Analysis of the birefringence of the smooth muscle anococcygeus of the rat, at rest and in contraction. I.

The birefringence of the rat anococcygeus muscle was measured at rest and in contraction. A large increase (+30%) of the optical retardation was observed in muscles fully stimulated by Noradrenaline, in isometric conditions. This was associated with a reduction of the muscle thickness (-12%), so that the birefringence increased by 48%. These changes were reversed upon relaxation. The relationship between the birefringence increase and the mechanical response was investigated as a function of time and of Noradrenaline concentration. Possible origins for the birefringence increase are considered: an increased density of birefringent material, mainly filaments, seems the most likely explanation of the results.