The troponin I: inhibitory peptide uncouples force generation and the cooperativity of contractile activation in mammalian skeletal muscle
Hodges and his colleagues identified a 12 amino acid fragment of troponin I (TnI-ip) that inhibits Ca2+-activated force and reduces the effectiveness Ca2+ as an activator. To understand the role of troponin C (TnC) in the extended cooperative interactions of thin filament activation, we compared the effect of TnI-ip with that of partial troponin TnC extraction. Both methods reduce maximal Ca2+-activated force and increase [Ca2+] required for activation. In contrast to TnC extraction, TnI-ip does not reduce the extended cooperative interactions between adjacent thin filament regulatory units as assessed by the slope of the pCa/force relationship. Additional evidence that TnI-ip does not interfere with extended cooperativity comes from studies that activate muscle by rigor crossbridges (RXBs). TnI-ip increases both the cooperativity of activation and the concentration of RXBs needed for maximal force. This shows that TnI-ip binding to TnC increases the stability of the relaxed state of the thin filament. TnI-ip, therefore, uncouples force generation from extended cooperativity in both Ca2+ and RXB activated muscle contraction. Because maximum force can be reduced with no change—or even an increase—in cooperativity, force-generating crossbridges do not appear to be the primary activators of cooperativity between thin filament regulatory units of skeletal muscle.
KeywordsSkeletal muscle Troponin C Troponin I-inhibitory peptide Cooperativity Calcium activation Rigor crossbridge activation
We wish to thank Margaret Briggs for her assistance and comments in preparation of the manuscript, and Dr. Chris Wall for assistance in generating the bivariate fit for Fig. 1. This work was supported by grants from the U.S. National Institutes of Health to PWB (AR40300) and FS (AR39603).
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