TRANSEPIDERMAL POTENTIAL DIFFERENCE: DEVELOPMENT IN ANURAN LARVAE

Prolactin increases Na+ transport across adult bullfrog skin via stimulation of both ENaC and Na+/K+-pump

PRL is involved in osmoregulation in lower vertebrates. Its serum concentration starts to increase during the metamorphosis of bullfrog tadpoles. Adult bullfrog skin transports Na(+) from the apical to the basolateral side across the skin. PRL is involved in the regulation of this transport. We investigated the effect of ovine PRL on the epithelial Na(+) channel (ENaC), Na(+)/K(+)-pump, and basolateral K(+) channels, which regulate Na(+) transport across adult bullfrog skin, by measuring the short-circuit current (SCC). At 0.1 microg/ml, PRL had no effect on the SCC. PRL (1 microg/ml) was sufficient to stimulate the SCC since 1 and 10 microg/ml of PRL each increased SCC 1.8-fold. Current-fluctuation analysis revealed that PRL (10 microg/ml) increased the density of active ENaC almost 1.8-fold. The effect of PRL on the Na(+)/K(+)-pump was investigated using apically nystatin-permeabilized skin with Ca-free Na-Ringers' solution on each side. PRL (10 microg/ml) increased SCC in this condition around 1.1-fold, suggesting that PRL stimulates the Na(+)/K(+)-pump [although PRL (1 microg/ml) had no effect on this SCC]. The effect of PRL on basolateral K(+) channels was investigated using apically nystatin-permeabilized skin with high-K Ringer's solution on the apical side. PRL (10 microg/ml) had no effect on the SCC, suggesting that PRL does not affect basolateral K(+) channels. Thus, although PRL stimulates the Na(+)/K(+)-pump, this effect probably contributes less than that on ENaC to the regulation of Na(+) transport across adult bullfrog skin.

Larval bullfrog skin expresses ENaC despite having no amiloride-blockable transepithelial Na+ transport

Amiloride-blockable Na(+) transport, measured as an amiloride-blockable short-circuit current (Am-SCC), is mediated by the epithelial Na(+) channel (ENaC). Am-SCC is not normally present in bullfrog tadpole skin, but when such skin is cultured with corticoids an amiloride-blockable Na transport appears. Prolactin (PRL) inhibits its corticoid-induced development. Using specific PCR primers for adult frog ENaC and RT-PCR, we investigated whether corticoids can induce all three ENaC subunits, and whether this expression of ENaC subunit(s) can be blocked by adding PRL with the corticoids. We found that (1) the sequences of the RT-PCR products obtained using primers for alpha-ENaC were identical between larval and adult skins, (2) the mRNAs for all three ENaC subunits were expressed in larval skin under normal conditions despite no amiloride-blockable Na(+) transport being detectable, (3) all three subunits were expressed in larval skins whether they were cultured with corticoids (amiloride-blockable Na transport present) or with corticoids supplemented with PRL (no amiloride-blockable Na transport present). An antibody against a peptide from the alpha-ENaC of adult bullfrog was localized to the apical cells of both larval and adult skins. Since no amiloride-blockable Na transport exists across larval skin under these conditions, these results suggest that ENaC protein was expressed prior to the onset of transport. ENaC may be in the plasma membrane in an inactivated form or, alternatively, within vesicles waiting to be inserted.

Possible role of aldosterone and T(3) in development of amiloride-blockable SCC across frog skin in vivo

There are inconsistencies between the in vitro and in vivo effects of thyroid hormone and aldosterone (Aldo) on the development of an amiloride-blockable short-circuit current (SCC) across bullfrog skin [Takada, M., H. Yai, and K. Takayama-Arita. Am. J. Physiol. 268 (Cell Physiol. 37): C218-C226, 1995]. To address this issue, tadpoles were raised in Aldo + T(3). An amiloride-blockable SCC developed across the skin before forelimbs appeared. Noise analysis of the characteristics (single-channel current, blocking and unblocking rate coefficients, and apparent dissociation constant) of this amiloride-blockable Na(+) channel showed that it really was of the adult type. A similar SCC developed at stage XIX in the skin of tadpoles raised with Aldo alone. These results strongly support our hypothesis that the crucial hormone in the development of this SCC is Aldo but that a suppression mechanism attenuates its effect on SCC development until it is removed by the increase in the serum concentration of thyroid hormone (which starts at stages XVIII-XIX in vivo).

Effect of calcium on development of amiloride-blockable Na+ transport in axolotl in vitro

The axolotl, Ambystoma mexicanum, which has no specific calcium-containing sieve layer in the dermis, provides useful material for the study of the effect of Ca2+ on the development of amiloride-blockable active Na+ transport across the skin of amphibians. We raised axolotls in thyroid hormone or aldosterone or cultured the skin with corticoid plus one of several Ca2+ concentrations and found that 1) although the short-circuit current (SCC) was increased by both aldosterone and 3,3',5-triiodo-L-thyronine in vivo, only corticoid was necessary for such an increase in vitro; 2) the development of the SCC in vitro was both corticoid and Ca2+ dependent, because the SCC was well developed with over 100 microM Ca2+ but not with under 10 microM Ca2+ in the presence of corticoid, nor even with 300 microM Ca2+ without corticoid; and 3) Ca2+, but not corticoid, was necessary for the formation of cell-to-cell junctions, because the resistance of the skin was well developed with 300 microM Ca2+ without corticoid.

Corticoid-induced differentiation of amiloride-blockable active Na+ transport across larval bullfrog skin in vitro

The hormone-induced differentiation of an active Na+ transport across larval bullfrog skin during metamorphosis was investigated in vitro and in vivo. In in vitro experiments, EDTA-treated larval dorsal skin from which apical cells were removed was used. Even in the absence of thyroid hormone, corticoids induced the differentiation. Although aldosterone was the most potent hormone, hydrocortisone or corticosterone was also effective. Prolactin inhibited the corticoid-induced differentiation. The differentiation of the transport system coincided almost exactly with the appearance of adult features of the epidermis, namely, the epidermis at 7 days carried the human blood group antigen A, a specific molecular marker of adult-type bullfrog epidermis. The transport system appeared to develop in cells that had been newly generated from basal cells. On the contrary, in in vivo experiments, the effect of amiloride on the short-circuit current of the skin of tadpoles raised in the presence of aldosterone was very small, suggesting that a mechanism exists to inhibit the ability of aldosterone to induce the differentiation of the transport system in vivo.

Different sensitivity to amiloride of body and tail skins of Rana catesbeiana tadpoles during metamorphosis

Regional differences in potential difference and short-circuit current between the body (dorsal) and the tail skin during metamorphosis of Rana catesbeiana tadpoles were investigated. In body skin, the potential difference and the short-circuit current across the skin develop in two successive steps. At stage XX, the potential difference and the short-circuit current across the body skins were amiloride-insensitive (1st step). At stage XXII, however, amiloride-sensitive potential difference and the short circuit current appeared (2nd step). By contrast, in tail skin the potential difference and the short-circuit current remained amiloride-insensitive (1st step) even at stage XXIII. Since the tail regresses after stage XXIII, the appearance of the second step could not be followed in vivo. To determine whether or not the second step can be induced in the tail, tail skin was cultured under conditions where the skin survives for a much longer period than it does in normally developing tadpoles. Such cultured tail skin generated the amiloride-sensitive potential difference and the short-circuit current and cultured body skin also generated them. Therefore, development of the 2nd step in the tail skin may be delayed in vivo. To characterize the differences between body and tail skin, skins were mutually grafted between body and tail at stage XIII-XV. The body skin grafted on the tail underwent both the 1st and 2nd steps by stage XXII, whereas the tail skin grafted on the body only showed the 1st step by the same stage. These results suggest that the regional specificity of the skin is already established before the prometamorphic stage.

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.

Development of aldosterone-stimulation of short-circuit current across larval frog skin

The short-circuit current (SCC) across isolated skin from bullfrog larvae in developmental stage XXI was small and insensitive to amiloride. Overnight incubation of this tissue with 10(-6) M aldosterone stimulated the SCC from 1.35 +/- 0.55 to 14.55 +/- 4.12 microA.cm-2 with 11.18 +/- 4.46 microA.cm-2 being blocked by 100 microM amiloride. Histologic examination of aldosterone-treated skins revealed a separation of the apical cell layer from the underlying epidermis that was not seen in untreated preparations. The onset of amiloride-sensitive Na+ transport thus coincided with the exposure of the apical surface of newly differentiated epithelial cells. Similar results were obtained with skin from stage XXI larvae whose rate of metamorphosis had been stimulated by 10 micrograms.1-1 thyroxine (T4) but not with skin from T4-treated larvae in stages XIX and XX. Fluctuation analysis of the amiloride-sensitive SCC of the above preparations failed to show a consistent Lorentzian component in the power-density spectrum. Fluctuation analysis was possible on skins from larvae whose development had been accelerated by 7-9 days treatment with 10 micrograms.l-1 triiodothyronine (T3). Aldosterone treatment of these tissues resulted in a significant increase in Na+ channel density.

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.

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.

Toxicities of some heavy metals to the tadpoles of frog, Microhyla ornata (Dumeril & Bibron)

Static bioassays were conducted to determine the relative acute toxicities of five heavy metals (mercury, cadmium, copper, manganese and zinc) to 1-week-old and 4-week-old tadpoles of the frog, Microhyla ornata. Toxic effects were calculated on the basis of LC50 for 24 h, 48 h, 72 h and 96 h exposures at 25.5-26.0 degrees C. Mercury was the most toxic and zinc was the least toxic of the heavy metals tested. The sensitivity of the tadpoles to the heavy metals increased with increased age.

Embryonic chick allantois: functional isolation and development of sodium transport

By removing the shell membranes from the chorioallantoic membrane, the chorion is damaged, as visualized by electron microscopy, and rendered permeable, as evidenced by penetration of horseradish peroxidase and increased inhibition of the allantoic Na+-K+ pump by ouabain applied on the chorionic side. The short-circuit current (SCC) of this functionally isolated allantoic epithelium is augmented by nystatin, a channel-forming ionophore, when applied to the mucosal surface. Electrical parameters were determined for three age groups between 12 and 19 days of incubation. The SCC approximately doubled from the youngest (12-13 days) to the oldest (18-19 days) groups, whereas the transepithelial resistance (Re) of 700-900 omega X cm2 remained the same. Amiloride, an inhibitor of apical Na+ uptake, inhibited 98-100% of the SCC at 10(-4) M in both 15-16 and 18-19 day epithelia. In the 12- to 13-day preparation 20-25% of the SCC was insensitive to 10(-3) M amiloride. The Ki's for amiloride were similar in all preparations, at about 5 X 10(-7) M. Determination of the Hill coefficients for inhibition revealed a lower value (0.75 +/- 0.03) for the 12-13 day preparation compared with the two older preparations with coefficients not significantly different from unity. Replacing Na+ in the bathing solutions abolished the SCC of 18-19 day epithelia, whereas about 15% of the SCC remained at 12-13 days. Thus, during development, the SCC of the allantoic epithelium increases in magnitude and becomes increasingly (to 100%) amiloride-sensitive and Na+-dependent.

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.

The short-term effect of prolactin on the active Na transport system of the tadpole skin during metamorphosis

Prolactin (PRL) induced a transient increase in active Na transport measured as the short circuit current (SCC) of the tadpole skin. Increase in SCC by PRL accelerated as the stage advanced between stages XXII and XXV. Prolactin caused two types of effect on the electric parameters of the active Na transport. In type I, it increased both the electromotive force of the active Na current (ENa) and the resistance to the active Na current (RNa). In type II, it decreased both ENa and RNa. A second application of PRL had no effect on SCC; that is, desensitization to PRL was observed.

Stump currents in regenerating salamanders and newts

We report here that a variety of salamanders and newts from differing habitats all drive a steady ionic electric current out of the forelimb stump tip after forelimb amputation. Several hours after amputation the density of this stump current ranges from about 10 to 100 microA/cm2 in most species, and declines with time. In most cases, the magnitude of the stump current is dependent on the concentration of Na+ in the external medium (an artificial pondwater), suggesting that the well-known Na+ -dependent transcutaneous voltage described in amphibia (particularly frogs) is the EMF for this stump current. These measurements add to those previously reported for the North American red spotted newt (Notophthalmus viridescens), and suggest that electrical changes following amputation of urodele limbs are widespread among members of this group.

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.

A fluctuation analysis study of the development of amiloride-sensitive Na+ transport in the skin of larval bullfrogs (Rana catesbeiana)

In the presence of the Na+ -channel blocker amiloride, the short-circuit current across the skins of bullfrog tadpoles in metamorphic stages XIX-XXIV was subjected to fluctuation analysis. The resulting power spectra contained a Lorentzian component of which the plateau value (S0) decreased while the corner frequency (fc) increased as the mucosal amiloride concentration was increased from 0.5 to 24 microM. From the linear relationship between the fc values and the amiloride concentrations it was possible to determine the binding (k'01) and unbinding (k10) constants for amiloride to its receptor on the Na+ channel. With these parameters as well as short-circuit current and S0 values, the current through the individual Na+ channels (i) was calculated (average 0.58 pA). It did not increase significantly during late metamorphosis. The density of Na+ channels (M) in the apical membrane, on the other hand, increased significantly. It would appear that the increase in short-circuit current which occurs at this time is due primarily to an increase in amiloride-blockable Na+ channels. Unexpectedly, a Lorentzian component could be fitted to power spectra in amiloride-treated skins (stages XIX-XXI) which showed no amiloride-sensitive short-circuit current. Moreover, the typical increase in fc with the amiloride concentration did not occur in these animals.

Poorly selective cation channels in the skin of the larval frog (stage less than or equal to XIX)

The abdominal skin of bullfrog larvae (Rana catesbeiana) was placed in an Ussing-type chamber, and its transepithelial electrical parameters were recorded with mucosal solutions of different ionic composition. With "K+-like" cations (K+, NH+4, RB+, Cs+) the power spectra of the fluctuations in short-circuit current displayed a Lorentzian component (fc = 30 - 40 Hz). The relaxation noise could be suppressed by addition of the K+ -channel blockers Ba2+ and TEA to the mucosal solution. Also, in presence of the ionophore antibiotic nystatin the Lorentizian noise was abolished. The Na+ -channel probes amiloride and benzimidazolyl-2-guanidine (BIG) both enhanced the relaxation noise obtained with the K+-like cations but, with Na+ and Li+, also caused the rise of a relaxation component above the background noise. In presence of amiloride or BIG, the addition of Ba2+, TEA and nystatin still abolished the Lorentizian noise. It can be concluded that the relaxation-noise source is located in the apical cell membranes of the tadpole skin. These spontaneously fluctuating cation channels do not seem to strictly discriminate between K+-like ions (K+, NH+4, Rb+, Cs+) and Na+-like ions (Na+, Li+). On the other hand, well-known specific probes for K+ channels (Ba2+, TEA) and for Na+ channels (amiloride, BIG) interact with this apical cation channel. It is possible that the poorly selective channel plays a role in the ontogenesis of the specific Na+ transport in the maturing frog skin.

The direction of growth of differentiating neurones and myoblasts from frog embryos in an applied electric field

1. Disaggregated single neurones and myoblasts obtained from the neural tube and somites of Xenopus laevis embryos (stages 17-21) were cultured in the presence of steady small electric fields. 2. Neurites grew preferentially towards the negative pole, or cathode, in field strengths of 7-190 mV/mm. Many turned through considerable angles to do so. This effect disappeared below a threshold level of about 7 mV/mm. 3. Greater numbers of neurones sprouted neurites in cultures exposed to an electric field compared to control cultures. The difference could be as much as tenfold. The threshold level of this phenomenon was about 6-8 mV/mm. Other cell types such as pigment cells and fibroblasts were also stimulated to differentiate in culture by an electric field, although to a lesser extent than neurones. 4. Applied electric fields had no effect on the location of the origin of neurites on the cell body. 5. Spherical myoblasts cultured in applied electric fields (36-170 mV/mm) elongated with a bipolar axis of growth which was perpendicular to the electric field. The response was graded and disappeared at field strengths below 36 mV/mm. 6. It is suggested that in vivo, the direction of neural outgrowth from the neural tube and the strict spatial organization of somites might be under the control, in part, of endogenous electric fields. Possible sources of these are discussed.

Sulfatide content and (Na+ + K+)-ATPase activity of skin and gill during larval development of the Chilean frog, Calyptocephalella caudiverbera

The sulfatide content, phospholipid concentration, and (Na+ + K+)-ATPase activity from skin and gills of different stages of larval development of Calyptocephalella caudiverbera (a Chilean frog) were analyzed. Additionally, the short-circuit current in skin was studied. When skin and gills, depending on the stage of larval development, present (Na+ + K+)-ATPase activity, they have a high ratio of sulfatide to amount of membrane and the phosphatidylserine concentration remains unchanged. Sulfatide content and (Na+ + K+)-ATPase activity in skin are in direct relationship with the level of sodium flux present during development. The specific enzymatic hydrolysis of sulfatide with partially purified arylsulfatase of pig kidney inhibits 100% of the ouabain-sensitive (Na+ + K+)-ATPase. The ouabain-insensitive ATPase remains virtually unchanged with the treatment, even with a high concentration of arylsulfatase or with ouabain present in the medium. These experiments strongly suggest a role of sulfatides in the (Na+ + K+)-ATPase activity and, as a consequence, in sodium ion transport.

Prolactin receptors in Rana catesbeiana during development and metamorphosis

Specific binding of ovine prolactin was found in microsomal preparations of tail, gill, and kidney of the bullfrog Ran catesbeiana. Binding by larval and adult liver and by kidney before larval stage XVII was low or nondetectable. Renal binding increased during metamorphic climax and in response to treatment with thyroid hormone. The emergence of renal binding of prolactin may signify a shift in the hormone's participation in the control of hydromineral homeostasis from the gill, which is resorbed, to the kidney. A renal action of prolactin during climax may facilitate metamorphosis.

A quantitative analysis of plasma osmotic pressure during metamorphosis of the bullfrog, Rana catesbeiana

The plasma constituents contributing to osmotic pressure are, in decreasing order: Na+, Cl-, HCO3-, K+, glucose, amino acids, urea and protein. Plasma osmotic pressure increases from 180 mmoles/1 to 200 mmoles/1 throughout development.

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.

Intracellular and intercellular potentials in the early amphibian embryo

1. The sensitivity of the membrane potential of cells of early embryos of Xenopus laevis to variations in the extracellular concentrations of sodium and potassium has been determined.2. Alterations in the extracellular sodium concentration have little effect on the membrane potential at all pregastrular stages tested.3. Up to the 32 cell stage an increase in the concentration of potassium in the bathing medium causes a fall in cell membrane potential only when membrane newly synthesized in the furrow during cleavage is exposed at the embryo surface, during the second half of the cell division cycle.4. Beyond the morula stage (48 cells) a fall in membrane potential on raising external potassium can only be demonstrated when the seal which isolates the intercellular fluid from the bathing medium is broken so that cells lining the inner face of the embryo come into contact with the high potassium solution.5. The results suggest that the egg membrane has little selective permeability whereas membrane synthesized after fertilization is highly potassium permeable.6. No evidence could be obtained for any potential difference between the intercellular fluid and the external bathing medium.7. Dinitrophenol, sodium azide and cyanide prevented normal development only if they were injected into the intercellular cavity. Embryos reared in solutions containing these poisons turned into normal tadpoles.8. The formation of the intercellular cavity could be halted by injecting ouabain into the cavity while it was still small. Embryos reared in ouabain turned into normal tadpoles.9. The results suggest that the active transfer of sodium ions from the cells to the intercellular spaces is an integral part of the formation of the intercellular fluid. A hypothesis for the mechanism of formation of the cavity is put forward along these lines.

Fine structure of desmosomes. , hemidesmosomes, and an adepidermal globular layer in developing newt epidermis

The skin of late embryonic, larval, and young postmetamorphic newts, Taricha torosa, has been examined with particular reference to areas of cellular attachment. Stereo electron microscopic techniques and special staining methods for extracellular materials were utilized in addition to conventional avenues of ultrastructural study to investigate the fine architecture of desmosomes, hemidesmosomes, their associated filament systems, and extracellular materials. No evidence has been found that continuity of tonofilaments between adjacent cells exists at desmosomes. Rather, most of the tonofilaments which approach desmosomes (and perhaps also hemidesmosomes) course toward the "attachment plaque" and then loop, either outside the plaque or within it, and return into the main filament tracts of the cell. These facts suggest that the filamentous framework provides intracellular tensile support while adhesion is a product of extracellular materials which accumulate at attachment sites. Evidence is presented that the extracellular material is arranged as pillars or partitions which are continuous with or layered upon the outer unit cell membrane leaflets and adjoined in a discontinuous dense midline of the desmosome. A similar analysis has been made of extracellular materials associated with hemidesmosomes along the basal surface of epidermal cells. An adepidermal globular zone, separating the basal cell boundary from the underlying basal lamina and collagenous lamellae during larval stages, has been interpreted from enzyme and solvent extraction study as a lipid-mucopolysaccharide complex, the function of which remains obscure. These observations are discussed in relation to prevailing theories of cellular adhesion and epidermal differentiation. They appear consistent with the concept that a wide range of adhesive specializations exists in nature, and that the more highly organized of these, such as large desmosomes and hemidesmosomes, serve as strong, highly supported attachment sites, supplemental in function to a more generalized aggregating mechanism.