Expression and localization of muscarinic acetylcholine receptor subtypes in rat efferent ductules and epididymis

The expression of muscarinic acetylcholine receptor (mAChR) subtypes (M-1-M-5) was studied in the rat efferent ductules and epididymis at the mRNA and protein levels. The relative abundance of each mAChR transcript subtype differed depending on the tissue and the epididymal region analyzed. The M-1 mAChR mRNA level was more abundant in the efferent ductules than in the caput and cauda of the epididymis. The M-2 mAChR mRNA level was similar between the efferent ductules and caput of the epididymis and higher in the cauda region. The M-3 mAChR mRNA level was low in the efferent ductules and caput of the epididymis, but high levels were detected in the cauda region. mRNAs for M-4 and M-5 mAChRs were not detected in these tissues. Our studies indicated a variable degree of immunostaining for each mAChR subtype in a cell-type and tissue-specific pattern. M-3 mAChR was detected over the efferent ductule epithelium. M-2 and M-3 mAChRs were observed in the apical region of the ciliated cells. Apical and narrow cells of the initial segment showed distinct staining by M-1 antibody, whereas a supranuclear reaction was noted in the principal cells of the caput of the epididymis. In addition, staining for M-1 and M-2 mAChRs was visible in the apical membrane of some epithelial cells of the cauda region. M-3 mAChR was detected in the peritubular smooth muscle of the efferent ductules and epididymis. Functional studies suggested the involvement of this subtype in epididymal tubule contraction. Thus, the cell-specific expression of the various mAChR subtypes in the efferent ductules and epididymis suggests that these receptors play a role in the modulation of luminal fluid composition and smooth muscle contraction.