A volume-sensitive, IP3-insensitive Ca2+ store in vascular endothelial cells

The mechanism by which cell swelling and other physical forces increase the intracellular Ca2+ concentration ([Ca2+]i) is poorly defined. In vascular endothelial cells, the increase in [Ca2+]i after hypotonic swelling was independent of external Ca2+ and membrane potential, was not blocked by La3+ or Gd3+, and was prevented by thapsigargin, indicative of intracellular release. This release also occurred after depletion of agonist-sensitive Ca2+ stores. In cells in which the plasma membrane was permeabilized with saponin, hypotonic medium stimulated release of 45Ca2+ from a thapsigargin-sensitive site. Isosmotic substitutions with sucrose or urea revealed that this release was due specifically to swelling and not to changes in osmolarity or ion concentrations. This volume-sensitive release was inhibited by high concentrations of La3+ and Gd3+ in a time-dependent manner, suggesting inhibition from within the storage compartment. Release was not inhibited by ruthenium red or by prior stimulation with inositol 1,4,5-trisphosphate (IP3), indicating that the volume-sensitive storage site is distinct from mitochondria and from stores sensitive to ryanodine or IP3. The results suggest the presence of a novel, stretch-activated Ca2+ store in endothelial cells that could contribute to their mechanosensitivity.