Rabbit diaphragm: two types of fibres determined by calcium strontium activation and protein content.

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Abstract
Submaximum Ca2+- and Sr2+-activated tensions were measured in functionally skinned fibers from the rabbit diaphragm. The fibers could be classified into 2 types on the basis of activation by divalent cations. Type S fibers required a slightly higher [Ca2+] than [Sr2+] for half-maximal activation of tension. Type F fibers required 2 times higher [Ca2+] than type S and 8.5 times higher [Sr2+]. Sodium dodecyl sulfate [SDS] disc gels of type F fibers showed protein band patterns similar to those of fast-twitch muscle; those of type S fibers were identical to those of slow-twitch fibers. SDS disk gels of mixed, non-classified diaphragm fibers and an actual count of randomly selected fibers classified by Ca2+/Sr2+ tension characteristics showed the diaphragm to be composed of 60% fast-twitch fibers and 40% slow-twitch fibers. The protein stoichiometries of the 2 fiber types are consonant with the accepted stoichiometries for skeletal muscle. The percentages of type F and type S fibers in the diaphragm suggest that high ATPase activity shown by histochemical techniques is associated with type F fibers, and low activity with type S fibers.