Figure 3. Effects of unphosphorylated RcTn exchange on rate constant of force development (k_tr) in skinned rat cardiac myocytes and slow-twitch skeletal muscle fibers.

(A & B) A skinned cardiac myocyte preparation was first treated with PKA and the rate of force development was measured over a range of sarcomere lengths (green circles). Then the skinned myocyte preparation underwent exchange with unphosphorylated RcTn, which increased k_tr at all sarcomere lengths (A) and relative force levels (B). Inset in A shows force redevelopment traces in a myocyte preparation with unphosphorylated cTnI and PKA induced phosphorylated cTnI. (This experiment was performed on 4 myocyte preparations). (C & D) In a skinned slow-twitch skeletal muscle fiber, the sarcomere length (C) and relative force (D) dependence of k_tr was sequentially measured after an initial 12 hours of exchange with unphosphorylated RcTn (filled red squares), then after the fiber was treated with PKA (this decreased k_tr) (green circles), and then after an additional 12 hours of RcTn exchange (which returned k_tr to higher values) (open red squares). Inset shows an image of a slow-twitch skeletal muscle fiber and a force redevelopment trace after a slack-restretch maneuver. (This experiment was performed on 4 slow-twitch skeletal muscle fibers).