibians that show this property, but must have different function(s) in other species. It is suggested that the specific zonal distribution of MR cells in the species that were examined in this study could be due to ion exchange activity and water conservation in more terrestrial environments.

Antisense inhibition of expression of the light subunit (35 kDa) of the Gal/GalNac lectin complex inhibits Entamoeba histolytica virulence

One of the under-represented genes identified by cDNA representational difference analysis (RDA) between avirulent Entamoeba histolytica strain Rahman and virulent strain HM-1:IMSS was the amoebic light (35 kDa) subunit of the Gal/GalNac lectin complex. This lectin complex, which mediates the adhesion of the parasite to the target cell, also contains a heavy (170 kDa) subunit, which has the carbohydrate-binding domain. Stable transfectants of the virulent strain in which the expression of the 35 kDa subunit was inhibited by antisense RNA were not significantly affected in their adhesion activity to mammalian or bacterial cells but were strongly inhibited in their cytopathic activity, cytotoxic activity and in their ability to induce the formation of liver lesions in hamsters. These findings suggest that the 35 kDa subunit may have a specific function in the pathogenic pathway and provides a new insight into the role of this component of the Gal/GalNac lectin complex in amoebic virulence.

Lectin binding patterns in amphibian skin epithelium

Seven lectins (PNA, DBA, WGA, UEA-I, RCA, SBA, Con A) were used to localize glycoconjugates in the skin of 10 species of Amphibia, 7 anurans (Bufo marinus, Bufo bufo, Rana ridibunda, Rana pipiens, Hyla arborea, Pelobates syriacus and Xenopus laevis) and 3 urodeles (Salamandra salamandra, Triturus vulgaris and Ambystoma mexicanum). It was found that every lectin has a specific binding pattern in the skin of each species. No common pattern could be established, either among frogs or toads, nor for a particular lectin. Each lectin bound specifically and selectively to a particular epithelial component, which differed from one species to the other. A number of lectins showed selective binding to mitochondria-rich cells, but, again, a pattern in positivity could not be found. It is concluded that lectin histochemistry does correlate with cellular function. Our data can be applied in studies of epithelium and skin development, and of changes that occur during adaptation to the environment by amphibian species.

Elevated plasma osmotic concentration stimulates water absorption response in a toad

The water-seeking behavior (WR) of toads (Bufo viridis) was investigated. Fully hydrated toads that are allowed free choice of wet or dry filter paper voluntarily and spontaneously select to sit on water-soaked paper at a regular frequency during trials. Dehydration of bladder-emptied toads by 14% elicits WR in all animals. Injection of aldosterone or angiotensin-I reduced the dehydration threshold to 7% weight loss. WR frequency increased when plasma osmolality was elevated by injection of NaCl or other solutes (both ionic and non-ionic). Only urea, to which cell membranes are highly permeable, was the exception that did not produce this response. The increase in WR frequency induced by elevated plasma osmolality was augmented by injection of aldosterone or angiotensin-I. In vivo water uptake, measured in a water bath, was increased by an NaCl or oxytocin injection, but not by aldosterone. It is concluded that elevated plasma osmolality induces an increase in WR frequency that is separate and prior to the water uptake process. Different hormones are involved in each step.

Renal response of euryhaline toad (Bufo viridis) to acute immersion in tap water, NaCl, or urea solutions

Green toads (Bufo viridis) were acclimated to either tap water, 230 mOsmol NaCl kg-1 H2O (saline), 500 mOsmol NaCl kg-1 H2O (high saline), or 500 mmol L-1 urea. Renal functions for each acclimation group were studied on conscious animals that had one ureter chronically catheterized. Reciprocal immersion of tap-water- and saline-acclimated toads in the opposite solution did not stress the animals osmotically, and plasma osmolality increased or decreased by no more than 15%. However, urine osmolality and ionic composition changed immediately and profoundly on exposure to the other solution. Exposure of tap-water-acclimated toads to saline decreased urine flow by 30%, whereas the reciprocal immersion led to an increase of 30%. Immersion of tap-water-acclimated toads in high saline led to immediate cessation of urine flow, whereas immersion of 500 NaCl- or urea-acclimated toads in tap water led to a large increase in urine flow, with an overshoot that lasted 10 h (as a result of either salt or urea diuresis). Urine flow then stabilized at a level 5-6 times higher than the value attained at high-salt environment. On immersion of 500 urea-acclimated toads in 500 NaCl, urine flow doubled, accompanied by a change in ion composition, without change in the osmolality. In all experimental conditions, plasma potassium concentration was maintained within a narrow range. The results show that the toad's kidneys contributed efficiently both to osmo- and ionoregulation in a wide range of ambient solutions.

Trypsin inhibits voltage-activated chloride conductance of toad skin

The effect of trypsin on the voltage-activated chloride conductance (GCl) of toad skin was investigated. Serosal application of > 0.1 mg ml-1 trypsin decreased the voltage-activated GCl without notable delay. The maximal inhibition to 38% of the control values, exerted within 15 min, was in some experiments partly or completely reversible. Chymotrypsin had much lower effect than trypsin. Mucosal application of trypsin did not have any effect. Trypsin did neither interfere with the conductive pathway opened by supramaximal concentrations of cAMP nor with the inhibitory effect of epinephrine on the voltage-activated GCl. The effect of trypsin required influx of Ca2+ from the extracellular space. It is concluded that protease-activated receptors or trypsin-sensitive proteins in the basolateral membrane of toad skin epithelial cells interfere with regulative steps involved in the voltage-activation of GCl. This may be harmful for the segregation of epithelial cells using this enzyme.

alpha1-adrenoceptors antagonize activated chloride conductance of amphibian skin epithelium

The effects of adrenoceptor agonists on the transepithelial Cl- conductance (GCl) in the skin of several amphibian species, both toads and frogs, were studied. Epinephrine (Epi) from the serosal side selectively and reversibly inhibited the voltage-activated GCl in toad skin and the short-circuit GCl in frog skin. The main effects of activation of the adrenoceptors must reside in the skin epithelium and not in the glands, since measurements were made both from intact skins and split epithelia with essentially the same results. Effective concentrations of Epi were variable among individual tissues. GCl was reduced to 34+/-17% (n=46) with 1 micromol/l Epi, but in some tissues 0.1 micromol/l inhibited more than 80% of GCl, whereas some preparations were little influenced at >3 micromol/l Epi. The affected receptor type was identified by the use of the alpha1-agonist phenylephrine, which mimicked the response of Epi at concentrations above 30 micromol/l, whereas the alpha2-agonists xylazine and iodoclonidine had no effect at supramaximal concentrations. Prazosin, a specific alpha1-antagonist, reduced or eliminated the inhibition by Epi, but the response pattern suggests a low affinity. The alpha2-antagonist yohimbine, at concentrations </=0.3 micromol/l, had a minimal effect, but reduced the inhibition by Epi at concentrations of 1-10 micromol/l. This might indicate affinity to alpha1-adrenoceptors in amphibian skin. Activation of beta-adrenoceptors by isoproterenol (0.1-5 micromol/l) led to a transient increase of the baseline inactivated component of GCl with a slight reduction of the voltage-activated GCl at the higher concentrations, but the inhibitory effect of Epi was not altered. Epi, on the other hand, neither prevented nor reversed the induction of a voltage-insensitive GCl in toad skin caused by application of cAMP at supramaximal concentrations (>100 micromol/l CPT-cAMP). Preincubation of the serosal medium with Ca2+-free solution (in the presence of 2 mmol/l EGTA) for extended periods of time (>30 min) eliminated the response to Epi. It is concluded that alpha1-adrenoceptors participate in the physiological control of voltage-activated Cl- conductance in amphibian skin epithelium via modulation of intracellular Ca2+, presumably by efflux from intracellular stores.

Glyconeogenesis and urea synthesis in the toad Bufo viridis during acclimation to water restriction

Metabolic consequences of osmotic stress were investigated in the toad Bufo viridis. Toads were acclimated either to terrestrial conditions in the absence of free water or to being partially immersed in 250 mmol L-1 NaCl, which was achieved by gradually increasing the salinity of the bath. This slow acclimation evoked little metabolic response, whereas the immediate osmotic challenge of water restriction resulted in a significant increase in the concentration of urea in the plasma and in liver glycogen. Urea accumulation, involving a transient increase in its rate of synthesis, allows the toads to lower their body water potential and thereby to absorb soil-bound water. The metabolic cost of this response is reduced by conserving the resulting by-product, glucose, as glycogen stored in the liver for future use.

Effects of NEM on voltage-activated chloride conductance in toad skin

The regulation of the voltage-activated chloride current conductance (GCl) in toad skin was investigated by the use of the SH reagents N-ethylmaleimide (NEM) and p-chloro-mercuricbenzenesulfonic acid PCMBS. This anion pathway is controlled by a voltage-sensitive gating regulator. Mucosal application of NEM decreased the voltage-activation in a time and concentration dependent manner, half-maximal inhibition being exerted at a concentration of 30 microM within 20 min. At concentrations higher than 100 microM, the voltage-activated GCl was near-completely and irreversibly inhibited in less than 10 min. Resting, deactivated conductance was essentially unaffected. NEM had no effect on active sodium transport (measured as Isc) under conditions, which fully dissipated the voltage-activated GCl. After complete inhibition of the voltage-activated GCl with NEM, chloride conductance could still be stimulated by CPT-cAMP as in control tissues. Under these conditions, NEM at concentrations above 1 mm decreased GCl reversibly. Mucosal application of PCMBS at 500 microM inhibited the activated conductance by 35%, which was slightly reversible. Inhibition of voltage-activated GCl, which was observed after mucosal addition of the membrane-impermeable NEM analogue, eosin-5-maleimide, was completely reversible after washout. This suggests that the binding site for the maleimide is not accessible from the external face of the apical membrane. Brief application of NEM at lower concentrations (1-3 min, </= 100 microM) led to partial inhibition of GCl, followed by occasionally complete recovery upon washout of NEM. Recovery of voltage-activated GCl was progressively attenuated and eventually disappeared after subsequent brief applications of NEM. This could reflect recruitment of permeation/control sites from a finite pool. The data are discussed in the frame of a working model for the voltage-activated Cl--pathway, that contains two principle components, i.e., an anion-selective permeation path which is controlled by regulatory protein(s).

Salt and water balance in the toad Bufo viridis during recovery from two different osmotically stressful conditions

Toads, Bufo viridis, were subjected to two modes of osmotic stress: on soil of water potential approximately 5 atm and continuous partial immersion in 250 mmol/l NaCl solution. In both conditions, plasma osmolality was greatly elevated involving a large increase in urea concentration and was maintained hyperosmotic to the external environment. After acclimation to either condition, toads were allowed access to tap water, and the concentrations of body fluids and gross body weight were followed over a 7-day period. The toads bathed only until preacclimation gross weight was regained, although plasma osmolality remained elevated. Weight remained stable thereafter. Excess plasma Na+ and Cl- were eliminated within a few days, whereas the urea level diminished very slowly. K+ was closely controlled throughout, both during acclimation and recovery. The results suggest that B. viridis is equipped with a series of set points for osmotic pressure that enable it to maintain different steady states according to the prevailing conditions, the magnitude of the shift depending on the level of the imposed osmotic stress.

Lectin binding pattern and band 3 localization in toad skin epithelium and the effect of salt acclimation

Seven lectins were employed to localize glycoconjugates in the skin of a toad (Bufo viridis). Each of the lectins exhibited a particular, specific and selective binding pattern. Peanut lectin (PNA) and WGA bound to mitochondria-rich (MR) cells, but WGA bound also abundantly, in the dermis. Band 3-like protein, as indicated by the reaction with polyclonal anti band 3 antibody, was localized exclusively in MR cells. Ionic acclimation (200 mmol/L NaCl, or 50 mmol/L KCl) affected profoundly the binding pattern of the lectins. High NaCl acclimation resulted also in diminishing anti band 3 antibody binding, whereas in skins of KCl-acclimated toads the staining remained similar to the control. The binding of WGA but not PNA, corresponded with the same cells that stained with anti band 3 antibody. PNA in concentration of > 10 micrograms/mL reduced reversibly, both the resting and activated Cl- conductance by 25-30%. Based on differential binding of band 3, WGA and PNA, these observations provide conclusive verification of the presence of at least two populations of MR cells in the toad skin epithelium. It is suggested that the PNA positive MR cells may correspond to a beta-type MR cell. The information can be used to study molecular mechanisms that are involved in ionic acclimation.

Different temperature relations of two species of toads that coexist at the border of their geographical distributions

Abstract

Temperature relations of two species of toads (Bufo viridis and Bufo regularis) that overlap at the border of their geographical distributions were studied in the laboratory in temperatures between 25°C and 2°C. Bufo regularis, a tropical species, could not survive at low temperatures that eventually led to its death in 1-2 days. The two species did not differ in their cooling and heating rates. At temperatures below 8-10°C, Bufo viridis maintained a body temperature that was higher than ambient by 1-2°C, while body temperature of Bufo regularis equaled that of the environment. In Bufo regularis heart beats ceased at low temperatures (< 4°C), whereas in Bufo viridis at the same temperature, a rate of 6-9 beats/min was sustained. Our experiments suggest that Bufo viridis is endowed with particular capacities that allow it to remain active at low temperatures, while in Bufo regularis, a temperature-sensitive central mechanism seems to be damaged at low temperature. The ability of Bufo viridis to endure low temperatures and to remain active in this condition is specific to the species, and is not related directly to the immediate ecological situation of the animal.

Effects of cyclic AMP and theophylline on chloride conductance across toad skin

1. The effects of the phosphodiesterase inhibitors theophylline and isobutylmethylxanthine (IBMX) on baseline and voltage-activated Cl- conductance (gCl) of toad skin were compared with those of the potent 2-chlorophenylthio analogue of cAMP (CPT-cAMP). 2. Using intact and split skins of Bufo viridis we confirmed that theophylline and IBMX raised the voltage-activated gCl with a pattern identical to that seen under control conditions. This effect was small or missing if gCl was already high in the control. 3. CPT-cAMP, in contrast, increased the Cl(-)-specific conductance by up to 6 mS cm-2 at short circuit. The characteristic time-dependent, slow activation of gCl by serosa-positive clamp potentials was completely lost under these conditions. 4. Coinciding with the loss of voltage activation of gCl the plateau value of the Lorentzian component of fluctuation in current at serosa-positive clamp potentials decreased by almost 50%. The corner frequencies were not notably different. 5. After CPT-cAMP, the sigmoidal voltage-conductance relation that is characteristic of control conditions or after theophylline disappeared; the patterns were variable and incompatible with voltage activation. 6. The voltage-activated gCl under control conditions and with theophylline was blocked by mucosal NO3-, I- or SCN-, the last two being almost equally effective. In the presence of CPT-cAMP, mucosal NO3- had minimal influence on tissue conductance, whereas the effects of I- and SCN- were essentially unchanged. Br- on the mucosal side could substitute for Cl- under all conditions. 7. The results suggest that protein phosphorylation by supramaximal concentrations of cAMP induces maximal conductance through anion-specific routes, while the voltage sensitivity of this pathway is lost. The effects of theophylline and IBMX on the voltage-activated Cl-conductance of toad skin cannot be explained solely by inhibition of the phosphodiesterase.

Renal function at steady state in a toad (Bufo viridis) acclimated in hyperosmotic NaCl and urea solutions

Kidney function of the euryhaline toad Bufo viridis was studied in animals acclimated to tap water and solutions of NaCl (230 and 500 mosmol.kg-1 H2O) and urea (500 mmol.l-1) in steady-state conditions. An ureter was catheterized for continuous urine collection and blood was sampled from an iliac artery. A single injection of 3H-inulin served for estimation of glomerular filtration rate: this was in the range of 15-27 ml.kg-1.h-1 and did not differ significantly in any of the acclimation conditions. Urine flow, on the other hand, varied considerably and was highest in tap water (18.2 +/- 3.2 ml.kg-1.h-1; urine/plasma inulin ratio = 0.88), lower in 230 mosmol.kg-1 H2O NaCl solution (13.5 +/- 3.9 ml.kg-1.h-1; u/p inulin ratio = 1.73) and lowest in 500 mosmol.kg-1 H2O NaCl or urea acclimation solutions (5-7 ml.kg-1.h-1; u/p inulin = 3.7-4.2). Clearance of free water was high in the tap water group, lower in 230 mosmol.kg-1 H2O NaCl solution, and much lower in the hyperosmotic acclimation conditions. Clearances of both Na+ and Cl- were similar under our experimental conditions, but changed independently in accordance to the composition of the acclimation solution. Potassium clearance was similar in all acclimation conditions, and a constant plasma K+ concentration was maintained. Urea clearance was high in tap water and 500 mmol.l-1 urea acclimation groups and low in the NaCl acclimations.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamics and density of mitochondria-rich cells in toad skin epithelium

The dynamics of change in mitochondria-rich (MR) cells density in the skin epithelium of Bufo viridis was studied on skin biopsies taken in vivo, throughout experimental periods lasting up to 3 months. When the bathing solution contained Cl-, MR cells' density (Dmrc) greatly decreased. There was one exception, when the acclimation solution was KCl, Dmrc in the skin increased. The rate of decrease in Dmrc depended on the mode of acclimation. When bath NaCl concentration was elevated slowly in small increments, the change in Dmrc was very slow. A regression line was calculated for the rate of decrease in the density of MR cells. An equation in the form of y = 1574 - 10.23x (where x = days; R2 = 0.626) was obtained with bath NaCl that was elevated from 30 to 200 mmol/l, in 45 days. Oxytocin (60 mU/ml) increased sodium transport, independently and without effect on Cl- conductance. Theophylline (1 mmol/l), which leads also to elevation of cellular cAMP in contrast, increased Na+ transport, but elevated Cl- conductance 3-4 times as well. Cl- conductance that is activated by transepithelial potential was much lower in skin from hyperosmotic NaCl-acclimated toads, as compared with that in skin from tap water-acclimated animals. Our experiments confirm that MR cells are a major pathway for Cl- conductance, as suggested earlier. However, the density of these cells in the skin epithelium of B viridis depends not only on bath NaCl concentration, but also on the mode of acclimation of the animals. Since transport functions other than gCl reside in the amphibian skin MR cells, the density of MR cells must also depend on these functions. These functions, and the mechanisms responsible for the down and up regulation of MR cells' density, remain to be established.

Ion transport across the early chick embryo: I. Electrical measurements, ionic fluxes and regional heterogeneity

The chick blastoderm at the stage of late gastrula is a flat disc formed by three cell layers and exhibiting epithelial properties. Blastoderms were cultured in miniature chambers and their electrophysiological characteristics were determined under Ussing conditions. Under open-circuit condition and identical physiological solutions on both sides, spontaneous transblastodermal potential difference (Voc) of -7.5 +/- 3.3 mV (ventral side positive) was measured. Under short-circuit condition (transblastodermal delta V = 0 mV), the blastoderm generated short-circuit current (Isc) of 21 +/- 8 microA/cm2, which was entirely dependent on extracellular sodium, sensitive to ouabain applied ventrally and independent of extracellular chloride. The net transblastodermal Na+ flux fully accounted for the measured Isc, both under control conditions and with ouabain. The total transblastodermal resistance (Rtot) was 390 +/- 125 omega cm2. Frequently, the Voc, Isc and Rtot showed spontaneous oscillations with a period of 4-5 min. Removal of endoderm and mesoderm did not significantly affect the electrical properties, indicating that the electrogenic sodium transport is generated by the ectoderm. The Voc and Isc measured in the area pellucida (-1.3 +/- 0.8 mV, 9.3 +/- 4.4 microA/cm2) and extraembryonic area opaca (-7.8 +/- 1.1 mV, 31.2 +/- 12.7 microA/cm2) were significantly different. Such a heterogeneous distribution of electrical properties can explain the presence in the blastoderm of extracellular electrical currents found by using a vibrating probe.

Ion transport across the early chick embryo: II. Characterization and pH sensitivity of the transembryonic short-circuit current

The ectoderm of the one-day chick embryo generates dorsoventrally oriented short-circuit current (Isc) entirely dependent on extracellular sodium. At the dorsal cell membrane, the Isc was modified reversibly and in a concentration-dependent manner by: amiloride (60% decrease at 1 mM, with 2 apparent IC50S: 0.13 and 48 microM), phlorizin (0.1 mM) or removal of glucose (30% decrease, additive to that of amiloride), SITS (1 mM, 13% decrease). Acidification of alkalinization of the dorsal (but not ventral) superfusate produced, respectively, decrease or increase of Isc with a pH50 of 7.64. Ba2+ (0.1-1 mM) from either side of the ectoderm decreased the Isc by 30%. Anthracene-9-carboxylic acid, furosemide and inducers of cAMP had no effect on electrophysiological properties of the blastoderm. The chick ectoderm is therefore a highly polarized epithelium containing, at the dorsal membrane, the high and low affinity amiloride-sensitive Na+ channels, Na(+)-glucose cotransporter, K+ channels and pH sensitivity, and, at the ventral membrane, the Na+, K(+)-ATPase and K+ channels. The Na+ transport reacts to pH, but lacks the cAMP regulatory system, well known in many epithelia. The active Na+ transport drives glucose and fluid into the intraembryonic space, across and around the blastoderm which, in the absence of blood circulation, could secure renewal of extracellular fluid and disposal of wastes and thus maintain the cell homeostasis.

Differential processing of provasotocin: relative increase of hydrin 2 (vasotocinyl-Gly) in amphibians able to adapt to an arid environment

Hydrin 2 (vasotocinyl-Gly) is an intermediate in pro-vasotocin processing found, along with vasotocin, only in the neurohypophysis of anuran amphibians. It increases cutaneous water permeability in the frog and is likely involved in neuroendocrine control of osmoregulation. The relative amounts of vasotocin and hydrin 2 stored in neurohypophysis have been measured on the one hand in amphibian species known not to adapt in dry areas, on the other hand in two species, Bufo regularis (Africa) and Bufo viridis (Near-East) able to survive in an arid environment. In the first group, the proportions of the two peptides are approximately equal whereas in the two toads the molar ratio hydrin 2 to vasotocin reaches 2. The ratio does not appear to vary significantly when these toads are either submitted to dehydration or placed in saline solutions. Predominance of hydrin 2 suggests an adaptive decrease of the activity of the alpha-amidating enzymatic system involved in the conversion of vasotocinyl-Gly into mature amidated vasotocin.

Mechanisms of hyperosmotic acclimation in Xenopus laevis (salt, urea or mannitol)

The acclimation of the clawed toad Xenopus laevis to hyperosmotic solutions of NaCl (balanced solution of sea salt), urea or mannitol was studied. The animals could not be acclimated to salt solutions more concentrated than 400 mosm.1-1. Urea was tolerated till 500 mmol.1-1. Plasma osmolality was always hyperosmotic to the environmental solution, but with diminished osmotic gradient at the highest tolerated solutions. Plasma urea concentration approached 90 mmol.1-1, similar in the three solutions of acclimation. Urine volume was very small under all conditions. Serum aldosterone and corticosterone did not differ significantly, although there was a slight tendency towards lower aldosterone in the NaCl solution. In vivo water uptake in tap water acclimated animals was very small, and was higher in the other groups. Only the salt- and urea-acclimated, but not the tap water and mannitol-acclimated groups responded with a clear increase following injection of oxytocin or theophylline. In vitro urea fluxes were similar and invariable in both directions under all conditions. No significant effect of theophylline was observed. Sodium transport measured by the short-circuit technique in vitro was lower in salt- and mannitol-acclimation conditions, and was stimulated significantly under all conditions in response to serosal oxytocin or theophylline. It is concluded that Xenopus laevis can osmoregulate at a limited range of external solutions. It is limited in the increase of its plasma urea concentration; the transport properties of the skin do not change very much upon acclimation, except for the hydroosmotic response to oxytocin.