CA2+-ACTIVATED K+ CHANNELS IN RAT THYMIC LYMPHOCYTES - ACTIVATION BY CONCANAVALIN-A

1. The role of ion channels in the mitogenic response of rat thymic lymphocytes to concanavalin A (ConA) was studied using single-channel patch-clamp recordings and measurements of membrane potential with the fluorescent probe bis-oxonol. 2. ConA (20-mu-g ml-1) evoked a rapid membrane hyperpolarization; Indo-1 measurements indicated a concurrent increase in [Ca2+]i. The hyperpolarization was blocked by cytoplasmic loading with the Ca2+ buffer BAPTA (bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), or charybdotoxin, a component of scorpion venom known to block K+ channels in lymphocytes. 3. Cell-attached patch-clamp recordings showed that both ConA and the Ca2+ ionophore ionomycin activated channels with high selectivity for K+. Two conductance levels were observed - 6-7 pS and 17-18 pS - measured as inward chord conductance at 60 mV from reversal potential (E(rev)) with 140 mM-KCl in the pipette. The current-voltage relationship for the larger channel displayed inward rectification and channel open probability was weakly dependent upon membrane potential. 4. These experiments provide the first direct evidence for mitogen-activated Ca2+-gated K+ channels (I(K(Ca))) in lymphocytes. This conductance is relatively inactive in unstimulated rat thymocytes but following the intracellular Ca2+ rises induced by ConA, I(K(Ca)) channels are activated and produce a significant hyperpolarization of the cell potential.