Capacitative Ca2+ entry is closely linked to the filling state of internal Ca2+ stores:: A study using simultaneous measurements of ICRAC and intraluminal [Ca2+]

I-CRAC (the best characterized Ca2+ current activated by store depletion) was monitored concurrently for the first time with [Ca2+] changes in internal stores, To establish the quantitative and kinetic relationship between these two parameters, we have developed a novel means to clamp [Ca2+] within stores of intact cells at any level. The advantage of this approach, which is based on the membrane-permeant low-affinity Ca2+ chelator N,N,N',N'-tetrakis (2-pyridylmethyl)ethylene diamine (TPEN), is that [Ca2+] within the ER can be lowered and restored to its original level within 10-15 s without modifications of Ca2+ pumps or release channels. Using these new tools, we demonstrate here that Ca2+ release-activated Ca2+ current (I-CRAC) is activated (a) solely by reduction of free [Ca2+] within the ER and (b) by any measurable decrease in [Ca2+](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.