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CO 2 permeability and bicarbonate transport in microperfused interlobular ducts isolated from guinea‐pig pancreas
Author(s) -
Ishiguro H.,
Naruse S.,
Kitagawa M.,
Suzuki A.,
Yamamoto A.,
Hayakawa T.,
Case R. M.,
Steward M. C.
Publication year - 2000
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2000.00305.x
Subject(s) - chemistry , epithelial polarity , cotransporter , bicarbonate , biophysics , lumen (anatomy) , apical membrane , intracellular ph , dids , ion transporter , medicine , endocrinology , membrane , intracellular , biochemistry , sodium , biology , organic chemistry
1 Permeabilities of the luminal and basolateral membranes of pancreatic duct cells to CO 2 and HCO 3 − were examined in interlobular duct segments isolated from guinea‐pig pancreas. Intracellular pH (pH i ) was measured by microfluorometry in unstimulated, microperfused ducts loaded with the pH‐sensitive fluoroprobe 2′7′‐bis(2‐carboxyethyl)‐5(6)‐carboxyfluorescein (BCECF). 2 When HCO 3 − /CO 2 was admitted to the bath, pH i decreased transiently as a result of CO 2 diffusion and then increased to a higher value as a result of HCO 3 − uptake across the basolateral membrane by Na + ‐HCO 3 − cotransport. 3 When HCO 3 − /CO 2 was admitted to the lumen, pH i again decreased but no subsequent increase was observed, indicating that the luminal membrane was permeable to CO 2 but did not allow HCO 3 − entry to the cells from the lumen. Only when the luminal HCO 3 − concentration was raised above 125 m m was HCO 3 − entry detected. The same was true of duct cells stimulated with forskolin. 4 Recovery of pH i from an acid load, induced by exposure to an NH 4 + pulse, was dependent on basolateral but not luminal Na + and could be blocked by basolateral application of methylisobutylamiloride and H 2 DIDS. This indicates that the Na + ‐H + exchangers and Na + ‐HCO 3 − cotransporters are located exclusively at the basolateral membrane. 5 In the presence of HCO 3 − /CO 2 , substitution of basolateral Cl − with glucuronate caused larger increases in pH i than substitution of luminal Cl − . This suggests that the anion exchanger activity in the basolateral membrane is greater than that in the luminal membrane. 6 We conclude that the luminal and basolateral membranes are both freely permeable to CO 2 , but while the basolateral membrane has both uptake and efflux pathways for HCO 3 − , the luminal membrane presents a significant barrier to the re‐entry of secreted HCO 3 − , largely through the inhibition of the luminal anion exchanger by high luminal HCO 3 − concentrations.