Plasma membrane IP₃ receptors

IP₃Rs (inositol 1,4,5-trisphosphate receptors) are expressed in the membranes of non-mitochondrial organelles in most animal cells, but their presence and role within the plasma membrane are unclear. Whole-cell patch–clamp recording from DT40 cells expressing native or mutated IP₃Rs has established that each cell expresses just two or three functional IP₃Rs in its plasma membrane. Only approx. 50% of the Ca²⁺ entry evoked by stimulation of the B-cell receptor is mediated by store-operated Ca²⁺ entry, the remainder appears to be carried by the IP₃Rs expressed in the plasma membrane. Ca²⁺ entering the cell via just two large-conductance IP₃Rs is likely to have very different functional consequences from the comparable amount of Ca²⁺ that enters through the several thousand low-conductance store-operated channels.