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.
Arteaga
G M
GM
Departments of Physiology and Biophysics and Pediatrics, College of Medicine, University of Illinois at Chicago, Chicago, IL 60612, USA.
Palmiter
K A
KA
Leiden
J M
JM
Solaro
R J
RJ
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