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Thin filament near‐neighbour regulatory unit interactions affect rabbit skeletal muscle steady‐state force‐Ca 2+ relations
Author(s) -
Regnier Michael,
Rivera Anthony J.,
Wang ChienKao,
Bates Mandy A.,
Chase P. Bryant,
Gordon Albert M.
Publication year - 2002
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.2001.013179
Subject(s) - biophysics , actin , skeletal muscle , chemistry , tropomyosin , biochemistry , anatomy , biology
The role of cooperative interactions between individual structural regulatory units (SUs) of thin filaments (7 actin monomers : 1 tropomyosin : 1 troponin complex) on steady‐state Ca 2+ ‐activated force was studied. Native troponin C (TnC) was extracted from single, de‐membranated rabbit psoas fibres and replaced by mixtures of purified rabbit skeletal TnC (sTnC) and recombinant rabbit sTnC (D27A, D63A), which contains mutations that disrupt Ca 2+ coordination at N‐terminal sites I and II (xxsTnC). Control experiments in fibres indicated that, in the absence of Ca 2+ , both sTnC and xxsTnC bind with similar apparent affinity to sTnC‐extracted thin filaments. Endogenous sTnC‐extracted fibres reconstituted with 100 % xxsTnC did not develop Ca 2+ ‐activated force. In fibres reconstituted with mixtures of sTnC and xxsTnC, maximal Ca 2+ ‐activated force increased in a greater than linear manner with the fraction of sTnC. This suggests that Ca 2+ binding to functional Tn can spread activation beyond the seven actins of an SU into neighbouring units, and the data suggest that this functional unit (FU) size is up to 10–12 actins. As the number of FUs was decreased, Ca 2+ sensitivity of force (pCa 50 ) decreased proportionally. The slope of the force‐pCa relation (the Hill coefficient, n H ) also decreased when the reconstitution mixture contained < 50 % sTnC. With 15 % sTnC in the reconstitution mixture, n H was reduced to 1.7 ± 0.2, compared with 3.8 ± 0.1 in fibres reconstituted with 100 % sTnC, indicating that most of the cooperative thin filament activation was eliminated. The results suggest that cooperative activation of skeletal muscle fibres occurs primarily through spread of activation to near‐neighbour FUs along the thin filament (via head‐to‐tail tropomyosin interactions).