Calcium-dependent potentiation of store-operated calcium channels in T lymphocytes

Abstract

The depletion of intracellular Ca2+ stores triggers the opening of Ca2+ releaseactivated Ca2+ (CRAC) channels in the plasma membrane of T lymphocytes. We have investigated the additional role of extracellular Ca2+ (Ca(o)/2+) in promoting CRAC channel activation in Jurkat leukemic T cells. Ca2+ stores were depleted with 1 μM thapsigargin in the nominal absence of Ca(o)/2+ with 12 mM EGTA or BAPTA in the recording pipette. Subsequent application of Ca(o)/2+ caused I(CRAC) to appear in two phases. The initial phase was complete within 1 s and reflects channels that were open in the absence of Ca(o)/2+. The second phase consisted of a several fold exponential increase in current amplitude with a time constant of 5-10 s; we call this increase Ca2+ dependent potentiation, or CDP. The shape of the current-voltage relation and the inferred single-channel current amplitude are unchanged during CDP, indicating that CDP reflects an alteration in channel gating rather than permeation. The extent of CDP is modulated by voltage, increasing from ~ 50% at + 50 V to ~ 350% at - 75 m V in the presence of 2 mM Ca(o)/2+. The voltage dependence of CDP also causes I(CRAC) to increase slowly during prolonged hyperpolarizations in the constant presence of Ca(o)/2+. CDP is not affected by exogenous intracellular Ca2+ buffers, and Ni2+, a CRAC channel blocker, can cause potentiation. Thus, the underlying Ca2+ binding site is not intracellular. Ba2+ has little or no ability to potentiate CRAC channels. These results demonstrate that the store-depletion signal by itself triggers only a small fraction of capacitative Ca2+ entry and established Ca2+ as a potent cofactor in this process. CDP confers a previously unrecognized voltage dependence and slow time dependence on CRAC channel activation that may contribute to the dynamic behavior of I(CRAC).