Short title: Parathyroid hormones and estradiol in sea bream Key words: fish, sea bream, calcium, phosphate, estradiol, PTHrP

Estradiol (E2) increases circulating calcium and phosphate levels in fish, thus acting as a hypercalcemic and hyperphosphatemic factor during periods of high calcium requirements such as during vitellogenesis. Since, parathyroid hormone related protein (PTHrP) has been shown to be calciotropic in fish, we hypothesized that the two hormones could be mediating the same process. Sea bream, Sparus auratus, juveniles receiving a single intraperitoneal injection of piscine PTHrP(1-34) showed an elevation in calcium plasma levels within 24 h. In contrast injections of the PTH/PTHrP receptor antagonist PTHrP(7-34) decreased circulating levels of calcium in the same period. Intra-peritoneal implants of E2 (10 μg/g) evoked significant increases of circulating plasma levels of calcium and phosphorus and a sustained increases of circulating plasma levels of PTHrP. However, a combined treatment E2 and PTHrP(7-34) evoked a markedly lower calcium response compared to E2 alone. We conclude that PTHrP or a related peptide that binds the PTH/PTHrP receptor mediates at least in part the hypercalcemic effect of E2 in calcium and phosphate balance in fish. Page 2 of 18