Ca²⁺ activation of myofilaments from transgenic mouse hearts expressing R92Q mutant cardiac troponin t

The functional consequences of the R92Q mutation in cardiac troponin T (cTnT), linked to familial hypertrophic cardiomyopathy in humans, are not well understood. We have studied steady- and pre-steady-state mechanical activity of detergent-skinned fiber bundles from a transgenic (TG) mouse model in which 67% of the total cTnT in the heart was replaced by the R92Q mutant cTnT. TG fibers were more sensitive to Ca²⁺ than nontransgenic (NTG) fibers [negative logarithm of half maximally activating molar Ca²⁺ (pCa₅₀) = 5.84 ± 0.01 and 6.12 ± 0.01 for NTG and TG fibers, respectively]. The shift in pCa₅₀ caused by increasing the sarcomere length from 1.9 to 2.3 μm was significantly higher for TG than for NTG fibers (ΔpCa₅₀ = 0.13 ± 0.01 and 0.29 ± 0.02 for NTG and TG fibers, respectively). The relationships between rate of ATP consumption and steady-state isometric tension were linear, and the slopes were the same in NTG and TG fibers. Rate of tension redevelopment was more sensitive to Ca²⁺ in TG than in NTG fibers (pCa₅₀ = 5.71 ± 0.02 and 6.07 ± 0.02 for NTG and TG fibers, respectively). We concluded that overall cross-bridge cycling kinetics are not altered by the R92Q mutation but that altered troponin-tropomyosin interactions could be responsible for the increase in myofilament Ca²⁺ sensitivity in TG myofilaments.