科研之友
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摘要
Divergent effects of alpha- and beta-myosin heavy chain (MHC) isoforms on contractile behavior arise mainly because of their impact on thin filament cooperativity. The N terminus of cardiac troponin T (cTnT) also modulates thin filament cooperativity. Our hypothesis is that the impact of the N terminus of cTnT on thin filament activation is modulated by a shift from alpha-to beta-MHC isoform. We engineered two recombinant proteins by deleting residues 1-43 and 44-73 in rat cTnT (RcTnT): RcTnT(1-43 Delta) and RcTnT(44-73 Delta), respectively. Dynamic and steady-state contractile parameters were measured at sarcomere length of 2.3 mu m after reconstituting proteins into detergent-skinned muscle fibers from normal (alpha-MHC) and propylthiouracil-treated (alpha-MHC) rat hearts. alpha-MHC attenuated Ca2+-activated maximal tension (similar to 46%) in RcTnT(1-43 Delta) fibers. In contrast, alpha-MHC decreased tension only by 19% in RcTnT(1-43 Delta) fibers. Both alpha- and beta-MHC did not affect tension in RcTnT(44-73 Delta) fibers. The instantaneous muscle fiber stiffness measurements corroborated the divergent impact of alpha- and beta-MHC on tension in RcTnT(1-43 Delta) fibers. pCa(50) (-log of [Ca2+](free) required for half-maximal activation) decreased significantly by 0.13 pCa units in alpha-MHC + RcTnT(1-43 Delta) fibers but remained unaltered in beta-MHC + RcTnT(1-43 Delta) fibers, demonstrating that beta-MHC counteracted the attenuating effect of RcTnT(1-43 Delta) on myofilament Ca2+ sensitivity. beta-MHC did not alter the sudden stretch-mediated recruitment of new cross-bridges (E-R) in RcTnT(1-43 Delta) fibers, but alpha-MHC attenuated E-R by 36% in RcTnT(1-43 Delta) fibers. The divergent impact of alpha- and beta-MHC on how the N terminus of cTnT modulates contractile dynamics has implications for heart disease; alterations in cTnT and MHC are known to occur via changes in isoform expression or mutations.
- 出版日期2013-10
