Microsomal CA2+ uptake in human platelets is stimulated by calmodulin

Calcium accumulating vesicles were prepared from washed human platelets. Ca2+ uptake could be stimulated with calmodulin. The stimulation of Ca2+ uptake was not abolished by the adenylate cyclase inhibitor 2',5'-dideoxyadenosine, thus excluding enhanced cAMP formation as a possible cause for the calmodulin effect. Our findings suggest that, in addition to cAMP, calmodulin is also involved in the regulation of platelet free Ca2+ concentrations, i.e. Ca2+ homeostasis in platelets is under dual control.