Hormone- or neurotransmitter-evoked cytosolic Ca2+ signals are general
ly thought to be initiated by an increase in inositol trisphosphate (I
P3) concentration that in turn opens Ca2+ release channels in the endo
plasmic reticulum. Recent data challenge this simple model and indicat
e IP3 receptors in several organelles and an additional intracellular
messenger system for Ca2+ regulation.