Time-resolved measurements of structural changes of troponin and intracellular Ca concentrations during twitch of frog skeletal muscle

Matsuo, Tatsuhito; Iwamoto, Hiroyuki*; Yagi, Naoto*

To elucidate the behavior of troponin during twitch of frog skeletal muscle, structural changes of troponin and changes of the intracellular Ca concentration ([Ca]i) were monitored by X-ray diffraction and fluo3-AM at the sarcomere length (SL) of 2.8 m and 4.0 m. [Ca]i was converted to the concentration of Ca bound to troponin ([CaTn]). At 4.0 m SL, the troponin reflection intensity at 38.5 nm began to increase at 3 ms after the stimulus when [CaTn] has already reached a half of its peak, and returned to the resting level more quickly than [CaTn]. At 2.8 m SL, the X-ray intensity showed a drastic decrease and the decay of [CaTn] was slower. These results indicate that (1) structures of troponin molecules begin to change only after many of them have bound Ca, and return to its resting conformation even when some still bind Ca in no filament overlap state, (2) the dissociation of Ca appears to be slowed down and conformations are largely affected by crossbridges.