Cardiac troponin I threonine 144: role in myofilament length dependent activation.

Myofilament length-dependent activation is the main cellular mechanism responsible for the Frank-Starling law of the heart. All striated muscle display length-dependent activation properties, but it is most pronounced in cardiac muscle and least in slow skeletal muscle. Cardiac muscle expressing slow skeletal troponin (ssTn)I instead of cardiac troponin (cTn)I displays reduced myofilament length-dependent activation. The inhibitory region of troponin (Tn)I differs by a single residue, proline at position 112 in ssTnI versus threonine at position 144 in cTnI. Here we tested whether this substitution was important for myofilament length-dependent activation; using recombinant techniques, we prepared wild-type cTnI, ssTnI, and 2 mutants: cTnI(Thr>Pro) and ssTnI(Pro>Thr). Purified proteins were complexed with recombinant cardiac TnT/TnC and exchanged into skinned rat cardiac trabeculae. Force-Ca2+ relationships were determined to derive myofilament Ca2+ sensitivity (EC50) at 2 sarcomere lengths: 2.0 and 2.2 microm (n=7). Myofilament length-dependent activation was indexed as deltaEC50, the difference in EC50 between sarcomere lengths of 2.0 and 2.2 microm. Incorporation of ssTnI compared with cTnI into the cardiac sarcomere reduced deltaEC50 from 1.26+/-0.30 to 0.19+/-0.04 micromol/L. A similar reduction also could be observed when Tn contained cTnI(Thr>Pro) (deltaEC50=0.24+/-0.04 micromol/L), whereas the presence of ssTnI(Pro>Thr) increased deltaEC50 to 0.94+/-0.12 micromol/L. These results suggest that Thr144 in cardiac TnI modulates cardiac myofilament length-dependent activation.