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 (IP(3)Rs) 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 IP(3)Rs. 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 IP(3)Rs 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 IN production, blocking IP(3)Rs, 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(Ca)3.1) 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 K(Ca)3.1 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.