Measurement of Na--K--ATPase in the separated epidermis of Rana catesbeiana frogs and tadpoles

Vascular aspects of water uptake mechanisms in the toad skin: perfusion, diffusion, confusion

Blood cell flow (BCF) in the water absorbing "seat patch" region of toad skin was measured with laser Doppler flow cytometry. BCF of dehydrated toads increased by a factor of 6-8 when water contact was made and declined gradually as toads rehydrated. Water absorption was initially stimulated and declined in parallel with BCF. Water absorption measured during the initial rehydration period did not correlate with BCF and hydrated toads injected with AVT increased water absorption without an increase in BCF indicating the lack of an obligate relation between blood flow and water absorption. Aquaporins 1-3 were characterized by RT-PCR analysis of seat patch skin. AQP 1 was localized in the endothelium of subepidermal capillaries and serves as a pathway for water absorption in series with the apical and basolateral membranes of the epithelium. Dehydrated toads rehydrated more rapidly from dilute NaCl solutions than from deionized water despite the reduced osmotic gradient. BCF of toads rehydrating on 50 mM NaCl was not different than on deionized water and blocking Na+ transport with 100 microM amiloride did not reduce water absorption from 50 mM NaCl. Thus, neither circulation nor solute coupling explains the greater absorption from dilute salt solutions. Rehydration from 10 mM CaCl2 was stimulated above that of DI water by a similar degree as with 50 mM NaCl suggesting the anion might control water permeability of the skin.

Effects of hydrin 2 on cutaneous electrical properties of Rana pipiens

Electrical properties were measured in isolated skin patches from leopard frogs (Rana pipiens) to explore the possibility that hydrin 2 (vasotocinyl-gly) may regulate cutaneous ion transport. Hydrin 2 is a neurohypophysial peptide commonly implicated in rehydration of anurans. Experimental frogs were given either one or two intraperitoneal (i.p.) injections of hydrin 2 in saline solution, each calculated to achieve a final blood concentration of 81 ng/ml. After a 2-h equilibration period, the frogs were euthanized and abdominal skin removed for study in a Ussing chamber. Results of this study confirm that exposure of R. pipiens to hydrin 2 promotes a highly significant increase in transepithelial potential (TEP) and short-circuit current (Isc), with both treatment and treatment x time interactions in doubly injected frogs (repeated measures ANOVA). The apparent decrease in cutaneous R of experimental animals was not significant. Supplemental additions of hydrin 2 to the serosal bath provoked changes in TEPs, Iscs, and Rs which are difficult to interpret. In a dose-response study, 10-fold increases in serosal hydrin 2 concentration triggered significant changes in TEP and Isc, with maximal responses observed at 10 ng/ml. Because hydrin 2 is known to facilitate "cutaneous drinking" in anurans, this study suggests a correlation between ion and water transport across the skin.

Osmoregulatory-like mitochondria-rich cells in the developing pancreatic ducts of young anuran tadpoles

Pancreatic ducts of young posthatching Rana temporaria tadpoles are the main component of the developing pancreas. At this stage (free-swimming tadpoles with internal gills), duct cells display a high degree of development of basal and lateral outfoldings of the cell membrane with extensive interdigitation, and numerous mitochondria are present throughout the cytoplasm. Wide intercellular spaces also exist, sometimes forming canaliculi-like structures. Since these traits are characteristic of cells engaged in osmotic regulation, we suggest the possibility that this temporary duct system participates in such control. Duct cells in tadpoles with well-developed hindlegs have diminished interdigitation, and mitochondria are localized apically.

Differentiation of frog skin active Na+ transport during metamorphosis is induced by thyroid hormone

Hormonal control of differentiation of the active Na+ transport system across the skin of the Rana catesbeiana tadpole during metamorphosis was investigated. Active Na+ transport in the tadpole does not operate before climax stages (stage XX) because of the lack of a Na+ channel, even though the skin already has a Na+ pump. Injection of aldosterone (200 nmol/kg body wt), corticosterone (500 nmol/kg body wt), or hydrocortisone (300 nmol/kg body wt) at stages XIII-XV and administered for 2 weeks neither induced differentiation of the Na+ channel nor stimulated the Na+ pump. On the other hand, differentiation of the Na+ channel (increase in active Na+ transport) was induced by thyroid hormone without supplementary mineralicorticoid or glucocorticoid treatment. Triiodothyronine (10 nmol/kg body wt every other day for 2 weeks) increased Na+ channel density, even when the mineralicorticoid antagonist spironolactone (20 mumol/kg body wt) or glucocorticoid antagonist metyrapon (442 nmol/kg body wt) were injected. The skin active Na+ transport system acquires aldosterone sensitivity only after differentiation of the Na+ channel is induced by thyroid hormone.

Effect of prolactin on the active Na transport across Rana catesbeiana skin during metamorphosis

The effect of long-term application of prolactin (PRL) on active Na transport across the abdominal skin of Rana catesbeiana tadpoles during metamorphosis was investigated. At Taylor-Kollros stage XX, the potential difference (PD) and short-circuit current (SCC) across the skin were absent and resistance to an active Na current (RNa) was infinite. At stage XXI, the PD and SCC clearly appeared and increased thereafter. The RNa after stage XXI remained at a relatively constant value of about 5-10 k omega.cm2, whereas the electromotive force of the active Na current (ENa) greatly increased in stages XXI-XXII. It thus appears that Na channels are first formed at stage XXI, and that the Na pump rapidly develops during stages XXI-XXII. Long-term application of PRL was started at both stage XXI (when PD, SCC, and ENa are still low) and stage XXV (when PD, SCC, and ENa are high). Prolactin (20 micrograms/g body wt) was injected every other day for 2 weeks. Since the PD, SCC, and ENa remained low after the injection was started at stage XXI, early treatment with PRL apparently inhibits the differentiation of the Na pump (i.e., ENa). By contrast, treatment with PRL starting at stage XXV decreased the PD and SCC and increased the RNa, but was without effect of the ENa of this group. It appears that PRL has no effect on the Na pump already differentiated, although it inhibits the maintenance of Na channels formed in earlier stages.

Polarization of plasma membrane glycoconjugates in amphibian epidermis during metamorphosis

Wheat germ agglutinin (WGA) binding sites have been examined in tadpole epidermal cells at the level of both light and electron microscopy using the WGA-ovomucoid-gold technique. In premetamorphic tadpoles the reaction was observed on the plasma membranes of epithelial cells showing a gradient from inner to outer membranes. These glycoconjugates were polarized during development, and at the end of metamorphic climax they were only located in plasma membranes of stratum corneum. The existence of an apical cell surface coat is needed to facilitate the absorption of water through the adult epidermis. The possible implications of this polarization process are discussed.

Thyroxin specifically stimulates anuran larvae epidermal sodium pump during metamorphosis

1. Sodium pump, measured as K+-dependent, ouabain sensitive pNPPase, but not the non-specific, ouabain insensitive pNPPase, was stimulated by thyroxin in epidermis of tadpoles of Rana perezi. 2. Epidermal K+-pNPPase of thyroxin treated tadpoles was only stimulated in those stages already showing activity and reached levels similar to adult frogs in tadpoles at metamorphic climax.

Changes in cell membrane fluidity affect the sodium transport across frog skin and its sensitivity to amiloride

1. 1-5 mM n-hexanol added to the outer (mucosal) medium of isolated skin of the frog Rana temporaria increases the short circuit current (Isc) across it. 2. This effect shows a saturable dependency on the outer sodium concentration, also when NaCl is replaced by Na2SO4. 3. n-Hexanol at a concentration of 1 mM, and cold acclimation of the frogs, which increases the fluidity of epidermal cell membranes, do not affect the sensitivity of Isc to the inhibiting effect of amiloride. 4. n-Hexanol at a concentration (5 mM) which causes a fluidization of cell membrane preparations from isolated frog epidermis also increases the sensitivity of Isc to amiloride. 5. The effects of low concentrations of n-hexanol and of cold acclimation probably depend on an increase of the permeability of apical membranes of epidermal cells to sodium caused by membrane fluidization. At higher concentrations of n-hexanol, a further disordering of the membrane structure occurs with a better access of amiloride to its action sites.

Lectin inhibition and kinetics of microsomal K+-dependent p-nitrophenyl phosphatase of frog epidermis

The specific activity of K+-dependent p-NPPase (paranitrophenylphosphatase) from frog (Rana ridibunda) epidermis microsomal preparation was determined. The activity was proportional to time of incubation and protein concentrations under our assays conditions. Optimal phosphatase activity was at pH from 8 to 9 and over 35 degrees C. 10(-3) M ouabain inhibited 100% of the activity and the Ki was estimated about 5 X 10(-5) M. The Km for p-NPP was 3.8 mM and 2.1 for K+. The lectins GSI and GSII produced 80-90% of non-competitive inhibition of the activity. 50% of inhibition by GSI was obtained at 2 micrograms/ml. The Km for p-NPP did not change but the Vmax of activity was clearly reduced for both GSI and GSII lectins.

Interrelationships among epidermal Na-K ATPase, developmental stage and length of Rana catesbeiana tadpoles

Sodium and potassium activated adenosine triphosphatase (Na-K ATPase) was extracted from the skin of Rana catesbeiana tadpoles and adults. Using carefully staged tadpoles, it was noted that the enzyme level increases two or three stages before there is a perceptible potential difference generated across the skin. The level of non-ouabain-inhibitable ATPase remains constant throughout the life cycle. It was also concluded that Rana catesbeiana tadpoles cannot be reliably staged using length as a key trait.

Active transepithelial potassium transport in frog skin via specific potassium channels in the apical membrane

In frog skin bathed in Cl--Ringer's solution the short circuit current (SCC) is equal to the net Na+ flux. In the present study Na+ and K+ transport across frog skin have been investigated in skins bathed in a solution where all Cl- has been substituted by the impermeable anion gluconate. In this solution the net Na+ flux (9.22 +/- 0.72 nmole/cm2/min) was significantly higher than the SCC (7.61 +/- 0.63) nmole/cm2/min). Measurement of the transepithelial K+ influx and K+ efflux showed that the discrepancy between the net Na+ flux and the SCC was caused by an active outwards going transepithelial K+ transport. The K+ but not the Na+ transport could be blocked by adding the K+ channel blocking agent Ba++ to the apical solution. Thus, the K+ transport occurs via a K+ specific pathway in the apical membrane. Ouabain blocked both the Na+ and the K+ transport, whereas the presence of the Na+ channel blocking agent amiloride in the apical solution blocked the Na+ transport and reduced the K+ transport. In the presence of amiloride in the apical solution the SCC and the transepithelial potential difference (PD) reversed so that the outside (the apical side) of the frog skin became positive with respect to the basolateral side. The inverted SCC was carried by an active transepithelial K+ transport, this K+ transport required the presence of Na+ in the basolateral solution. The experiments show that frog skin can insert or activate K+ channels in the apical membrane, indicating that the frog may regulate its K+ content by varying the K+ permeability of the apical membrane.

Corticosteroids in serum of Rana catesbeiana during development and metamorphosis

Radioimmunoassays (RIA) for aldosterone (ALDO), corticosterone (B), and cortisol (F) were used to measure corticosteroids in serum of Rana catesbeiana tadpoles and of young and adult frogs. Because of the uncertain identity of the cortisol-like material in the RIA, it was designated "F." Serum ALDO was detectable in tadpoles by stage VII and it remained at about 2 ng/ml in most stages until midclimax; then its concentration rose to high levels in froglets and frogs. Serum B appeared later in development (by stage XII) and its concentration increased rapidly to an initial peak at stage XVII. The B concentration increased again after midclimax coincident with the rise in ALDO. After stage XV serum B concentration was about 10 times higher than ALDO in most samples. Serum "F" was detectable by stage V and the concentration rose gradually to stage XIV; then it increased more rapidly until midclimax, whereupon its concentration fell precipitously to reach low levels in froglets and adults. A single injection of ACTH (0.1 IU/tadpole) failed to increase serum ALDO, B, or "F" in premetamorphic larvae, but it caused a significant elevation of the two glucocorticoids in prometamorphic animals. Chronic treatment with ACTH or thyroxine (T4) increased the serum levels of the three steroids. Treatment with ACTH plus T4 markedly increased ALDO and B responses to the ACTH but the "F" response was diminished. We interpret the results to indicate that low levels of thyroid hormones (TH) sensitize the interrenal to stimulation by ACTH. Higher levels of TH and/or longer exposure to these hormones further enhance the ALDO and B responses while inhibiting the "F" response. The TH may also alter peripheral metabolism and/or clearance of the steroids to produce the changes which were observed. In either case, our results indicate that TH-induced maturation of the hypothalamo-hypophyseal-interrenal axis contributes to the control of development and metamorphosis in anurans. The pattern of serum B and "F" appears to be related to developmental events, such as hind leg growth and gut and tail regression, but the serum pattern of ALDO did not show any such relationship.

Coupled transepithelial sodium and potassium transport across isolated frog skin: effect of ouabain, amiloride and the polyene antibiotic filipin

Addition of the polyene antibiotic filipin (50 microM) to the outside bathing solution (OBS) of the isolated frog skin resulted in a highly significant active outward transport of K+ because filipin per se increases the nonspecific Na+ and K+ permeability of the outward facing membrane. The K+ transport was calculated from the chemically determined changes in K+ concentrations in the solution bathing the two sides of the skin. The active transepithelial K+ transport required the presence of Na+ in the OBS, but not in the inside bathing solution (IBS), and it was inhibited by the Na+, K+-ATPase inhibitor ouabain. The addition of Ba++ to the IBS in the presence of filipin in the OBS resulted in an activation of the transepithelial K+ transport and in an inhibition of the active Na+ transport. This is in agreement with the notion that Ba++ decreases the passive K+ permeability of the inward facing membrane. In the presence of amiloride (which blocks the specific Na permeability of the outward facing membrane) and Ba++ there was a good correlation between the active Na+ and K+ transport. It is concluded that the active transepithelial K+ transport is carried out by a coupled electrogenic Na-K pump, and it is suggested that the pump ratio (Na/K) is 1.5.

Temperature compensation of sodium transport and ATPase activity in frog skin

Na+ transport across frog skin, measured as short-circuit current (SCC) shows perfect temperature compensation in frogs acclimated to 6 degrees, 12 degrees, and 23 degrees C as SCC values observed at the acclimation temperatures are equal (about 13 muA/cm2). Reacclimation experiments show that this is not a starvation effect. While very little temperature compensation is seen in the activity of Na+, K+-ATPase in epidermal homogenates from frog skins, the activity of Mg2+-ATPase shows inverse compensation at assay temperatures from 4 degrees to 48 degrees C. This ATPase is apparently activated either by Mg2+ or by Ca2+ and it probably controls the passive permeability of epidermal cells. It is suggested that the inverse temperature compensation in the activity of this enzyme is the main mechanism by which the observed perfect temperature compensation of Na+ transport across frog skin occurs.

Effects of high hydrostatic pressures on the activity of the membrane ATPases of some organs implicated in hydromineral regulation

1. The effects of high hydrostatic pressures have been studied on the ATPases extracted from tissues implicated in iono- and osmoregulation of a frog and various teleostean fishes. Pressure affects enzyme activity in the same qualitative way, whatever the tissue and the species considered. 2. The Mg2+ ATPase activity is maximally enhanced at 250 kg/cm2. A slight inhibition is observed at higher pressures up to 1000 kg/cm2. 3. The (Na+ + K+)ATPase is little affected by low pressures but strongly inhibited at 500 kg/cm2 and more. 4. The results are discussed in terms of pressure effects on the recently described protein-lipid interaction linked to ATPase activity.

The action of thyroid hormone

Thyroid hormone affects both developmental and metabolic processes. It has a relatively specific effect on the synthesis of a number of enzymes and other proteins. The fundamental cellular mechanism of action seems to be at the level of genetic regulation. It involves interaction with nuclear receptors, leading to an activation of the protein synthesizing machinery. How binding to receptors is coupled to genetic activation is completely unknown. At least part of the metabolic effects of thyroid hormone could be mediated through an interaction with mitochondria and cell membrane, and with some enzymatic systems such as adenylcyclase.

The permeability of the skin of a neotenous urodele amphibian, the mudpuppy Necturus maculosus

1. The permeability of the isolated skin of a neotenous urodele amphibian, the mudpuppy Necturus maculosus, to Na, Cl, urea and water was measured. 2. Unidirectional transcutaneous flux measurements and the action of ouabain and amiloride, showed that there was normally no active Cl or Na transport, nor a Cl/Cl exchange diffusion process. 3. Amphotericin B initiated a transcutaneous potential difference and short-circuit current, which could be inhibited by ouabain. 4. The short-circuit current was nearly equivalent to the net Na Transport and this was also inhibited by ouabain. 5. A transcutaneous active Na transport mechanism thus appears to be incipient in the mudpuppy but is limited by a low permeability of the outer barrier of the cells. 6. Vasotocin increased the skin's diffusion permeability for water but had no effect on the influx of Na or urea. 7. The function of Necturus skin is in several respects unique compared to that of other amphibians.

Lithium transport across isolated frog skin epithelium

Transepithelial Li+ influx was studied in the isolated epithelium from abdominal skin of Rana catesbeiana. With Na+-Ringer's as inside medium and Li+-Ringer's as outside medium, the Li+ influx across the epithelium was 15.6 muA/cm2. This influx was considerably reduced by removal of either Na+ or K+ from the inside bath or by the addition of ouabain or amiloride. Epithelial K+ or Na+ concentration was respectively lower in epithelia bathed in K+-free Ringer's or Na+-free Ringer's. In conditions of negligible Na+ transport, a 20 mM Li+ gradient (outleads toin) produced across the short-circuited epithelium a Li+ influx of 11.8 muA/cm2 and a mean short-circuit current of 10.2 muA/cm2. The same Li+ gradient in the opposite direction produced a Li+ outflux of only 1.9 muA/cm2. With equal Li+ concentration (10.3 and 20.6 mM) on both sides of the epihelium, plus Na+ in the inside solution only, a stable Li+-dependent short-circuit current was observed. Net Li+ movement (outleads toin) was also indirectly determined in the presence of an opposing Li+ gradient. Although Li+ does not substitute for Na+ as an activator to the (Na+ +K+)-ATPase from frog skin epithelium, Li+ influx appears to be related to Na+-K+ pump activity. It is proposed that the permeability of the "outer barrier" to Na+ and Li+ is regulated by the electrical gradient produced by electrogenic Na+-K+ pumps located in the membrane of the deeper epithelial cells.

The mechanism of the calorigenic action of thyroid hormone. Stimulation of Na plus + K plus-activated adenosinetriphosphatase activity

In an earlier study, we proposed that thyroid hormone stimulation of energy utilization by the Na(+) pump mediates the calorigenic response. In this study, the effects of triiodothyronine (T(3)) on total oxygen consumption (Q(OO2)), the ouabain-sensitive oxygen consumption [Q(OO2)(t)], and NaK-ATPase in liver, kidney, and cerebrum were measured. In liver, approximately 90% of the increase in Q(OO2) produced by T(3) in either thyroidectomized or euthyroid rats was attributable to the increase in Q(OO2)(t). In kidney, the increase in Q(OO2)(t) accounted for 29% of the increase in Q(OO2) in thyroidectomized and 46% of the increase in Q(OO2) in euthyroid rats. There was no demonstrable effect of T(3) in euthyroid rats on Q(OO2) or Q(OO2)(t) of cerebral slices. The effects of T(3) on NaK-ATPase activity in homogenates were as follows: In liver +81% from euthyroid rats and +54% from hypothyroid rats. In kidney, +21% from euthyroid rats and +69% from hypothyroid rats. T(3) in euthyroid rats produced no significant changes in NaK-ATPase or Mg-ATPase activity of cerebral homogenates. Liver plasma membrane fractions showed a 69% increase in NaK-ATPase and no significant changes in either Mg-ATPase or 5'-nucleotidase activities after T(3) injection. These results indicate that thyroid hormones stimulate NaK-ATPase activity differentially. This effect may account, at least in part, for the calorigenic effects of these hormones.