TROPONIN T MODULATES SARCOMERE LENGTH DEPENDENT RECRUITMENT OF CROSSBRIDGES IN CARDIAC MUSCLE

¹ Washington State University
² Washinton State University

^* To whom correspondence should be addressed. E-mail: murali{at}vetmed.wsu.edu.

Submitted on October 31, 2005
Revised on December 12, 2005
Accepted on 12 January 2006

	Abstract

The heterogenic nature of troponin T (TnT) isoforms in fast skeletal and cardiac muscle suggests important functional differences. Dynamic features of rat cardiac TnT (cTnT) and rat fast skeletal TnT (fsTnT) reconstituted cardiac muscle preparations were captured by fitting the force response of small amplitude (0.5%) muscle length changes to the recruitment-distortion model. The recruitment of force-bearing crossbridges (XB) by increases in muscle length was favored by cTnT. The recruitment magnitude was approximately 1.5 times greater for cTnT than for fsTnT reconstituted muscle fibers. The speed of muscle length-mediated XB recruitment (b) was 0.57 to 0.50 times as fast as fsTnT reconstituted fibers (3.05 vs. 5.32 s^-1 at SL of 1.9 µm and 4.16 vs. 8.36 s^-1 at SL of 2.2 µm). Due to slowing of b in cTnT reconstituted fibers, the frequency of minimum stiffness(f_min) was shifted to lower frequencies of length changes (at SL of 1.9 µm, 0.64 Hz and 1.16 Hz for cTnT and fsTnT reconstituted fibers, respectively; at SL of 2.2 µm, 0.79 Hz and 1.11 Hz for cTnT and fsTnT reconstituted fibers, respectively). Our model simulation of the data implicates TnT as a participant in the process by which SL and XB-regulatory unit cooperative interactions activate thin filaments. Our data suggest that the amino acid sequence differences in cTnT may confer a heart-specific regulatory role. cTnT may participate in tuning the heart muscle by decreasing the speed of XB recruitment so that the heart beats at a rate commensurate with f_min.

Key Words: Length-dependent activation, crossbridge cycling, myofilaments, regulatory proteins, sarcomere dynamics, thin filaments

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