Electrophoretic and ultrastructural analysis of the rabbit's striated external urethral sphincter.

The male rabbit's external urethral sphincter was investigated with O'Farrell's 2-dimensional electrophoretic analysis of myosin light chains and electron microscopy. Its pattern of myosin light chains was different from that of the soleus (predominantly slow twitch muscle) but was very similar to that of the psoas (predominantly fast twitch muscle). Ultrastructurally it was shown to be red muscle resembling the soleus. Therefore the fiber type of the rabbit's external urethral sphincter was determined to be the red (fast) type.