SINGER, T.D.*; SCHULTE, P.M.; University of Waterloo*; University of British Columbia: Osmoregulation and control of CFTR expression in teleost fish

Homologues of the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) play an important role in osmoregulation of seawater adapted teleost fish. CFTR, an apically located chloride channel, interacts with basolaterally located Na⁺,K⁺-ATPase and a Na⁺K⁺2Cl^– co-transporter in gill chloride cells to maintain ion balance. We have examined the expression patterns of CFTR in several teleosts capable of regulating ion secretion and adapting to changes in salinity. The euryhaline killifish Fundulus heteroclitus, which is capable of rapid movement between freshwater and seawater, expresses kfCFTR in gill, opercular epithelium, intestine and brain. Following abrupt transfer from freshwater to seawater, gill kfCFTR mRNA abundance increased several fold by 8 h, peaking at 24 h and subsided to levels slightly elevated from freshwater by 2 weeks. This increase was preceded by a sharp transient rise in plasma cortisol, a hormone thought to play a key role in seawater adaptation. In a second teleost, the Atlantic salmon, a distinct juvenile developmental phase known as smoltification occurs prior to seawater entry. During the peak of smoltification, Atlantic salmon CFTR I and II isoforms showed distinct gill expression patterns following abrupt seawater transfer. Cortisol implantation of smolts also had specific effects on expression of each of the two isoforms. In contrast, gradual seawater transfer resulted in no significant changes in plasma cortisol and modest changes, if any, in CFTR mRNA abundance in the rainbow trout. The ability of euryhaline teleosts to regulate CFTR expression during salinity adaptation makes them an exceptional model for expression studies.