In this study we investigated the physiological role of the cardiac troponin T (cTnT) isoforms in the presence of human slow skeletal troponin I (ssTnI). ssTnI is the main troponin I isoform in the fetal human heart. In reconstituted fibers containing the cTnT isoforms in the presence of ssTnI, cTnT1-containing fibers showed increased Ca2+ sensitivity of force development compared with cTnT3- and cTnT4-containing fibers. The maximal force in reconstituted skinned fibers was significantly greater for the cTnT1 (predominant fetal cTnT isoform) when compared with cTnT3 (adult TnT isoform) in the presence of ssTnI. Troponin (Tn) complexes containing ssTnI and reconstituted with cTnT isoforms all yielded different maximal actomyosin ATPase activities. Tn complexes containing cTnT1 and cTnT4 (both fetal isoforms) had a reduced ability to inhibit actomyosin ATPase activity when compared with cTnT3 (adult isoform) in the presence of ssTnI. The rate at which Ca2+ was released from site II of cTnC in the cTnI-cTnC complex (122/s) was 12.5-fold faster than for the ssTnI·cTnC complex (9.8/s). Addition of cTnT3 to the cTnI·cTnC complex resulted in a 3.6-fold decrease in the Ca2+ dissociation rate from site II of cTnC. Addition of cTnT3 to the ssTnI-cTnC complex resulted in a 1.9-fold increase in the Ca2+ dissociation rate from site II of cTnC. The rate at which Ca2+ dissociated from site II of cTnC in Tn complexes also depended on the cTnT isoform present. However, the TnI isoforms had greater effects on the Ca2+ dissociation rate of site II than the cTnT isoforms. These results suggest that the different N-terminal TnT isoforms would produce distinct functional properties in the presence of ssTnI when compared with cTnI and that each isoform would have a specific physiological role in cardiac muscle.
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