Slow kinetics of InsP3-induced Ca2+ release: differences between uni- and bi-directional 45Ca2+ fluxes

The effects of a long-lasting stimulation with inositol 1,4,5-trisphosphate (InsP3) have been studied in monolayers of permeabilized A7r5 cells. When measured under unidirectional 45Ca2+ efflux conditions, i.e. in the presence of 2 microM thapsigargin, an initial fast release was observed which then progressively slowed down into a slow phase which persisted for up to 20 min. When measured under bidirectional 45Ca2+ flux conditions with functional Ca2+ pumps, a transient phase of re-uptake occurred between the initial fast and the subsequent slow release phase. These kinetics are compatible with intrinsic inactivation of the InsP3 receptor. However, this inactivation did not prevent the slow release component. The slow component was not due to the accumulation of an InsP3 metabolite nor to a GTP-dependent translocation of Ca2+ between stores. The slow release phase was more pronounced when the Ca2+ pumps were active than when they were inhibited. This observation is compatible with other findings indicating that the InsP3 receptor is controlled by luminal Ca2+. The decreasing effectiveness of a 20 min lasting InsP3 challenge in mobilizing Ca2+ from less filled stores is most likely due to a progressive depletion of the store and cannot be considered as an experimental artifact caused by a preferential emptying of InsP3-sensitive Ca2+ stores. We conclude that the InsP3 receptor can intrinsically inactivate but that this inactivation is unable to prevent the slow release, which is especially pronounced when Ca2+ pumps are active.