CALMODULIN-ANTAGONISTS INHIBIT VESICULAR CA2+ UPTAKE IN DICTYOSTELIUM

Chemotactic stimulation by cAMP elicits Ca2+ entry across the plasma m embrane and uptake into intracellular Ca2+ stores. In order to better understand Ca2+ regulation in Dictyostelium we measured Ca-45(2+) upta ke in homogenates of aggregation competent cells. Besides the InsP(3)- sensitive store the acidosomes are responsible for Ca2+ transport. Abo ut 50% of the vesicular Ca-45(2+) accumulation was inhibited by the ca lmodulin antagonist W-7 and 14% by the less efficacious analogue W-5. Half maximal inhibition by W-7 occurred at 37 mu M concentration. Calm odulin antagonised the activity of W-7, and a monoclonal antibody agai nst Dictyostelium calmodulin inhibited Ca2+ sequestration as did calmo dulin antagonists of different classes. 100 mu M BHQ - a SERCA-type Ca 2+ transport ATPase blocker - inhibited most of the W-7 sensitive comp artment and oxalate increased Ca2+ uptake into this compartment indica ting that intracellular Ca2+ stores are the target of W-7. Ca2+/calmod ulin thus seems to provide for a feedback regulation of Ca2+ sequestra tion.