COOPERATIVE EFFECTS OF RIGOR AND CYCLING CROSSBRIDGES ON CALCIUM BINDING TO TROPONIN C
Marie E. Cantino 1* and Abraham Quintanilla 2
1 University of Connecticut
2 Amgen Inc.
* To whom correspondence should be addressed. E-mail: marie.cantino{at}uconn.edu.
Submitted on July 20, 2006
Revised on August 29, 2006
Accepted on 21 September 2006
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Abstract |
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The effects of rigor and cycling cross-bridges on distributions of calcium (Ca) bound within sarcomeres of rabbit psoas muscle fibers were compared using electron probe x-ray microanalysis. Calcium in the overlap region of rigor fibers, after correction for that bound to thick filaments, was significantly higher than in the I-band at all pCa levels tested between 6.9 and 4.8, but the difference was greatest at pCa 6.9. With addition of MgATP, differences were significant at high levels of activation (pCa 5.6 and 4.9); near and below the threshold for activation, Ca was the same in I-band and overlap regions. Comparison of Ca and mass profiles at the A-I junction showed elevation of Ca extending 55 to 110 nm (up to three regulatory units) into the I-band. Extraction of TnC reduced I-band and overlap Ca in rigor fibers at pCa 5.6 to the same levels found in unextracted fibers at pCa 8.9, suggesting that variations reported here reflect changes in Ca bound to troponin C (TnC). Taken together, these observations provide evidence for near-neighbor cooperative effects of both rigor and cycling crossbridges on Ca2+ binding to TnC.
Key Words:
Calcium regulation, electron probe x-ray microanalysis, vertebrate striated muscle
