TROPONIN C-MEDIATED CALCIUM SENSITIZATION INDUCED BY LEVOSIMENDAN DOES NOT IMPAIR RELAXATION

Levosimendan is a novel positive inotropic drug targeted to increase c ontraction force of the heart through its calcium-dependent binding to troponin C (cTnC). We investigated the calcium-sensitizing effect of levosimendan on contractile proteins as well as its positive inotropic and lusitropic effects in paced guinea pig papillary muscle. We also studied the effect on energy consumption of myosin-actin crossbridges in a myosin ATPase assay. The calcium sensitization induced by levosim endan in fibers skinned with saponin was dependent on the perforation velocity of cell membranes. Levosimendan was almost ineffective in slo wly perforated fibers, but was the most potent calcium sensitizer in f ibers with rapidly perforated cells. The perforation-dependent calcium sensitization was probably due to changes in phosphorylation state of contractile proteins during the slow dissection of fibers. It is note worthy that the calcium-sensitizing effect of levosimendan was not aff ected by acidic pH. Levosimendan at therapeutically relevant (0.3-10 m u M) concentrations markedly increased calcium sensitivity both at pH 6.7 and 7.0, being more potent than EMD 53998, pimobendan, and MCI-154 . The lack of effect of levosimendan on maximum tension supports the h ypothesis that levosimendan increases calcium sensitivity through its action on cTnC. Unlike EMD 53998, levosimendan did not increase myosin ATPase activity, indicating that it did not increase the cycling rate of myosin-actin crossbridges. In paced papillary muscles, levosimenda n induced positive inotropic effect without changing relaxation time. Thus, levosimendan was devoid of the main negative factors described f or calcium sensitizers.