Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, C87
Fibre size and oxidative capacity of the external anal and urethral sphincters
M. Buffini1, C. O'Herlihy1,2, R. O'Connell1,3, J. Jones1
1. School of Medicine and Medical Science, UCD, Dublin, Ireland. 2. Obstetrics and Gynaecology, National Maternity Hospital, Holles Street, Dublin, Ireland. 3. Professorial Surgical Unit, St. Vincent's University Hospital, Dublin, Ireland.
The external urethral (EUS) and anal (EAS) sphincters are two striated muscles which play a fundamental role in the maintenance of urinary and faecal continence. Despite their common embryological origin and synchronised electrical activity in vivo, these muscles display different contractile properties. The aim of this study was to explain the findings from in vitro functional experiments by examining some of the structural characteristics of the EAS and EUS. The fatigue indices of sphincters from eight female Wistar rats (200-250g) were measured. The EAS and EUS were mounted as ring preparations in tissue baths containing Tyrode’s solution maintained at 37οC and bubbled with 100% O2. Tubocurarine (10-4M) was added to the bath solution to ensure nerve independent muscle contraction. The fatigue stimulation protocol consisted of 50 200ms trains at 50Hz which were 4 seconds apart. Data are expressed as mean ± SEM and statistically analysed with a students t-test. The EAS was much more susceptible to fatigue than the EUS. At the end of the fatigue protocol the final contraction of the EAS had fatigued to 41.86 ± 2.73% of the initial contractile force. Conversely, the contractions of the EUS were relatively unaffected by this particular stimulation protocol, the final contraction of the EUS was 94.93 ± 3.09% of the first, p<0.001 (n=8). The absolute force produced by the muscle per cross sectional area or the specific force was measured. At 50Hz the specific force of the EAS was twice that of the EUS, 4.7 ± 0.6 mN.mm-2 vs. 2.4 ± 0.3 mN.mm-2, p=0.014. This suggested that the EAS is composed of larger muscle fibres than the EUS. In order to determine the average area of the striated muscle fibres in the EAS and EUS, the sphincters from three animals were oriented so that the muscle fibres were cut transversely. These sections were then incubated with antibodies against laminin (1:500) to highlight the diameter of each fibre. The areas of 450 fibres were measured form each sphincter muscle. The cross sectional area of fibres in the EAS was on average 376 ± 5 μm2 while in the urethra it was 187 ± 4 μm2 (P<0.0001). The smaller specific force and fibre size of the urethra coupled with its fatigue resistance indicates that it has a higher oxidative capacity than the anal sphincter. This was confirmed by succinate dehydrogenase (SDH) histochemistry. In conclusion, the voluntary muscle of urinary continence in the rat is composed of small, fatigue resistant muscle fibres in comparison to the more powerful but fatigable voluntary sphincter of the anal canal.
Where applicable, experiments conform with Society ethical requirements