Fractional Ca2+ release from the endoplasmic reticulum activates Ca2+ entry in freshly isolated rabbit aortic endothelial cells.

The purpose of the present investigation was to examine the correlation between rates of endoplasmic reticulum (ER) depletion and activation of store operated channels (SOC) in freshly isolated rabbit aortic endothelial cells. We investigated the effects of 10 microM cyclopiazonic acid (CPA), 10 microM ryanodine, and 10 mM caffeine on the rate of Ca2+ depletion from the ER and on Ca2+ and Mn2+ influx using fura-2 fluorescence. We observed that the spontaneous loss of the ACh-sensitive pool is slow. Activation of ryanodine receptors (caffeine, ryanodine) or inhibition of the ER Ca2+ pump (CPA) increased the rate of Ca2+ loss from the ACh-sensitive pool. CPA stimulated Mn2+ influx, while caffeine and ryanodine did not. Our results show non linear correlation between ER depletion and activation of divalent cation entry. In the case of CPA, less than 20% depletion of the ACh sensitive store was required for full activation of SOC, while caffeine and ryanodine deplete over 50% of the ACh sensitive store without activating any influx. These data suggest that only a small compartment of the ER is involved in regulation of Ca2+ entry.