Hormonal Control of Osmoregulatory Proteins in the Intestine of Atlantic Killifish (Fundulus heteroclitus)

The Atlantic killifish (Fundulus heteroclitus) is a euryhaline fish, and thus able to maintain ionic and osmotic homeostasis in a wide range of salinities. When exposed to the dehydrating marine environment, the lost water is replenished through drinking seawater (SW) which is processed and absorbed in the intestine through the involvement of various cellular effectors. Intestinal water transport is osmotically driven by the active removal of NaCl from the gut lumen to the blood coupled to the precipitation of divalent Mg2+ and Ca2+ due to the secretion of bicarbonate. These processes allow for successive removal of water through aquaporins or via a claudin-controlled paracellular path. The control mechanisms for acclimation are still largely unknown, but hormonal regulation is one likely mechanism to induce long-term physiological changes. Prolactin, a freshwater (FW) acclimation hormone, and cortisol, primarily a SW acclimation hormone that may play a small role in FW acclimation, are two examples of such regulatory hormones. This study compared the mRNA expression of various cellular effectors of water handling in the intestine of Atlantic killifish, including components involved in salt transport (nkcc2, ecac), bicarbonate secretion (slc4a4a) and water handling (aqp1, aqp8, cldn15a, and cldn15b), in response to salinity acclimation and hormonal treatment with cortisol and prolactin. A comparison of intestinal mRNA expression of the chosen targets was performed on fish acclimated for 21 days to FW, brackish water (BW) and SW. A more than 2-fold higher level of nkcc2 mRNA was observed in the intestine of BW and SW fish compared to FW fish. No significant difference was observed in the other targets. Anterior intestine explants were treated with prolactin, cortisol, and a combination of both, and qPCR analysis indicated a 3-fold increase in nkcc2 mRNA in the intestine treated with cortisol, while no effects of prolactin was observed. No significant effects on other targets were observed.