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. Ca 2+ 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 (K Ca3.1) channel also colocalized to the endothelial projections, and blockage of this channel caused an 8-mV depolarization. Inhibition of Ca 2+ pulsars also depolarized to a similar extent, and blocking K Ca3.1 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. © 2008 by The National Academy of Sciences of the USA.