In the kidneys of mammals,
cyclooxygenase type 2 (COX2) is expressed in medullary interstitial cells, the macula densa and epithelial cells of the cortical thick ascending limb where it generates
prostaglandins that regulate
hormone secretion, inhibit ion transport, and support cell survival during
salt loading and
dehydration. In teleosts, the gills are in direct contact with an aquatic environment and are the dominant site of osmoregulation. During transfers between salinities, specialized cells in the gills (
chloride cells) rapidly regulate NaCl secretion for systemic osmoregulation while they simultaneously are exposed to acute osmotic shock. This study was conducted to determine if COX2 is expressed in the gills, and if so, to evaluate its function in cellular and systemic osmoregulation. Degenerate primers, reverse transcription-PCR and rapid amplification of
cDNA ends were used to deduce the complete
cDNA sequence of a putative COX2
enzyme from the gills of the euryhaline killifish (Fundulus heteroclitus). The 2,738 base pair
cDNA includes a coding region for a 610
amino acid protein that is over 70% identical to mammalian COX2. A purified antibody generated against a conserved region of mouse COX2 labeled
chloride cells, suggesting that the
enzyme may control NaCl secretion as an autocrine agent. Real-time PCR was then used to demonstrate that
mRNA expression of the COX2 homologue was threefold greater in gills from chronic seawater killifish than in gills from chronic freshwater killifish. Expression of Na(+)/K(+)/2Cl(-) cotransporter and the
cystic fibrosis transmembrane conductance regulator were also greater in seawater, suggesting that chronic COX2 expression in the gills is regulated in parallel to the key ion transporters that mediate NaCl secretion. Real-time PCR was also used to demonstrate that acute transfer from seawater to freshwater and from freshwater to seawater led to rapid, transient inductions of COX2 expression. Together with previous physiological evidence, the present molecular and immunological data suggest that constitutive branchial COX2 expression is enhanced in seawater, where
prostaglandins can regulate NaCl secretion in
chloride cells. Our data also suggest that branchial COX2 expression may play a role in cell survival during acute osmotic shock.