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This study evaluated the mechanism of the negative inotropic effect ofanisoquinoline alkaloid derivative,1-(4-dimethylamyphenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (F-24)using electrically stimulated rat left ventricular papillary muscle. F-24produced a pronounced negative inotropic effect in a dose-dependent manner.This effect of F-24 was dependedon the Ca2+ concentration inthe bathing mediaand markedly reducedinthe presence of nifedipine. Moreover, the negative inotropic effect of F-24 wasdecreasedin the presence oflidocaineand in external media with high KCl(26mM). Furthermore, F-24 significantly decreasedcontraction induced by low Na+ solution and ouabain, as well as, markedly inhibitedpost-rest potentiation of contraction.We conclude that the negative inotropic effect of F-24 may bemediatedby multiplemechanisms involvingeither direct or indirect modulation ofCa2+ handling in cardiomyocytes resulting ina reduction in the amount of Ca2+ released from the SR and suppression of the contraction force.