Further characteristics of the Ca(2+)-inactivated Cl(-) channel in Xenopus laevis oocytes.
Removal of extracellular Ca(2+) activates ion channels in the plasma membrane of defolliculated oocytes of the South Africa clawed toad Xenopus laevis. At present, there is controversy about the nature of the Ca(2+)-inactivated ion channels. Recently, we identified one of these channels as a Ca(2+)-inactivated Cl(-) channel (CaIC) using single channel analysis. In this work we confirm and extend previous observations on the CaIC by presenting a decisive extension of the regulation and inhibition profile. CaIC current is reversibly blocked by the divalent and trivalent cations Zn(2+) (half-maximal blocker concentration, K(1/2) = 8 muM), and Gd(3+) (K(1/2) = 20 muM). Furthermore, CaIC is inhibited by the specific Cl(-) channel blocker NPPB (K(1/2) approximately 3 muM). Interestingly, CaIC-mediated currents are further sensitive to the cation channel inhibitor amiloride (500 muM) but insensitive to its high affinity analogue benzamil (100 muM). An investigation of the pH-dependence of the CaIC revealed a reduction of currents in the acidic range. Using simultaneous measurements of membrane current (I(m)), conductance (G(m)) and capacitance (C(m)) we demonstrate that Ca(2+) removal leads to instant activation of CaIC already present in the plasma membrane. Since C(m) remains constant upon Ca(2+) depletion while I(m) and G(m) increase drastically, no exocytotic transport of CaIC from intracellular pools and functional insertion into the plasma membrane is involved in the large CaIC currents. A detailed overview of applicable blockers is given. These blockers are useful when oocytes are utilized as an expression system for foreign proteins whose investigations require Ca(2+)-free solutions and disturbances by CaIC currents are unwanted. We further compare and discuss our results with data of Ca(2+)-inactivated cation channels reported by other groups.
Amasheh
S
S
Institute for Clinical Physiology, Freie Universität Berlin, UKBF, Hindenburgdamm 30, D-12200 Berlin, Germany.
Weber
W
W
D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D018118:Chloride Channels / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D003300:Copper / Q000494:pharmacology; D004553:Electric Conductivity; D005260:Female; D005682:Gadolinium / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating / Q000187:drug effects*; D009579:Nitrobenzoates / Q000494:pharmacology; D009865:Oocytes / Q000166:cytology / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000378:metabolism; D015290:Second Messenger Systems / Q000187:drug effects; D019311:Staurosporine / Q000494:pharmacology; D014982:Xenopus laevis; D015032:Zinc / Q000494:pharmacology
D018118:Chloride Channels; D009579:Nitrobenzoates; D047428:Protein Kinase Inhibitors; C013407:benzamil; C058176:5-nitro-2-(3-phenylpropylamino)benzoic acid; D003300:Copper; D000584:Amiloride; D005682:Gadolinium; D011494:Protein Kinases; D019311:Staurosporine; D015032:Zinc; D002118:Calcium