Calcium balance in sea bream (Sparus aurata):: the effect of oestradiol-17β

In all teleost fishes vitellogenesis is triggered and maintained by cestradiol-17beta (E-2) 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 mug/g E-2. Treatment with E, induced the production of circulating vitellogenin, a 2.5-told 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 E-2. Combined infiux and unchanged efflux rates resulted in a significant 31% increase in net calcium uptake. There was no evidence for an effect of E-2 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 fresh\water 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 E-2-dependent calcium regulation between freshwater and marine teleosts.