This Article Full Text Full Text (PDF) All Versions of this Article: biophysj.106.093757v1 92/2/525    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Cantino, M. E. Articles by Quintanilla, A. Search for Related Content PubMed PubMed Citation Articles by Cantino, M. E. Articles by Quintanilla, A.

Cooperative Effects of Rigor and Cycling Cross-Bridges on Calcium Binding to Troponin C

Department of Physiology and Neurobiology, University of Connecticut, Storrs, Connecticut

Correspondence: Address reprint requests to Marie E. Cantino, E-mail: marie.cantino{at}uconn.edu.

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 55–110 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 cross-bridges on Ca²⁺ binding to TnC.