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 (Ifast) and a slow one (Islow). In response to a subsequent application of the same dose of Ca2+, Ifast and Islow 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 Ifast was more significant than that to Islow. Both Ifast and Islow 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, beta-phorbol dibutyrate, caused full inhibition of Ifast without any change in Islow. 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.
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Department of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv, Israel.
Singer
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Dascal
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D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002463:Cell Membrane Permeability; D018118:Chloride Channels; D002712:Chlorides / Q000378:metabolism*; D004553:Electric Conductivity; D005260:Female; D007473:Ion Channels / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D009865:Oocytes; D015240:Phorbol 12,13-Dibutyrate / Q000494:pharmacology; D011493:Protein Kinase C / Q000378:metabolism*; D014982:Xenopus laevis
D018118:Chloride Channels; D002712:Chlorides; D007473:Ion Channels; D008565:Membrane Proteins; D015240:Phorbol 12,13-Dibutyrate; D000001:Calcimycin; D011493:Protein Kinase C; D002118:Calcium