Control of ion transport by mitochondrion-rich chloride cells of eurythermic teleost fish: Cold shock vs. cold acclimation

Abstract

Seawater-acclimated eurythermic mummichogs (Fundulus heteroclitus L.) were acclimated to cold and warm conditions (5 and 20°C, 4weeks). 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, Isc). Cold exposure to 10°C slowed Cl− secretion (Q10=1.62±0.204 95% CI) and OEs responded rapidly and reversibly to hypotonic shock, but below 8.0°C a sharp decrease (Q10=5.63±0.736) occurred and the tissue was unresponsive to hypotonicity. By immunocytochemistry, Focal Adhesion Kinase (FAK) phosphorylated at tyrosine-407 (pY407) 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.