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

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Abstract
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 constitutent muscle fibers of the EUS with aging may play a role in human genuine stress incontinence.

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