Functional architecture of inositol 1,4,5-trisphosphate signaling in restricted spaces of myoendothelial projections

Calcium (Ca²⁺) release through inositol 1,4,5-trisphosphate receptors (IP₃Rs) regulates the function of virtually every mammalian cell. Unlike ryanodine receptors, which generate local Ca²⁺ events ("sparks") that transmit signals to the juxtaposed cell membrane, a similar functional architecture has not been reported for IP₃Rs. Here, we have identified spatially fixed, local Ca²⁺ release events ("pulsars") in vascular endothelial membrane domains that project through the internal elastic lamina to adjacent smooth muscle membranes. Ca²⁺ pulsars are mediated by IP₃Rs in the endothelial endoplasmic reticulum of these membrane projections. Elevation of IP₃ by the endothelium-dependent vasodilator, acetylcholine, increased the frequency of Ca²⁺ pulsars, whereas blunting IP₃ production, blocking IP₃Rs, or depleting endoplasmic reticulum Ca²⁺ inhibited these events. The elementary properties of Ca²⁺ pulsars were distinct from ryanodine-receptor-mediated Ca²⁺ sparks in smooth muscle and from IP₃-mediated Ca²⁺ puffs in Xenopus oocytes. The intermediate conductance, Ca²⁺-sensitive potassium (KCa3.1) channel also colocalized to the endothelial projections, and blockage of this channel caused an 8-mV depolarization. Inhibition of Ca²⁺ pulsars also depolarized to a similar extent, and blocking KCa3.1 channels was without effect in the absence of pulsars. Our results support a mechanism of IP₃ signaling in which Ca²⁺ release is spatially restricted to transmit intercellular signals.