Ca2+ affinity of cardiac troponin C (TnC) is regulated by the active cross-bridges (downstream-dependent mechanism). In the present study, we showed one of the methods to evaluate the downstream-dependent change in the Ca2+ affinity of TnC during contraction using the aequorin-injected ferret papillary muscle. For this purpose, the tension-dependent change in the extra-Ca2+ (a transient increase in the intracellular Ca2+ concentration ([Ca2+]i) in response to a quick length reduction) was measured under various conditions. We examined whether the regression line between the magnitude of tension reduction and the magnitude of the normalized extra-Ca2+ (the extra-Ca2+ was divided by [Ca2+]i immediately before length change) (the normalized extra-Ca2+-tension relation) in twitch contraction can be used for the estimation of the downstream-dependent change in the Ca2+ affinity of TnC. The normalized extra-Ca2+ -tension relation became shallow by EMD 57033 (EMD) (one of the Ca2+ sensitizers) and by an increase in Ca2+ concentration in the solution ([Ca2+]o) in a concentration-dependent manner. However, 2,3-butanedione monoxime (BDM) (one of the desensitizers) antagonized the effects of EMD and higher [Ca2+]o in a concentration-dependent manner. These effects of EMD and BDM were also observed in the normalized extra-Ca2+-tension relation in tetanic contraction. The normalized extra-Ca2+-tension relation became steep by shortening the initial muscle length before contraction in tetanic contraction. Length-tension relation in twitch contraction was significantly shifted upward by higher [Ca2+]o and EMD, but BDM showed the opposite effects on them in a concentration-dependent manner. Thus, the downstream-dependent change in the Ca2+ affinity of TnC which physiologically functions in intact cardiac muscle can be evaluated using the normalized extra-Ca2+-tension relation.
Bibliographical noteFunding Information:
We thank Dr. N. Beier (E. Merck) for providing the compounds of EMD 57033, Mrs. Mary Beth Sibuya for reading the manuscript, N. Fukuda and K. Komukai for significant discussion, N. Tomizawa for technical assistance. This work was partly supported by a Grant-in-Aid from the Ministry of Education, Culture and Science, and by grants from The Vehicle Racing Commemorative Foundation, from Japanese Private School Promotion Foundation, and Uehara Memorial Science Foundation to S.K., and also supported by Ueda Memorial Foundation to T.I.
- Ca affinity of troponin C
- Cardiac muscle
- Length-tension relation