The action of nitric oxide on the striated anal sphincter in the rat

Karen Griffin, Colm O’Herlihy, Ronan O’ Connell, James Jones

University College Dublin, Dublin, Ireland

KM Griffin1, C O’Herlihy 1 2, PR O’ Connell 1 3, JFX Jones1

1School of Medicine and Medical Science, University College Dublin, Dublin 4, Ireland. 2 National Maternity Hospital, Holles Street, Dublin 2, Ireland. 3 Academic Surgical Unit, St Vincent’s University Hospital, ElmPark, Dublin 4, Ireland.

Faecal continence is maintained by contraction of smooth and striated sphincters. Nitric oxide (NO) relaxes the smooth muscle of the internal anal sphincter (IAS) (O’Kelly, et al. 1992). However little is known about the effects of NO the striated external anal sphincter (EAS) hence the present investigation.

Experiments were carried out on 16 female Wistar rats (200g – 250g). Each rat was killed humanely by a blow to the head and cervical dislocation. The anal canal was removed en bloc. The sphincter was mounted as a ring preparation in a 100ml organ bath containing Tyrode’s solution and tubocurarine (104M) bubbled with 100% O2 and maintained at 37°C. The baseline force was set to 10 mN and the EAS muscle was directly stimulated by a silver-silver chloride electrode using supra-maximal voltages. The stimulation protocol consisted of 100 bursts of trains of stimuli 200 ms in duration, at a frequency of 25Hz every 50 seconds. The NO synthase inhibitor L-NNA (10-4M) or TRIM (10-4M) was added to the water bath and 10 min later, the NO donor sodium nitroprusside (10-3M; SNP) was added to the organ bath. Student’s t-tests were used to assess statistical significance between groups. All values are expressed as means ± S.E.M. P < 0.05 was considered to be significant.

On addition of L-NNA, the force of contraction of the striated muscle of the anal sphincter was significantly decreased from 22 ± 1.97 to 18.54 ± 1.68 mN. With the addition of TRIM a small non-significant decrease in the force of the striated muscle from 13.13 ± 1.23 to 12.05 ± 1.06 mN) was recorded. On the addition of SNP to the organ bath containing L-NNA, the striated muscle force decreased slightly further from 18.55 ± 1.69 to 18.25 ± 2.01. However, a significant decrease was seen with the addition of SNP following TRIM from 12.05 ± 1.06 to 10.75 ± 1.10.

Both NO synthase inhibitors produced a decrease in the force of the striated muscle, suggesting basal NO release acts to facilitate EAS tone, however this effect could not be reversed with the NO donor, SNP. In fact SNP acted to inhibit the EAS as we originally hypothesised. The inhibitory effect of TRIM and L-NNA would appear not to be due to modulation of NO.

This work is supported by Health Research Board.

O’Kelly T, et al. Gut. 34(5): May. 1993; 689–693