Glycyrrhizic acid (GA) is one of the main active components in licorice and has often been reported to have cardioprotective effects. However, the underlying cellular mechanisms remain unclear. The aim of this study is to verify the protective effects of GA against isoproterenol (ISO)-induced myocardial ischemia injury in rats. Another aim is to explore the cellular mechanisms based on the L-type Ca2+ channel, myocardial cell contraction, and intracellular Ca2+ ([Ca2+](i)) transient. The results show that GA reduced the ST segment elevation, decreased the heart rate, prevented ISO-induced QT-interval shortening, improved heart morphology, and decreased the activity of CK and LDH. GA blocked ICa-L in a dose-dependent manner. The concentration for 50% of the maximal effect (EC50) of GA was 145.54 mu g/mL, and the maximal inhibition was 47.43 +/- 0.75% at 1000 mu g/mL. However, GA did not affect the dynamical properties of the Ca2+ channel. GA reversibly reduced the amplitude of cell contraction in a dose-dependent manner and slowed down its deflection and recovery, as well as the [Ca2+](i) transient. The data demonstrate that GA inhibits L-type Ca2+ channels, decreases the [Ca2+](i) transient, and shows a negative cardiac inotropic effect in the ventricular myocardial cells of adult rats. It also protects the myocardia from ischemia injury induced by ISO.