Prolactin regulates luminal bicarbonate secretion in the intestine of the sea bream (Sparus auratus L.).

The pituitary hormone prolactin (PRL) is a pleiotropic endocrine factor that plays a major role in the regulation of ion balance in fish, with demonstrated actions mainly in the gills and kidney. The role of PRL in intestinal ion transport remains little studied. In marine fish, that have high drinking rates epithelial bicarbonate secretion in the intestine produces luminal carbonate aggregates believed to play a key role in water and ion homeostasis. The present study was designed to establish the putative role of PRL in the regulation of intestinal bicarbonate secretion (BCS) in a marine fish. Basolateral addition of PRL to the anterior intestine of the sea bream mounted in Ussing chambers causes a rapid (<20 min) decrease of BCS measured by pH-stat. A clear inhibitory dose response curve is obtained, with a maximal inhibition of 60-65% of basal BCS. The threshold concentration of PRL with a significant effect on BCS is 10 ng.ml(-1), which is comparable with putative plasma levels in seawater fish. The effect of PRL on apical BCS is independent of the generation route for bicarbonate, as shown in a preparation devoid of basolateral HCO(3)(-) /CO(2) buffer. In addition, specific inhibitors of JAK2 (AG-490, 50μM), PI3K (LY-294002, 75μM) or MEK (U-012610, 10μM) cause a 50-70% reduction in the effect of PRL on BCS, and demonstrate the involvement of PRL receptors. In addition to rapid effects, PRL actions are mediated at a genomic level. Incubation of intestinal explants of anterior intestine of the sea bream in vitro for 3 hours demonstrates a specific effect of PRL on expression of Slc4a4A (Na(+)-HCO(3)(-) co-transporter), but not on Slc26a6A or Slc26a3B (Cl(-)/ HCO(3)(-)- exchangers). We propose a new role for PRL in the regulation of BCS, an essential function for ion/water homeostasis in the intestine of marine fish