Depressant effects of chloroquine on the isolated guinea-pig heart

Some anti-malarials have deleterious effects upon the heart. The actions of two of these, chloroquine and quinacrine, were compared on isolated guinea-pig atria and Langendorff preparations to assess their effects on several calcium pools. Both compounds decreased developed tension in a concentration (chloroquine 10-100 microM; quinacrine 3-100 microM) and time-dependent manner, with quinacrine being twice as potent as potent as chloroquine. Ventricular muscle was much more sensitive to chloroquine than was atrial muscle. Concentrations of chloroquine, comparable to that found in the serum of patients ingesting toxic doses, caused significant inhibition of developed tension. The effects of chloroquine were almost completely reversed on washout; quinacrine, however, was less readily reversible. Chloroquine also had a direct negative chronotropic effect, substantially reduced force-frequency relationships and developed tension in partially depolarized atrial preparations; while post-rest contraction and the positive inotropic effect of ouabain were unaffected. Increases in extracellular calcium antagonized the negative inotropic effect. Quinacrine had a marked effect on post-rest contraction and attenuated the positive inotropic action of ouabain. It is concluded that the action of chloroquine may involve a superficial calcium pool, while quinacrine may act upon several calcium pools.