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 At higher [Ca2+]i, the effect of K+ channels on Em is superseded by opening of nonselective cation channels, producing depolarization.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.