Ca2+ induces charybdotoxin-sensitive membrane potential changes in rat lymphocytes.
There is disagreement regarding the existence of Ca2+-activated K+ channels in lymphocytes. Depolarization, hyperpolarization, or little change in membrane potential (Em) has been reported following elevation of free cytosolic Ca2+ concentration ([Ca2+]i). Patch-clamping studies have demonstrated inhibition of voltage-gated K+ channels, but Ca2+-activated K+ channels have not been detected. We used charybdotoxin (CTX), a potent inhibitor of Ca2+-activated K+ channels, to assess their presence in rat thymic lymphocytes. Fluorescent probes were used to measure Em and [Ca2+]i in cell suspensions treated with ionomycin. At basal [Ca2+]i, CTX had no effect on Em, suggesting that Ca2+-activated K+ channels do not contribute importantly to the resting potential. Elevation of [Ca2+]i in the submicromolar range induced a hyperpolarization that was dependent on the outward K+ gradient. The shape and duration of the Em change closely followed the elevation of [Ca2+]i. This hyperpolarization was inhibited by nanomolar concentrations of CTX. When [Ca2+]i approached or exceeded 1 microM, a biphasic Em change was recorded. A transient, CTX-sensitive hyperpolarization was followed by a sustained depolarization. The latter was greatly reduced when external Na+ was omitted. The data suggest that thymic lymphocytes possess Ca2+-sensitive K+ channels, which are activated by moderate increases in [Ca2+]i, resulting in hyperpolarization. At higher [Ca2+]i, the effect of K+ channels on Em is superseded by opening of nonselective cation channels, producing depolarization. Variations in the level of [Ca2+]i attained in earlier studies can explain existing discrepancies.
Grinstein
S
S
Division of Cell Biology, Hospital for Sick Children, Toronto, Canada.
Smith
J D
JD
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002478:Cells, Cultured; D018999:Charybdotoxin; D002712:Chlorides / Q000502:physiology; D003600:Cytosol / Q000378:metabolism; D004533:Egtazic Acid / Q000031:analogs & derivatives; D004987:Ethers / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D015759:Ionomycin; D007700:Kinetics; D008214:Lymphocytes / Q000187:drug effects / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D051381:Rats; D011919:Rats, Inbred Strains; D012604:Scorpion Venoms / Q000494:pharmacology*
D002712:Chlorides; D004987:Ethers; D012604:Scorpion Venoms; D018999:Charybdotoxin; D004533:Egtazic Acid; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D015759:Ionomycin; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium