Attenuation of length dependence of calcium activation in myofilaments of transgenic mouse hearts expressing slow skeletal troponin I

1. We compared sarcomere length (SL) dependence of the Ca²+-force relation of detergent-extracted bundles of fibres dissected from the left ventricle of wild-type (WT) and transgenic mouse hearts expressing slow skeletal troponin I (ssTnI-TG). Fibre bundles from the hearts of the ssTnI-TG demonstrated a complete replacement of the cardiac troponin I (cTnI) by ssTnI. 2. Compared to WT controls, ssTnI-TG fibre bundles were more sensitive to Ca²+ at both short SL (1· ± 0·1 μm) and long SL 2·3 ± 0·1 μm). However, compared to WT controls, the increase in Ca²+ sensitivity (change in half-maximally activating free Ca²+; ΔEC₅0) associated with the increase in SL was significantly blunted in the ssTnI-TG myofilaments. 3. Agents that sensitize the myofilaments to Ca²+ by promoting the actin-myosin reaction (EMD 57033 and CGP-48506) significantly reduced the length-dependent ΔEC₅₀ for Ca²+ activation, when SL in WT myofilaments was increased from 1·9 to 2·3 μm. 4. Exposure of myofilaments to calmidazolium (CDZ), which binds to cTnC and increases its affinity for Ca²+, sensitized force developed by WT myofilaments to Ca²+ at SL 1·9 μm and desensitized the WT myofilaments at SL 2·3 μm. There were no significant effects of CDZ on ssTnI-TG myofilaments at either SL. 5. Our results indicate that length-dependent Ca²+ activation is modified by specific changes in thin filament proteins and by agents that promote the actin-myosin interaction. Thus, these in vitro results provide a basis for using these models to test the relative significance of the length dependence of activation in situ.