Inactivation of calcium-activated chloride conductance in Xenopus oocytes: roles of calcium and protein kinase C

Inactivation of Ca²⁺ -induced Cl^- currents was studied in Xenopus oocytes using the two-electrode voltage-clamp technique. In oocytes permeabilized to Ca²⁺ by treatment with the ionophore A23187, Ca²⁺ influx caused by the addition of 2.5-5 mM Ca²⁺ to the extracellular solution elicited Cl^- currents consisting of two components: a fast, transient one (I_fast) and a slow one (I_slow). In response to a subsequent application of the same dose of Ca²⁺, I_fast and I_slow were reduced (inactivation phenomenon). The inactivation did not depend on the direction of current flow, but did depend on the duration of the first exposure to Ca²⁺. The extent of inactivation of I_fast was more significant than that to I_slow. Both I_fast and I_slow fully recovered from inactivation in less than 30 min. Intracellular injections of 100-400 pmol CaCl₂ evoked large inward currents but did not reduce the amplitude of currents evoked by Ca²⁺ influx. The activator of protein kinase C, β-phorbol dibutyrate, caused full inhibition of I_fast without any change in I_slow. H-7 (1,5-isoquinolinesulfonyl-1,2 methylpiperazine), an inhibitor of protein kinases, strongly reduced the extent of inactivation. Our results suggest that elevation of intracellular Ca²⁺ by Ca²⁺ influx through the plasma membrane causes inactivation of the Ca²⁺ -dependent Cl^- conductance via activation of a Ca²⁺ -dependent protein kinase, possibly protein kinase C, whereas Ca²⁺ arriving at the membrane from inside the cell does not initiate the processes leading to inactivation.