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.