A deletion mutation Delta K210 in cardiac troponin T (cTnT) was recently found to cause familial dilated cardiomyopathy (DCM). To explore the effect of this mutation on cardiac muscle contraction under physiological conditions, we determined the Ca(2+)-activated force generation in permeabilized rabbit cardiac muscle fibers into which the mutant and wild-type cTnTs were incorporated by using our TnT exchange technique. The free Ca(2+) concentrations required for the force generation were higher in the mutant cTnT-exchanged fibers than in the wild-type cTnT-exchanged ones, with no statistically significant differences in maximal force-generating capability and cooperativity. Exchanging the mutant cTnT into isolated cardiac myofibrils also increased the free Ca(2+) concentrations required for the activation of ATPase. In contrast, a deletion mutation Delta E160 in cTnT that causes familial hypertrophic cardiomyopathy (HCM) decreased the free Ca(2+) concentrations required for force generation, just as in the case of the other HCM-causing mutations in cTnT. The results indicate that cTnT mutations found in the two distinct forms of cardiomyopathy (i.e., HCM and DCM) change the Ca(2+) sensitivity of cardiac muscle contraction in opposite directions. The present study strongly suggests that Ca(2+) desensitization of force generation in sarcomere is a primary mechanism for the pathogenesis of DCM associated with the deletion mutation Delta K210 in cTnT.