A fast Ca2+-induced Ca2+-release mechanism in Dictyostelium discoideum

In vertebrate cells calcium-induced calcium release (CICR) is thought to be responsible for rapid cytosolic Ca2+ elevations despite the occurrence of strong Ca2+ buffering within the cytosol. In Dictyostelium, a CICR mechanism has not been reported. While analyzing Ca2+ regulation in a vesicular fraction of Dictyostelium rich in Ca2+-flux activity, containing contractile vacuoles (CV) as the main component of acidic Ca2+ stores and ER, we detected a rapid Ca2+ change upon addition of Ca2+ (CIC). CIC was three times larger in active stores accumulating Ca2+ than before Ca2+ uptake and in inactivated stores. Ca2+ release was demonstrated with the calmodulin antagonist W7 that inhibits the V-type H+ATPase activity and Ca2+ uptake of acidic Ca2+ stores. W7 caused a rapid and large increase of extravesicular Ca2+ ([Ca2+]e), much faster and larger than thapsigargin (Tg), a Ca2+-uptake inhibitor of the ER. W7 treatment blocked CIC indicating that a large part of CIC is due to Ca2+ release. The height of CIC depended on the filling state of the Ca2+ stores. CIC was virtually unchanged in the iplA− strain that lacks a putative IP3 or ryanodine receptor thought to be located at the endoplasmic reticulum. By contrast, CIC was reduced in two mutants, HGR8 and lvsA−, that are impaired in acidic Ca2+-store function. Purified Ca2+ stores enriched in CV still displayed CIC, indicating that CV are a source of Ca2+-induced Ca2+ release. CIC-defective mutants were altered in their oscillatory properties. The irregularity of the HGR8 oscillation suggests that the principal oscillator is affected in this mutant.