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General and Comparative Endocrinology 152 (2007) 273

Discussion

Endocrinology of salt and water balance

Yoshio Takei a, Neil Hazon b

a Laboratory of Physiology, Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, 164-8639 Tokyo, Japanb School of Biology, Gatty Marine Laboratory, University of St. Andrews, St. Andrews KY16 8LB, Scotland, UK

The symposium started with two State-of-the-Art lec-tures by Yoshio Takei on ‘Exploring novel body fluid reg-ulating hormones in fish’ and by Geoff Coast on ‘Theendocrine control of salt balance in insects’. The lectureby Yoshio Takei was dedicated to the late Professor IanChester Jones, one of the founding fathers of comparativeendocrinology, and discussed a new approach to identifynovel hormones by utilizing recently established genomedatabases of several teleost species. Since hypotensive andion-extruding hormones are highly diversified in fishescompared with terrestrial animals, excellent opportunitiesare now available to identify both new hormones in fishand, potentially, also enable discovery of new hormonesin mammals. In the second State-of-the-Art Lecture Coastreviewed hormonal regulation of ion transport in insectMalpighian tubule by two distinct classes of neuropeptides(diuretics and antidiuretics). In addition, he introducedrecent in vivo studies on the effect of diuretic factors onthe urine composition in blood sucking insects.

Tes Toop introduced her recent studies on the mRNAexpression of natriuretic peptide receptors in osmoregula-tory tissues of salmonids, that suggest the involvement ofnatriuretic peptides in adaptation to different osmotic envi-ronments. Gordon Cramb reviewed recent pioneeringmolecular studies on the guanylin peptides and their recep-tors (GC-C) in the eel. This was followed by Tatsuya

Sakamoto who proposed new functions for the classicalosmoregulatory hormones, prolactin and cortisol, in theintestine as apoptotic/proliferative factors involved in tis-sue reorganization for adaptation to different osmotic envi-ronments. Akihisa Urano summarized extensive studies onhormonal gene expression in the migratory salmon,Oncorhynchus keta, particularly in relation to freshwateradaptation. Hiroko Nishimura presented data on develop-mental studies on the role of vasotocin-sensitive waterchannel (AQP2) in chicken nephrogenesis in relation to

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reduced nutritional supply. Gordon Cramb presented apaper for Chris Cutler reporting the effects of cortisol onaquaporin (AQP1 and AQP3) expression in the osmoregu-latory tissues of eels and discussed their roles in seawateradaptation. Minoru Uchiyama summarized comprehensivestudies on hormonal control of osmoregulation in anuransin different osmotic habitats, from totally aquatic Xenopus

to terrestrial species. Peter Klaren discussed low salinityacclimation of the euryhaline seawater teleost, gilthead sea-bream, Sparus auratus, in which thyroid hormone metabo-lizing enzyme activities were examined in osmoregulatoryorgans such as the gills and kidney. Adelino Canario

reported a possible function of GnRH receptors in osmo-regulation in the seabass, Dicentrarchus labrax. At least 5functional GnRH receptor subtypes exist in the genomeof seabass, and some of their gene expression in osmoreg-ulatory tissues change after transfer to hypoosmotic media.

A general discussion examined the relevance of usinggenetically-modified animals in comparative endocrinol-ogy of osmoregulation. Gene knock-in or knock-out ani-mals have been infrequently employed in osmoregulatorycontexts, even though the technique has been establishedin non-mammalian species including fish. This is mostprobably because homeostatic regulation such as osmo-regulation cannot be achieved by a single hormone andmany endocrine factors may interact to ensure effectiveregulation. However, it is also possible that there is a piv-otal hormone that governs the whole process of adapta-tion to an osmotic environment, as shown by prolactinfor freshwater adaptation in some teleost species and byantidiuretic hormone for adaptation to terrestrial environ-ment. Therefore, such an essential hormone may also existfor seawater adaptation in teleost fish. Confirmation ofthis essential role by gene-modified animals at the organ-ismal level may prove to be more important in the post-genome era.