Parathyroid hormone-related protein: a calcium regulatory factor in sea bream (Sparus aurata L.) larvae

The effects of an N-terminal peptide (amino acids 1-38) of Fugu parathyroid hormone-related protein (PTHrP 1-38) on calcium regulation of larval sea b ream were investigated in seawater (36 parts per thousand) and after transf er to dilute seawater (12 parts per thousand). Exposure to PTHrP 1-38 evoke d a 1.5-fold increase in calcium influx in both full-strength and dilute se awater. Calcium influx in dilute seawater-adapted larvae was roughly one-ha lf that observed in full-strength seawater controls. PTHrP 1-38 also reduce d drinking of fish in seawater but, at all concentrations tested, was witho ut effect in dilute seawater. The amount of water imbibed was 55% lower in dilute seawater than in seawater. PTHrP 1-38 exposure affected the calcium influx route: the main contribution of calcium uptake shifted from intestin al absorption to extraintestinal uptake, probably by the induction of a dos e-dependent increase in branchial (active) transport. Moreover, seawater-ad apted fish exposed to 1 nM and 10 mM PTHrP 1-38 experienced a 2.5-fold redu ction in overall calcium efflux. Overall, the calciotropic action of PTHrP 1-38 resulted in a dose-dependent increase in net calcium balance.