Elementary events of agonist-induced Ca2+ release in vascular endothelial cells

The subcellular spatial and temporal organization of agonist-induced Ca2+ signals was investigated in single cultured vascular endothelial cells. Extracellular application of ATP initiated a rapid increase of intracellular Ca2+ concentration ([Ca2+](i)) in peripheral cytoplasmic processes from where activation propagated as a [Ca2+](i) wave toward the central regions of the cell. The average propagation velocity of the [Ca2+](i) wave in the peripheral processes was 20-60 mu m/s, whereas in the central region the wave propagated at <10 mu m/s. The time course of the recovery of [Ca2+](i) depended on the cell geometry. In the peripheral processes (i.e., regions with a high surface-to-volume ratio) [Ca2+](i) declined monotonically, whereas in the central region [Ca2+](i) decreased in an oscillatory fashion. Propagating [Ca2+](i) waves were preceded by small, highly localized [Ca2+](i) transients originating from 1- to 3-mu m-wide regions. The average amplitude of these elementary events of Ca2+ release was 23 nM, and the underlying flux of Ca2+ amounted to similar to 1-2 x 10(-18) mol/s or similar to 0.3 pA, consistent with a Ca2+ flux through a single or small number of endoplasmic reticulum Ca2+-release channels.