Seasonal changes in the activation of crossbridge motions of isolated thick filament from Limulus striated muscle

Summary

In dynamic light scattering, measurements of the intensity-intensity time correlation function from a suspension of rod-like particles of length L could reveal dynamical information related to translational and internal motions of those particles. For a suspension of thick filaments isolated from the myosin-regulated, striated muscles of Limulus at KL>1 (where K is the scattering vector), the average characteristic linewidth ($\bar \Gamma$) increased with the addition of Ca²⁺ or with the depletion of ATP. The increase in the $\bar \Gamma$ with the addition of Ca²⁺ could be due to the presence of energy-requiring, high-frequency motions of the crossbridges activated by Ca²⁺. The increase in $\bar \Gamma$ which occurred with the depletion of ATP was assumed to be mainly due to the thermal motions of the crossbridges after they had moved radially away from the filament backbone. The percentage increase in $\bar \Gamma$ following the addition of Ca²⁺ was found to be seasonal, i.e., values of $\bar \Gamma$ obtained from thick filaments isolated between the middle of June and the middle of September were smaller than those obtained during the rest of the year. The effect of temperature on the percentage increase in $\bar \Gamma$ was also different. The increase showed a maximum at about 35°C during the summer and at about 25°C at other times. However, the percentage increase in $\bar \Gamma$ developed under ATP-depleted conditions showed no temperature-related maximum. The number of bound Ca²⁺ per myosin molecule was 1 during the summer and 2 at other times.