Attenuation of length dependence of calcium activation in myofilaments of transgenic mouse hearts expressing slow skeletal troponin I.
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. Compared to WT controls, ssTnI-TG fibre bundles were more sensitive to Ca2+ at both short SL (1.9 +/- 0.1 micrometer) and long SL (2.3 +/- 0.1 micrometer). However, compared to WT controls, the increase in Ca2+ sensitivity (change in half-maximally activating free Ca2+; DeltaEC50) associated with the increase in SL was significantly blunted in the ssTnI-TG myofilaments. Agents that sensitize the myofilaments to Ca2+ by promoting the actin-myosin reaction (EMD 57033 and CGP-48506) significantly reduced the length-dependent DeltaEC50 for Ca2+ activation, when SL in WT myofilaments was increased from 1.9 to 2.3 micrometer. 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 micrometer and desensitized the WT myofilaments at SL 2.3 micrometer. There were no significant effects of CDZ on ssTnI-TG myofilaments at either SL. 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.
Departments of Physiology and Biophysics and Pediatrics, College of Medicine, University of Illinois at Chicago, Chicago, IL 60612, USA.
G M
GM
Arteaga
K A
KA
Palmiter
J M
JM
Leiden
R J
RJ
Solaro
D008841:Actin Cytoskeleton / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D000199:Actins / Q000187:drug effects / Q000378:metabolism; D000818:Animals; D001392:Azocines / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002316:Cardiotonic Agents / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007093:Imidazoles / Q000494:pharmacology; D066298:In Vitro Techniques; D051379:Mice; D008822:Mice, Transgenic; D009119:Muscle Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D020033:Protein Isoforms / Q000096:biosynthesis / Q000235:genetics; D011804:Quinolines / Q000494:pharmacology; D012518:Sarcomeres / Q000187:drug effects / Q000378:metabolism; D013829:Thiadiazines / Q000494:pharmacology; D019210:Troponin I / Q000096:biosynthesis* / Q000235:genetics
D000199:Actins; D001392:Azocines; D002316:Cardiotonic Agents; D004791:Enzyme Inhibitors; D007093:Imidazoles; D010726:Phosphodiesterase Inhibitors; D020033:Protein Isoforms; D011804:Quinolines; D013829:Thiadiazines; D019210:Troponin I; C070609:EMD 53998; C095056:BA 41899; C031938:calmidazolium; D002118:Calcium