The role of thin filament cooperativity in cardiac length-dependent calcium activation.
Length-dependent activation (LDA) is a prominent feature of cardiac muscle characterized by decreases in the Ca(2+) levels required to generate force (i.e., increases in Ca(2+) sensitivity) when muscle is stretched. Previous studies have concluded that LDA originates from the increased ability of (strong) cross-bridges to attach when muscle is lengthened, which in turn enhances Ca(2+) binding to the troponin C (TnC) subunit of the troponin complex. However, our results demonstrate that inhibition of strong cross-bridge attachment with blebbistatin had no effect on the length-dependent modulation of Ca(2+) sensitivity (i.e., EC(50)) or Ca(2+) cooperativity, suggesting that LDA originates upstream of cross-bridge attachment. To test whether LDA arises from length dependence of thin-filament activation, we replaced native cTnC with a mutant cTnC (DM-TnC) that is incapable of binding Ca(2+). Although progressive replacement of native cTnC with DM-TnC caused an expected monotonic decrease in the maximal force (F(max)), DM-TnC incorporation induced much larger increases in EC(50) and decreases in Ca(2+) cooperativity at short lengths than at long lengths. These findings support the conclusion that LDA arises primarily from the influence of length on the modulation of the Ca(2+) cooperativity arising from interaction between adjacent troponin-tropomyosin complexes on the thin filament.
Center for Cardiovascular Research, Department of Physiology and Biophysics, University of Illinois at Chicago, Chicago, Illinois, USA.
Gerrie P
GP
Farman
Edward J
EJ
Allen
Kelly Q
KQ
Schoenfelt
Peter H
PH
Backx
Pieter P
PP
de Tombe
D008841:Actin Cytoskeleton / Q000378:metabolism*; D019943:Amino Acid Substitution; D000818:Animals; D001483:Base Sequence; D055592:Biophysical Phenomena; D020013:Calcium Signaling / Q000502:physiology*; D017931:DNA Primers / Q000235:genetics; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D018613:Microscopy, Confocal; D008955:Models, Cardiovascular; D016297:Mutagenesis, Site-Directed; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D014335:Tropomyosin / Q000378:metabolism; D019209:Troponin C / Q000235:genetics / Q000378:metabolism
D017931:DNA Primers; D050505:Mutant Proteins; D014335:Tropomyosin; D019209:Troponin C