Calmodulin-antagonists inhibit vesicular Ca2+ uptake in Dictyostelium

Chemotactic stimulation by cAMP elicits Ca2+ entry across the plasma membrane and uptake into intracellular Ca2+ stores. In order to better understand Ca2+ regulation in Dictyostelium we measured 45Ca2+ uptake in homogenates of aggregation competent cells. Besides the InsP3-sensitive store the acidosomes are responsible for Ca2+ transport. About 50% of the vesicular 45Ca2+ accumulation was inhibited by the calmodulin antagonist W-7 and 14% by the less efficacious analogue W-5. Half maximal inhibition by W-7 occurred at 37 microM concentration. Calmodulin antagonised the activity of W-7, and a monoclonal antibody against Dictyostelium calmodulin inhibited Ca2+ sequestration as did calmodulin antagonists of different classes. 100 microM BHQ-a SERCA-type Ca2+ transport ATPase blocker-inhibited most of the W-7 sensitive compartment and oxalate increased Ca2+ uptake into this compartment indicating that intracellular Ca2+ stores are the target of W-7. Ca2+/calmodulin thus seems to provide for a feedback regulation of Ca2+ sequestration.