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Bupivacaine exhibits a cardiodepressant effect, the molecular mechanism(s) of which have yet to be fully understood. Bupivacaine may directly act on contractile proteins and thereby decrease myofibrillar Ca2+ sensitivity.Rat ventricular muscle was used. First, the effect of bupivacaine was examined on tetanic contractions in isolated intact myocytes. Next, Triton X-100–treated ventricular trabeculae were used to investigate the effect of bupivacaine on the pCa (= −log [Ca2+])–tension relation as well as on maximal Ca2+-activated tension. Furthermore, to test whether bupivacaine inhibits the pathway downstream from Ca2+ binding to troponin C, tension was elicited in the skinned preparations by lowering the Mg-adenosine triphosphate (MgATP) concentration in the absence of Ca2+. The effect of bupivacaine on the pMgATP (= −log [MgATP])–tension relation was examined.In myocytes, 3 μm bupivacaine significantly (P < 0.01) increased intracellular Ca2+ concentration required for 5% cell shortening from the resting cell length. In skinned preparations, bupivacaine shifted the pCa–tension relation to the lower pCa side; the midpoint of the pCa curve (pCa50) was significantly (P < 0.05) changed by 10 and 100 μm bupivacaine. A highly correlated linear relation (R = 0.81;P < 0.0005) was present between pCa50 and maximal Ca2+-activated tension. Bupivacaine (10 and 100 μm) significantly (P < 0.05) shifted the midpoint of the pMgATP–tension relation to the higher pMgATP side.Bupivacaine decreases myofibrillar Ca2+ sensitivity in ventricular muscle, and this is coupled with the compound's inhibitory effect on the pathway beyond Ca2+ binding to troponin C, possibly on the actomyosin interaction. The current results may partly explain the overall cardiodepressant effect of bupivacaine in vivo.