Fig. 4. Correlation between S1 and S2 myosin heavy chain
(MHC) isoforms and troponin T1 (TnT1) found in slow
fibers from the distal region of the cutter claw. (A) Representative fibers
demonstrate that some fibers contain only the S2 MHC isoform (a,b),
some fibers contain both the S1 and S2 isoforms (c,d)
and some fibers contain only the S1 isoform (e,f). Rabbit MHC
(Sigma) is the double band to the left of lane a. (B) Western blot probed with
anti-TnT from the fibers shown in A. In general, the fibers containing larger
amounts of the S2 MHC isoform also contain a large proportion of
TnT1. Exceptions to this trend are observed in the fiber in lane f,
which possesses only the S1 MHC isoform but also contains
TnT1, and the fiber in lane d, which contains the S2 MHC
isoform but only has the TnT3 isoform. (C) Correlation between the
percentage of TnT existing as the T1 isoform and the percentage of
the S2 MHC isoform found in different slow fibers. A significant
correlation exists between TnT1 and the S2 isoform [%
TnT1=1.56+0.29(% S2), r2=0.59,
P<0.0009]. While a correlation exists between these two markers
for S2 fibers, the relationship is not absolute and accounts for
only 
60% of the variability in the proportion of TnT1 in the
fibers. These data support the notion that S1 and S2
fibers exist in a continuum between pure S1 and S2
fibers. It is also apparent that the TnT1 isoform is never present
by itself but, at most, accounts for about 50% of the TnT protein in a
fiber.