Key Points
- To investigate how near-neighbour interactions among thin filament regulatory units govern force development and kinetics in demembranated cardiac muscle compared with skeletal muscle.
- Exchanged native cardiac troponin in skinned cardiac muscle with mixtures of wild-type troponin and an inactive mutant (cTnC D65A) at varying ratios.
- Measured Ca2+ sensitivity, maximal Ca2+-activated force, and rate of force redevelopment (ktr) in the presence of cross-bridge modulators (butanedione monoxime or 2-deoxy-ATP).
- Quantified and compared Ca2+ dissociation rates between isolated whole cardiac and skeletal troponin complexes.
- Maximal Ca2+-activated force increased sublinearly with wild-type troponin content in cardiac muscle, indicating activation of fewer than 7 actins per functional unit, contrasting with the supralinear (>7 actins) response in skeletal muscle.
- Ca2+ sensitivity of force and ktr shifted leftward by 0.1 to 0.2 pCa units as wild-type troponin increased, while the force-pCa slope and maximal ktr remained unaffected by loss of near-neighbour interactions.
- Cardiac troponin exhibited a greater than 2-fold faster Ca2+ dissociation rate than skeletal troponin, and cross-bridge modulation exerted no enhanced effect when cardiac neighbor interactions were disrupted.
Structured PICO
PPopulationDemembranated (skinned) cardiac and skeletal muscle
IInterventionExchange of native cardiac troponin (cTn) with different ratio mixtures of wild-type (WT) cTn and cTn containing WT cardiac troponin T/I + cardiac troponin C (cTnC) D65A (a site II inactive cTnC mutant)
CComparatorSkeletal muscle fibers (using sTnC D28A, D65A) and varying ratios of WT cTn
OOutcomeMaximal Ca(2+)-activated force (F(max)) and rate of force redevelopment (k(tr))surrogate
The study demonstrates that in cardiac muscle, Ca(2+) binding to individual troponin complexes is insufficient to completely activate their corresponding regulatory units, highlighting a fundamental difference from skeletal muscle.