Can inorganic ions be used to modulate tone in sheep isolated internal anal sphincter?

Sarah Rayment1, Colin Melia2, John Scholefield1, Vince Wilson3. 1Division of Surgery, University of Nottingham, NG7 2UH, United Kingdom, 2School of Pharmacy, University of Nottingham, NG7 2RD, United Kingdom, 3School of Biomedical Sciences, University of Nottingham, NG7 2UH, United Kingdom.

Progressive weakening of the anal sphincter complex with age, is a common cause of faecal incontinence (FI). In man, α1-adrenoceptor agonists such as phenylephrine (PE) and L-erythromethoxamine (LEM) have been shown to increase sphincter pressure in FI patients although not without side effects (Cheetham, Kamm and Phillips, 2001; Nisar et al., 2007). The aim of our study was to investigate (i) whether sodium orthovanadate (SOV) can increase tone in a sheep isolated internal anal sphincter (SIAS) model by inhibition of tyrosine phosphatase and (ii) whether lithium ions can modulate responses to α1-adrenoceptor agonists by inhibiting the recycling of intracellular signalling molecules (IP3).

Using a similar method to Mundey et al. (2000), sheep tissue that had been acquired from an abattoir was dissected to isolate strips of SIAS. Tissue strips that generated spontaneous myogenic tone were used to construct cumulative contraction curves to SOV or α1-adrenoceptor agonists. Both the magnitude and duration of responses to the α1-adrenoceptor agonists PE and LEM were compared in the presence and absence of lithium ions. To measure the duration of the response to α1-adrenoceptor agonist, measurements were taken at 5 minute intervals for a total of 60 minutes after administration of the final dose (which was designated as time 0).

SOV, but not lithium, caused a concentration-dependent contraction of SIAS muscle strips (pEC50 3.48 ± 0.12; n = 8). The maximal response to SOV did not differ significantly from that observed with α1-adrenoceptor agonists using a one-way ANOVA with Bonferroni post-test (Rmax SOV 140 ± 28% of basal tone, n = 8; Rmax PE 129 ± 37%, n = 6; LEM 113 ± 19%, n = 9). Based on paired t-tests, the presence of 1mM lithium chloride did not significantly affect the pEC50 or maximal responses to either PE or LEM. Under control conditions, responses to both agonists declined over the 60 minute period: the responses reduced to 56 ± 13% (PE; n = 6) and -20 ± 14% (LEM; n = 8) of the response at time 0 respectively. Based on unpaired t-tests, the presence of 1mM LiCl significantly attenuated the reduction in tone observed with agonists within 15 minutes. And, at 60 minutes responses in the presence of lithium were 123 ± 8% (PE; n = 7; p = 0.0007) and 62 ± 8% (LEM; n = 8; p = 0.0002) of the response at time 0. Similar observations were made in the presence of 0.5mM lithium sulphate but not 1mM rubidium chloride.

This work demonstrates that, in a SIAS model, inorganic ions can be used to influence sphincter tone either alone, as in the case of SOV, or to enhance the duration of responses to α1-adrenoceptor agonists.

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