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 time 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 (1.9 + 0.1 mu m) and long SL (2.3 + 0.1 mu m). However, compared to WT controls, the increase in Ca2+ sensitivity (change in half-maximally activating free Ca2+; Delta EC50) 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 (EMI) 57033 and CGP-48506) significantly reduced the length-dependent Delta EC50 for Ca2+ activation, when SL in WT myofilaments was increased from 1.9 to 2.3 mu m. 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 mu m and desensitized the WT myofilaments at SL 2.3 mu m. There were no significant effects of CDZ on ssTnI-TG myofilaments at either XL. 5. Our results indicate that length-dependent Ca2+ activation is modified by specific changes in thill 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.