Birefringence of glycerinated crab muscle fiber under various conditions

By using glycerinated single fibers of crab muscle (Sesarma haematocheir) which has long sarcomeres, the birefringence of the I band, H band and the overlapping region between thin and thick filaments was measured separately, under various environmental conditions. At the resting length, the birefringence of the fiber was decreased by the addition of Ca2+ in the absence of ATP, by about 0.35%. This birefringence decrease was found to take place in the overlapping region. The decrease corresponded to about 2% of the birefringence of thin filaments in this region. The birefringence of the fiber was increased by the addition of ATP in the absence of Ca2+, by about 6%. This birefringence increase also took place mostly in the overlapping region. The increase of birefringence by pyrophosphate was about half of that by ATP. The birefringence of the fiber was decreased by the increase of the ionic strength from 0.12 to 0.20. The origin of the observed changes of birefringence is discussed.