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Estrogen is an essential hormone for many reproductive and non-reproductive functions. The function of estrogen in the reproductive cycle of seabream (Sparus aurata), a protandrous hermaphrodite teleost fish, is complex but it is understood to be involved in sex inversion, a process that occurs in some individuals during the second reproductive season. Estrogen action is mediated by two estrogen receptor (ER) subtypes designated alpha and beta. As a step to understanding the mechanisms of estrogen action during natural and induced sex reversal in seabream, we have isolated two cDNAs encoding distinct forms of ER homologous to mammalian ERalpha and ERbeta. The seabream ERalpha clone (sbERalpha1), which was truncated in the A/B domain, corresponded to a variant differing in five amino acids from another recently cloned sbERalpha. The ERbeta clone (sbERbeta) encoded a protein 559 amino acids long and showed only 40% identity to sbERalpha. Northern blot analysis of liver and ovary mRNA indicated the presence of several transcripts of the two receptor subtypes. PCR analysis showed that the two receptors differed in their expression pattern. sbERalpha had a more restricted distribution, occurring mainly in testis, liver and heart, and sbERbeta was present in most tissues, being more abundant in ovary, testis, liver, intestine and kidney. The presence in seabream of two ERs with several ER transcripts and their pattern of distribution are consistent with the widespread effects of estrogen in different tissues.

In all teleost fishes vitellogenesis is triggered and maintained by oestradiol-17beta (E2) and is accompanied by an increase of blood plasma calcium and phosphate. The action of this hormone on calcium metabolism was investigated by treating fast-growing immature juvenile sea bream (Sparus aurata) with coconut butter implants alone (control) or implants containing 10 microg/g E2. Treatment with E2 induced the production of circulating vitellogenin, a 2.5-fold increase in plasma ionic Ca2+ and a 10-fold increase in plasma total calcium, largely bound to protein. In contrast to freshwater species, which obtain most of their calcium from the environment directly through the gills, the intestinal component of calcium uptake of the salt water-living sea bream represented up to 60-70% of the total uptake. The whole body calcium uptake, expressed as the sum of calcium obtained via intestinal and extra-intestinal (likely branchial) routes increased significantly in response to E2. Combined influx and unchanged efflux rates resulted in a significant 31% increase in net calcium uptake. There was no evidence for an effect of E2 on the calcium and phosphate content of the scales or the tartrate-resistant acid phosphatase activity (an index for bone/scale osteoclast activity). While most freshwater fish appear to rely on internal stores of calcium, i.e. bone and/or scales to increase calcium availability, the marine sea bream accommodates calcium-transporting mechanisms to obtain calcium from the environment and preserve internal stores. These observations suggest that a fundamental difference may exist in the E2-dependent calcium regulation between freshwater and marine teleosts.