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

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Abstract
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.