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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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Oubain-sensitive HCO 3 - secretion & acid absorption by the marine teleost fish play a role in osmoregulation


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slide1

Oubain-sensitive HCO3- secretion & acid absorption by the marine teleost fish play a role in osmoregulation

M. Grosell & J. Genz

study goals
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
slide4

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-

results
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
extracellular hco 3 vs endogenous co 2
Extracellular HCO3- vs. Endogenous CO2
  • HEPES vs. 5 mM HCO3- saline
  • When exposed to HCO3-/CO2, base secretion increased twofold

Figure 4

of endogenous co 2
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
role of ca
Role of CA
  • Pharmacological test

- Etoxzolamide

Figure 5

what happens to h
What happens to H+?
  • Must be removed to
    • Maintain constant intracellular pH
    • Prevent reversal of CA-catalyzed hydration
slide11
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.

altering serosal ph
directly measure h transport
pharmacology
Pharmacology
  • Not inhibited by EIPA or Amiloride
    • Does not discount NHE
  • Significant reduction when exposed to oubain (NKA inhibitor)
slide15
ca mg ratio in precipitate
further discussion
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?