Effects of phosphatidylinositol kinase inhibitors on the activation of the store-operated calcium current I _CRAC in RBL-1 cells

Abstract.

In electrically non-excitable cells, Ca²⁺ entry is mediated predominantly through the store-operated Ca²⁺ influx pathway, which is activated by emptying the intracellular Ca²⁺ stores following an increase in the levels of the second messenger inositol 1,4,5-trisphophate (InsP₃). InsP₃ is generated from the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP₂). Recently, roles for other phosphoinositides (PIs) in store-operated Ca²⁺ influx have been suggested because inhibitors of PI kinases reduce Ca²⁺ influx when the latter is triggered independent of PIP₂ hydrolysis. Using the whole-cell patch-clamp technique to record the store-operated Ca²⁺ current I _CRAC in RBL-1 cells, we examined whether PIs are involved in linking store depletion to activation of CRAC channels. Of several structurally distinct PI kinase inhibitors, only one (LY294002) was able to reduce partially the extent of activation of I _CRAC although this could not be reversed by exogenous phosphatidylinositol 3,4,5-trisphosphate (PIP₃). Our findings suggest that, if a PI kinase is involved in activation of I _CRAC in RBL-1 cells, it has a unique pharmacological profile. Alternative explanations for the results are discussed.