RESUMEN
Seawater-acclimated eurythermic mummichogs (Fundulus heteroclitus L.) were acclimated to cold and warm conditions (5 and 20 °C, 4 weeks). Opercular epithelia (OE) from 20 °C-acclimated animals, containing numerous mitochondrion-rich chloride cells were mounted in Ussing-style membrane chambers, cooled to 16, 13, 10, 5 and 2.5 °C, then subjected to hypotonic shock that normally inhibits Cl(-) secretion (as short-circuit current, I(sc)). Cold exposure to 10 °C slowed Cl(-) secretion (Q(10)=1.62 ± 0.204 95% CI) and OEs responded rapidly and reversibly to hypotonic shock, but below 8.0 °C a sharp decrease (Q(10)=5.63 ± 0.736) occurred and the tissue was unresponsive to hypotonicity. By immunocytochemistry, Focal Adhesion Kinase (FAK) phosphorylated at tyrosine-407 (pY(407)) colocalized with CFTR in apical membrane and dephosphorylated with hypotonic shock at 20 °C but failed to dephosphorylate at 5 °C, while opercular epithelia from cold-acclimated fish at 5 and 20 °C responded normally to hypotonic shock. Cold-shock of warm-acclimated OEs also stimulated covering over of mitochondrion- rich cell apical crypts, detected by SEM. Cold-acclimation increased C18:1 and decreased C18:0 fatty acids in liver, indicating homeoviscous adaptation. Eurythermic fish acclimate osmoregulatory systems to cold by maintaining membrane fluidity and preserving complex transport regulation pathways.