Electrophoretic and functional identification of two troponin C isoforms in toad skeletal muscle fibers

The differential sensitivity of frog twitch and slow-tonic fibers to Ca2+ and Sr2+ suggests that these two fiber types express different troponin C (TnC) isoforms. To date, only one TnC isoform from anurans ( resembling the mammalian fast-twitch isoform) has been isolated and characterized. In this study, we examined the possibility that anuran striated muscle contains more than one TnC isoform. Toward this end, we determined the TnC isoform composition of 198 single fibers from the rectus abdominis of the cane toad ( a mixed slow-tonic and twitch muscle) and of toad cardiac muscle using a method that enables the identification of TnC isoforms on the basis of the effect of Ca2+ on their electrophoretic mobility. The fibers were typed according to their myosin heavy chain (MHC) isoform composition. The data indicate that striated muscle of the cane toad contains two TnC isoforms, one of which (TnC-t) is present in all fibers displaying only twitch MHC isoforms and the other of which (TnC-T/c) is present in fibers displaying the tonic MHC isoform and in cardiac muscle. For a subpopulation of 15 fibers, the TnC isoform composition was also compared with Ca2+ and Sr2+ activation characteristics. Fibers containing the TnC-T/c isoform were similar to 3-fold more sensitive to Ca2+, similar to 40-fold more sensitive to Sr2+, and responded to a similar to 4.6-fold broader range of [Ca2+] than did fibers containing the TnC-t isoform. The Ca2+ activation properties of toad fibers containing the TnC-T/c isoform appear to be consistent with the previously reported physiological characteristics of amphibian slow-tonic muscle fibers.