LUMINAL CA2-TRISPHOSPHATE FOR ITS RECEPTOR( INCREASES THE AFFINITY OFINOSITOL 1,4,5)

Luminal Ca2+ has been proposed to regulate the sensitivity of intracel lular Ca2+ stores to InsP3 and perhaps thereby to contribute to the me chanisms responsible for regenerative intracellular Ca2+ signals. Sinc e Ca2+ release from intracellular stores is accompanied by K+ influx i nto the stores, InsP3-stimulated Ca2+ mobilization can be inhibited by substantially reducing the [K+] of incubation media. By measuring [H- 3]InsP3 binding to permeabilized hepatocytes in K+-deficient media, th ereby preventing InsP3-stimulated Ca2+ mobilization, we have examined the effects of Ca2+ within the intracellular stores on equilibrium bin ding of InsP3 to its receptor. Our results demonstrate a small, but si gnificant (P < 0.05, n = 8-9), effect of luminal Ca2+ on InsP3 binding ; the concentration of InsP3 causing half-maximal displacement of [H-3 ]InsP3 (1.25 nM) was 3.5 nM for empty stores and 2.1 nM for stores con taining Ca2+. Our results suggest that at least part of the stimulator y effect of luminal Ca2+ on InsP3-stimulated Ca2+ mobilization may res ult from an effect of luminal Ca2+ on InsP3 binding to its receptor.