Key Points
- Determine whether sequence variations in loop 1 drive differences in nucleotide turnover and contraction kinetics between rat and pig cardiac beta-myosin heavy chains.
- Obtained 100% beta-MHC from normal pig myocardium and hypothyroid rat myocardium induced via thyroidectomy and propylthiouracil treatment.
- Assessed unloaded shortening velocity via slack tests in skinned myocytes at 15 °C alongside solution measurements of actin-activated ATPase activity.
- Applied stopped-flow techniques to quantify rates of MgATP-induced acto-HMM dissociation, ADP dissociation, and ATP cleavage.
- Rat skinned myocytes exhibited faster unloaded shortening velocity (1.1 ± 0.8 muscle lengths/s vs 0.35 ± 0.05 muscle lengths/s) and 2.9-fold higher actin-activated ATPase activity compared to pig myocytes.
- The ADP dissociation rate was approximately 3.8-fold faster in rat beta-myosin (135 s⁻¹) than in pig beta-myosin (35 s⁻¹), and ATP cleavage was ~30% faster despite nearly identical loop 1 sequences.
Structured PICO
PPopulationRat and pig beta-myosin heavy chains (beta-MHC) from myocardium (pig normally expresses 100% beta-MHC, rat induced by surgical thyroidectomy and propylthiouracil)
IInterventionAssessment of kinetic properties (unloaded shortening velocity, actin-activated ATPase activity, acto-myosin dissociation rates)
OOutcomeKinetic properties including unloaded shortening velocity, actin-activated ATPase activity, and rates of acto-myosin dissociationsurrogate
Despite identical loop 1 sequences, rat and pig beta-myosin heavy chains exhibit large differences in turnover kinetics, suggesting other parts of the MHC backbone are responsible.