Chloride transport activation by plasma osmolarity during rapid adaptation to high salinity of Fundulus heteroclitus

Abstract

Transition from low salt water to sea water of the euryhaline fish, Fundulus heteroclitus, involves a rapid signal that induces salt secretion by the gill chloride cells. An increase of 65 mOsm in plasma osmolarity was found during the transition. The isolated, chloride-cell-rich opercular epithelium of sea-water-adapted Fundulus exposed to 50 mOsm mannitol on the basolateral side showed a 100% increase in chloride secretion, which was inhibited by bumetanide 10(-4) M and 10(-4) M DPC (N-Phenylanthranilic acid). No effect of these drugs was found on apical side exposure. A Na+/H+ exchanger, demonstrated by NH4Cl exposure, was inhibited by amiloride and its analogues and stimulated by IBMX, phorbol esters, and epithelial growth factor (EGF). Inhibition of the Na+/H+ exchanger blocks the chloride secretion increase due to basolateral hypertonicity. A Cl-/HCO3- exchanger was also found in the chloride cells, inhibited by 10(-4) M DIDS but not involved in the hyperosmotic response. Ca2+ concentration in the medium was critical for the stimulation of Cl- secretion to occur. Chloride cell volume shrinks in response to hypertonicity of the basolateral side in sea-water-adapted operculi; no effect was found on the apical side. Fresh-water-adapted fish chloride cells show increased water permeability of the apical side. It is concluded that the rapid signal for adaptation to higher salinities is an increased tonicity of the plasma that induces chloride cell shrinkage, increased chloride secretion with activation of the Na+K+2Cl- cotransporter, the Na+/H+ exchanger and opening of Cl- channels.

Metabolic substrates are not mobilized from the osmoregulatory organs (gills and kidney) of the estuarine pufferfishes Sphoeroides greeleyi and S. testudineus upon short-term salinity reduction

“…The results also fell within the range reported for other marine/estuarine teleosts (e.g., Maren et al, 1992;Claiborne et al, 1994;Zadunaisky et al, 1995;Zadunaisky, 1996;Jensen et al, 1998;Marshall et al, 1999). When facing salinity changes, plasma osmolality and chloride concentrations directly reflect the fishes capacity of extracellular fluid homeostasis.…”

Aquaporin-3 expressed in the basolateral membrane of gill chloride cells in Mozambique tilapia Oreochromis mossambicus adapted to freshwater and seawater

“…It is more likely, however, that increased environmental salinity results in a slight increase in internal osmolality, which could be detected by the basolateral membrane of chloride cells. In fact, changes in osmolality on the basolateral side have been shown to affect the rate of Cl -secretion by opercular epithelia of killifish (Zadunaisky et al, 1995;Marshall et al, 2000).…”

Branchial ionocyte organization and ion-transport protein expression in juvenile alewives acclimated to freshwater or seawater

“…Compared with ionocytes, accessory cells are different in that they have a lower abundance of Na + /K + -ATPase (referred to hereafter as NKA) and are ultrastructurally distinct; however, they are similar in that they are rich in mitochondria and have a tubular network. The tight junctions between an ionocyte and accessory cell are shallow and might represent a low-resistance pathway for Na + extrusion in seawater (Silva et al, 1977;Zadunaisky et al, 1995;Towle, 1990). …”