Ishiguro H, Naruse S, Kitagawa M, Suzuki A, Yamamoto A, Hayakawa T, Case R M, Steward M C
Internal Medicine II, Nagoya University School of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya 466-8550, Japan.
J Physiol. 2000 Oct 15;528 Pt 2(Pt 2):305-15. doi: 10.1111/j.1469-7793.2000.00305.x.
Permeabilities of the luminal and basolateral membranes of pancreatic duct cells to CO2 and HCO3- were examined in interlobular duct segments isolated from guinea-pig pancreas. Intracellular pH (pHi) was measured by microfluorometry in unstimulated, microperfused ducts loaded with the pH-sensitive fluoroprobe 2'7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). When HCO3-/CO2 was admitted to the bath, pHi decreased transiently as a result of CO2 diffusion and then increased to a higher value as a result of HCO3- uptake across the basolateral membrane by Na+-HCO3- cotransport. When HCO3-/CO2 was admitted to the lumen, pHi again decreased but no subsequent increase was observed, indicating that the luminal membrane was permeable to CO2 but did not allow HCO3- entry to the cells from the lumen. Only when the luminal HCO3- concentration was raised above 125 mM was HCO3- entry detected. The same was true of duct cells stimulated with forskolin. Recovery of pHi from an acid load, induced by exposure to an NH4+ pulse, was dependent on basolateral but not luminal Na+ and could be blocked by basolateral application of methylisobutylamiloride and H2DIDS. This indicates that the Na+-H+ exchangers and Na+-HCO3- cotransporters are located exclusively at the basolateral membrane. In the presence of HCO3-/CO2, substitution of basolateral Cl- with glucuronate caused larger increases in pHi than substitution of luminal Cl-. This suggests that the anion exchanger activity in the basolateral membrane is greater than that in the luminal membrane. We conclude that the luminal and basolateral membranes are both freely permeable to CO2, but while the basolateral membrane has both uptake and efflux pathways for HCO3-, the luminal membrane presents a significant barrier to the re-entry of secreted HCO3-, largely through the inhibition of the luminal anion exchanger by high luminal HCO3- concentrations.
在从豚鼠胰腺分离的小叶间导管段中，检测了胰腺导管细胞腔膜和基底外侧膜对二氧化碳和碳酸氢根的通透性。在未受刺激、用pH敏感荧光探针2'7'-双（2-羧乙基）-5（6）-羧基荧光素（BCECF）加载的微灌注导管中，通过微荧光测定法测量细胞内pH（pHi）。当向浴槽中加入碳酸氢根/二氧化碳时，由于二氧化碳扩散，pHi短暂下降，然后由于通过钠-碳酸氢根共转运体跨基底外侧膜摄取碳酸氢根而升高到更高值。当向管腔中加入碳酸氢根/二氧化碳时，pHi再次下降，但随后未观察到升高，表明腔膜对二氧化碳可通透，但不允许碳酸氢根从管腔进入细胞。仅当管腔碳酸氢根浓度升高到125 mM以上时，才检测到碳酸氢根进入。用福斯可林刺激的导管细胞也是如此。由暴露于铵脉冲诱导的酸负荷后pHi的恢复依赖于基底外侧而非管腔的钠，并且可被基底外侧应用甲基异丁基阿米洛利和H2DIDS阻断。这表明钠-氢交换体和钠-碳酸氢根共转运体仅位于基底外侧膜。在存在碳酸氢根/二氧化碳的情况下，用葡糖醛酸盐替代基底外侧的氯离子比替代管腔的氯离子导致pHi升高幅度更大。这表明基底外侧膜中的阴离子交换体活性大于管腔膜中的活性。我们得出结论，腔膜和基底外侧膜对二氧化碳均自由通透，但基底外侧膜既有摄取也有排出碳酸氢根的途径，而腔膜对分泌的碳酸氢根的再进入呈现出显著屏障，这主要是由于管腔中高浓度的碳酸氢根对管腔阴离子交换体的抑制作用。
