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 ⁴⁵Ca²⁺. Results in GA were compared with those obtained in a rat liver endoplasmic reticulum (ER) enriched fraction. The addition of IP₃ at concentrations ranging from 100 nm to 100 μm, 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 IP₃ concentration, reaching about 60% in both GA and ER fractions at 100 μm IP₃. 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 μm, 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. Received: 13 March 2000/Revised: 13 July 2000