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Osmoregulation in American shad, Alosa sapidissima, and the role of teleost chloride cells in ion movement
The osmoregulatory physiology of American shad, Alosa sapidissima, was investigated. Tolerance to full strength seawater developed at the larval-juvenile transition (45 d post-hatch) three months prior to seaward migration. Increased seawater tolerance was associated with gill development, proliferation of chloride cells and increased gill Na$\sp+$,K$\sp+$-ATPase activity. Shad lose the ability to osmoregulate in fresh water during autumnal migration (a possible cue) evidenced by declines in plasma chloride (20%) observed in wild juveniles. Gill Na$\sp+$,K$\sp+$-ATPase activity increases during this period. These changes were observed in the laboratory under natural conditions. Plasma chloride dropped 68% and gill Na$\sp+$,K$\sp+$-ATPase activity increased three-fold. Decreased plasma chloride was associated with increased mortality. Chloride cells (on both the primary filament and secondary lamellae in fresh water) increased during autumn as temperature declined. Changes in physiology and chloride cells are delayed and of a lower magnitude when shad in fresh water were held at constant (24$\sp\circ$C) temperature. In seawater, chloride cells on the secondary lamellae (likely ion uptake cells) declined to less than 2% of fresh water levels. Chloride cells (in both fresh and seawater acclimated shad) were shown to be rich in mitochondria and Na$\sp+$,K$\sp+$-ATPase by developing a technique for using specific fluorescent dyes in fixed tissue. In order to differentiate between the mechanisms of branchial ion uptake and excretion, radioligand binding methods were used to quantify Na$\sp+$,K$\sp+$,2Cl$\sp-$ cotransporters in the gill tissue of Atlantic salmon (Salmo salar). However, no high affinity binding was measured. The Na$\sp+$,K$\sp+$,2Cl$\sp-$ cotransporter was immuno-histochemically colocalized with Na$\sp+$,K$\sp+$-ATPase to chloride cells in fresh water and seawater acclimated shad. Western blot analysis was used to characterize a 170-190 kDa protein (Na$\sp+$,K$\sp+$,2Cl$\sp-$ cotransporter) present in greater quantities in seawater acclimated shad and Atlantic salmon than in fresh water acclimated fish. The presence of the cotransporter in the chloride cells of seawater acclimated shad supports the accepted model of ion excretion but the immunolocalization of the cotransporter in chloride cells of fresh water acclimated American shad is unexpected and remains poorly understood. ^
Animal Physiology|Aquatic sciences
Zydlewski, Joseph, "Osmoregulation in American shad, Alosa sapidissima, and the role of teleost chloride cells in ion movement" (1998). Doctoral Dissertations Available from Proquest. AAI9909237.