Inhibition of Calmodulin-activated Cyclic Nucleotide Phosphodiesterase: Multiple Binding-sites for Tricyclic Drugs on Calmodulin

A cyclic nucleotide phosphodiesterase from guinea-pig heart is activated by calmodulin in the presence of calcium ions. Activation was measured over a range of calmodulin concentrations, and is antagonised by several tricyclic psychotropic drugs including trifluoperazine, imipramine, chlorpromazine and amitriptyline. When the concentration of amitriptyline was increased, its apparent inhibition constant for binding to calmodulin decreased. This was due in part to binding of amitriptyline to glass surfaces; but after correction for this the discrepancy was still significant. It is proposed that this is due to two sites on calmodulin for amitriptyline, with binding to either site being sufficient to prevent calmodulin from activating phosphodiesterase.