Role of the inositol 1,4,5-trisphosphate receptor in Ca2+ feedback inhibition of calcium release-activated calcium current (Icrac)

We examined the activation and regulation of calcium release-activated calcium current (I-crac) in RBL-1 cells in response to various Ca2+ store-depleting agents. With [Ca2+](i) strongly buffered to 100 nM, I-crac was activated by ionomycin, thapsigargin, inositol 1,4,5-trisphosphate (IP,), and two metabolically stable IP3 receptor agonists, adenophostin A and L-alpha-glycerophospho-D-myoinositol-4,5-bisphosphate (GPIP(2)). With minimal [Ca2+](i) buffering, with [Ca2+](i) free to fluctuate I-crac was activated by ionomycin, thapsigargin, and by the potent IP3 receptor agonist, adenophostin A, but not by GPIP(2) or IP3 itself. Likewise, when [Ca2+](i) was strongly buffered to 500 nM, ionomycin, thapsigargin, and adenophostin A did and GPIP(2) and IP3 did not activate detectable I-crac. However, with minimal [Ca2+](i) buffering, or with [Ca2+](i) buffered to 500 nM, GPIP(2) was able to fully activate detectable I-crac if uptake of Ca2+ intracellular stores was first inhibited. Our findings suggest that when IP3 activates the IP3 receptor, the resulting influx of Ca2+ quickly inactivates the receptor, and Ca2+ is re-accumulated at sites that regulate I-crac. Adenophostin A, by virtue of its high receptor affinity, is resistant to this inactivation, Comparison of thapsigargin-releasabIe Ca2+ pools following activation by different IP3 receptor agonists indicates that the critical regulatory pool of Ca2+ may be very small in comparison to the total IP3-sensitive component of the endoplasmic reticulum, These findings reveal new and important roles for IP3 receptors located on discrete IP3-sensitive Ca2+ pools in calcium feedback regulation of I-crac and capacitative calcium entry.