Sk. Bortolotto et al., MHC isoform composition and Ca2+- or Sr2+-activation properties of rat skeletal muscle fibers, AM J P-CELL, 279(5), 2000, pp. C1564-C1577
Chemically skinned single fibers from adult rat skeletal muscles were used
to test the hypothesis that, in mammalian muscle fibers, myosin heavy chain
(MHC) isoform expression and Ca2+- or Sr2+-activation characteristics are
only partly correlated. The fibers were first activated in Ca2+- or Sr2+-bu
ffered solutions under nearphysiological conditions, and then their MHC iso
form composition was determined electrophoretically. Fibers expressing only
the MHC I isoform could be appropriately identified on the basis of either
the Ca2+- or Sr2+-activation characteristics or the MHC isoform compositio
n. Fibers expressing one or a combination of fast MHC isoforms displayed no
significant differences in their Ca2+- or Sr2+-activation properties; ther
efore, their MHC isoform composition could not be predicted from their Ca2- or Sr2+-activation characteristics. A large proportion of fibers expressi
ng both fast- and slowtwitch MHC isoforms displayed Ca2+- or Sr2+-activatio
n properties that were not consistent with their MHC isoform composition; t
hus both fiber-typing methods were needed to fully characterize such fibers
. These data show that, in rat skeletal muscles, the extent of correlation
between MHC isoform expression and Ca2+- or Sr2+-activation characteristics
is fiber-type dependent.