DOI: 10.1530/JOE-16-0162 http://joe.endocrinology-journals.org © 2016 Society for Endocrinology Printed in Great Britain Published by Bioscientifica Ltd. Journal of Endocrinology 325–337 J P BREVES and others Prolactin and cortisol regulate aquaporin 3 Research 230:3 Abstract Aquaporins (Aqps) are expressed within key osmoregulatory tissues where they mediate the movement of water and selected solutes across cell membranes. We leveraged the functional plasticity of Mozambique tilapia (Oreochromis mossambicus) gill epithelium to examine how Aqp3, an aquaglyceroporin, is regulated in response to osmoregulatory demands. Particular attention was paid to the actions of critical osmoregulatory hormones, namely, prolactin (Prl), growth hormone and cortisol. Branchial aqp3 mRNA levels were modulated following changes in environmental salinity, with enhanced aqp3 mRNA expression upon transfer from seawater to freshwater (FW). Accordingly, extensive Aqp3 immunoreactivity was localized to cell membranes of branchial epithelium in FW-acclimated animals. Upon transferring hypophysectomized tilapia to FW, we identifed that a pituitary factor(s) is required for Aqp3 expression in FW. Replacement with ovine Prl (oPrl) was suffcient to stimulate Aqp3 expression in hypophysectomized animals held in FW, an effect blocked by coinjection with cortisol. Both oPrl and native tilapia Prls (tPrl 177 and tPrl 188 ) stimulated aqp3 in incubated gill flaments in a concentration-related manner. Consistent with in vivo responses, coincubation with cortisol blocked oPrl-stimulated aqp3 expression in vitro. Our data indicate that Prl and cortisol act directly upon branchial epithelium to regulate Aqp3 in tilapia. Thus, within the context of the diverse actions of Prl on hydromineral balance in vertebrates, we defne a new role for Prl as a regulator of Aqp expression. Hormonal regulation of aquaporin 3: opposing actions of prolactin and cortisol in tilapia gill Jason P Breves 1 , Mayu Inokuchi 2,3 , Yoko Yamaguchi 2 , Andre P Seale 2 , Bethany L Hunt 1 , Soichi Watanabe 3 , Darren T Lerner 2,4 , Toyoji Kaneko 3 and E Gordon Grau 2 1 Department of Biology, Skidmore College, Saratoga Springs, New York, USA 2 Hawai‘i Institute of Marine Biology, University of Hawai‘i at Mānoa, Kāne‘ohe, Hawai‘i, USA 3 Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, University of Tokyo, Bunkyo, Tokyo, Japan 4 University of Hawai‘i Sea Grant College Program, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, USA Journal of Endocrinology (2016) 230, 325–337 Key Words f aquaporin f prolactin f cortisol f gill f osmoregulation Correspondence should be addressed to J P Breves Email jbreves@skidmore.edu Introduction Aquaporin (Aqp) proteins are important mediators of transepithelial fuid transport and cell volume regulation in vertebrate tissues (Boury-Jamot et al. 2006, Verkman 2012). Spanning the plasma membrane, Aqps facilitate passive movements of water and, in some cases, small nonionic compounds. Diverse activities, such as cell migration, neural signaling, fat metabolism and osmotic balance, rely upon the regulated movement of water and solutes via Aqps. Accordingly, misregulated or genetic interruption of Aqp function underlies a host of