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Characterization of Muscarinic Acetylcholine Receptors in the Rat Epididymis¹

^a Section of Experimental Endocrinology, Department of Pharmacology, Universidade Federal de São Paulo, Escola Paulista de Medicina, São Paulo, SP 04044-020, Brazil

ABSTRACT

The aim of the present study was to characterize the muscarinic acetylcholine receptor subtypes present in the caput and cauda of rat epididymis. The specific binding of [³H]quinuclidinyl benzilate ([³H]QNB) to epididymal membranes was time dependent, temperature dependent, and saturable. The cauda epididymis showed higher affinity to [³H]QNB and higher muscarinic receptor density when compared to the caput region. The [³H]QNB binding was tested in competition studies with different muscarinic receptor antagonists. Each antagonist tested displaced [³H]QNB bound to caput and cauda epididymal membrane with similar affinity. Correlation among the negative logarithm of inhibition constant values (pK_i) for these antagonists obtained in the epididymis with their correspondent published pK_i values obtained in tissues that expressed each receptor subtype (M₁, M₂, M₃, and M₄) indicated that the muscarinic receptors present in caput and cauda epididymis belong to the muscarinic M₂ receptor subtype. When reverse transcription-polymerase chain reaction was used to identify muscarinic receptor mRNA subtypes in the epididymis, only m2 transcripts were detected in the caput region, while both m2 and m3 mRNA subtypes were observed in the cauda region. In conclusion, these results demonstrate that muscarinic receptors are present in the rat epididymis, with expression levels dependent on the region of the epididymis analyzed. Thus, the cholinergic neurotransmitter in the epididymis may be a factor controlling contractility and/or the luminal fluid microenvironment.

FOOTNOTES

First decision: 31 October 2000.

¹ This study was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (grant 97/2056-9), Brazil. T.W. Fogarty International (grant 5R37HDO4466-26, subcontract UNC 5-53284). Master fellowship supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (E.M.) and scholarship supported by FAPESP (E.F.G.); research fellowship supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (M.C.W.A. and C.S.P.).

² Correspondence: Catarina Segreti Porto, Section of Experimental Endocrinology, Department of Pharmacology, Rua Três de maio 100, São Paulo, SP 04044-020, Brazil. FAX: 55 115 576 4448; porto.farm{at}infar.epm.br

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