Electrical and transport characteristics of skin of larval Rana catesbeiana

A lethal fungal pathogen directly alters tight junction proteins in the skin of a susceptible amphibian

Bacterial and viral pathogens can weaken epithelial barriers by targeting and disrupting tight junction (TJ) proteins. Comparatively, however, little is known about the direct effects of fungal pathogens on TJ proteins and their expression. The disease, chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), is threatening amphibian populations worldwide. Bd is known to infect amphibian skin and disrupt cutaneous osmoregulation. However, exactly how this occurs is poorly understood. This study considered the impact of Bd infection on the barrier properties of the Australian green tree frog (Litoria caerulea) epidermis by examining how inoculation of animals with Bd influenced the paracellular movement of FITC-dextran (4 kDa, FD-4) across the skin in association with alterations in the mRNA and protein abundance of select TJ proteins of the epidermal TJ complex. It was observed that Bd infection increased paracellular movement of FD-4 across the skin linearly with fungal infection load. In addition, Bd infection increased transcript abundance of the tricellular TJ (tTJ) protein tricellulin (tric) as well as the bicellular TJ (bTJ) proteins occludin (ocln), claudin (cldn) -1, -4 and the scaffolding TJ protein zonula occludens-1 (zo-1). However, while Tric protein abundance increased in accord with changes in transcript abundance, protein abundance of Cldn-1 was significantly reduced and Ocln protein abundance was unchanged. Data indicate that disruption of cutaneous osmoregulation in L. caerulea following Bd infection occurs, at least in part, by an increase in epidermal paracellular permeability in association with compromised integrity of the epidermal TJ complex.

Stretch-activated cation channel from larval bullfrog skin

Cell-attached patches from isolated epithelial cells from larval bullfrog skin revealed a cation channel that was activated by applying suction (-1 kPa to -4.5 kPa) to the pipette. Activation was characterized by an initial large current spike that rapidly attenuated to a stable value and showed a variable pattern of opening and closing with continuing suction. Current-voltage plots demonstrated linear or inward rectification and single channel conductances of 44-56 pS with NaCl or KCl Ringer's solution as the pipette solution, and a reversal potential (-V(p)) of 20-40 mV. The conductance was markedly reduced with N-methyl-D-glucamide (NMDG)-Cl Ringer's solution in the pipette. Neither amiloride nor ATP, which are known to stimulate an apical cation channel in Ussing chamber preparations of larval frog skin, produced channel activation nor did these compounds affect the response to suction. Stretch activation was not affected by varying the pipette concentrations of Ca(2+) between 0 mmol l(-1) and 4 mmol l(-1) or by varying pH between 6.8 and 8.0. However, conductance was reduced with 4 mmol l(-1) Ca(2+). Western blot analysis of membrane homogenates from larval bullfrog and larval toad skin identified proteins that were immunoreactive with mammalian TRPC1 and TRPC5 (TRPC, canonical transient receptor potential channel) antibodies while homogenates of skin from newly metamorphosed bullfrogs were positive for TRPC1 and TRPC3/6/7 antibodies. The electrophysiological response of larval bullfrog skin resembles that of a stretch-activated cation channel characterized in Xenopus oocytes and proposed to be TRPC1. These results indicate this channel persists in all life stages of anurans and that TRP isoforms may be important for sensory functions of their skin.

Effects of arginine vasotocin and mesotocin on the activation and development of amiloride-blockable short-circuit current across larval, adult, and cultured larval bullfrog skins

Amphibian skin has osmoregulatory functions, with Na(+) crossing from outside to inside. Na(+) transport can be measured as the short-circuit current (SCC). We investigated the short-term and long-term effects of arginine vasotocin (AVT) and mesotocin (MT) (which modulate Na(+) transport) on the activation and development of an amiloride-blockable SCC (adult-type feature) in larval, adult, and corticoid-cultured larval bullfrog skins. We found: (1) AVT-receptor (AVT-R) and MT-receptor (MT-R) mRNAs could be detected in both larval and adult skins, (2) in the short term (within 60 min), the larval SCC (amiloride-stimulated SCC) was increased by AVT, forskolin, and MT, suggesting that AVT and MT did not activate the inactive ENaC (epithelial sodium channel) protein thought to be expressed in larval skin, (3) in the short term (within 90 min), AVT, forskolin, and MT stimulated the adult SCC (amiloride-blockable SCC), (4) AVT and MT increased both the larval and adult SCC via receptors insensitive to OPC-21268 (an antagonist of the V(1)-type receptor), OPC-31260 (an antagonist of the V(2)-type receptor), and ([d(CH(2))(5),Tyr(Me)(2),Thr(4),Orn(8),des-Gly-NH (2) (9) ]VT) (an antagonist of the oxytocin receptor), (5) culturing EDTA-treated larval skin with corticoids supplemented with AVT (1 microM) or MT (1 microM) for 2 weeks (long-term effects of AVT and MT) did not alter the corticoid-induced development of an amiloride-blockable SCC (adult-type feature). AVT and MT thus have the potential to stimulate SCC though channels that are already expressed, but they may not influence the development of the amiloride-blockable SCC (an adult-type feature) in larval 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.

Growth hormone is a weaker candidate than prolactin for the hormone responsible for the development of a larval-type feature in cultured bullfrog skin

Prolactin (PRL) has, for some years, been considered to be the 'juvenile hormone' in amphibians. Recently, growth hormone (GH) has been proposed as another candidate, because in the larval stages the expression of the mRNA GH is high but it is downregulated in the climax stages of metamorphosis or following treatment with thyroid hormone. In the present study, we investigated whether GH promotes the development of one particular larval-type feature of bullfrog tadpole skin in vitro. The amiloride-, acetylcholine- and ATP-stimulated short-circuit current (SCC) is a physiological marker of larval-type bullfrog skin. These types of ligand-stimulated SCC (1) developed when EDTA-treated tadpole skin was cultured with corticoids supplemented with PRL or GH and (2) were not significantly different between skin cultured with PRL and intact tadpole skin. However, the amiloride-induced SCC response in skin cultured with GH differed in its kinetics from that of the intact (control) tadpole. On this basis, PRL seems a better candidate than GH for the juvenile hormone, at least with regard to the development of amiloride-stimulated non-selective cation channels.

Prolactin increases open-channel density of epithelial Na+ channel in adult frog skin

The short-term effect of prolactin on the skin of the adult tree frog Hyla arborea japonica was investigated using current-fluctuation analysis. Basolateral application of ovine prolactin (10 microg ml(-1)) (1) increased the amiloride-blockable short-circuit current (SCC) across the skin 2.6+/-0.4-fold and (2) increased the open-channel density (M) of the epithelial Na(+) channel 6.1+/-1.2-fold but decreased the single-channel current i to 0.4+/-0.1 times the control value (N=9). The increase in SCC induced by prolactin was thus due to an increase in M, not i. Apparently, in amphibians prolactin has not only a counteracting effect on metamorphosis but also a stimulatory effect on the development of adult-type features, such as this amiloride-blockable SCC.

Cloning and characterization of a functional P2X receptor from larval bullfrog skin

ATP activates an apical-to-basolateral nonselective cation current across the skin of larval bullfrogs (Rana catesbeiana) with similarities to currents carried by some P2X receptors. A functional P2X receptor was cloned from tadpole skin RNA that encodes a 409-amino acid protein with highest protein homology to cP2X(8). RT-PCR showed that this transcript was found in skin, heart, eye, brain, and skeletal muscle of tadpoles but not in skin, brain, or heart of adults. After transcribed RNA from this clone was injected into Xenopus oocytes, application of ATP activated a transient current similar to other P2X receptors and the ATP-activated transient in short-circuit current (I(sc)) across intact skin. The agonists 2-methylthio-ATP and adenosine-5'-O-(thiotriphoshate) also activated transient currents. alpha,beta-Methylene-ATP and ADP were poor agonists of this receptor. Suramin and pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS) were potent antagonists, and PPADS showed an irreversible blockade of this receptor to agonist activation. Under external Na(+)-free, Ca(2+)/Mg(2+)-free conditions (N-methyl-D-glucamine replacement, 0.5 mM EGTA), ATP activated a steadily increasing inward current. Fluorescence microscopy showed that propidium was entering the cells, suggesting that a relatively large pore size was formed under zero divalent conditions. This clone has some characteristics consistent with previously described ATP-activated I(sc) in the tadpole skin. Because the clone is not found in adult skin, it may have some exclusive role in the tadpole such as sensory reception by the skin or triggering apoptosis at metamorphosis.

Sodium-dependent short-circuit current across the yolk sac membrane during embryonic development in normal and shell-less cultured chicks

The transepithelial electrical characteristics of the isolated yolk sac membrane of normal in ovo or shell-less cultured chick embryos were investigated. In normal chicks the potential difference (blood side positive relative to yolk side) and short-circuit current of the membrane increased during development. Ouabain (10(-4) M) on the blood side (basolateral side, serosal side) significantly decreased potential difference and short-circuit current but was without effect on the yolk side (brush border side, mucosal side). Substitution of choline for Na+ in the bathing solutions abolished the potential difference and the short-circuit current; when Na+ replaced choline this effect was reversed. Amiloride added to both sides of the yolk sac membrane had no effect on potential difference or short-circuit current. Injection of aldosterone (50 micrograms) and T3 (10 microM) into yolk did not induce amiloride sensitivity. The short-circuit current was not altered by addition of either glucose or alanine to the bath. The short-circuit current of the yolk sac membrane of shell-less cultured embryos was significantly lower than that of normal controls. Addition of Ca2+ to the serosal bathing medium did not reverse the foregoing condition, but decreased the short-circuit current. It is concluded that the yolk sac short-circuit current is Na+ dependent and increases with developmental age in the chick embryo.

Different modes of electrogenic Na+ absorption in the coprodeum of the chicken embryo: role of extracellular Ca2+

Transepithelial electrogenic Na+ transport (INa) was investigated in the coprodeum of 20-days-old chicken embryos in Ussing chambers. Short circuit current (Isc) and transepithelial resistance (Rt) were 14.7 +/- 4.8 microA.cm-2 (n = 12) and 0.53 +/- 0.09 k omega.cm-2 (n = 12), respectively. INa was calculated from changes in Isc by substitution of mucosal Na+ by (N-methyl-D-glucamine) (NMDG). Isc inversed during Na+ removal, and INa was found to be 27.8 +/- 4.7 microA.cm-2 (n = 12). Amiloride (100 mumol.l-1) inhibited only about 60% of INa. Analysis of Isc fluctuations revealed a Lorentzian component in the power density spectrum with a corner frequency of about 57 Hz. This component was not correlated to INa, and its origin is still unclear. Removal of mucosal Ca2+ increased INa about 2.5-fold due to an increase of the amiloride-insensitive component of INa in additionally investigated adult tissues. The results clearly show that this is due to a non-selective cation channel with an "apparent" order of selectivity Cs+ greater than Na+ = K+ greater than Rb+ greater than Li+. The Ca2+ concentration required to block 50% of the Isc was about 18 mumol.l-1. The IscCa could also be suppressed by other divalent cations such as Mg2+ and Ba2+. Additionally, an INa-linked Lorentzian component occurred which dominated the control spectrum with a significantly higher corner frequency (about 88 Hz). The results indicate that Na+ absorption in the coprodeum of the chicken embryo is more complex than in adult hens.(ABSTRACT TRUNCATED AT 250 WORDS)