Oubain-sensitive HCO3- secretion

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

• M. Grosell J. Genz

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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

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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-
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Results

• Resting tissue
• Viable and stable for gt5 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

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Extracellular HCO3- vs. Endogenous CO2

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

Figure 4
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of endogenous CO2

• 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)

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Role of CA

• Pharmacological test
• - Etoxzolamide

Figure 5
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What happens to H?

• Must be removed to
• Maintain constant intracellular pH
• Prevent reversal of CA-catalyzed hydration

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• 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.

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Altering serosal pH
pH Reduction
7.4 33
7.0 44
6.6 47
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Directly measure H transport
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Pharmacology

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

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• Net acid/base flux

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CaMg ratio in precipitate
Intestinal
Salinity (ppt) 9 33 50
Ca2 (mmol/g) -0.20 0.14 0.44 0.14 0.65 0.04
Mg2 (mmol/g) -0.02 0.02 4.13 1.96 0.47 0.02
CaMg ratio 6.54 1.95 0.16 0.03 1.36 0.08
Rectal
Salinity (ppt) 9 33 50
Ca2 (mmol/g) 0.15 0.05 0.08 0.03 0.14 0.07
Mg2 (mmol/g) 0.03 0.01 0.13 0.03 0.19 0.03
CaMg ratio 4.92 1.23 -2.36 3.15 0.45 0.29
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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?