Intestinal transport following transfer to increased salinity in an anadromous fish (Oncorhynchus mykiss).

The ability to transition from freshwater to seawater environments is an intrinsic requirement of the life history of some fish species, including the anadromous rainbow trout (Oncorhynchus mykiss). The differences between hyper- and hypoosmoregulation are developed quickly (in hours to days), and at all scales, from gene expression to organ function. In this study, intestinal ion and water transport was examined in O. mykiss following acute transfer from freshwater (FW) to 70% seawater (SW). Plasma [Mg²+] increased at 24h post-transfer but recovered by 72 h. In the intestinal fluids, total CO₂ was found to increase with SW exposure/acclimation, while [Na+] decreased after 24h of SW exposure. Overall, in vitro experiments demonstrated the importance of base secretion to epithelial water uptake, and suggested that the primary physiological adjustments occurred 24-72 h after acute SW transfer. The mRNA expression of ion transporters important for intestinal osmoregulation and maintenance of acid-base balance was also investigated. A Na+/H+ exchanger (NHE2) and anion exchanger (SLC26a6) were hypothesized to be involved in the transport of acid-base equivalents, Na+, and Cl⁻, but were not uniformly expressed across tissue samples, and expression, where present, did not change following salinity transfer. NHE1, however, was expressed in all examined tissues (gill, kidney, anterior intestine, and pyloric cecae), but exhibited no changes in expression following acute salinity transfer.