Proportions of fiber types in the external urethral sphincter of young nulliparous and old multiparous rabbits.

The proportions of fast and slow myosin molecules in specimen's of the external urethral sphincter (EUS) from 6 young nulliparous (6-month-old) and 6 old multiparous (2-year-old) rabbits were studied using myosin heavy chain electrophoresis. The percentages of fast and slow myosin molecules were 80.4 +/- 4.5% and 19.6 +/- 4.5% in the EUS from nulliparas, and 68.7 +/- 6.3% and 31.3 +/- 6.3% in the EUS from multiparas. The difference between the two groups was significant (P < 0.01). We suggest that a selective decrease in the volume of type 2 (fast) muscle fibers and/or conversion of type 2 to type 1 (slow) muscle fibers had taken place in the EUS of old multiparas. The proportional change in the constituent muscle fibers of the EUS with aging may play a role in human genuine stress incontinence.