Sodium, net acid and ammonia fluxes in freshwater-adapted European flounder (Platichthys flesus L.). Pharmacological inhibition and effects on gill ventilation volume

Abstract

Net branchial sodium uptake, and net acid and ammonia excretion, together with gill ventilation volumes were measured in euryhaline European¯ounder Platichthys¯esus, adapted to freshwater. External amiloride or acetazolamide (0.1 mmol l -1 ) caused signi®cant reductions (P < 0.05) in sodium uptake and net acid excretion, while 0.05 mmol l -1 thiocyanate had no effect on any of the¯uxes measured. Ethoxzolamide (0.1 mmol l -1 ) however, caused signi®cant reductions in all¯uxes measured and also caused a 32% reduction in the volume of water exhaled by the gills. In this study, it appears that sodium uptake and net acid excretion are indirectly linked, probably by a proton pump arrangement which is thought to be a mechanism for sodium uptake in trout gills.

Acclimation of ion regulatory capacities in gills of marine fish under environmental hypercapnia

“…Again, the mechanisms involved may differ between freshwater and seawater fish. Studies in freshwater fish have proposed active export of protons by an apical V-type H ϩ -ATPase, coupled to an import of Na ϩ ions via an ENaC (epithelial Na ϩ -channel)-like channel (10,28). In seawater fish, use of the Na ϩ /H ϩ exchanger and the existing Na ϩ gradient is energetically cheaper.…”

Modulation of Rh glycoproteins, ammonia excretion and Na+ fluxes in three freshwater teleosts when exposed chronically to high environmental ammonia

“…Current hypotheses include a direct coupling through Na + /NH 4 + exchangers, coupling with H + excretion by an apical Na + /H + exchanger (NHE), and coupling with H + excretion via an apical H + pump, which energizes Na + uptake through a putative Na + channel. The last two mechanisms would acidify the gill boundary layer, thereby enhancing the 'diffusion-trapping' of NH 3 as NH 4 + and maintaining the partial pressure gradient for diffusive NH 3 efflux, thus manifesting as an apparent Na + /NH 4 + exchange (Clarke and Potts, 1998;Wilson et al, 1994).…”

Isolated filament potentials and branchial ion fluxes in the European flounder (Platichthys flesus L.). Evidence for proton pump mediated sodium uptake

“…Proton equivalent excretion in this study was found to be sensitive to vanadate, and also to the carbonic anhydrase inhibitor acetazolamide (Clarke & Potts, 1998). The titratable component measured in this study did indeed increase upon sodium channel inhibition before eventually declining, although these changes were not signi®cant possibly due to the small sample size.…”

“…Proton pumps (H + -ATPase) remove protons from the cell generating a negative potential on the inner side of the apical membrane. There is also preliminary evidence that euryha-line¯ounder Platichthys¯esus also adopt this mechanism of sodium uptake when adapted to freshwater (Clarke & Potts, 1998). The location of such pumps in ®sh gills, however, remains unclear, and may be associated with the pavement epithelial cells (Laurent, Goss & Perry, 1994;Sullivan, Fryer & Perry, 1995, 1996, the mitochondria rich cells (Lin & Randall, 1991) or both cell types (Lin & Randall, 1995).…”