Figure 5.

SRX is present in active cardiac muscle but not active skeletal muscle. Muscles were relaxed in mantATP and were chased with either a relaxing solution containing 4mM ATP (red) or an activating solution containing an additional 3 mM CaCl₂ in the presence of 5 mM EGTA, pCa = 5.7 (blue). (A) In skeletal fibers, the fluorescence decays slowly in a relaxing solution (upper curve), but quickly in an activating solution (lower curve). This shows that in activated skeletal fibers the myosin heads transition rapidly out of the SRX. (B) In cardiac muscles, the decay of fluorescence is not changed between chase with relaxing and activating solutions. This shows that myosin heads in the SRX remain in that state when adjacent myosin heads are interacting with actin and generating force. Parameters of the fits to the data shown here are provided in the Supporting Material.