Absence of a relationship between arterial pH and pancreatic bicarbonate secretion in the isolated perfused cat pancreas

Mechanisms of bicarbonate secretion in the pancreatic duct

In many species the pancreatic duct epithelium secretes HCO3- ions at a concentration of around 140 mM by a mechanism that is only partially understood. We know that HCO3- uptake at the basolateral membrane is achieved by Na+-HCO3- cotransport and also by a H+-ATPase and Na+/H+ exchanger operating together with carbonic anhydrase. At the apical membrane, the secretion of moderate concentrations of HCO3- can be explained by the parallel activity of a Cl-/HCO3- exchanger and a Cl- conductance, either the cystic fibrosis transmembrane conductance regulator (CFTR) or a Ca2+-activated Cl- channel (CaCC). However, the sustained secretion of HCO3- into a HCO- -rich luminal fluid cannot be explained by conventional Cl-/HCO3- exchange. HCO3- efflux across the apical membrane is an electrogenic process that is facilitated by the depletion of intracellular Cl-, but it remains to be seen whether it is mediated predominantly by CFTR or by an electrogenic SLC26 anion exchanger.

Influence of epidermal growth factor on bovine pancreatic duct cell bicarbonate

BACKGROUND AND AIMS

Epidermal growth factor (EGF) is secreted in pancreatic juice and its receptor is expressed on pancreatic duct cells (PDCs), suggesting a physiological role which has yet to be defined. Here we examined the effects of EGF on bicarbonate production and carbonic anhydrase (CA) activity in a PDC explant model.

METHODS

Bovine main PDCs were prepared and maintained in culture as explants. Levels of CA expression, phosphorylation, and enzymatic activity were measured in resting cells and compared to that of cells exposed to 10 nM secretin, 10 nM EGF, or both. Bicarbonate production was measured using the autoburette pH titration technique.

RESULTS

CA protein levels were unchanged with any treatment, but enzyme activity increased by 180% with secretin treatment and was reduced by 54% with EGF. The combination treatment led to a synergistic increase 240% above basal. EGF alone did not affect bicarbonate secretion, but the normal increase observed with secretin stimulation (1.3 +/- 0.4 to 2.9 +/- 0.6 micromol/h/cm(2)) was abolished by acute EGF pretreatment. On the other hand, EGF pretreatment for 24 h significantly increased basal and stimulated secretion (2.2 +/- 0.5 and 3.8 +/- 0.5, respectively) compared to controls.

CONCLUSIONS

EGF exerts a regulatory role on bicarbonate secretion by the pancreatic duct epithelium, independent of its effect on CA activity. Its inhibition of stimulated bicarbonate secretion could play a protective role in the setting of pancreatic inflammation, where increased levels of EGF are associated with reduced pancreatic juice production.

CO2 permeability and bicarbonate transport in microperfused interlobular ducts isolated from guinea-pig pancreas

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.

Expression and distribution of the Na(+)-HCO(-)(3) cotransporter in human pancreas

The cellular mechanisms of HCO(-)(3) secretion in the human pancreas are unclear. Expression of a Na(+)-HCO(-)(3) cotransporter (NBC) mRNA has been observed recently, but the distribution and physiological role of the NBC protein are not known. Here we examined the expression and localization of NBC in human pancreas by Northern blot, immunoblot, and immunofluorescence microscopy. Rat kidney NBC probes detected a single 9.5-kb band by Northern blot. On immunoblots, two polyclonal antisera directed against different epitopes of rat kidney NBC identified a single approximately 130-kDa protein. In cryosections of normal human pancreas, both antisera labeled basolateral membranes of large, morphologically identifiable ducts and produced a distinct labeling pattern in the remainder of the parenchyma. In double-labeling experiments, NBC immunoreactivity in the parenchyma colocalized with the Na(+)-K(+) pump, a basolateral marker. In contrast, NBC and cystic fibrosis transmembrane conductance regulator, an apical membrane marker, were detected within the same histological structures but at different subcellular localizations. The NBC antisera did not label acinar or islet cells. Our observations suggest that secretion of HCO(-)(3) by human pancreatic duct cells involves the basolateral uptake of Na(+) and HCO(-)(3) via NBC, an electrogenic Na(+)-HCO(-)(3) cotransporter.

Bicarbonate secretion in interlobular ducts from guinea-pig pancreas

1. The transport of HCO3- across the luminal membrane of pancreatic duct cells was studied by monitoring the luminal pH of isolated guinea-pig interlobular ducts after microinjection of an extracellular fluoroprobe, the dextran conjugate of 2'7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF-dextran). Luminal Cl- concentration was also measured by microfluorometry following microinjection of the dextran conjugates of 6-methoxy-N-(4-aminoalkyl)quinolinium bromide (ABQ-dextran) and Cl-NERF (Cl-NERF-dextran). 2. When HCO3-/CO2 was admitted to the bath, a transient acidification of the duct lumen was observed, followed by a marked alkalinization. The latter was abolished when the luminal Cl- concentration was reduced to 25-35 mM by replacement with glucuronate and may, therefore, be attributed to Cl(-)-HCO3- exchange at the luminal membrane. 3. Secretin, forskolin and acetylcholine stimulated HCO3- secretion into the lumen even when the luminal Cl- concentration was reduced to approximately 7 mM. Furthermore, agonist-evoked HCO3- secretion was not inhibited by luminal glibenclamide, dihydro-4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (H2DIDS) or 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB). These observations are not easily reconciled with HCO3- transport across the luminal membrane being mediated by Cl(-)-HCO3- exchange in parallel with a Cl- conductance. 4. Agonist-stimulated HCO3- secretion was blocked by omitting Na+ from the bath but not by addition of N-methyl-N-isobutylamiloride (MIA) or bafilomycin A1. This supports our previous conclusion that HCO3- entry into duct cells from the extracellular fluid requires Na+ but is not dependent on Na(+)-H+ exchange or vacuolar-type H(+)-ATPase activity. 5. The three actions of secretin on guinea-pig pancreatic duct cells described in this and the accompanying paper - stimulation of a relatively Cl(-)-insensitive luminal HCO3- efflux pathway, stimulation of basolateral Na(+)-HCO3- cotransport, and lack of effect on intracellular pH- require the current model of pancreatic HCO3- secretion to be modified.

Accumulation of intracellular HCO3- by Na(+)-HCO3- cotransport in interlobular ducts from guinea-pig pancreas

1. Short segments of interlobular duct were microdissected from guinea-pig pancreas following enzymatic digestion. After overnight culture, intracellular pH (pH1) and Na+ concentration ([Na+]i) were measured by microfluorometry in duct cells loaded with either the pH-sensitive fluoroprobe 2'7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) or the sodium-binding benzofuran isophthalate (SBFI). 2. The transporters responsible for maintaining pHi above equilibrium were investigated by using the NH4Cl pulse technique to acid load the cells. In the absence of HCO3-/CO2, the recovery of pH1 was Na+ dependent, abolished by 0.2 mM amiloride and by 10 microM N-methyl-N-isobutylamiloride and was therefore attributed to Na(+)-H+ exchange. 3. In the presence of HCO3-/CO2, amiloride only partially inhibited the recovery from acid loading. The amiloride-insensitive component was abolished by 0.5 mM H2DIDS and unaffected by depletion of intracellular Cl- and was therefore attributed to Na(+)-HCO3- cotransport. 4. Stimulation with 10 nM secretin did not cause a significant change in pH1 despite a significant increase in HCO3- efflux. However, in the presence of secretin, addition of 0.5 mM H2DIDS caused a decline in pH1 that was three times more rapid than that obtained with 0.2 mM amiloride. 5. In secretin-stimulated ducts, Na+ uptake increased when HCO3-/CO2 was added to the bath and this increase was strongly inhibited by 0.5 mM H2DIDS. 6. We conclude that Na(+)-HCO3- cotransport contributes approximately 75% of the HCO3- taken up by guinea-pig pancreatic duct cells during stimulation with secretin. It is proposed that electrical coupling between HCO3- efflux at the luminal membrane and electrogenic Na(+)-HCO3- cotransport at the basolateral membrane explains why secretin causes little change in pH1.

Effects of arterial pH and carbon dioxide on pancreatic exocrine H+/HCO3- secretion and secretin-dependent translocation of cytoplasmic vesicles in pancreatic duct cells

To elucidate why arterial pH and carbon dioxide (PaCO2) modify the pancreatic H+/HCO3- secretory response to secretin stimulation, experiments were performed on anaesthetized pigs, recording the effects of arterial pH and PaCO2 on exocrine H+/HCO3- secretion and on morphology of pancreatic duct cells. Duct cells contained numerous cytoplasmic vesicles at secretory rest. Their number more than doubled during elevation of PaCO2 from 5.5 to 11.0 kPa. At arterial pH 7.40, maximal secretin stimulation cleared the cytoplasm of duct cells of more than 90% of the vesicles. At high PaCO2, this was accompanied by doubling the basolateral plasma membrane area and a 30% higher secretion rate than at PaCO2 5.5 kPa. Lowering arterial pH to 7.0 more than halved the secretin-induced vesicle clearance of duct-cell cytoplasm as well as exocrine H+/HCO3- secretion and abolished the secretin-dependent basolateral membrane area changes. Supramaximal secretin stimulation did not reverse the inhibitory effect of severe metabolic acidosis on secretion. It is concluded that PaCO2 and arterial pH may modify the secretory response to secretin through determining the incorporation of cytoplasmic vesicle material into the basolateral plasma membrane of duct cells.