Local regulation of epididymal anion secretion by pituitary adenylate cyclase-activating polypeptide W L Zhou, P S Leung, T P Wong, N J Dun 1 , P Y D Wong and H C Chan Department of Physiology, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, NT, Hong Kong and 1 Department of Anatomy and Neurobiology, Medical College of Ohio, Toledo, USA (Requests for offprints should be addressed to H Chang Chan, Department of Physiology, The Chinese University of Hong Kong, Shatin, NT, Hong Kong) Abstract The present study investigated the role of pituitary adenyl- ate cyclase-activating polypeptide (PACAP) in the regu- lation of electrogenic anion secretion by the rat cauda epididymal epithelium. PACAP38, which has been shown to affect secretory function in various exocrine and endo- crine tissues, gave rise to a concentration-dependent increase in the short-circuit current (Isc). The PACAP38 effect was restricted to the apical aspect of the epididymal cells. The Isc response to PACAP38 was abolished in Cl  -free solution and completely inhibited by the Cl  channel blocker, diphenylamine-dicarboxylic acid. The Isc response to PACAP38 was also suppressed by pretreat- ment of the cells with the adenylate cyclase inhibitor, MDL12330A. The localization of PACAP38 was further investigated using an immunohistochemical technique. PACAP38 immunoreactivity was observed in the cauda epididymal tubules as well as in the cultured epithelium, indicating its epithelial origin. The present results suggest that Cl  secretion in the epididymis may be regulated by PACAP38, which could be locally synthesized and released by the epithelial cells, in a paracrine or autocrine fashion. Journal of Endocrinology (1997) 154, 389–395 Introduction In addition to neurohumoral control, epididymal Cl  secretion is also subject to local regulation. Local factors may be synthesized and released by the epithelium thereby stimulating Cl  secretion in a paracrine or autocrine fashion. Recent studies have demonstrated that calcitonin gene-related peptide (CGRP) (Leung et al. 1992), and angiotensin II (Wong et al. 1989b, Zhao et al. 1996) are produced locally by the epididymis and act on the epi- didymal epithelium to stimulate Cl  secretion. This is thought to provide long-term regulation of the epithelium to ensure the maintenance of a stable microenvironment for the maturation and storage of spermatozoa. The pituitary adenylate cyclase-activating polypeptide (PACAP) has been found to be widely distributed in the body and to affect secretory processes in various endocrine and exocrine tissues, for instance, the hypothalamic– pituitary complex (Propato-Mussafiri et al. 1992) and the pancreas (Mungan et al. 1991, Fridolf et al. 1992, Naruse et al. 1992, Bertrand et al. 1996) as well as the respiratory tract (Uddman et al. 1991). Apart from its well-known neuroendocrine functions, PACAP38 has been shown to affect epithelial ion transport. In a Cl  -secreting human colonic cell line, T 84 , for example, PACAP38 has been shown to potently stimulate enterocyte ion transport via mechanisms mediated by the vasoactive intestinal peptide (VIP) receptor and cAMP-dependent signaling (Nguyen et al. 1992). PACAP and its mRNA have also been found in the developing but not mature germ cells of the rat testis (Shioda et al. 1994). Receptor binding assays and receptor autoradiography have revealed two high affinity binding sites for PACAP localized in the testis and epididymis, at least on the germinal cells (Shivers et al. 1991). These findings suggest that PACAP may function as a paracrine or autocrine agent in the male reproductive tract. The present study investigated the role of PACAP38 in the regulation of epididymal Cl  secretion by examining its stimulatory effect on the Cl  -dependent short-circuit current (Isc) and its localization in the epididymal epi- thelium. The rat cauda epididymis was used for both electrophysiological and immunohistochemical studies because this region of the rat epididymis had been best characterized electrophysiologically, as compared with the caput and corpus, and the effects of many neuropeptides on the Cl  secretion by the cauda epididymal epithelium had also been studied previously. 389 Journal of Endocrinology (1997) 154, 389–395  1997 Journal of Endocrinology Ltd Printed in Great Britain 0022–0795/97/0154–0389 $08.00/0