The activation of vertebrate development at fertilization relies on IP₃-dependent Ca²⁺ release, a pathway that is sensitized during oocyte maturation. This sensitization has been shown to correlate with the remodeling of the endoplasmic reticulum into large ER patches, however the mechanisms involved are not clear. Here we show that IP₃ receptors within ER patches have a higher sensitivity to IP₃ than those in the neighboring reticular ER. The lateral diffusion rate of IP₃ receptors in both ER domains is similar, and ER patches dynamically fuse with reticular ER, arguing that IP₃ receptors exchange freely between the two ER compartments. These results suggest that increasing the density of IP₃ receptors through ER remodeling is sufficient to sensitize IP₃-dependent Ca²⁺ release. Mathematical modeling supports this concept of 'geometric sensitization' of IP₃ receptors as a population, and argues that it depends on enhanced Ca²⁺-dependent cooperativity at sub-threshold IP₃ concentrations. This represents a novel mechanism of tuning the sensitivity of IP₃ receptors through ER remodeling during meiosis.
