Tricyclic Antidepressants Inhibit the Ca2+-Dependent ATPase Activity from Plasma Membrane

Tricyclic antidepressants are moderately potent inhibitors of the plasma membrane Ca2+-ATPase activity measured in erythrocyte ghosts. For the calmodulin-activated activity, half-maximal inhibition was observed in the presence of 0.25 mM clomipramine. Desipramine, imipramine, and trimipramine show half-maximal inhibition in the range of 0.8 to 1 mM. The inhibition dependence on clomipramine concentration is the same whether the enzyme is activated by exogenous calmodulin or by tryptic digestion. A similar behavior was observed for desipramine. The inhibition mechanisms utilized by clomipramine and desipramine are different. The clomipramine effect is associated with the Ca2+-bound enzyme conformation and can be attributed to a decrease in the rate of phosphorylation by ATP. The desipramine effect appears more related to the Ca2+-free conformation, since the partial reaction involved in the release of inorganic phosphate is perturbed by this drug. There is also little or no effect of tricyclics on the enzyme's affinity for ligand (Ca2+ or ATP) binding.