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Effect of cycloheximide on urea facilitated transport through toad gallbladder epithelium
No full textTransepithelial urea outfluxes across toad gallbladder were determined before and after the addition of cycloheximide. The drug inhibits the movement of urea but has no effect on thiourea and antipyrine outfluxes. The inhibition of amide transport is time dependent as also shown in counterflow experiments. These results are consistent with the hypothesis that cycloheximide inhibits the synthesis of membrane proteic sites involved in urea mediated transport
Permeability pathways for non-electrolytes through Bufo bufo gall-bladder
No full textAmphotericin B treatment increases the thiourea, D-xylose and mannitol fluxes and lowers those of urea, N-methyl-urea, acetamide, formamide, and N-N'-dimethyl-thiourea. The degree of flux inhibition is related to the cellular permeability of these compounds. Most probably Amphotericin B increases the permeability of all those molecules across the luminal plasma membrane, but simultaneously elicits a cellular swelling, which reduces the diffusion across the lateral plasma membranes. This effect masks the polyene effect especially for molecules showing a mainly cellular permeation pathway such as amides and lipid soluble molecules
Pesticides alter ion transport across frog (Pelophylax kl. esculentus) skin
No full textWe have measured, in the edible frog (Pelophylax kl. esculentus), the effect of two fungicides (8-hydroxyquinoline and captan), and four herbicides (DCMU, glyphosate, paraquat, and propachlor) on the short-circuit current, whose value gives an estimate of the net ion transport taking place across isolated skin. Glyphosate and paraquat treatment produced a modest increase in short-circuit current, corresponding to 2.6±0.7 and 4.6±0.8 μA·cm−2, whereas the other substances had a more sustained effect, ranging from 9.1±0.6 (propachlor) to 14.8±0.9 μA·cm−2 (captan), which is mainly attributable to an increase in the Na+ absorption, and, to a lesser extent, Cl− secretion. The increase in short-circuit current after pesticide treatment, was partially abolished by AF12198, indomethacin, SC58125, SQ 22536, and W7; these results suggest that pesticides, independently from their chemical structure, induce the release of interleukin-1, which triggers the activity of cyclooxygenase-2, whose products, via a concentration in intracellular cAMP and Ca2+ concentration, increase Na+ absorption. The resulting Na+ disequilibrium must be compensated for by other epithelia, with the only consequence being the dissipation of energy. However, our results are important because they indicate that pesticides interact with the basic cellular machinery, which is responsible for the myriad of biological functions of different cell types
Facilitated transport of urea across the gall-bladder luminal membrane
No full textCounterflow experiments demonstrate the existence of urea counter-transport on the epithelium luminal surface. This phenomenon disappears when 10(-4) M phloretin is added to the perfusion fluid. Moreover counterflow experiments made using thiourea as elicitor, demonstrate that the phenomenon is specific for the urea
Na+ and Cl- net absorption by the isolated skin of Rana esculenta
No full textIn the last five years, several measurements of 22Na+ influx (Ji) and outflux (Jo) across symmetrical parts of the isolated skin of Rana esculenta, under permanent short circuitation, were performed in our Institute. The mean value of the 22Na+ net fluxes (Ji-Jo) exceeded the mean value of the short circuit current measurements (1.14 +/- 0.04 versus 0.98 +/- 0.02 microE.cm-2.h-1, 253 experiments). Since this discrepancy could be due to a concomitant Cl- net absorption, 36Cl- unidirectional fluxes were detected under similar experimental conditions. The Cl- net flux mean value was 0.11 +/- 0.02 microE.cm-2.h-1 (316 experiments) which accounts for 70% of the discrepancy between the Na+ net flux and short circuit current. This Cl- net absorption occurred in the absence of electrochemical gradients and was very likely maintained by a Na+/K+/2Cl- cotransport located at the outermost membrane of the epithelium. In fact bumetanide challenge (10(-5) M in the external fluid) strongly inhibited 36Cl- influx and 22Na+ influx across this tissue and cleared off the discrepancy between short circuit current and sodium net flux
Action of capsaicin and related peptides on the ionic transport across the skin of Rana esculenta
No full textCapsaicin at low concentrations increases the short circuit current (SCC) across frog skin. Simultaneous measurements of both transepithelial fluxes of 22Na or 36Cl demonstrate that the SCC increase is due to stimulation of sodium active absorption. Capsaicin acts through the liberation of several peptides; thus these peptides were tested on the SCC across frog skin. Those more active are, in order of potency: Cyclic Calcitonin Gene Related Peptide (CGRP), Kassinin and Eledoisin, Substance P (SP) and Neurokinin A. Neurokinin B and Vasoactive Intestinal Peptide (VIP) have no effect. Also the actions of SP and CGRP are due mainly to stimulation of Na+ active absorption. A strict parallelism regarding the sensitivity to inhibitors (Naproxen, SQ22536 and CP96345) between SP, CGRP and Capsaicin strengthens the hypothesis that SP and CGRP are liberated by Capsaicin in this tissue
Action of physalaemin on the ionic transport across the frog skin
No full textThe effects of the non-mammalian tachykinin physalaemin were studied on the short circuit current (SCC) and on both influx (J(i)) and outflux (J(o)) of 36Cl- and 22Na+ across the isolated skin of Rana esculenta. Physalaemin, added to the internal bathing fluid, increased SCC in a dose-dependent manner with a maximal effect at 1 μM. This increase was due to a stimulation of both Na+ absorption and Cl- secretion. Bumetanide (20 μM in the internal fluid), an inhibitor of the Na+/K+/2Cl- cotransporter, reduced the action of physalaemin on SCC by 46%. Furthermore diphenylamine-2-carboxylic acid (DPC, 0.1 mM in the external fluid), an inhibitor of Cl- channels, decreased the effect of the peptide on SCC by 48%. It is concluded that physalaemin activates the Na+/K+/2Cl- cotransporter at the basolateral membrane, accumulating Cl- in the cells and favouring its exit through Cl- channels at the outermost membrane of the epithelium. An inhibitor of cyclooxygenases, i.e. naproxen, strongly inhibited the physalaemin effect on SCC, whereas 5,8,11-eicosatriynoic acid (ETI), an inhibitor of lipooxygenases was without effect. Therefore, it is proposed that prostaglandins (probably PGE2) are the cellular mediators of this action. An antagonist of NK1 receptors for tachykinins, CP 99,994, inhibited the physalaemin action on SCC, whereas challenge with SR 48,968, an antagonist of NK2 receptors, had no effect on physalaemin action. It is concluded that physalaemin effect on SCC in frog skin is mediated by its interaction with NK1 receptors
Facilitated transport of urea across the toad gallbladder
No full textThe toad gallbladder epithelium is much more selective than that of the rabbit especially as to the permeability of two molecules like urea and thiourea. These observations can probably be attributed to different permeation mechanisms of the 2 molecules. Neither active transport nor solvent drag can explain these phenomena. 10(-4) M phloretin strongly inhibits urea movement, but does not alter either thiourea fluxes or isotonic net water transport: these results suggest that a specific mechanism is involved in urea movement. The urea transport shows saturation kinetic which is consistent with the presence of a facilitated mechanism
Permeability of three herbicides across the isolated frog and pig skins
No full textAim:
Although it is generally accepted that the animal skin represents a barrier to the permeation of pesticides in the living organism, experimental evidences are lacking. For this reason we measured the transcutaneous permeability of three heavily used herbicides (atrazine, paraquat, glyphosate) across the skin of frog and pig.
Methods:
The ventral skin, from Rana esculenta, and the ear skin, from pig, were isolated and mounted in a specially constructed perfusion apparatus preventing the edge damage interference. Skin was perfused on both sides with a Ringer solution. 1 mCi of the 14C- labelled chemical under investigation was added to one compartment. From the opposite compartment, 4 ml were collected after 6 or 24 h, for frog or pig, respectively. Radioactivity was measured with a scintillation counter.
Results:
Permeability values of frog skin were higher than those measured in pig skin. For example, in the case of atrazine, values were 57,68 10-3 cm h-1 and 0,92 10-3 cm h-1, respectively. In both cases the rank order of permeability was: atrazine > paraquat > glyphosate.
Conclusion:
The low permeability of herbicides across the pig skin confirms that the skin is not an important site of permeation of xenobiotics in mammals. On the other hand, the high permeability value of atrazine across the frog skin accounts for its toxic effect on the whole organism and justifies its banning by several countries in Europe
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