Abstract
The whole-cell patch-clamp technique has been used to study membrane currents in cultured rabbit medullary thick ascending limb (MTAL) epithelial cells. A Ca2+-activated K+ current was characterized by its voltage-dependent and Ca2+-dependent properties. When the extracellular K+ ion concentration was increased from 2 to 140 mm, the rereversal potential (Ek) was shifted from −85 to 0 mV with a slope of 46 mV per e-fold change. The Ca2+-activated K+ current is blocked by charybdotoxin (CTX) in a manner similar to the apical membrane Ca2+-activated K+ channel studied with the single channel patch-clamp technique. The results suggest that the Ca2+-activated K+ current is the predominant, large conductance and Ca2+-dependent K+ pathway in the cultured MTAL cell apical membrane. The biophysical properties and physiological regulation of a Cl− current were also investigated. This current was activated by stimulation of intracellular cAMP using forskolin and isobutyl-1-methylxanthine (IBMX). The current-voltage (I–V) relationship of the Cl− current showed an outward-rectifying pattern in symmetrical Cl− solution. The Cl− selectivity of the whole-cell current was confirmed by tail current analysis in different Cl− concentration bath solutions. Several Cl− channel blockers were found to be effective in blocking the outward-rectifying Cl− current in MTAL cells. The cAMP-dependent Cl− transport in MTAL cells was further confirmed by measuring changes in the intensity of Cl− sensitive dye using fluorescence microscopy. These results suggest that the Cl− channel in the apical or basolateral membrane of MTAL cells may be regulated by cAMP-dependent protein-kinase-induced phosphorylation.
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This study was supported by the National Institutes of Health grants GM46834 to L.L. and DK32753 to W.B.G., and by a Grant-in-Aid from the American Heart Association of Ohio to L.L.
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Lu, L., Markakis, D. & Guggino, W.B. Identification and regulation of whole-cell Cl− and Ca2+-activated K+ currents in cultured medullary thick ascending limb cells. J. Membarin Biol. 135, 181–189 (1993). https://doi.org/10.1007/BF00231443
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DOI: https://doi.org/10.1007/BF00231443