Dual effects of α 2 ‐adrenoceptors in modulating myogenic tone in sheep isolated internal anal sphincter
Abstract Background The role of α ‐adrenoceptors in promoting continence through modulation of sphincter tone has focused primarily on the effects of α 1 ‐adrenoceptors. We have used three clinically available agents, which are selective for α 2 ‐adrenoceptors, to investigate their role in contracti...
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Published in | Neurogastroenterology and motility Vol. 26; no. 8; pp. 1095 - 1103 |
---|---|
Main Authors | , , , , , , , , |
Format | Journal Article |
Language | English |
Published |
01.08.2014
|
Online Access | Get full text |
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Abstract | Abstract
Background
The role of
α
‐adrenoceptors in promoting continence through modulation of sphincter tone has focused primarily on the effects of
α
1
‐adrenoceptors. We have used three clinically available agents, which are selective for
α
2
‐adrenoceptors, to investigate their role in contractile and neurogenic responses on the internal anal sphincter (
IAS
).
Methods
IAS
strips, which had spontaneously generated tone, were used to investigate the contractile effect of lofexidine, brimonidine, and dexmedetomidine on muscle tone in the presence or absence of subtype selective antagonists. The effect of brimonidine on the magnitude and time course of neurogenic responses generated by electrical field stimulation (
EFS
) was also examined. The affinity of test compounds at
α
1
‐ and
α
2
‐adrenoceptors was established by competition binding with [3H]‐prazosin and [3H]‐
RX
821002.
Key Results
All agonists caused concentration‐dependent contraction of the
IAS
and lofexidine demonstrated an enantiomeric difference in potency with a 10‐fold difference between the (−) and (+) isomers. Responses to lofexidine and dexmedetomidine were inhibited in the presence of the
α
1
‐adrenoceptor selective antagonist prazosin, but not in the presence of
RX
811059 (
α
2
‐adrenoceptor selective antagonist); brimonidine responses were inhibited by
RX
811059 and, to a lesser extent, by prazosin. Brimonidine affected both magnitude and duration of neurogenic responses, which was reversed in the presence of
RX
811059.
Conclusions & Inferences
We conclude that
α
2
‐adrenoceptors can mediate contraction of
IAS
, although this effect is most evident with efficacious imidazoline agonists rather than the most selective ligand. In addition, this receptor subtype can directly inhibit noradrenergic contractile responses to
EFS
and, indirectly, enhance nitrergic relaxatory responses. |
---|---|
AbstractList | The role of alpha -adrenoceptors in promoting continence through modulation of sphincter tone has focused primarily on the effects of alpha 1-adrenoceptors. We have used three clinically available agents, which are selective for alpha 2-adrenoceptors, to investigate their role in contractile and neurogenic responses on the internal anal sphincter (IAS). IAS strips, which had spontaneously generated tone, were used to investigate the contractile effect of lofexidine, brimonidine, and dexmedetomidine on muscle tone in the presence or absence of subtype selective antagonists. The effect of brimonidine on the magnitude and time course of neurogenic responses generated by electrical field stimulation (EFS) was also examined. The affinity of test compounds at alpha 1- and alpha 2-adrenoceptors was established by competition binding with [3H]-prazosin and [3H]-RX821002. All agonists caused concentration-dependent contraction of the IAS and lofexidine demonstrated an enantiomeric difference in potency with a 10-fold difference between the (-) and (+) isomers. Responses to lofexidine and dexmedetomidine were inhibited in the presence of the alpha 1-adrenoceptor selective antagonist prazosin, but not in the presence of RX811059 ( alpha 2-adrenoceptor selective antagonist); brimonidine responses were inhibited by RX811059 and, to a lesser extent, by prazosin. Brimonidine affected both magnitude and duration of neurogenic responses, which was reversed in the presence of RX811059. We conclude that alpha 2-adrenoceptors can mediate contraction of IAS, although this effect is most evident with efficacious imidazoline agonists rather than the most selective ligand. In addition, this receptor subtype can directly inhibit noradrenergic contractile responses to EFS and, indirectly, enhance nitrergic relaxatory responses. In sheep IAS, alpha 2-adrenoceptors can exhibit dual effects in modulating myogenic tone. alpha 2-adrenoceptor agonists may mediate contraction of IAS, or directly inhibit noradrenergic contractile responses to EFS and, indirectly, enhance nitrergic relaxatory responses. Abstract Background The role of α ‐adrenoceptors in promoting continence through modulation of sphincter tone has focused primarily on the effects of α 1 ‐adrenoceptors. We have used three clinically available agents, which are selective for α 2 ‐adrenoceptors, to investigate their role in contractile and neurogenic responses on the internal anal sphincter ( IAS ). Methods IAS strips, which had spontaneously generated tone, were used to investigate the contractile effect of lofexidine, brimonidine, and dexmedetomidine on muscle tone in the presence or absence of subtype selective antagonists. The effect of brimonidine on the magnitude and time course of neurogenic responses generated by electrical field stimulation ( EFS ) was also examined. The affinity of test compounds at α 1 ‐ and α 2 ‐adrenoceptors was established by competition binding with [3H]‐prazosin and [3H]‐ RX 821002. Key Results All agonists caused concentration‐dependent contraction of the IAS and lofexidine demonstrated an enantiomeric difference in potency with a 10‐fold difference between the (−) and (+) isomers. Responses to lofexidine and dexmedetomidine were inhibited in the presence of the α 1 ‐adrenoceptor selective antagonist prazosin, but not in the presence of RX 811059 ( α 2 ‐adrenoceptor selective antagonist); brimonidine responses were inhibited by RX 811059 and, to a lesser extent, by prazosin. Brimonidine affected both magnitude and duration of neurogenic responses, which was reversed in the presence of RX 811059. Conclusions & Inferences We conclude that α 2 ‐adrenoceptors can mediate contraction of IAS , although this effect is most evident with efficacious imidazoline agonists rather than the most selective ligand. In addition, this receptor subtype can directly inhibit noradrenergic contractile responses to EFS and, indirectly, enhance nitrergic relaxatory responses. |
Author | Eames, T. Acheson, A. G. Simpson, J. A. D. Gruss, H. Rayment, S. J. Henry, Y. Dashwood, M. R. Scholefield, J. H. Wilson, V. G. |
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References_xml | – ident: e_1_2_10_9_1 doi: 10.1007/PL00005221 – ident: e_1_2_10_17_1 doi: 10.1038/sj.bjp.0703322 – ident: e_1_2_10_23_1 doi: 10.1038/sj.bjp.0701296 – ident: e_1_2_10_21_1 doi: 10.1111/j.1365-2125.1982.tb01393.x – ident: e_1_2_10_10_1 doi: 10.1213/ANE.0b013e31822b8629 – ident: e_1_2_10_29_1 doi: 10.1097/00000542-200212000-00022 – volume: 264 start-page: 1187 year: 1993 ident: e_1_2_10_20_1 article-title: Discrimination and pharmacological characterization of I2‐imidazoline sites with [3H]‐Idazoxan and alpha‐2 adrenoceptors with [3H]RX821002 (2‐methoxy Idazoxan) in the human and rat brains publication-title: J Pharm Exp Ther contributor: fullname: Miralles A – volume: 272 start-page: G1075 year: 1997 ident: e_1_2_10_27_1 article-title: Effects of transmural field stimulation in isolated smooth muscle of human rectum and internal anal sphincter publication-title: Am J Physiol contributor: fullname: Glavind EB – ident: e_1_2_10_8_1 doi: 10.2165/00003088-198814050-00002 – ident: e_1_2_10_13_1 doi: 10.1345/aph.1M347 – ident: e_1_2_10_19_1 doi: 10.1038/sj.bjp.0703183 – ident: e_1_2_10_25_1 doi: 10.1111/j.1476-5381.1988.tb11669.x – ident: e_1_2_10_31_1 doi: 10.1111/j.1365-2036.2010.04391.x – volume: 53 start-page: 963 year: 1998 ident: e_1_2_10_22_1 article-title: Protean agonism at alpha2A‐adrenoceptors publication-title: Mol Pharmacol contributor: fullname: Jansson CC – ident: e_1_2_10_28_1 doi: 10.1172/JCI114728 – ident: e_1_2_10_30_1 doi: 10.1046/j.1365-2036.2000.00757.x – volume: 42 start-page: 2049 year: 2001 ident: e_1_2_10_11_1 article-title: Potent alpha(2A)‐adrenoceptor‐mediated vasoconstriction by brimonidine in porcine ciliary arteries publication-title: Invest Ophthalmol Vis Sci contributor: fullname: Wikberg‐Matsson A – ident: e_1_2_10_3_1 doi: 10.1046/j.1365-2168.1999.01292.x – ident: e_1_2_10_5_1 doi: 10.1002/bjs.5171 – ident: e_1_2_10_24_1 doi: 10.1124/jpet.110.170688 – ident: e_1_2_10_6_1 doi: 10.1111/j.1476-5381.2010.00842.x – ident: e_1_2_10_4_1 doi: 10.1136/gut.48.3.356 – ident: e_1_2_10_14_1 doi: 10.1021/jm00157a011 – ident: e_1_2_10_16_1 doi: 10.1111/j.1365-2982.2008.01250.x – ident: e_1_2_10_2_1 doi: 10.1136/gut.52.2.264 – ident: e_1_2_10_12_1 doi: 10.1136/bjo.2006.096693 – ident: e_1_2_10_7_1 doi: 10.1038/bjp.2008.217 – ident: e_1_2_10_18_1 doi: 10.1111/j.1476-5381.1992.tb14450.x – volume: 273 start-page: 18 year: 1985 ident: e_1_2_10_15_1 article-title: Interference of enantiomers of lofexidine with alpha‐adrenoceptors publication-title: Arch Int Pharmacodyn Ther contributor: fullname: Willffert B – ident: e_1_2_10_26_1 doi: 10.1038/sj.bjp.0705249 |
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Snippet | Abstract
Background
The role of
α
‐adrenoceptors in promoting continence through modulation of sphincter tone has focused primarily on the effects of
α
1... The role of alpha -adrenoceptors in promoting continence through modulation of sphincter tone has focused primarily on the effects of alpha 1-adrenoceptors. We... |
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Title | Dual effects of α 2 ‐adrenoceptors in modulating myogenic tone in sheep isolated internal anal sphincter |
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