Interactions between inositol trisphosphate and Ca 2+ dependent Ca 2+ release mechanisms on the endoplasmic reticulum of permeabilised bovine aortic endothelial cells

Abstract

In this paper we describe data from cultured bovine aortic endothelial (BAE) cells demonstrating a Ca 2+ induced Ca 2+ release (CICR) process which appears to have pharmacological properties different from CICR mechanisms in other cell types. CICR was measured in saponin permeabilised cells in which the internal stores had been preloaded with 45Ca 2+. Step increases in the free Ca 2+ concentration of the bathing solution, from 10 nM up to 10 μM were found to increase 45Ca 2+ loss. This process was completely inhibited by ruthenium red. Caffeine induced a small release of 45Ca 2+ and the response to a subsequent stimulation with a Ca 2+ step was reduced. In intact cells, ryanodine activated small oscillations in intracellular Ca 2+ in the presence, but not the absence, of external Ca 2+. However, in permeabilised cells, ryanodine had no effect on either basal efflux or the increased efflux of 45Ca 2+ seen following a step increase in free Ca 2+. These data suggest the operation of a ruthenium red sensitive but ryanodine insensitive CICR mechanism on the endoplasmic reticulum (ER) which may also be modulated by caffeine. An IP 3 dependent 45Ca 2+ release was also observed. In the presence of ruthenium red, the IP 3 induced 45Ca 2+ release was reduced suggesting that CICR may operate to amplify the magnitude of the IP 3 response. The Ca 2+ dependence of the IP 3 induced release was also measured. Co-operativity between IP 3 and Ca 2+ could not be detected between 100–300 nM Ca 2+. The results suggest that the regulation of IP 3 induced Ca 2+ release may be different in BAE cells, and point to the operation of a ‘novel’ CICR process and to complex interactions between Ca 2+ release systems in BAE cells.