A Ca2+-activated channel from Xenopus laevis oocyte membranes reconstituted into planar bilayers.

Abstract

Plasma membrane fractions from X. laevis oocytes were incorporated into planar lipid bilayers. There are numerous Ca2+-activated nonspecific channels that are more permeable to anions. These channels are activated by Ca2+ at micromolar concentration but not by Mg2+, Zn2+ or M2+, even at millimolar concentrations. Decreasing Ca2+ concentration to less than 1 .mu.M decreases the time of channel opening until channels close completely in the absence of Ca2+ and in the presence of EGTA [ethylene glycol bis(.beta.-aminoethyl ether)N,N,N'',N''-tetracetic acid]. I- and Br- are more permeable through this channel than Cl-. The time during which the channels remain open is also voltage-dependent, with the channels switching off at higher voltages in both polarities. Single-channel activity shows a conductance of 380 pS in 1 M NaCl and 1 mM CaCl2, with an averavge open lifetime of 1.5 s at 40 mV. Similar channels are found in different stages of oocyte maturation. The hypothesis that an increase in oocyte-free Ca2+ directly activates these channels, and the resultant Cl- efflux forms the ionic basis for the fertilization potential in X. laevis is supported.