XENOPUS-LAEVIS OOCYTES CONTAIN ENDOGENOUS LARGE-CONDUCTANCE CA2-ACTIVATED K+ CHANNELS()

Xenopus laevis oocytes have become a pre-eminent tool for studying clo ned ion channels, primarily because they intrinsically express low lev els of most types of ion channels. However, when these cells are used for single channel studies, it is essential to determine whether or no t oocytes contain even low levels of endogenous ion channels with prop erties similar to the channel being investigated. We show here that X. laevis oocytes express endogenous large-conductance Ca2+-activated K channels with properties similar to mammalian isoforms of this channe l. The endogenous channels exhibit a voltage-dependence of 12-14 mV pe r e-fold change in open probability (p(o)), can be activated by microm olar Ca2+ concentrations, and have a single channel conductance of app roximately 200 pS in symmetrical 110 mM K+ solutions. Patch clamp expe riments indicate that this endogenous channel is present at low densit ies (similar to 1 channel/3000 mu m(2)). If endogenous channel subunit s can form functional tetramers with other exogenous potassium channel subunits, then they will give rise to the expression of a heterogeneo us channel population. Therefore, studies involving the heterologous e xpression of large-conductance Ca2+-activated K+ channels in Xenopus l aevis oocytes require careful analysis and interpretation. Copyright ( C) 1996 Elsevier Science Ltd