The prolactin cell of a euryhaline fish, the tilapia, Oreochromis mossambicus: a model for osmoreception

Prolactin (PRL) plays a central role in fresh water (FW) adaptation in teleost fish. Consistent with the freshwater-adapting actions of prolactin (PRL), the release of this hormone from the tilapia pituitary is stimulated as extracellular osmolality is reduced both in vitro and in vivo. The main objective of this research was to investigate the mechanisms involved in the mediation between an osmotic stimulus and PRL release. The present studies also provided evidence of the unique osmosensitivity of the PRL cells when compared to the other pituitary cell types such as GH cells. Evidence now suggests that growth hormone (GR) acts in the seawater (SW) adaptation in some euryhaline fish. The release and content of PRL in response to long-term (days) changes in medium osmolality were markedly different than those of GH and corticotropin (ACTH) measured from the same pituitaries. Repeated blood withdrawal and transfer from SW to FW increased circulating PRL levels, whereas GH levels were unchanged. PRL release was not only more sensitive to a decrease in extracellular osmolality but long-lasting, when compared with the response of GH cells in dispersed cells and whole pituitaries. The tilapia PRL cell provides an excellent model to investigate osmoreception. Working with this model, a technique was developed for examining changes in cell volume and PRL release from the same preparation of cells, as well as measurements of [Ca2+]i. These approaches were utilized to demonstrate that the rapid increase in PRL release in response to reductions in medium osmolality is largely dependent on extracellular calcium. Studying the relationships between cell volume, [Ca2+] and PRL release in response to varying osmolalities, depolarizing conditions and ion channel blockers, provided evidence that stretch-activated calcium-permeant ion channels are responsible for the transduction of reduced extracellular osmolality into increased PRL release. The present findings support the proposed signal transduction model for osmotic stimuli in PRL cells. A decrease in extracellular osmolality leads to an increase in cell volume. Cell swelling increases the open probability of stretch-activated ion channels, and results in the entrance of extracellular calcium into the cell and stimulation of PRL release.