Evidence for Na-H exchange in the rabbit pancreas

Secretin causes H+/HCO3- secretion from pig pancreatic ductules by vacuolar-type H(+)-adenosine triphosphatase

BACKGROUND/AIMS

Secretin stimulates pancreatic ductules to secrete HCO3- into pancreatic juice and H+ into interstitial fluid. The aim of the present study was first to examine whether ductular H+ secretion is inhibited by micromolar concentrations of bafilomycin A1, which blocks vacuolar H(+)-adenosine triphosphatase by specific action, and secondly to test for evidence of ductular Na+/HCO3- cotransport.

METHODS

Ductular H+ secretion was estimated from the rate of intracellular pH recovery after acid-loading (24 mmol/L NH4Cl) microdissected pancreatic ductules from pig, mounted in a flow-through perfusion chamber on the stage of a fluorescent microscope. Intracellular pH was measured using the fluorescent pH indicator 2'7'-bis (carboxyethyl)-5,6-carboxyfluorescein and dual-wave-length excitation of fluorescence. The ducts were superfused perfused with either HCO3(-)-free HEPES-containing buffers or HCO3(-)-containing buffers.

RESULTS

Secretin (10(-8) mol/L) induced a net H+ secretion of 1.87 +/- 0.23 mumol.mL cell vol-1.min-1 that was blocked by 10(-6) mol/L bafilomycin A1 and was unaffected by Na+ substitution with choline using HEPES superfusion buffers. Secretin-stimulated ductules superfused with bicarbonate-containing, Cl(-)-free buffers showed Na(+)-dependent and 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid-inhibitable alkalinization of intracellular pH.

CONCLUSIONS

Secretin causes H+/HCO3- secretion from pancreatic ductules by a mechanism involving vacuolar-type H(+)-adenosine phosphatase. Pancreatic ductules also show Na+/HCO3- cotransport, which may account for a small fraction of secreted bicarbonate.

A model for fluid secretion in the exocrine pancreas

Fluid secretion by the isolated rabbit pancreas is strongly dependent on the presence of Na+ in the bathing medium. Substitution of Na+ by another cation such as Li+ or K+ causes an inhibition of fluid secretion rate and a change in the composition of the secreted fluid which is dependent on the nature of the substituent cation. Stimulation of the pancreas by CCK-8 or carbachol increases paracellular ion permeability and, in some cases, also fluid secretion rate. We present a simple, quantitative model for ion and water secretion which accounts for the effects observed upon Na+ substitution and stimulation. The main features are active, Na+-dependent transcellular HCO3- transport and passive, paracellular cation and anion permeation. The activity of the HCO3- pump is dependent on the energy status of the cell and on the Na+ concentration in the bathing medium, and is competitively inhibited by K+. The paracellular ion permeabilities can be modulated by stimulatory agonists. We examine the extent to which, according to the model, fluid secretion is controlled by the various system parameters such as ion permeabilities and ion pump activity, and by external parameters such as the ion concentrations in the bathing medium. In addition, calculation of the effects of changes in these parameters are carried out in order to gain more insight in the mechanisms of secretion.

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.

Properties of the luminal membrane of isolated perfused rat pancreatic ducts. Effect of cyclic AMP and blockers of chloride transport

The aim of the present study was to investigate by what transport mechanism does HCO-3 cross the luminal membrane of pancreatic duct cells, and how do the cells respond to stimulation with dibutyryl cyclic AMP (db-cAMP). For this purpose a newly developed preparation of isolated and perfused intra- and interlobular ducts of rat pancreas was used. Responses of the epithelium to inhibitors and agonists were monitored by electrophysiological techniques. Addition of HCO-3/CO2 to the bath side of nonstimulated ducts depolarized the PD across the basolateral membrane (PDbl) by about 9 mV, as also observed in a previous study [21]. This HCO-3 effect was abolished by Cl- channel blockers or SITS infused into the lumen of the duct: i.e. 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB, 10(-5) M) hyperpolarized PDbl by 8.2 +/- 1.6 mV (n = 13); 3',5-dichlorodiphenylamine-2-carboxylic acid (DCl-DPC, 10(-5) M) hyperpolarized PDbl by 10.3 +/- 1.7 mV (n = 10); and SITS hyperpolarized PDbl by 7.8 +/- 0.9 mV (n = 4). Stimulation of the ducts with db-cAMP in the presence of bath HCO-3/CO2 resulted in depolarization of PDbl, the ductal lumen became more negative and the fractional resistance of the luminal membrane decreased. Together with forskolin (10(-6) M), db-cAMP (10(-4) M) caused a fast depolarization of PDbl by 33.8 +/- 2.5 mV (n = 6). When db-cAMP (5 x 10(-4) M) was given alone in the presence of bath HCO-3/CO2, PDbl depolarized by 25.3 +/- 4.2 mV (n = 10).(ABSTRACT TRUNCATED AT 250 WORDS)

NN'-dicyclohexylcarbodiimide (DCCD) reduces pancreatic NaHCO3 secretion without changing pancreatic tissue ATP levels

To study the mechanism responsible for pancreatic NaHCO3 secretion, the inhibitor NN'-dicyclohexylcarbodiimide (DCCD) was administered to six secretin-infused, anaesthetized pigs. Pancreatic juice was collected from a catheter in the main pancreatic duct. Secretion rate was measured at several arterial pH values in each animal, both before and after DCCD. 15 (14-30) mumol kg-1 body wt DCCD, intra-arterially, reduced pancreatic NaHCO3 secretion from 296 (234-398) to 181 (134-237) mumol min-1 at arterial pH 7.43 (7.42-7.47). Similar fractional reductions of secretion occurred at lower arterial pH. Pancreatic tissue ATP concentration, 1.8 (1.4-2.0) mumol g-1 wet wt, was not changed by DCCD. DCCD, less than or equal to 10(-4) mol l-1, did not change Na,K-ATPase nor carbonic anhydrase activities in separate in vitro assay systems. It is concluded that DCCD reduced pancreatic NaHCO3 secretion by a mechanism not involving ATP depletion nor inhibition of Na,K-ATPase nor carbonic anhydrase activities in pancreatic cells. Because DCCD inhibits proton pumps, DCCD may have reduced NaHCO3 secretion through interfering with a proton pump involved in extruding H+ from HCO-3 secreting cells to interstitial fluid in the pancreas.

Lack of effect of amiloride, furosemide, bumetanide and triamterene on pancreatic NaHCO3 secretion in pigs

This study examines the effects of the Na entry blockers amiloride, furosemide, bumetanide and triamterene on pancreatic NaHCO3 secretion. The purpose of the study was to elucidate whether Na flux through a Na-H-antiport, a Na,K,Cl-symport or a Na-K(H)-antiport respectively, may be rate limiting for pancreatic NaHCO3 secretion. The effect of each drug on secretion was tested in four groups of six anaesthetized pigs (20-25 kg body wt) during supramaximal, intravenous secretin infusion (3.0 CU kg-1 body wt. h-1), following laparotomy and cannulation of the pancreatic duct. Secretion was measured over a wide range of arterial pH values, both before and after drug administration in each animal. Pancreatic NaHCO3 secretion averaged 268 (219-308) mumol min-1 at physiological arterial pH 7.40 in the control period. Neither amiloride (90 mg kg-1 body wt), furosemide (10 mg kg-1 body wt), bumetanide (0.6 mg kg-1 body wt) nor triamterene (25 mg kg-1 body wt) affected the rate of secretin-dependent NaHCO3 secretion or the relation between NaHCO3 secretion and arterial pH. It is concluded that Na flux through neither a Na-H-antiport, a Na,K,Cl-symport nor a Na-K(H)-antiport appears to be rate limiting for pancreatic secretion of NaHCO3.

Effects of digitoxin and hypokalaemia on pancreatic NaHCO3 secretion and pancreatic Na,K-ATPase activity

To study the role played by Na,K-ATPase in the pancreatic secretion of NaHCO3, experiments were performed in 20 anaesthetized, secretin-infused pigs (3.0 clinical units X kg b. wt. X h-I). The relationship between pancreatic NaHCO3 secretion and arterial pH was obtained before and during Na,K-ATPase inhibition by digitoxin and hypokalaemia. Na,K-ATPase activity in pancreatic tissue homogenate averaged 5.45 (5.02-6.68) mumol Pi X mg X protein X h-I. Retrograde injection of 0.5 ml 1.4 X 10(-4) mol X l-I digitoxin into pancreatic ducts reduced pancreatic Na,K-ATPase activity by 3I(I8-47)%, while intra-arterial injection of 0.2 mg X kg b. wt-I digitoxin reduced pancreatic Na,K-ATPase activity by 50(45-56)%. Digitoxin and hypokalaemia reduced the rate of pancreatic NaHCO3 and shifted the normal, proportional relationship between NaHCO3 secretion and arterial pH towards higher pH. Hypokalaemia reduced Na,K-ATPase activity and NaHCO3 secretion in proportion. These effects indicate that Na,K-ATPase helps to sustain the requisite electrochemical potential gradients for driving H+ ions, and hence HCO-3 ions, out of secretory cells.

Plasma Na+-iron concentration or pH as regulator of pancreatic HCO-3 secretion

Pancreatic HCO-3 secretion is caused by proton flux from pancreatic cells to interstitial fluid which, hypothetically, may be derived by a Na-pump or a proton pump. A Na-pump would reabsorb protons from pancreatic duct in proportion with plasma Na+ concentration (PNa+). A proton pump would cause passive HCO-3 flux into pancreatic ducts that would vary with pancreatic jice HCO-3 concentration (CHCO-3). Because pancreatic ducts are water-permeable, CHCO-3 varies with plasma osmolarity. This phenomenon allows testing of the two hypotheses. Intravenous infusion of hypotonic salt solution to 5 anesthetized, secretin infused pigs (2.7 C.U./kg b. wt. H-1) lowered PNa+ by 20 +/- 2 mmol/l and CHCO-3 by 20 +/- 4 mmol/l and increased pancreatic HCO-3 secretion by 71 +/- 5 mumol/min. Intravenous infusion of hypertonic salt solutions to 5 other pigs raised PNa+ by 52 +/- 3 mmol/l and CHCO-3 by 54 +/- 3 mmol/l and reduced HCO-3 secretion by 86 +/0 26 mumol/min. Isotonic glucose infusion lowered PNa+ by 27 +/- 2 mmol/l and did not change CHCO-3 nor HCO-3 secretion rate in 5 pigs. These findings comply with the proton pump hypothesis and are at variance with the Na-pump hypothesis of pancreatic HCO-3 secretion.

Localization of sodium pump sites in cat pancreas

1. Cat pancreases were exposed to [3H]ouabain either by perfusion, or by incubation of slices, with Krebs--Henseleit bicarbonate solution containing [3H]ouabain, and [3H]ouabain binding sites were localized by light microscopical autoradiography. 2. Acinar cells were found to possess only a few [3H]ouabain binding sites located on the basolateral plasma membranes. 3. The epithelial cells lining the intercalated and interlobular ducts were heavily labelled. The [3H]ouabain binding sites were located on the basolateral membranes. 4. Since secretion of pancreatic juice can be abolished by ouabain, it is concluded that the duct cells, especially those of the numerous small (intercalated) and intermediate-sized (interlobular) ducts are responsible for pancreatic secretion of electrolytes and water. 5. The localization of the (Na+ + K+)-activated ATPase to the basolateral membranes of the cells excludes a direct role of the transport enzyme in the secretion of Na+. However, a model is proposed where the Na+ + K+)-activated ATPase plays a crucial, though indirect role in pancreatic secretion of electrolytes and water.

The role of sodium ions in pancreatic fluid secretion in the rabbit

1. NaCl in the bathing medium of the isolated rabbit pancreas has been gradually replaced by isotonic amounts of either sucrose, choline chloride or KCl. 2. The replacement of NaCl always leads to a reduction in pancreatic flow. This reduction is largest with sucrose as replacing agent and smallest with KCl. 3. The Na+ concentration in the secreted fluid decreases only slightly when NaCl is replaced by sucrose. In the experiments with choline chloride, the Na+ concentration in the secreted fluid decreases slightly when less than 60 mM-NaCl has been replaced, but decreases markedly after more extensive NaCl replacement. Upon replacement of NaCl by KCl, the Na+ concentration in the secreted fluid is always equal to that in the bathing medium. 4. The K+ concentration in the secreted fluid increases by 8 mM upon replacing NaCl by either sucrose or choline chloride. Upon replacement by KCl the K+ concentration in the secreted fluid is always approximately equal to that in the bathing medium. 5. There is a linear relationship between the ratio of the K+ concentrations in the secreted fluid and the bathing medium and the ratio of the Na+ concentrations in these fluids. 6. Replacement of NaCl in the bathing medium does not lead to a change in the transepithelial potential. 7. The secreted fluid is always isotonic with that of the bathing medium; in the case of an extensive replacement of NaCl by either sucrose or choline chloride isotonicity is maintained by a higher cation concentration and an increased appearance of the replacing agents in the secreted fluid. 8. These studies confirm previous findings that extracellular Na+ is essential for pancreatic fluid secretion. 9. They also indicate that the monovalent cations reach the secreted fluid through a paracellular pathway. 10. These findings are incorporated in a model for the electrolyte movements involved in the pancreatic fluid secretion by ductular as well as by acinar cells.

Membrane adenosine triphosphatase activities in rat pancreas

The membrane ATPase activities present in rat pancreas were studied to investigate the possible role of ATPase enzymes in HCO3(-) secretion in the pancreas. It was found that all the HCO3(-)-sensitive (anion-sensitive) ATPase activity was accountable as pancreatic mitochondrial ATPase, thus supporting the view that a distinct plasma membrane 'bicarbonate-ATPase' is not involved in HCO3(-) secretion in pancreas. A remarkably high Mg+- and CA2+-requiring ATPase activity (30 mumol ATP hydrolysed/min per mg) was found in the plasma membrane fraction (rho = 1.10-1.13). This activity has been characterized in some detail. It is inhibited by p-fluorosulfonylbenzoyladenosine, an affinity label analogue of ATP and the analogue appears to label covalently a protein of Mr approximately 35 000. The (Ca2+ + Mg2+)-ATPase activity did not form a 'phosphorylated-intermediate' and was vanadate-insensitive. These and other tests have served to demonstrate that the (Ca2+ + Mg2+)-ATPase activity is different in properties from (Na+ + K+)-ATPase, Ca2+-ATPase, (H+ + K+)-ATPase or mitochondrial H+-ATPase. Apart from the (Ca2+ + Mg2+)-ATPase of plasma membrane and mitochondrial ATPase, the only other membrane ATPase activities noted were (Na+ + K+)-ATPase, which occurred in the same fractions as the (Ca2+ + Mg2+)-AtPase at rho = 1.10-1.13 and was of surprisingly low activity, and an ATPase activity in light membrane fractions (rho - 1.08-1.09) derived from zymogen granule membranes. At this time, therefore, there is no obvious candidate for an ATPase activity at the luminal surface of pancreatic cells which is directly involved in ion transport, but the results presented here direct attention to the high activity (Ca2+ + Mg2+)-ATPase in the plasma membrane fraction.

Relationship between plasma pH and pancreatic HCO3- secretion at different intravenous secretin infusion rates

The relationship between pancreatic HCO3- secretion and plasma pH during acute systemic acid-base changes was investigated in 6 anesthetized, artificially ventilated pigs (20-25 kg) at 2 different, i.v. secretion infusion rates. At 0.45 C.U./kg b. wt. h-1 secretin infusion and plasma pH 7.40 +/- 0.01 pancreatic HCO3- secretion averaged 61+/- 12 mumol/min. Stepwise lowering of plasma pH through i.v. infusion of HCl and CO2 administration to inspired air proportionately reduced secretion rate; estimated zero HCO3- secretion occurring at plasma pH 7.01. Subsequent i.v. secretin infusion at 2.70 C.U./kg b. wt. h-1 increased HCO3- secretion to 249 +/- 42 mumol/min at plasma pH 7.33 + 0.04; stepwise lowering of plasma pH proportionately reduced HCO3- secretion to estimated zero at plasma pH 6.71. A reduction of plasma pH by 0.1 pH unit reduced HCO3- secretion during low and high rate of i.v. secretin infusion by 18 +/- 3 mumol/min and 35 +/- 8 mumol/min, respectively. Secretin infusion rate did not affect pancreatic chloride excretion. These findings support the view that secretin increases HCO3- secretion, and hence proton transport to the interstitial fluid, by augmenting the proton motive force developed by HCO3- secreting cells.

Importance of bicarbonate in bile salt independent fraction of bile flow

The bile salt independent fraction (BSIF) of canalicular bile flow from the isolated rat liver perfused with bicarbonate-free perfusate is 50% of that from the liver perfused with bicarbonate-containing perfusate. HCO3-excretion is nearly eliminated and Na+ and Cl- excretion is reduced 50%. Replacement of HCO3- into perfusate increased bile flow by 0.3 microliter/g.min without changing bile acid excretion rate. 5.5-Dimethyl-2,4-oxazolidinedione (DMO) produced a similar effect. DMO was passively distributed between bile and plasma. The data indicate that a bicarbonate transport mechanism is responsible for production of up to 50% of the BSIF. Another weak acid, N-5[5-(2-methoxyethoxy)-2-pyrimidinyl]sulfamoylbenzene (glymidine), was rapidly excreted into bile and increased bile flow by over 2.0 microliter/g.min. Glymidine is probably excreted by an independent organic anion transport mechanism, and any effect on the bicarbonate transport mechanism is obscured. Canaliculus-enriched hepatocyte membrane fractions contained no HCO3-stimulated ATPase activity. Either this enzyme is unimportant in hepatocyte bicarbonate transport or transport occurs across membranes other than the bile canalicular membrane.

A micropuncture investigation of the whole tissue mechanism of electrolyte secretion by the in vitro rabbit pancreas

Micropuncture techniques have been used to examine electrolyte secretion by the in vitro rabbit pancreas. The concentration profiles of the major secreted ions and digestive protein and the electrical potential profile within the pancreatic ductal system have been determined during spontaneous and secretin-stimulated secretion. The active transport of both Na and HCO(3) are the rate-controlling steps in primary secretion. Spontaneous secretion is produced primarily within the intralobular ducts. The anion composition of this primary secretion depends on the secretion rate with HCO(3) ranging from about 70 meq/liter at low rates to about 110 meq/liter at high rates. With secretin stimulation the smaller extralobular ducts also secrete and this extralobular secretion has a higher HCO(2) content than that of the intralobular secretion. In the main collecting duct the anion composition of the juice is modified further by Cl-HCO(3) exchange which appears to be a passive process depending on the average residence time of the juice in the main duct.