The inhibition of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase by macrocyclic lactones and cyclosporin A

The pharmacology of macrocyclic lactones is varied, with many beneficial effects in treating disease processes. FK-506, rapamycin and ascomycin have been utilized as immunosuppressant agents. Ivermectin is typically used to treat parasitic worm infections in mammals. Another immunosuppressant, cyclosporin A, is a cyclic oligotide that has similar immunosuppressant properties to those exerted by macrocyclic lactones. Here we report on the inhibition by these compounds of sarcoplasmic/endoplasmic-reticulum Ca²⁺-ATPase (SERCA) Ca²⁺ pumps. Ivermectin, cyclosporin A and rapamycin all inhibited the skeletal muscle sarcoplasmic reticulum Ca²⁺-ATPase (SERCA1). In addition, although ivermectin inhibited brain microsomal endoplasmic reticulum (type 2b) Ca²⁺-ATPase, cyclosporin A and rapamycin did not. As cyclosporin A also did not inhibit cardiac Ca²⁺-ATPase activity, this would suggest that it could be an isoform-specific inhibitor. Ivermectin was shown to be the most potent Ca²⁺-ATPase inhibitor of the macrocyclic lactones (IC₅₀ = 7μM). It appears to show a ‘competitive’ inhibition with respect to high concentrations of ATP by increasing the regulatory binding site K_m but without affecting the catalytic site K_m. In addition, ivermectin stabilizes the ATPase in an E1 conformational state, and inhibits Ca²⁺ release from the enzyme during turnover. This would suggest that ivermectin inhibits Ca²⁺ release from the luminal binding sites of the phosphoenzyme intermediate, a step that is known to be accelerated by high [ATP].