THE ANTIANGINAL AGENT RANOLAZINE IS A WEAK INHIBITOR OF THE RESPIRATORY COMPLEX-I, BUT WITH GREATER POTENCY IN BROKEN OR UNCOUPLED THAN IN COUPLED MITOCHONDRIA

Ranolazine (RS-43285) has shown antianginal effects in clinical trials and cardiac anti-ischaemic activity in several in vivo and in vitro a nimal models, but without affecting haemodynamics. Its mechanism is th ought to mainly involve a switch in substrate utilisation from fatty a cids to glucose to, thus, improve efficiency of O-2 use; however, its precise molecular target(s) are unknown. In studies to investigate its action further, using isolated rat heart mitochondria, ranolazine was found to weakly inhibit (pIC(50) values > 300 mu M) respiration by co upled mitochondria provided with NAD(+)-linked substrates but not with succinate. With broken mitochondrial membranes or submitochondrial pa rticles, ranolazine inhibited NADH but not succinate oxidation and wit h pIC(50) values in the lower range of 3-50 mu M. Studies with differe nt electron accepters and respiratory inhibitors indicated that it inh ibits respiratory Complex I at a site between ferricyanide and menadio ne and ubiquinone-l reduction (i.e. at a similar locus to rotenone). H owever, unlike rotenone, ranolazine was an uncompetitive inhibitor wit h respect to ubiquinone-l. Ranolazine inhibition of Complex I was reve rsible and occurred also with mitochondria from pig, guinea pig, and h uman heart, and rat liver. Further studies using rat heart mitochondri a in different energisation states (i.e. broken, uncoupled, or coupled ) showed a 50-100-fold shift to greater potency of ranolazine in the b roken compared to the coupled; with the uncoupled it was about 2-fold less potent than the broken, These shifts in potency were not found wi th rotenone or amytal. Studies with radiolabelled ranolazine showed th at it bound to mitochondrial membranes with greater affinity in the br oken compared to the coupled or uncoupled conditions. Rotenone displac ed radiolabelled ranolazine from its binding site. This property of ra nolazine may play some role in its anti-ischaemic activity.