Investigation of the distribution and function of alpha-adrenoceptors in the sheep isolated internal anal sphincter.
OBJECTIVE
We have investigated the distribution of alpha-adrenoceptors in sheep internal anal sphincter (IAS), as a model for the human tissue, and evaluated various imidazoline derivatives for potential treatment of faecal incontinence.


METHODS
Saturation and competition binding with (3)H-prazosin and (3)H-RX821002 were used to confirm the presence and density of alpha-adrenoceptors in sheep IAS, and the affinity of imidazoline compounds at these receptors. A combination of in vitro receptor autoradiography and immunohistochemistry was used to investigate the regional distribution of binding sites. Contractile activity of imidazoline-based compounds on sheep IAS was assessed by isometric tension recording.


RESULTS
Saturation binding confirmed the presence of both alpha(1)- and alpha(2)-adrenoceptors, and subsequent characterization with sub-type-selective agents, identified them as alpha(1A)- and alpha(2D)-adrenoceptor sub-types. Autoradiographic studies with (3)H-prazosin showed a positive association of alpha(1)-adrenoceptors with immunohistochemically identified smooth muscle fibres. Anti-alpha(1)-adrenoceptor immunohistochemistry revealed similar distributions of the receptor in sheep and human IAS. The imidazoline compounds caused concentration-dependent contractions of the anal sphincter, but the maximum responses were less than those elicited by l-erythro-methoxamine, a standard non-imidazoline alpha(1)-adrenoceptor agonist. Prazosin (selective alpha(1)-adrenoceptor antagonist) significantly reduced the magnitude of contraction to l-erythro-methoxamine at the highest concentration used. Both prazosin and RX811059 (a selective alpha(2)-adrenoceptor antagonist) reduced the potency (pEC(50)) of clonidine.


CONCLUSIONS
This study shows that both alpha(1)- and alpha(2)-adrenoceptors are expressed in the sheep IAS, and contribute (perhaps synergistically) to contractions elicited by various imidazoline derivatives. These agents may prove useful in the treatment of faecal incontinence.
Rayment
S J
SJ
Centre for Integrated Systems Biology and Medicine, Department of Surgery, The University of Nottingham Medical School, Queen's Medical Centre, Clifton Boulevard, Nottingham, UK.
Eames
T
T
Simpson
J A D
JA
Dashwood
M R
MR
Henry
Y
Y
Gruss
H
H
Acheson
A G
AG
Scholefield
J H
JH
Wilson
V G
VG
D058668:Adrenergic alpha-1 Receptor Antagonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D001003:Anal Canal / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D000911:Antibodies, Monoclonal / Q000494:pharmacology; D001345:Autoradiography; D001667:Binding, Competitive; D004305:Dose-Response Relationship, Drug; D005242:Fecal Incontinence / Q000378:metabolism; D006801:Humans; D019329:Idazoxan / Q000031:analogs & derivatives / Q000494:pharmacology; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000187:drug effects; D011224:Prazosin / Q000494:pharmacology; D011485:Protein Binding; D011869:Radioligand Assay; D018340:Receptors, Adrenergic, alpha-1 / Q000096:biosynthesis* / Q000378:metabolism; D018341:Receptors, Adrenergic, alpha-2 / Q000096:biosynthesis* / Q000378:metabolism; D012756:Sheep / Q000378:metabolism
D058668:Adrenergic alpha-1 Receptor Antagonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D000911:Antibodies, Monoclonal; D018340:Receptors, Adrenergic, alpha-1; D018341:Receptors, Adrenergic, alpha-2; C058525:2-methoxyidazoxan; D011224:Prazosin; D019329:Idazoxan