Capacitative Ca(2+) Entry Is Closely Linked to the Filling State of Internal Ca(2+) Stores: A Study Using Simultaneous Measurements of I(CRAC) and Intraluminal [Ca(2+)]

I(CRAC) (the best characterized Ca(2+) current activated by store depletion) was monitored concurrently for the first time with [Ca(2+)] changes in internal stores. To establish the quantitative and kinetic relationship between these two parameters, we have developed a novel means to clamp [Ca(2+)] within stores of intact cells at any level. The advantage of this approach, which is based on the membrane-permeant low-affinity Ca(2+) chelator N,N,N′,N′-tetrakis (2-pyridylmethyl)ethylene diamine (TPEN), is that [Ca(2+)] within the ER can be lowered and restored to its original level within 10–15 s without modifications of Ca(2+) pumps or release channels. Using these new tools, we demonstrate here that Ca(2+) release–activated Ca(2+) current (I(CRAC)) is activated (a) solely by reduction of free [Ca(2+)] within the ER and (b) by any measurable decrease in [Ca(2+)](ER). We also demonstrate that the intrinsic kinetics of inactivation are relatively slow and possibly dependent on soluble factors that are lost during the whole-cell recording.