VOLUME-SENSITIVE, IP3-INSENSITIVE CA2-CELLS( STORE IN VASCULAR ENDOTHELIAL)

The mechanism by which cell swelling and other physical forces increas e the intracellular Ca2+ concentration ([Ca2+](i)) is poorly defined. In vascular endothelial cells, the increase in ([Ca2+](i)) after hypot onic swelling was independent of external Ca2+ and membrane potential, was not blocked by La3+ or Gd3+, and was prevented by thapsigargin, i ndicative of intracellular release. This release also occurred after d epletion of agonist-sensitive Ca2+ stares. In cells in which the plasm a membrane was permeabilized with saponin, hypotonic medium stimulated release of Ca-45(2+) from a thapsigargin-sensitive site. Isosmotic su bstitutions with sucrose or urea revealed that this release was due sp ecifically to swelling and not to changes in osmolarity or ion concent rations. This volume-sensitive release was inhibited by high concentra tions of La3+ and Gd3+ in a time-dependent manner suggesting inhibitio n from within the storage compartment. Release was not inhibited by ru thenium red or by prior stimulation with inositol 1,4,5-trisphosphate (IP3), indicating that the volume-sensitive storage site is distinct f i om mitochondria and from stores sensitive to ryanodine or IP3. The r esults suggest the presence of a novel, stretch-activated Ca2+ store i n endothelial cells that could contribute to their mechanosensitisity.