日本鰻Anguilla japonica礦物性皮質素受體(minerolocorticoid receptor)參與低張環境離子調節

Abstract

Japanese eels (Anguilla japonica) are catadromous with a delicate hormonal system to maintain normal ionic levels under different salinity. Cortisol is the major corticosteroid in teleost and is considered to be a dual regulator as glucocorticoid and mineralocorticoid. On the other hand, mineralocorticoid receptor (MR) function of ion-retention and its protection from excess cortisol binding by 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) are well known in mammals. Since the ancestral MR appeared in cartilaginous fish, the MR may be functional in teleosts. In ion deficient environment, teleosts maintain their osmolarity by ion-absorptive mitochondria-rich cells (MRCs) with highly-regulated Na+, K+-ATPase (NKA) activity. The cDNA sequence of eel MR has been cloned in this thesis and 11β-HSD2 homolog, e11β-HSDsf was retrived from database. To testify the ion-retention role of MR, I induced proliferation of ion-absorptive MRCs in the gill of Japanese eel, Anguilla japonica under distilled water (DW). In DW transfer, lamellar MRCs distributed abundantly after 48 hours and lamellar MRCs cells distributed on the whole lamella after 24 hours in the DDW transfer. By absolute quantification of RT-qPCR, the MR and e11β-HSDsf mRNA levels were up-regulated in same patterns during time course in DW in the gill, intestine and kidney. There were also more copy numbers of e11β-HSDsf than copy numbers of MR. It is possible that e11β-HSDsf protects MR in eels. The result of in situ hybridization showed that the mRNA of MR localized in the MRCs. NKA activity is an indicator of osmoregulation in both fish and mammals. The relations between MR and NKA activity was investigated by time-course and antagonist treatment. NKA activity decrease to freshwater (FW) level in MR antagonist, spironolactone treatment under the presence of lamellar MRCs. This study suggest the protection of MR by e11β-HSDsf and establish the MR function in ion-retention under hypotonic environment in Japanese eels.