Inositol 1,4,5-trisphosphate but not ryanodine-receptor agonists induces calcium release from rat liver Golgi apparatus membrane vesicles

We investigated the direct effect of inositol 1,4,5-trisphosphate (IP3) and ryanodine receptor agonists on Ca2+ release from vesicles of a rat liver Golgi apparatus (GA) enriched fraction, which were actively loaded with Ca-45(2+). Results in GA were compared with those obtained in a rat liver endoplasmic reticulum (ER) enriched fraction. The addition of IF, at concentrations ranging from 100 nM to 100 mu M, in the presence of thapsigargin, a specific inhibitor of sarcoplasmic/endoplasmic reticulum Ca2+-ATPases, promoted a rapid decrease in the Ca2+ content of GA vesicles. The amount of Ca2+ released from the vesicles was a function of IF, concentration, reaching about 60% in both GA and ER fractions at 100 mu M IF,. Calcium release was inhibited by heparin, an antagonist of IP3 receptors. Calcium exhibited a bell-shaped effect on IP3-dependent Ca2+ released from GA vesicles: it activated Ca2+ release at concentrations up to 1 mu M, and inhibited it at higher concentrations. In contrast to that found in the endoplasmic reticulum fraction, none of the ryanodine receptor agonists tested (cyclic ADP-ribose, caffeine and ryanodine) significantly induced Ca2+ release from GA fraction vesicles in the presence of thapsigargin. Our results indicate the presence of an IP3-sensitive Ca2+ release mechanism in the Golgi apparatus membrane analogous to that of the ER. However, a Ca2+ release mechanism sensitive to ryanodine receptor agonists like that of ER is not evident in the GA membrane.