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Thin‐filament regulation of force redevelopment kinetics in rabbit skeletal muscle fibres
Author(s) -
MorenoGonzalez Alicia,
Gillis Todd E.,
Rivera Anthony J.,
Chase P. Bryant,
Martyn Donald A.,
Regnier Michael
Publication year - 2007
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2006.124164
Subject(s) - chemistry , skeletal muscle , biophysics , actin , kinetics , anatomy , biochemistry , biology , physics , quantum mechanics
Thin‐filament regulation of isometric force redevelopment ( k tr ) was examined in rabbit psoas fibres by substituting native TnC with either cardiac TnC (cTnC), a site I‐inactive skeletal TnC mutant (xsTnC), or mixtures of native purified skeletal TnC (sTnC) and a site I‐ and II‐inactive skeletal TnC mutant (xxsTnC). Reconstituted maximal Ca 2+ ‐activated force (r F max ) decreased as the fraction of sTnC in sTnC: xxsTnC mixtures was reduced, but maximal k tr was unaffected until r F max was <0.2 of pre‐extracted F max . In contrast, reconstitution with cTnC or xsTnC reduced maximal k tr to 0.48 and 0.44 of control ( P < 0.01), respectively, with corresponding r F max of 0.68 ± 0.03 and 0.25 ± 0.02 F max . The k tr –pCa relation of fibres containing sTnC: xxsTnC mixtures (r F max > 0.2 F max ) was little effected, though k tr was slightly elevated at low Ca 2+ activation. The magnitude of the Ca 2+ ‐dependent increase in k tr was greatly reduced following cTnC or xsTnC reconstitution because k tr at low levels of Ca 2+ was elevated and maximal k tr was reduced. Solution Ca 2+ dissociation rates ( k off ) from whole Tn complexes containing sTnC (26 ± 0.1 s −1 ), cTnC (38 ± 0.9 s −1 ) and xsTnC (50 ± 1.2 s −1 ) correlated with k tr at low Ca 2+ levels and were inversely related to r F max . At low Ca 2+ activation, k tr was similarly elevated in cTnC‐reconstituted fibres with ATP or when cross‐bridge cycling rate was increased with 2‐deoxy‐ATP. Our results and model simulations indicate little or no requirement for cooperative interactions between thin‐filament regulatory units in modulating k tr at any [Ca 2+ ] and suggest Ca 2+ activation properties of individual troponin complexes may influence the apparent rate constant of cross‐bridge detachment.