Investigation of the distribution and function of α‐adrenoceptors in the sheep isolated internal anal sphincter

BACKGROUND AND PURPOSE We have investigated the distribution of α-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. EXPERIMENTAL APPROACH Saturation and competition binding with 3H-prazosin and 3H-RX821002 were used to confirm the presence and density of α-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. KEY RESULTS Saturation binding confirmed the presence of both α1- and α2-adrenoceptors, and subsequent characterization with sub-type-selective agents, identified them as α1A- and α2D-adrenoceptor sub-types. Autoradiographic studies with 3H-prazosin showed a positive association of α1-adrenoceptors with immunohistochemically identified smooth muscle fibres. Anti-α1-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 α1-adrenoceptor agonist. Prazosin (selective α1-adrenoceptor antagonist) significantly reduced the magnitude of contraction to l-erythro-methoxamine at the highest concentration used. Both prazosin and RX811059 (a selective α2-adrenoceptor antagonist) reduced the potency (pEC50) of clonidine. CONCLUSIONS AND IMPLICATIONS This study shows that both α1- and α2-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.