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 tro ponin I (ssTnI-TG). Fibre bundles from the hearts of the ssTnI-TG demonstra ted a complete replacement of the cardiac troponin I (cTnI) by ssTnI. 2. Compared to WT controls, ssTnI-TG fibre bundles were more sensitive to C a2+ 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 i n SL was significantly blunted in the ssTnI-TG myofilaments. 3. Agents that sensitize the myofilaments to Ca2+ by promoting the actin-my osin reaction (EMI) 57033 and CGP-48506) significantly reduced the length-d ependent Delta EC50 for Ca2+ activation, when SL in WT myofilaments was inc reased 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 myofila ments 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 b y specific changes in thill filament proteins and by agents that promote th e actin-myosin interaction. Thus, these in vitro results provide a basis fo r using these models to test the relative significance of the length depend ence of activation in situ.