The origin and secretion of pancreatic juice bicarbonate

The Contribution of Phytate-Degrading Enzymes to Chicken-Meat Production

The contribution that exogenous phytases have made towards sustainable chicken-meat production over the past two decades has been unequivocally immense. Initially, their acceptance by the global industry was negligible, but today, exogenous phytases are routine additions to broiler diets, very often at elevated inclusion levels. The genesis of this remarkable development is based on the capacity of phytases to enhance phosphorus (P) utilization, thereby reducing P excretion. This was amplified by an expanding appreciation of the powerful anti-nutritive properties of the substrate, phytate (myo-inositol hexaphosphate; IP₆), which is invariably present in all plant-sourced feedstuffs and practical broiler diets. The surprisingly broad spectra of anti-nutritive properties harbored by dietary phytate are counteracted by exogenous phytases via the hydrolysis of phytate and the positive consequences of phytate degradation. Phytases enhance the utilization of minerals, including phosphorus, sodium, and calcium, the protein digestion, and the intestinal uptakes of amino acids and glucose to varying extents. The liberation of phytate-bound phosphorus (P) by phytase is fundamental; however, the impacts of phytase on protein digestion, the intestinal uptakes of amino acids, and the apparent amino acid digestibility coefficients are intriguing and important. Numerous factors are involved, but it appears that phytases have positive impacts on the initiation of protein digestion by pepsin. This extends to promoting the intestinal uptakes of amino acids stemming from the enhanced uptakes of monomeric amino acids via Na⁺-dependent transporters and, arguably more importantly, from the enhanced uptakes of oligopeptides via PepT-1, which is functionally dependent on the Na⁺/H⁺ exchanger, NHE. Our comprehension of the phytate-phytase axis in poultry nutrition has expanded over the past 30 years; this has promoted the extraordinary surge in acceptance of exogenous phytases, coupled with the development of more efficacious preparations in combination with the deflating inclusion costs for exogenous phytases. The purpose of this paper is to review the progress that has been made with phytate-degrading enzymes since their introduction in 1991 and the underlying mechanisms driving their positive contribution to chicken-meat production now and into the future.

The ranked importance of dietary factors influencing the performance of broiler chickens offered phytase-supplemented diets by the Plackett-Burman screening design

1.The objective of the present study was to rank the importance of the following dietary factors; canola meal, wheat, whole barley, digestible lysine, phytate-P, calcium, available P, sodium and three NSP-degrading feed enzymes. Their influence on growth performance, gastro-intestinal tract parameters, energy utilisation, ileal N digestibility and disappearance rates were determined via the Plackett-Burman design in broiler chickens offered phytase-supplemented diets. 2. The eleven dietary factors were assigned two levels in the Plackett-Burman design matrix. The resulting twelve dietary treatments were offered to six replicates per treatment (six birds per cage) with a total of 468 male Ross 308 broiler chicks from 7 to 28 d post-hatch. 3. Increasing digestible lysine levels improved weight gain by 15.6% (P < 0.001) and gain:feed by 9.36% (P < 0.001). Increasing calcium levels reduced weight gain by 6.36% (P < 0.001) and gain:feed by 2.60% (P < 0.001). The high calcium level increased gizzard pH from 2.78 to 3.01 (P < 0.005). Whole barley significantly increased relative gizzard weights and contents, pancreas weights and both ileal N digestibility coefficients (0.774 versus 0.803; P < 0.001) and ileal N disappearance rates (23. 3 versus 24.5 g/bird/day; P < 0.001). 4. Overall, digestible lysine level and calcium level were identified as the most influential dietary factors to influence growth performance of broilers offered phytase-supplemented diets, which hold implications for practical diet formulations.

Progress in comprehending the phytate–phytase axis in chicken-meat production

After an extended delay, the level of acceptance of exogenous phytases by the global chicken-meat industry is now almost complete. Contemporary bacterial phytases degrade phytate primarily in the gizzard. The extent of phytate degradation determines the extent to which phytate-bound phosphorus (P) is liberated; however, studies designed to investigate phytate degradation along the digestive tract have generated some confusing outcomes. This may be related to the reactivity of the phytate moiety, coupled with problems with inert dietary markers and perhaps a lack of complete and uniform extractions of phytate from digesta due to variations in digesta pH and phytate solubility. Quite recently, phytase was shown to have profound impacts on sodium (Na) digestibility coefficients in four segments of the small intestine. This has obvious implications for intestinal uptakes of glucose and amino acids via their respective Na+-dependent transport systems and it is possible that phytate and phytase have reciprocal impacts on ‘sodium pump’ (Na+, K+-ATPase) activity. It has been recently demonstrated unequivocally that phytase has the capacity to increase amino acid digestibility coefficients to the extent that phytase may generate a ‘proximal shift’ in the sites of amino acid absorption. The impact of phytase on starch digestibility is more equivocal and phytase responses may stem more from enhanced glucose absorption rather than starch digestion. The acceptance of phytase is hardly surprising, given its capacity to increase P utilisation coupled with numerous other positive influences that are still being properly realised.

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.

The stoichiometry of the electrogenic sodium bicarbonate cotransporter pNBC1 in mouse pancreatic duct cells is 2 HCO(3)(-):1 Na(+)

The electrogenic sodium bicarbonate cotransporter pNBC1 is believed to play a major role in the secretion of bicarbonate by pancreatic duct cells, by transporting bicarbonate into the cell across the basolateral membrane. Thermodynamics predict that this function can be achieved only if the reversal potential of the cotransporter is negative to the cell's membrane potential, or equivalently that the HCO3-:Na+ stoichiometry is not larger then 2:However, there are no data available on either the reversal potential or the HCO3-:Na+ stoichiometry of pNBC1 in pancreatic cells. We studied pNBC1 function in mouse pancreatic duct cells. RT-PCR analysis of total RNA revealed that these cells contain the message for pNBC1, but not for kNBC1, NBC2 or NBC3. To measure cotransporter activity, mouse pancreatic duct cells were grown to confluence on a porous substrate, mounted in an Ussing chamber, and the apical plasma membrane permeabilized with amphotericin B. Ion flux through pNBC1 was achieved by applying Na+ concentration gradients across the basolateral plasma membrane. The current through the cotransporter was isolated as the difference current due to the reversible inhibitor dinitrostilbene disulfonate (DNDS). Current-voltage relationships for the cotransporter, measured at three different Na+ concentration gradients, were linear over a range of about 100 mV. The reversal potential data, obtained from these current-voltage relationships, all corresponded to a 2 HCO3-:1 Na+ stoichiometry. The data indicate that pNBC1 is functionally expressed in mouse pancreatic duct cells. The cotransporter operates with a 2 HCO3-:1 Na+ stoichiometry in these cells, and mediates the transport of bicarbonate into the cell across the basolateral membrane.

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.

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.

Effect of bicarbonate on potassium conductance of isolated perfused rat pancreatic ducts

The aim of this study was to investigate the role of the K+ conductance in unstimulated and stimulated pancreatic ducts and to see how it is affected by provision of exogenous HCO3-/CO2. For this purpose we have applied electrophysiological techniques to perfused pancreatic ducts, which were dissected from rat pancreas. The basolateral membrane potential PDbl of unstimulated duct cells was between -60 mV and -70 mV, and the cells had a relatively large K+ conductance in the basolateral membrane as demonstrated by (a) 20-22 mV depolarization of PDbl in response to increase in bath K+ concentration from 5 mmol/l to 20 mmol/l and (b) the effect of a K+ channel blocker, Ba2+ (5 mmol/l), which depolarized PDbl by 30-40 mV. These effects on unstimulated ducts were relatively independent of bath HCO3-/CO2. The luminal membrane seemed to have no significant K+ conductance. Upon stimulation with secretin or dibutyryl cyclic AMP, PDbl depolarized to about -35 mV in the presence of HCO3-/CO2. Notably, the K+ conductance in the stimulated ducts was now only apparent in the presence of exogenous HCO3-/CO2 in the bath solutions. Upon addition of Ba2+, PDbl depolarized by 13 +/- 1 mV (n = 7), the fractional resistance of the basolateral membrane, FRbl increased from 0.66 to 0.78 (n = 6), the specific transepithelial resistance, Rte, increased from 52 +/- 13 omega cm2 to 59 +/- 15 omega cm2 (n = 11), and the whole-cell input resistance, Rc, measured with double-barrelled electrodes, increased from 20 M omega to 26 M omega (n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Disturbance of fluid and electrolyte transport in cystic fibrosis epithelia

Studies of the disease CF suggest that the basic defect is related to impaired electrolyte movement in the epithelia of a variety of organs with exocrine function. The disturbances of electrolyte secretion in the organs classically involved in CF range from 1) a decrease in secretion- or uptake of chloride ions in all the organs studied; 2) an increase in sodium uptake in nasal airway epithelium and 3) a decrease in bicarbonate output of the pancreas. In this review an overview is presented of the expression of the CF defect, the abnormalities of fluid and electrolyte secretion in each CF affected organ are considered in more detail with particular emphasis on the hormonal and neuronal (dys)regulation of iron transport systems in epithelial cells.

Pathophysiology of the exocrine pancreas in cystic fibrosis

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Impaired chloride secretion, as well as bicarbonate secretion, underlies the fluid secretory defect in the cystic fibrosis pancreas

Pancreatic fluid and electrolyte secretion was assessed in 56 patients with cystic fibrosis (CF) and 56 non-CF control subjects undergoing pancreatic function testing while stimulated with cholecystokinin and secretin. Both CF patients and control subjects exhibited a wide range of pancreatic function. Fluid and trypsin outputs were positively correlated in both groups. Fluid output in CF subjects was significantly lower, however, than that of control subjects at any given level of trypsin output. Sodium, bicarbonate, and chloride secretions were all significantly decreased in CF subjects. Bicarbonate and chloride were important determinants of fluid secretion, but at any given bicarbonate or chloride output CF subjects secreted significantly less fluid than control subjects. When bicarbonate and chloride were analyzed as simultaneous predictor variables, adjusted fluid secretion was not significantly different in CF and control subjects. Diminished fluid secretion in CF subjects is therefore caused by impaired chloride, as well as bicarbonate, secretion.

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)

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

1. The secretion rate of bicarbonate by the isolated saline-perfused cat pancreas was linearly related to the bicarbonate concentration of the arterial inflow at constant PCO2 and at high volume rates of secretion. 2. Pancreatic bicarbonate secretion was independent of arterial inflow pH at constant bicarbonate concentrations when the pH was manipulated by alterations in the PCO2 at high volume rates of secretion. 3. A small but statistically significant linear relationship existed between the pH of the arterial inflow and bicarbonate secretion at constant PCO2 after inhibition of carbonic anhydrase by acetazolamide. Under the same conditions no relationship was found between bicarbonate secretion and arterial inflow pH when the perfusate bicarbonate concentration was kept constant and the PCO2 varied. 4. When the volume rate of secretion was reduced by about 60-70% of maximum no relationship was found to exist between arterial inflow pH and bicarbonate secretion at constant bicarbonate concentration in the perfusate. There was also no relationship between inflow pH and bicarbonate secretion at constant PCO2 down to a pH of 7.3 until the bicarbonate concentration of the perfusate was reduced below 10 mM, when the secretion rate fell off rapidly. 5. A linear relationship was found to exist between the volume rate of secretion and the PCO2 of the pancreatic juice and the output of lactate both in the isolated saline-perfused gland and the blood-perfused pancreas in situ. 6. At high rates of secretion the PCO2 of the pancreatic juice was always higher than that of either the arterial inflow or the venous outflow. There is therefore no gradient for the passive movement of carbon dioxide between the arterial inflow and the pancreatic juice. 7. Inhibition of secretion with acetazolamide caused a fall in the PCO2 of pancreatic juice and increased the output of lactate. The secretion of lactate was not due to hypoxia as it also occurred in the blood-perfused gland in situ which had normal haemoglobin concentrations and oxygen saturation. 8. It is concluded that the secretion of bicarbonate is independent of arterial pH but critically dependent upon the arterial concentration of the bicarbonate ion. These experiments do not support the concept that the secretion of protons over the basolateral membrane is the major primary event in pancreatic secretion of bicarbonate.

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.

Ex-vivo isolated perfusion of the pancreas in the Syrian golden hamster

The technique of ex-vivo isolated pancreatic perfusion has been a valuable method for investigation of the physiology of the exocrine and endocrine pancreas. We have adapted the technique of isolated pancreatic perfusion for use in the Syrian golden hamster, an animal used widely in studies of pancreatic carcinogenesis. The technique involves surgical harvest of the pancreas with its aortic and portal venous blood supply intact and perfusion of the pancreas with a modified Krebs buffer at 37 degrees C. Physiological function of the perfused pancreas system was examined in 27 Syrian hamsters. In tests of endocrine function, the perfused pancreas responded by increasing insulin secretion within 1 min of elevating perfusate glucose concentration, and also secreted insulin promptly in response to 10 mM arginine. In exocrine studies, the flow of pancreatic juice was stimulated by the addition of 0.8 X 10(-9) M secretin to the perfusate, and amylase output was significantly increased by the addition of 0.2 X 10(-9) M cholecystokinin (CCK-8). The ex-vivo isolated perfused pancreas in the hamster thus appears to respond appropriately to physiological stimuli and is a valuable additional tool for studies of the hamster pancreas.

The effect of hypothermia on exocrine pancreatic secretion in rabbits

The effects of experimentally induced hypothermia on exocrine pancreatic secretion in rabbits were investigated. During hypothermia the flow of pancreatic juice decreased to 50% of basal values and recovered after rewarming. Hypothermia scarcely affected HCO-3, Cl- and Na+ concentrations but did cause significant alterations in K+ concentrations. During hypothermia and later normothermia a parallel secretion in the enzymes amylase, chymotrypsin and trypsin was seen to take place. Enzyme secretion decreased throughout the experimental period in the rabbits undergoing hypothermia and later normothermia, as in the case of the control animals.

The sodium and bicarbonate dependence of bile secretion in the guinea-pig

The effects of the total substitution of sodium by lithium, and of bicarbonate by a mixture of chloride and phosphate, on the flow and composition of bile was studied in the isolated perfused guinea-pig liver using a single-pass perfusion system. 60% of secretion was bicarbonate dependent and 80% specifically required the presence of sodium. The secretion of bicarbonate in bile against a concentration gradient was inhibited by the presence of lithium. In contrast to the results of similar studies in the rat, lithium is not an effective substitute for sodium in maintaining bile secretion.

The mechanism of fluid secretion in the rabbit pancreas studied by means of various inhibitors

In order to increase our understanding of the mechanism of pancreatic fluid secretion we have studied the effects of various transport inhibitors on this process in the isolated rabbit pancreas. In this preparation, a high rate of unstimulated fluid secretion occurs, which probably originates from the ductular cells. Inhibitory are ouabain, furosemide, bumetanide, piretanide, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) and acetazolamide, with their half-inhibitory concentrations: 2 X 10(-6) M (ouabain), 1.3 X 10(-3) M (furosemide), 2.2 X 10(-3) M (bumetanide and piretanide) and 1.4 X 10(-4) M (SITS). With acetazolamide a maximal inhibition of only 20% is found at 10(-3) M. Amiloride (10(-3) M) has no effect on pancreatic fluid secretion. The inhibitory effects on HCO-3 output are always larger and those on Cl- output lower than those on fluid secretion. The results suggest that the ouabain-sensitive (Na+ + K+)-ATPase system provides the energy for a Na+-gradient-driven Cl--HCO-3-exchange transport system, sensitive to the loop diuretics furosemide, bumetanide and piretanide and to SITS. This system would drive the transcellular transport of HCO-3 and secondarily that of cations, Cl- and water.

The anionic basis of fluid secretion by the rabbit mandibular salivary gland

The role played by anions in salivary secretion has been studied in experiments on the isolated, perfused mandibular gland of the rabbit, in which perfusate Cl- and/or HCO3- were replaced by other anions. Replacement of Cl- with Br- had no significant effect on salivary secretion rate, but replacement with the other anions tested caused secretory rate to fall, by 38% (I-), 50% (NO3-), 61% (isethionate, ise -), and 66% ( CH3SO4 -), respectively. Replacement of perfusate Cl- with ise - or CH3SO4 - caused the salivary HCO3- concentration to rise up to 4-fold. Replacement with Br- or I- seemed to have little effect on salivary HCO3- concentration but, in contrast to ise -, Br- and I- entered the saliva in concentrations comparable to those of Cl- during control perfusion. In glands perfused with HCO3- and ise -, the addition of methazolamide, an inhibitor of carbonic anhydrase, caused a further 60% drop in secretory rate, but the saliva remained rich in HCO3-. Replacement of perfusate HCO3- with Cl- or ise - had no effect on salivary secretion or composition. Replacement of both HCO3- and Cl- in the perfusate with ise - reduced salivary secretion to less than 2% of control levels. In control glands (i.e. perfused with both HCO3- and Cl-), administration of furosemide, an inhibitor of Na+/Cl- co-transport, reduced the secretion rate and increased salivary HCO3- in a manner indistinguishable from that seen when perfusate Cl- was replaced with ise -. In control perfused glands, administration of SITS (4-acetamido-4'- isothio cyano-2,2'-disulphonic acid stilbene), an inhibitor of Cl-/HCO3- antiports , did not cause any change in salivary HCO3- concentration. Unexpectedly, it induced a significant increase in salivary secretory rate. The results show that salivary secretion depends on two independent transport systems. One is a Cl- -dependent, furosemide-sensitive system, probably a Na+/Cl- symport. The other is an HCO3- -dependent, methazolamide-sensitive system, and is probably an Na+/H+ antiport.

Pancreatic O2 consumption and CO2 output during secretin-induced, exocrine secretion from the pancreas in the anesthetized dog

Secretin stimulates pancreatic water and CO2 excretion as well as pancreatic blood flow. It has been questioned whether the production (i.e. water and CO2 excretion) is reflected in the input-output difference of nutrients. In pentobarbital anesthetised dogs, pancreatic exocrine secretion was stimulated by secretin (Karolinska), 1 U/kg injected as an i.v. bolus. Secretion was maximally increased at 2 min after the secretin shot and returned to a basal value at between 16 and 32 min after secretin. Blood flow was also maximally increased at 2 min, but decreased to the basal value at between 8 and 16 min. O2 extraction first decreased (at 2 min) and then gradually increased until it was higher than the basal value (at 16 min) and then returned to the basal level (at 32 min). O2 consumption increased quickly, reached a plateau, lasting from 1 to 16 min, and then decreased to the basal level (32 min). CO2 transfer from blood to tissue reached a maximum at 4 min and then decreased to the basal value (at between 16 and 32 min). The curves for CO2 transfer from tissue to pancreatic secretion and for CO2 in the secretion had the same shape. It is concluded that the curve of production (of water and CO2 excretion) parallels the curve of O2 consumption fairly well. The O2 consumption curve did not correlate either with the blood flow curve or with the O2 extraction curve. About one quarter of the excreted CO2 originated from pancreatic metabolism and the remaining three quarters were transferred from blood, through the pancreatic tissue into the secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

The response of the pancreas of the anaesthetized cat to secretin before, during and after reversible vagal blockade

Cooling the cervical vagi of the anaesthetized splanchnectomized cat to 2 degrees C caused a 54.4 +/- 8.8% inhibition of pancreatic electrolyte secretion stimulated submaximally with pure secretin. On rewarming the vagi there was a prolonged increase in secretion rate over and above the control rate which existed before cooling. The increase lasted about 90 min. There were no changes in acid/base status due to interference of the lung inflation reflex which could account for the inhibition of secretion and the subsequent rebound. Cold block of the cervical vagi increased the transpancreatic electrical conductance, indicating that vasodilation had occurred and therefore eliminated a vasomotor cause for the inhibition. Electrolyte secretion was also inhibited by bilateral vagal section. Atropine only partially prevented the inhibitory response to vagal cooling. A cholinergic mechanism, therefore, accounted for some but not all of the response to vagal cooling. It is concluded that even in the fasted, anaesthetized animal vagal impulses facilitate the action of secretin on the pancreas. This facilitation is only partially cholinergic; the major part of the response is due to some non-cholinergic transmitter substance. Such a mechanism may be necessary to potentiate the action of the very small amounts of secretin which appear to be released during a meal.

Abolished relationship between pancreatic HCO-3 secretion and arterial pH during carbonic anhydrase inhibition

After acetazolamide administration, CO2 hydration in pancreatic cells would be slow and might become a rate-limiting factor to pancreatic HCO-3 secretion. Correspondingly, pancreatic HCO-3 secretion-normally pH dependent-would become slow and pH-independent. However, acetazolamide would not be expected to interfere with the capacity of the secretory mechanism to generate a proton potential gradient between pancreatic cells and interstitial fluid. These predictions were examined in 5 anesthetized, secretion infused (2.7 C. U./kg b.wt. h-1) pigs. Pancreatic juice was collected from a catheter in the pancreatic duct. Arterial pH was varied through i.v. HCl and NaHCO3 infusions and CO2 addition to inspired air. Before acetazolamide, HCO-3 secretion varied with plasma pH and averaged 298 +/- 30 mumol/min at control arterial pH. Acetazolamide (150 mg/kg, i.v.) reduced HCO3 secretion to 84 +/- 12 mumol/min and rendered secretion independent of arterial pH between pH 7.6 and pH 7.0. It is concluded that acetazolamide imposes a pH-independent transport maximum on pancreatic HCO-3 secretion, but does not reduce the capacity of the secretory mechanism to sustain a proton potential gradient between cells and interstitial fluid.

The role of buffer anions and protons in secretion by the rabbit mandibular salivary gland

1. The role of extracellular HCO3- and H+ in the formation of primary saliva and its subsequent modification by the glandular ducts has been investigated in the isolated perfused mandibular salivary gland of the rabbit. 2. Variation of extracellular HCO3- concentration between 12.5 and 50.0 mmol/l was without effect on salivary flow rate or on Na+ and K+ excretion, even though salivary HCO3- (and Cl-) content altered with changes in the extracellular concentration of the two anions. 3. Complete replacement of perfusate HCO3- by Cl- reduced fluid secretion by 34% and almost abolished ductal Na+ absorption. However, when extracellular pH was controlled by replacing HCO3- with the hydrophilic HEPES buffer, fluid secretion but not ductal Na+ absorption was restored to normal. 4. Complete replacement of exogenous HCO3- with acetate increased fluid secretion by 110% and also stimulated ductal Na+ absorption. This effect did not appear to be related to changes in cell pH and remains unexplained. Acetate entered the saliva in concentrations comparable to those seen for HCO3- in control experiments. 5. Salivary secretion showed an almost linear dependence on extracellular pH, rising from 14% of control (pH 7.4) levels at pH 6.2 to 130% at pH 7.8. Ductal Na+ absorption showed similar pH dependence. 6. Carbonic anhydrase inhibitors did not affect fluid secretion rates (except when supramaximal doses of ACh were used to evoke secretion) but they did cause a large reduction in salivary HCO3- output. In glands perfused with acetate rather than HCO3-, carbonic anhydrase inhibitors had no effect on excretion of fluid, acetate or metabolically derived HCO3-. Duct perfusion studies suggested that the effect of the inhibitors on HCO3- output was at the site of primary secretion rather than at the ductal site of HCO3- transport.

The reflexion coefficient as a measure of transepithelial permeability in the isolated rabbit pancreas

1. The reflexion coefficients of a number of non-electrolytes and electrolytes have been determined in the isolated rabbit pancreas. 2. The reflexion coefficients of the following non-electrolytes were: urea, -0.02; glycerol, 0.06; erythritol, 0.11; sorbitol, 0.41; mannitol, 0.42; arabinose, 0.72; xylose, 0.74, assuming a value of 1.00 for sucrose. 3. These values are equal within the experimental error to values previously obtained with a tracer technique for the same preparation, but they are significantly lower than those reported by other investigators for the isolated perfused cat pancreas. 4. Addition of 100 mM-sucrose to the bathing medium resulted in proportionally increased Na+ and K+ concentrations in the secreted fluid. The secreted fluid remained isotonic with the bathing medium under all circumstances. 5. Addition of 10(-5) M-carbachol to the bathing medium led to a reduction in the reflexion coefficient of sucrose from 1 to 0.85, but only when 25 mM-sucrose was used. 6. The reflexion coefficients of electrolytes were: NaCl, 0.50; KCl, 0.51; NaHCO3, 0.52 and choline chloride, 1.02. 7. It is concluded that the isolated rabbit pancreas is highly permeable, both to electrolytes and to small non-electrolytes, probably being more leaky than any other epithelium studied so far.

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.

Exocrine ductal pCO2 in the rabbit pancreas

An improved pCO2 microelectrode has been evaluated and used to investigate whether a significant barrier to diffusion of CO2 exists in the rabbit pancreas. The results of this study show the improved Carter and Caflisch pCO2 microelectrode to be an accurate and reliable tool for measuring pancreatic venous and ductal pCO2. The similarities between pCO2 values from the pancreatic ducts and small pancreatic veins suggest that there is no barrier to CO2 diffusion between small veins and exocrine ducts in the rabbit pancreas, and that ductal pCO2 is probably strongly influenced by the CO2 tension of the small pancreatic blood vessels.

Effect of plasma H+-ion concentration on pancreatic HCO-3 secretion

The relationship between the rate of pancreatic HCO-3 secretion and plasma H+-ion concentration was investigated in 15 pentothal anesthetized, secretin infused pigs (1.8 C.U./kg b.w. h-1, intravenously) during acute metabolic and respiratory acid-base disturbances. Pancreatic HCO-3 secretion increasd to 196 +/- 10% of control during alkalosis and fell to 41 +/- 4% of control during acidosis. Partial metabolic compensation of respiratory acidosis restored HCO-3 secretion to 87 +/- 6% of control. A proportional relationship was found between HCO-3 secretion and plasma pH. Different, proportional relationships were found between HCO-3 secretion and plasma HCO-3 concentration during metabolic and respiratory acid-base changes. HCO-3 secretion was independent of H+-ion concentration in pancreatic juice. Plasma H+-ion concentration, therefore, seems to determine the rate of pancreatic HCO-3 secretion. This finding supports the hypothesis that a proton pump is responsible for pancreatic HCO-3 secretion.

Electrolyte secretion by the isolated cat pancreas during replacement of extracellular bicarbonate by organic anions and chloride by inorganic anions

1. The effect of replacing extracellular bicarbonate and chloride by other anions on the volume and composition of secretin-stimulated pancreatic juice has been analysed in the isolated, perfused cat pancreas. 2. The anions of some aliphatic carboxylic acids were able partially to substitute for bicarbonate in sustaining pancreatic secretion. The order of effectiveness was: acetate greater than proprionate greater than butyrate greater than formate. 3. The rate of secretion in the presence of 25 mM-acetate was 42% of that achieved with 25 mM-bicarbonate. The concentration of acetate in the secretion varied with flow rate, reaching a maximum of 120 mM at high flow rates and declining at lower flow rates, with reciprocal changes in chloride concentration. Bicarbonate was always present in the secretion at a concentration of 5--7 mM. 4. Inorganic anions were able totally or partially to substitute for chloride in sustaining secretion. In relation to chloride, their degree of effectiveness was: chloride = bromide = or greater than nitrate greater than iodide greater than sulphate greater than methyl sulphate greater than isethionate. Those anions which had no effect on secretion rate (i.e. bromide and nitrate) also had no effect on the bicarbonate concentration of the secretion and themselves appeared in the secretion in place of chloride. Those anions which inhibited secretion increased the bicarbonate concentration in the secretion in proportion to the degree of inhibition they caused (i.e. the increase was greatest with isethionate). 5. When perfusate chloride was only partially replaced by bromide or iodide the ratios of chloride: bromide and chloride: iodide in the secretion were approximately equal to those in the perfusate. 6. The carbonic anhydrase inhibitor acetazolamide reduced secretory rate and bicarbonate concentration when added to normal perfusion fluid or chloride-substituted fluids, but had no effect following replacement of perfusate bicarbonate by acetate. 7. These observations illustrate that an extracellular source of permeant anions is required for optimal pancreatic bicarbonate secretion to occur. This may indicate the participation of an anion exchange carrier in the transport events responsible for this secretory process.

Differentiation between the calcium-dependent effects of cholecystokinin-pancreaozymin and the bicarbonate-dependent effects of secretin in exocrine secretion of the rat pancreas

1. Differentiation between the secretory effects of pure natural cholecystokinin-pancreozymin (CCK-PZ) and those of synthetic secretin was studied in the isolated pancreas of the rat perfused with a standard solution containing both Ca2+ and HCO2-, a Ca2+ deficient solution, or a HCO3-deficient solution. 2. Secretin induced a dose-dependent increase in the flow of pancreatic juice and a slight but definite increase in amylase output which was also dependent upon the dose of secretin. 3. The increase in the flow of pancreatic juice, induced by continuous stimulation with secretin (5 Ivy dog m-u./ml.), was completely abolished during perfusion with a CHO3- deficient solution, but was only slightly suppressed during perfusion with a Ca2+-deficient solution. 4. The increase in flow, induced by continuous stimulation with CCK-PZ (5 Ivy dog m-u./ml.), was partly affected by the deprivation of HCO3-, but was strongly inhibited by the deprivation of Ca2+. The CCK-PZ-induced amylase output was not affected by the deprivation of HCO3-, but was significantly inhibited by the deprivation of Ca2+. 5. The present results favour the view that CCK-PZ acts on the acinar cells to increase both amylase output and juice flow, whereas secretin acts on centro-acinar and terminal duct cells to increase mainly juice flow.

Respiratory exchanges and acid-base balance during perfusion of ex vivo isolated pancreas

We used the technique of ex vivo isolated pancreas, perfused with whole heparinized blood. The organ was stimulated by secretin (G.I.H. Stockolm, 0.1-5.0 clinic units/hr), and/or carbamylcholine (100-200 mug/hr). Oxygen consumption was increased under stimulation. This increase was a function of the dose of secretin and also of the bicarbonate output in the juice. Oxygen uptake increased further when carbamylcholine was super-imposed on secretin. This extra increase was independent of hemodynamic conditions of the organ perfusion. Arteriovenous difference in oxygen saturation did not increase when the gland was stimulated. It tended to decrease when the stimulation resulted in a marked vasodilatation. Thus, oxygen needs seemed to be neither the limiting factor of the response to a given stimulation nor the triggering mechanism of functional vasodilatation. Values of pCO2 were spread over a wide range from one experiment to another. However, it could be observed that CO2 efflux into the vein decreased under stimulation by secretin; in most experiments, CO2 efflux was even replaced by an apparent consumption of CO2 during the perfusion of the stimulated gland. Furthermore, arteriovenous pH difference increased following secretin stimulation. This increase was dose-related to secretin. These facts are discussed under the background of theories recently proposed for bicarbonate secretion.

Extracellular bicarbonate ions and insulin secretion

The role of bicarbonate ions in insulin release was studied with incubated and perifused isolated rat islets of Langerhans. In the absence of NaHC03, the early phase of glucose-induced secretion was completely abolished and the second phase inhibited by approximately 65%. The insulinotropic effect of the sugar was totally restored after reintroduction of the ion in the medium. The monophasic secretory after reintroduction of the ion in the medium. The monophasic secretory response to tolbutamide was also markedly diminished by omission of NaHC03, WHereas the release evoked by a high concentration of K+ was very little affected. CO2 wwas unable to substitute for HC03minus, but small concentrations of the anion (3to 5mM) WEre sufficient to ensure a normal response to glucose.

Pancreatic acinar cells: ionic dependence of the membrane potential and acetycholine-induced depolarization

1. Intracellular recordings of membrane potentials have been made in vitro from the exocrine acinar cells of the mouse pancreas using glass micro-electrodes.2. The mean membrane potential of the acinar cells during superfusion with Krebs-Henseleit solution was -39.2 mV. Increasing [K](o) tenfold decreased the membrane potential by 28 mV when [K](o) was above 10 mM. This depolarization was not affected by atropine (1.4 x 10(-6)M). Strophanthin-G (10(-3)M) slowly depolarized the cells at about 10 mV hr(-1).3. Brief exposure to acetylcholine (ACh), 5.5 x 10(-5)M, or pancreozymin resulted in a short lasting depolarization of the acinar cells. Atropine (1.4 x 10(-6)M) blocked the depolarizing action of ACh but not that of pancreozymin. Adrenaline (5.5 x 10(-5)M) or cyclic AMP (10(-3)-10(-4)M) did not influence the membrane potential.4. The amplitude of the ACh-induced depolarization was not dependent on the presence of CO(2)/HCO(3) in the bathing fluid, but it was closely dependent on the extracellular Na concentration. However, ACh was still able to evoke a small depolarization even after prolonged exposure of the tissue to a Na-free solution.5. During exposure of the tissue to a Ca-free solution the resting membrane potential was decreased and the ACh-induced depolarization was significantly reduced. Some substances which are known in other tissues to inhibit membrane Ca(2+) currents, i.e. La(3+), D-600 and tetracaine, were able to reduce, but never abolish, the ACh-induced depolarization.6. These results suggest that the effect of ACh on the pancreatic acinar cell is to increase the permeability of the membrane to commonly occurring ions with a consequent Na-influx and a small Ca-influx.

Amylase secretion by the perfused cat pancreas in relation to the secretion of calcium and other electrolytes and as influenced by the external ionic environment

1. Amylase secretion from the perfused pancreas consists of two components: a small continuous basal secretion and a stimulated secretion in response to acetylcholine or cholecystokinin-pancreozymin. The response to small doses of either stimulant was repeatable over several hours.2. The calcium concentration of pancreatic juice, always less than that of the perfusate, was normally constant above secretory rates of 0.15 g/10 min. However, when the concentration of enzymes in the juice rose, either after stimulation or at very low secretory rates, the calcium concentration rose in parallel, suggesting that this calcium is bound to, or is a component of, pancreatic enzymes.3. Elevation of the perfusate calcium concentration resulted in a parallel increase in the calcium concentration of the pancreatic juice.4. Calcium-free solutions initially caused a small reduction in basal and stimulated amylase secretion and, after prolonged periods of perfusion, abolished stimulated secretion and caused a reduction in electrolyte secretion. The latter was completely reversed by calcium-rich perfusates but the effects on enzyme secretion were only partially reversible.5. Calcium-rich perfusates had no effect on the rate of electrolyte secretion but potentiated submaximally stimulated amylase secretion.6. Barium did not substitute for calcium in supporting pancreatic secretion.7. Alterations in the extracellular concentrations of sodium, potassium and magnesium had no direct effect on amylase secretion.8. The local anaesthetic tetracaine inhibited amylase secretion at a lower concentration than that required to inhibit electrolyte secretion.9. It is concluded (a) that calcium is secreted into the pancreatic juice in two fractions, one associated with enzymes and the other with the electrolyte component of the juice; and (b) that calcium ions play an important role in the stimulus-secretion coupling of pancreatic acinar cells, but that the effects of calcium depletion on electrolyte secretion may principally be due to alterations in the permeability of the duct system.

The effect of prolonged pancreatic secretion on blood acid-base balance in the conscious dog

Prolonged near maximal pancreatic secretion in conscious dogs has been found to result in a metabolic acidosis. This is mild and is accompanied by respiratory and other forms of compensation. Measurements of blood bicarbonate or base-excess changes cannot be used to estimate pancreatic bicarbonate output. The acidosis caused by pancreatic secretion cannot explain the changes in bicarbonate concentration seen in pancreatic juice during prolonged secretion.