Transepithelial chloride conductance in amphibian skin: regulatory mechanisms and localization

Abstract

The transepithelial transport of Na+ by amphibian skin must be accompanied by the corresponding anion, Cl-, and much effort has been devoted to the characterization of Cl- transport. The transepithelial Cl- conductance, G(Cl), is activated by voltage and adenosine 3',5'-cyclic monophosphate (cAMP), shows rectification, requires the presence of Cl- in the pathway and is influenced by factors modifying intracellular signalling cascades and by metabolic poisons such as cyanide (CN-). Until recently, these findings were interpreted as strong evidence for a transcellular path, for which, given the impermeability of the principal cells for Cl-, the mitochondria-rich cells (MRC) are the only candidate. This was supported by the apparent parallelism between G(Cl) and the density of MRC (D(mrc)). Data accumulated in recent years, however, raise serious doubts as to the validity of this concept. The single-channel conductance derived from various techniques is too small by an order of magnitude to account for the observed G(Cl), the very slow time course of conductance activation is not reconcilable with any known membrane channel gating processes, a more thorough examination of the relationship between G(Cl) and D(mrc) fails to show any consistent pattern and analysis of current density immediately above the transporting epithelium using the vibrating voltage probe shows current peaks associated with only a small fraction of MRC, and even so, these current peaks account for about 20% of the transepithelial current. The remaining 80% of the current cannot be localized to specific structures. Given the increasing evidence for close cellular control of tight-junction function, the foregoing findings are equally consistent with an additional, major, paracellular pathway for Cl-. A comprehensive description of Cl- transport must await the final resolution of the transport pathway(s).

Structure—function relationships in the integument of Salamandra salamandra during ontogenetic development

“…It was not possible to determine the transepithelial anion conductance in the low resistance larval integument (Table 1), whereas no finite Clconductance could be found in the high resistance skin of the adults following metamorphosis. It will be interesting to find out whether the voltage-activated conductance that characterizes toad skin (Larsen, 1991;Nagel et al, 2000) can be found in the skin of adult urodeles. Whatever may be the function of band 3 protein in MR-like cells, our experiments do not permit to make any conclusion on the possible association of MR cells and band 3 across the integument of S. salamandra to anion conductance (Larsen, 1991;Devuyst et al, 1993).…”

Skin epithelial transport and structural relationships in naturally metamorphosing Pelobates syriacus

“…Skin epithelium of adult amphibians is specialized in ions and water transport (Larsen, '91;Nagel et al, 2000 for reviews), particularly active Na þ transport. The latter is carried out through the principal cells compartment, and it has been analyzed down to the molecular level (reviewed in Garty and Palmer,'97).…”

α2-Adrenoceptors inhibit antidiuretic hormone-stimulated Na+ absorption across tight epithelia (Rana esculenta)

“…The absorptive system is located in the epithelium, the secretory system resides in the exocrine skin glands. Active transepithelial Na + absorption proceeds across the principal cells whereas Cl -follows passively via either the mitochondria-rich cells or the paracellular pathway [16,18]. The active uptake of Na + via the principal cells can be described by the two-membrane model [14].…”