Calcium Current Activated by Depletion of Calcium Stores in Xenopus Oocytes

Ca²⁺ currents activated by depletion of Ca²⁺ stores in Xenopus oocytes were studied with a two-electrode voltage clamp. Buffering of cytosolic Ca²⁺ with EGTA and MeBAPTA abolished I_Cl(Ca) and unmasked a current in oocytes that was activated by InsP₃ or ionomycin in minutes and by thapsigargin or the chelators themselves over hours. At −60 mV in 10 mM extracellular CaCl₂, the current was typically around −90 or −160 nA in oocytes loaded with EGTA or MeBAPTA, respectively. This current was judged to be a Ca²⁺-selective current for the following reasons: (a) it was inwardly rectifying and reversed at membrane potentials usually more positive than +40 mV; (b) it was dependent on extracellular [CaCl₂] with K_m = 11.5 mM; (c) it was highly selective for Ca²⁺ against monovalent cations Na⁺ and K⁺, because replacing Na⁺ and K⁺ by N-methyl-d-glucammonium did not reduce the amplitude or voltage dependence of the current significantly; and (d) Ca²⁺, Sr²⁺, and Ba²⁺ currents had similar instantaneous conductances, but Sr²⁺ and Ba²⁺ currents appeared to inactivate more strongly than Ca²⁺. This Ca²⁺ current was blocked by metal ions with the following potency sequence: Mg²⁺ << Ni²⁺ ≈ Co²⁺ ≈ Mn²⁺ < Cd²⁺ << Zn²⁺ << La³⁺. It was also inhibited by niflumic acid, which is commonly used to block I_Cl(Ca). PMA partially inhibited the Ca²⁺ current, and this effect was mostly abolished by calphostin C, indicating that the Ca²⁺ current is sensitive to protein kinase C. These results are the first detailed electrophysiological characterization of depletion-activated Ca²⁺ current in nondialyzed cells. Because exogenous molecules and channels are easy to introduce into oocytes and the distortions in measuring I_Cl(Ca) can now be bypassed, oocytes are now a superior system in which to analyze the activation mechanisms of capacitative Ca²⁺ influx.