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M. Grosell & J. Genz

Oubain-sensitive HCO 3 - secretion & acid absorption by the marine teleost fish play a role in osmoregulation. M. Grosell & J. Genz. Study Goals. Determine the contribution of endogenous metabolic CO 2 vs extracellular HCO 3 - as sources of secreted HCO 3 -

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M. Grosell & J. Genz

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  1. Oubain-sensitive HCO3- secretion & acid absorption by the marine teleost fish play a role in osmoregulation M. Grosell & J. Genz

  2. Study Goals • Determine the contribution of endogenous metabolic CO2 vs extracellular HCO3- as sources of secreted HCO3- • Determine role of carbonic anhydrase (CA) in hydrating endogenous CO2 • Determine fate of liberated H+ from CA-mediated hydration reaction

  3. Endogenous CO2 is hydrated in a reaction regulated by carbonic anhydrase (CA). The resulting HCO3- is actively transported across the apical membrane via Cl-/HCO3- exchange (AE). This is driven by the + potential inside the cell set up by NKA. Fig. 11 Cellular mechanisms in intestinal epithelial transport of HCO3-

  4. Results • Resting tissue • Viable and stable for >5 h • secretion rates are between 0.3-0.5 umol/cm2/h • TEP = -20 mV • Conductance = 10 mS/cm2 • HCO3- secretion decreases when deprived of O2 • Secretion is strongly temperature dependant

  5. Extracellular HCO3- vs. Endogenous CO2 • HEPES vs. 5 mM HCO3- saline • When exposed to HCO3-/CO2, base secretion increased twofold Figure 4

  6. Hydration of endogenous CO2 accounts for ~50% of total HCO3- secretion Percentage contributed by hydrated endogenous CO2 is species-dependant 30-60% CO2 in European flounder (Grosell et al 2001) and goby (Dixon & Loretz 1986) 100% extracellular HCO3- in Japanese eel (Ando & Subramanyam 1990) % of endogenous CO2

  7. Role of CA • Pharmacological test - Etoxzolamide Figure 5

  8. What happens to H+? • Must be removed to • Maintain constant intracellular pH • Prevent reversal of CA-catalyzed hydration

  9. Possible pathways for removal of H+ across the basolateral membrane is either Na+/H+ exchange (NHE) or an H+ pump. Na+ dependence (Fig. 8) suggests NHE.

  10. Altering serosal pH

  11. Directly measure H+ transport

  12. Pharmacology • Not inhibited by EIPA or Amiloride • Does not discount NHE • Significant reduction when exposed to oubain (NKA inhibitor)

  13. Net acid/base flux

  14. Ca:Mg ratio in precipitate Intestinal Rectal

  15. Further Discussion • Possibility of global impact of precipitates • Whole-animal acid/base balance • This process is similar to mammalian pancreatic secretion • Could intestinal HCO3- secretion be common to all vertebrates?

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