Secretion of fluid and amylase in the perfused rat pancreas

The roles of calcium and ATP in the physiology and pathology of the exocrine pancreas

This review deals with the roles of calcium ions and ATP in the control of the normal functions of the different cell types in the exocrine pancreas as well as the roles of these molecules in the pathophysiology of acute pancreatitis. Repetitive rises in the local cytosolic calcium ion concentration in the apical part of the acinar cells not only activate exocytosis but also, via an increase in the intramitochondrial calcium ion concentration, stimulate the ATP formation that is needed to fuel the energy-requiring secretion process. However, intracellular calcium overload, resulting in a global sustained elevation of the cytosolic calcium ion concentration, has the opposite effect of decreasing mitochondrial ATP production, and this initiates processes that lead to necrosis. In the last few years it has become possible to image calcium signaling events simultaneously in acinar, stellate, and immune cells in intact lobules of the exocrine pancreas. This has disclosed processes by which these cells interact with each other, particularly in relation to the initiation and development of acute pancreatitis. By unraveling the molecular mechanisms underlying this disease, several promising therapeutic intervention sites have been identified. This provides hope that we may soon be able to effectively treat this often fatal disease.

Cyclic AMP-dependent protein kinase A and EPAC mediate VIP and secretin stimulation of PAK4 and activation of Na+,K+-ATPase in pancreatic acinar cells

Rat pancreatic acinar cells possess only the p21-activated kinase (PAKs), PAK4 of the group II PAK, and it is activated by gastrointestinal hormones/neurotransmitters stimulating PLC and by a number of growth factors. However, little is known generally of cAMP agents causing PAK4 activation, and there are no studies with gastrointestinal hormones/neurotransmitters activating cAMP cascades. In the present study, we examined the ability of VIP and secretin, which stimulate cAMP generation in pancreatic acini, to stimulate PAK4 activation, the signaling cascades involved, and their possible role in activating sodium-potassium adenosine triphosphatase (Na⁺,K⁺-ATPase). PAK4 activation was compared with activation of the well-established cAMP target, cyclic AMP response element binding protein (CREB). Secretin-stimulated PAK4 activation was inhibited by KT-5720 and PKA Type II inhibitor (PKI), protein kinase A (PKA) inhibitors, whereas VIP activation was inhibited by ESI-09 and HJC0197, exchange protein directly activated by cAMP (EPAC) inhibitors. In contrast, both VIP/secretin-stimulated phosphorylation of CREB (pCREB) via EPAC activation; however, it was inhibited by the p44/42 inhibitor PD98059 and the p38 inhibitor SB202190. The specific EPAC agonist 8-CPT-2- O-Me-cAMP as well 8-Br-cAMP and forskolin stimulated PAK4 activation. Secretin/VIP activation of Na⁺,K⁺-ATPase, was inhibited by PAK4 inhibitors (PF-3758309, LCH-7749944). These results demonstrate PAK4 is activated in pancreatic acini by stimulation of both VIP-/secretin-preferring receptors, as is CREB. However, they differ in their signaling cascades. Furthermore, PAK4 activation is needed for Na⁺,K⁺ATPase activation, which mediates pancreatic fluid secretion. These results, coupled with recent studies reporting PAKs are involved in both pancreatitis/pancreatic cancer growth/enzyme secretion, show that PAK4, similar to PAK2, likely plays an important role in both pancreatic physiological/pathological responses. NEW & NOTEWORTHY Pancreatic acini possess only the group II p21-activated kinase, PAK4, which is activated by PLC-stimulating agents/growth factors and is important in enzyme-secretion/growth/pancreatitis. Little information exists on cAMP-activating agents stimulating group II PAKs. We studied ability/effect of cyclic AMP-stimulating agents [vasoactive intestinal polypeptide (VIP), secretin] on PAK4 activity in rat pancreatic-acini. Both VIP/secretin activated PAK4/CREB, but the cAMP signaling cascades differed for EPAC, MAPK, and PKA pathways. Both hormones require PAK4 activation to stimulate sodium-potassium adenosine triphosphatase activity. This study shows PAK4 plays an important role in VIP-/secretin-stimulated pancreatic fluid secretion and suggests it plays important roles in pancreatic acinar physiological/pathophysiological responses mediated by cAMP-activating agents.

Electrophysiological properties of anion exchangers in the luminal membrane of guinea pig pancreatic duct cells

The pancreatic duct epithelium secretes the HCO₃⁻-rich pancreatic juice. The HCO₃⁻ transport across the luminal membrane has been proposed to be mediated by SLC26A Cl⁻–HCO₃⁻ exchangers. To examine the electrophysiological properties of Cl⁻–HCO₃⁻ exchangers, we directly measured HCO₃⁻ conductance in the luminal membrane of the interlobular pancreatic duct cells from guinea pigs using an inside-out patch-clamp technique. Intracellular HCO₃⁻ increased the HCO₃⁻ conductance with a half-maximal effective concentration value of approximately 30 mM. The selectivity sequence based on permeability ratios was SCN⁻ (1.4) > Cl⁻ (1.2) = gluconate (1.1) = I⁻ (1.1) = HCO₃⁻ (1.0) > methanesulfonate (0.6). The sequence of the relative conductance was HCO₃⁻ (1.0) > SCN⁻ (0.7) = I⁻ (0.7) > Cl⁻ (0.5) = gluconate (0.4) > methanesulfonate (0.2). The current dependent on intracellular HCO₃⁻ was reduced by replacement of extracellular Cl⁻ with gluconate or by H₂DIDS, an inhibitor of Cl⁻–HCO₃⁻ exchangers. RT-PCR analysis revealed that the interlobular and main ducts expressed all SLC26A family members except Slc26a5 and Slc26a8. SLC26A1, SLC26A4, SLC26A6, and SLC26A10 were found to be localized to the luminal membrane of the guinea pig pancreatic duct by immunohistochemistry. These results demonstrate that these SLC26A Cl⁻–HCO₃⁻ exchangers may mediate the electrogenic HCO₃⁻ transport through the luminal membrane and may be involved in pancreatic secretion in guinea pig ducts.

Mechanism and synergism in epithelial fluid and electrolyte secretion

A central function of epithelia is the control of the volume and electrolyte composition of bodily fluids through vectorial transport of electrolytes and the obligatory H2O. In exocrine glands, fluid and electrolyte secretion is carried out by both acinar and duct cells, with the portion of fluid secreted by each cell type varying among glands. All acinar cells secrete isotonic, plasma-like fluid, while the duct determines the final electrolyte composition of the fluid by absorbing most of the Cl(-) and secreting HCO3 (-). The key transporters mediating acinar fluid and electrolyte secretion are the basolateral Na(+)/K(+) /2Cl(-) cotransporter, the luminal Ca(2+)-activated Cl(-) channel ANO1 and basolateral and luminal Ca(2+)-activated K(+) channels. Ductal fluid and HCO3 (-) secretion are mediated by the basolateral membrane Na(+)-HCO3 (-) cotransporter NBCe1-B and the luminal membrane Cl(-)/HCO3 (-) exchanger slc26a6 and the Cl(-) channel CFTR. The function of the transporters is regulated by multiple inputs, which in the duct include major regulation by the WNK/SPAK pathway that inhibit secretion and the IRBIT/PP1 pathway that antagonize the effects of the WNK/SPAK pathway to both stimulate and coordinate the secretion. The function of these regulatory pathways in secretory glands acinar cells is yet to be examined. An important concept in biology is synergism among signaling pathways to generate the final physiological response that ensures regulation with high fidelity and guards against cell toxicity. While synergism is observed in all epithelial functions, the molecular mechanism mediating the synergism is not known. Recent work reveals a central role for IRBIT as a third messenger that integrates and synergizes the function of the Ca(2+) and cAMP signaling pathways in activation of epithelial fluid and electrolyte secretion. These concepts are discussed in this review using secretion by the pancreatic and salivary gland ducts as model systems.

Prolonged stimulation of pancreatic serous secretions by bile and sodium taurocholate in anaesthetized rats

There have been numerous reports that infusion of either natural bile or bile salts into the duodenum evokes a rapid increase in pancreatic secretion through the release of the hormone secretin from the duodenal mucosa. We have extended this observation by the demonstration of an additional late increase in secretion which persisted for many hours and have sought to identify the processes underlying this increase. In anaesthetised rats, infusion of 20 mM taurocholate into the duodenum caused a staircase-like increase in the weight of pancreatic secretion which extended over many hours during which, the HCO[Formula: see text] and protein output of the secretion showed only minimal changes. This effect was also reproduced with intra-duodenal infusion of natural bile which was inferred to act though its taurocholate content. Since the stimulatory action was also obtained with superfusion of taurocholate or natural bile onto the small intestine and by intravenous injection of taurocholate, it was concluded that taurocholate acted by being absorbed into the bloodstream and then by exerting a stimulatory action on the exocrine pancreas. This action was inhibited by puromycin (a protein synthesis inhibitor), by furosemide (a Na( + )/K( + )/2Cl(-) cotransporter inhibitor), though not by SITS (an inhibitor of Cl(-)/HCO[Formula: see text] exchange). The long lasting increase in pancreatic serous secretion would be consistent with the possible activation of gene transcription by taurocholate leading to increased activity of the Na( + )/K( + )/2Cl(-) cotransporter through which the acinar cells increased their secretions.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

Suppressed glucose metabolism in acinar cells might contribute to the development of exocrine pancreatic insufficiency in streptozotocin-induced diabetic mice

High prevalence of exocrine pancreatic insufficiency has been observed in diabetic patients. However, the underlying mechanisms are not well known. Reduced cytosolic Ca(2+) signals in pancreatic acinar cells may contribute to lower digestive enzyme secretion. It is well known that adenosine triphosphate (ATP) regulates cytosolic Ca(2+) signals in acinar cells; however, little is known as to whether diabetes impairs glucose metabolism that produces ATP in acinar cells. Streptozotocin (STZ)-induced diabetic C57BL/6 mouse model was used. Four weeks after being diabetic, pancreatic acinar cells were isolated; and amylase secretion and contents, glucose utilization and oxidation, the activities of several key enzymes for glucose metabolism, and ATP and nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH) contents were determined. Compared with controls, diabetic mice had lower body weight. Cholecystokinin-8- and acetylcholine-stimulated amylase secretion was significantly impaired, and total amylase activity in acinar cells of STZ-diabetic mice was markedly reduced. Glucose utilization and oxidation were suppressed; measured enzyme activities for glucose metabolism and the ATP and NADPH contents were significantly reduced. These data indicate that glucose metabolism and ATP and NADPH productions are very important for maintaining acinar cell normal function. Reduction of ATP (reduces cytosolic Ca(2+) signals) and NADPH (reduces cell capability for antioxidative stress) productions may contribute to the development of exocrine pancreatic insufficiency in STZ-diabetic mice.

Fatty acids, alcohol and fatty acid ethyl esters: toxic Ca2+ signal generation and pancreatitis

Pancreatitis, a potentially fatal disease in which the pancreas digests itself as well as its surroundings, is a well recognized complication of hyperlipidemia. Fatty acids have toxic effects on pancreatic acinar cells and these are mediated by large sustained elevations of the cytosolic Ca(2+) concentration. An important component of the effect of fatty acids is due to inhibition of mitochondrial function and subsequent ATP depletion, which reduces the operation of Ca(2+)-activated ATPases in both the endoplasmic reticulum and the plasma membrane. One of the main causes of pancreatitis is alcohol abuse. Whereas the effects of even high alcohol concentrations on isolated pancreatic acinar cells are variable and often small, fatty acid ethyl esters--synthesized by combination of alcohol and fatty acids--consistently evoke major Ca(2+) release from intracellular stores, subsequently opening Ca(2+) entry channels in the plasma membrane. The crucial trigger for pancreatic autodigestion is intracellular trypsin activation. Although there is still uncertainty about the exact molecular mechanism by which this Ca(2+)-dependent process occurs, progress has been made in identifying a subcellular compartment--namely acid post-exocytotic endocytic vacuoles--in which this activation takes place.

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.

Basolateral anion transport mechanisms underlying fluid secretion by mouse, rat and guinea-pig pancreatic ducts

Fluid secretion by interlobular pancreatic ducts was determined by using video microscopy to measure the rate of swelling of isolated duct segments that had sealed following overnight culture. The aim was to compare the HCO(3)(-) requirement for secretin-evoked secretion in mouse, rat and guinea-pig pancreas. In mouse and rat ducts, fluid secretion could be evoked by 10 nm secretin and 5 microm forskolin in the absence of extracellular HCO(3)(-). In guinea-pig ducts, however, fluid secretion was totally dependent on HCO(3)(-). Forskolin-stimulated fluid secretion by mouse and rat ducts in the absence of HCO(3)(-) was dependent on extracellular Cl(-) and was completely inhibited by bumetanide (30 microm). It was therefore probably mediated by a basolateral Na(+)-K(+)-2Cl(-) cotransporter. In the presence of HCO(3)(-), forskolin-stimulated fluid secretion was reduced approximately 40% by bumetanide, approximately 50% by inhibitors of basolateral HCO(3)(-) uptake (3 microm EIPA and 500 microm H(2)DIDS), and was totally abolished by simultaneous application of all three inhibitors. We conclude that the driving force for secretin-evoked fluid secretion by mouse and rat ducts is provided by parallel basolateral mechanisms: Na(+)-H(+) exchange and Na(+)-HCO(3)(-) cotransport mediating HCO(3)(-) uptake, and Na(+)-K(+)-2Cl(-) cotransport mediating Cl(-) uptake. The absence or inactivity of the Cl(-) uptake pathway in the guinea-pig pancreatic ducts may help to account for the much higher concentrations of HCO(3)(-) secreted in this species.

Interactions between secretin and acetylcholine in the regulation of fluid secretion by isolated rat pancreatic ducts

1. Interlobular ducts were isolated from the rat pancreas and maintained in short-term tissue culture. Fluid secretion from these isolated ducts was measured using micropuncture techniques, intracellular calcium concentration ([Ca2+]i) by fura-2 microspectrofluorimetry, and cyclic AMP by radioimmunoassay. 2. Applying secretin and ACh simultaneously to ducts caused either a stimulation or an inhibition of fluid secretion depending on the doses employed. 3. The inhibitory effect of secretin and ACh could be relieved by atropine, and by the protein kinase C (PKC) inhibitors staurosporine and 1-(5-isoquinolinylsulphonyl)-2-methyl-piperazine (H-7). 4. Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) and phorbol 12, 13-dibutyrate (PDBu) inhibited secretin-evoked fluid secretion. 5. ACh and TPA also inhibited fluid secretion stimulated by the adenylate cyclase activator, forskolin. 6. Neither secretin nor the PKC activators and inhibitors had any effect on either the increase in [Ca2+]i evoked by ACh or the increase in intracellular cyclic AMP evoked by secretin and forskolin. 7. We conclude that the inhibitory effect of combined doses of secretin and ACh on ductal fluid secretion is probably mediated by PKC at a point in the secretory mechanism distal to the generation of intracellular messengers.

Na+, K+, and Cl- transport in resting pancreatic acinar cells

To understand the role of Na+, K+, and Cl- transporters in fluid and electrolyte secretion by pancreatic acinar cells, we studied the relationship between them in resting and stimulated cells. Measurements of [Cl-]i in resting cells showed that in HCO3(-)-buffered medium [Cl-]i and Cl- fluxes are dominated by the Cl-/HCO3- exchanger. In the absence of HCO3-, [Cl-]i is regulated by NaCl and NaK2Cl cotransport systems. Measurements of [Na+]i showed that the Na(+)-coupled Cl- transporters contributed to the regulation of [Na+]i, but the major Na+ influx pathway in resting pancreatic acinar cells is the Na+/H+ exchanger. 86Rb influx measurements revealed that > 95% of K+ influx is mediated by the Na+ pump and the NaK2Cl cotransporter. In resting cells, the two transporters appear to be coupled through [K+]i in that inhibition of either transporter had small effect on 86Rb uptake, but inhibition of both transporters largely prevented 86Rb uptake. Another form of coupling occurs between the Na+ influx transporters and the Na+ pump. Thus, inhibition of NaK2Cl cotransport increased Na+ influx by the Na+/H+ exchanger to fuel the Na+ pump. Similarly, inhibition of Na+/H+ exchange increased the activity of the NaK2Cl cotransporter. The combined measurements of [Na+]i and 86Rb influx indicate that the Na+/H+ exchanger contributes twice more than the NaK2Cl cotransporter and three times more than the NaCl cotransporter and a tetraethylammonium-sensitive channel to Na+ influx in resting cells. These findings were used to develop a model for the relationship between the transporters in resting pancreatic acinar cells.

Agonist-specific regulation of [Na+]i in pancreatic acinar cells

In a companion paper (Zhao, H., and S. Muallem. 1995), we describe the relationship between the major Na+,K+, and Cl- transporters in resting pancreatic acinar cells. The present study evaluated the role of the different transporters in regulating [Na+]i and electrolyte secretion during agonist stimulation. Cell stimulation increased [Na+]i and 86Rb influx in an agonist-specific manner. Ca(2+)-mobilizing agonists, such as carbachol and cholecystokinin, activated Na+ influx by a tetraethylammonium-sensitive channel and the Na+/H+ exchanger to rapidly increase [Na+]i from approximately 11.7 mM to between 34 and 39 mM. As a consequence, the NaK2Cl cotransporter was largely inhibited and the activity of the Na+ pump increased to mediate most of the 86Rb(K+) uptake into the cells. Secretin, which increases cAMP, activated the NaK2Cl cotransporter and the Na+/H+ exchanger to slowly increase [Na+]i from approximately 11.7 mM to an average of 24.6 mM. Accordingly, secretin increased total 86Rb uptake more than the Ca(2+)-mobilizing agonists and the apparent coupling between the NaK2Cl cotransport and the Na+ pump. All the effects of secretin could be attributed to an increase in cAMP, since forskolin affected [Na+]i and 86Rb fluxes similar to secretin. The signaling pathways mediating the effects of the Ca(2+)-mobilizing agonists were less clear. Although an increase in [Ca2+]i was required, it was not sufficient to account for the effect of the agonists. Activation of protein kinase C stimulated the NaK2Cl cotransporter to increase [Na+]i and 86Rb fluxes without preventing the inhibition of the cotransporter by Ca(2+)-mobilizing agonists. The effects of the agonists were not mediated by changes in cell volume, since cell swelling and shrinkage did not reproduce the effect of the agonists on [Na+]i and 86Rb fluxes. The overall findings of the relationships between the various Na+,K+, and Cl- transporters in resting and stimulated pancreatic acinar cells are discussed in terms of possible models of fluid and electrolyte secretion by these cells.

Regulation of fluid secretion and intracellular messengers in isolated rat pancreatic ducts by acetylcholine

1. We have studied the effects of acetylcholine (ACh) on fluid secretion and intracellular messengers in interlobular ducts isolated from the rat pancreas and maintained in short-term tissue culture. 2. Ductal fluid secretion was measured using micropuncture techniques. Intracellular free calcium ([Ca2+]i) and cyclic AMP concentrations were measured in single ducts using fura-2 microspectrofluorimetry and radioimmunoassay techniques respectively. Changes in the levels of these intracellular messengers were correlated with fluid secretion. 3. ACh stimulated ductal fluid secretion. The dose required for a half-maximal response was about 0.4 microM and maximal secretion was achieved with 10 microM ACh. These effects of ACh were blocked by atropine and by removal of extracellular Ca2+. 4. ACh was about four orders of magnitude less potent as an activator of ductal fluid transport than the hormone secretin; however, the maximal rates of fluid secretion evoked by these two agonists were similar. 5. ACh caused a dose-dependent rise in duct cell [Ca2+]i, but had no effect on cyclic AMP. In contrast, secretin increased duct cell cyclic AMP, but had no effect on [Ca2+]i. 6. The [Ca2+]i response evoked by ACh resulted from both mobilization of intracellular Ca2+ stores and influx of Ca2+ from the extracellular space. 7. The Ca2+ ionophore, ionomycin, mimicked the effect of ACh on ductal [Ca2+]i and fluid secretion. 8. We conclude that ACh stimulates fluid secretion from rat pancreatic duct cells by activating a 'Ca2+ pathway' which is distinct from the well documented 'cyclic AMP pathway' utilized by secretin.

Cerulein-induced acute pancreatitis in the rat. Study of pancreatic secretion and plasma VIP and secretin levels

A study was made with different doses of cerulein (2, 4, 10 and 20 micrograms/kg) administered subcutaneously to rats by four injections at intervals of 1 hr; the aim of this work was to study exocrine pancreatic secretion of the rat under cerulein-induced acute pancreatitis, analyzing enzyme and hydroelectrolyte secretion of pancreatic juice. A further aim was to study the relationship between the dose of cerulein and the plasma levels of peptides controlling hydroelectrolyte secretion of the pancreas, like secretin and vasoactive intestinal peptide (VIP). At the lowest dose schedule, the amounts of total protein and enzymes (amylase and trypsin) in pancreatic juice decreased significantly, plasma amylase increased, and the pancreas became edematous. Higher doses magnified these effects. By contrast, ductular function (flow and HCO3-) was well preserved in cerulein-treated rats, and this was probably due to the significant increase in plasma levels of immunoreactive secretin whereas VIP levels were unchanged. The secretin released by treatment with cerulein is able to palliate the lack of flow from acinar origin that is affected in the process of acute pancreatitis, being a beneficial response to the cerulein treatment.

Cytosolic Ca2+ gradients triggering unidirectional fluid secretion from exocrine pancreas

Exocrine gland cells secrete Cl(-)-rich fluid when stimulated by neurotransmitters or hormones. This is generally ascribed to a rise in cytosolic Ca2+ concentration ([Ca2+]i), which leads to activation of Ca2(+)-dependent ion channels. A precise understanding of Cl- secretion from these cells has been hampered by a lack of knowledge about the spatial distribution of the Ca2+ signal and of the Ca2(+)-dependent ion channels in the secreting epithelial cells. We have now used the whole-cell patch-clamp method and digital imaging of [Ca2+]i to examine the response of rat pancreatic acinar cells to acetylcholine. We found a polarization of [Ca2+]i elevation and ion channel activation, and suggest that this comprises a novel 'push-pull' mechanism for unidirectional Cl- secretion. This mechanism would represent a role for cytosolic Ca2+ gradients in cellular function. The cytosolic [Ca2+]i gradients and oscillations of many other cells could have similar roles.

Perifusion of isolated rat pancreatic acini: carbamylcholine-induced biphasic amylase release

To solve many problems in other in-vitro methods such as perfusion and static incubation, perifusion of isolated rat pancreatic acini was established. Continuous stimulation by carbamylcholine induced a biphasic secretory response, a sharp initial phase and a slow-varying second phase. The junction of these two phases occurred at almost the same time (in about 7 min of stimulation) at any concentration of carbamylcholine. The slow-rising former part of a second phase indicated that a biphasic pattern is produced by overlapping of two kinds of real phases and the beginning of a real second phase occurs at the time of the junction. The concentration dependence of amylase release showed a bell-shaped pattern and the maximal amylase release at an optimal concentration (10(-6)M) was 5.43 +/- 0.24% total/30 min. The sustained latter part of a second phase showed a slow decline at optimal or less concentrations but a plateau at supraoptimal concentrations.

Effect of secretin on pancreatic juice proteins in caerulein-induced acute pancreatitis in the rat

Nine hours after the start of treatment with caerulein in rats, an increase in the weight of the pancreas and an increase in serum amylase levels were observed. Likewise, a significant increase in endogenous secretin occurred in rats with acute pancreatitis. A dramatic reduction in the secretion of total protein and amylase was also observed. A partial recovery of this latter effect was achieved after an infusion of high doses of secretin. Under our experimental conditions, the volume of secretion did not vary in caerulein-treated rats wtih respect to controls, either in resting conditions or under secretin stimulation, which indicates that the ductular cells were not significantly affected. Isoelectrofocusing (IEF) and crossed-immunoelectrophoresis (CIE) studies revealed important alterations in the proteins of the pancreatic juice of rats with caerulein-induced acute pancreatitis. Trypsinogen appeared to be particularly affected, showing an increase in the T2 acidic form with an IEP of 4.4 and a decrease in the basic form T3 with an IEP of 8.0, which splits in other forms with a clear antigenic community. A hydrolase was also observed with an IEP of 6.2. In this sense, secretin administration may also be said to induce a significant improvement in established acute pancreatitis, since it tended to normalize the structure and proportion of the proteins secreted.

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)

Calcium and secretagogues-induced conductances in rat exocrine pancreas

The electrical properties of single acinar cells isolated from rat pancreas were studied with the whole-cell tight-seal recording method. Under resting conditions, the relative permeabilities of Cl and K were PCl/PK approximately equal to 3. At 1 microM internal calcium, a Ca and voltage-dependent Cl conductance was activated. At 10 microM internal calcium, the major conductance was selective for cations. It was not voltage-dependent. Acetylcholine and cholecystokinin induced an increase of internal Ca which in turn activated either only a Cl conductance or both Cl and cationic conductances. The secretagogue-induced conductance was increased to a variable extent by depolarisation. The absence of K channels activated by internal calcium indicates that, in pancreatic acinar cells, the mechanism of fluid secretion differs from that observed in other exocrine glands.

Neuroendocrine control of secretion in pancreatic and parotid gland acini and the role of Na+,K+-ATPase activity

The results of our investigations into the localization of Na+,K+-pump activity in pancreatic and parotid acinar cells and the effects of hormones and neurotransmitters on pump turnover can be integrated with data on other aspects of stimulus-response coupling to construct models of the neurohumoral control of protein, fluid, and electrolyte secretion (Fig. 23). In both tissues, Ca2+ and cyclic AMP serve as intracellular messengers. In pancreatic acinar cells, the Ca2+-dependent pathway activated by the occupation of CCK or cholinergic receptors provides the primary stimulus for digestive enzyme secretion. Cyclic AMP plays a comparatively minor role; VIP and secretin are much less effective stimulators of protein secretion. Conversely, cyclic AMP levels in parotid acinar cells, which are modulated primarily through occupation of beta-adrenergic receptors, are a major determinant of enzyme secretion. Activation of the Ca2+-dependent pathway by cholinergic or alpha-adrenergic agonists or substance P is less important. The presence of dual control processes in each gland suggests that the observed differences in effectiveness of cyclic AMP- versus Ca2+-dependent secretagogues may reflect not different mechanisms, but rather a shift in the relative emphasis placed on each pathway. This emphasis could conceivably result from subtle variations in the interaction between cellular protein kinases and phosphatases and their phosphoprotein substrates. Electrolyte secretion, on the other hand, appears to involve both discrete and common entities. In pancreatic acinar cells from rodent species, cholinergic or CCK receptor occupancy elicits a Ca2+-dependent increase in the open-state probability of nonselective cation channels in the basolateral plasma membrane. The resultant influx of Na+ and efflux of K+ is most probably the factor which activates Na+, K+-pumps. Based on electron probe studies of the effects of cholinergic agonists on acinar cell Na+ and K+ contents discussed earlier, a transient reduction in the intracellular K+/Na+ ratio of up to 4-fold may occur. A shift of this magnitude in the cytoplasmic microenvironment of the Na+, K+-pump clearly would have a stimulatory influence (see discussion by Jorgensen, 1980). In addition, Ca2+ itself may have direct effects on Na+,K+-pump activity. Calcium at levels much above 1 microM progressively inhibits Na+,K+-ATPase activity (Tobin et al., 1973; Yingst and Polasek, 1985). In unstimulated guinea pig pancreatic acinar cells, Ca2+i measured by quin-2 fluorescence was 161 +/- 13 nM (Hootman et al., 1985a) which increased to a maximal concentration of 803 +/- 122 nM following CCh stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Entero-pancreatic reflexes revealed by duodenal anesthesia in the dog

This study was designed to improve our understanding of duodeno-pancreatic reflexes, the existence of which was suggested by the previous observation of a reduction in secretin-stimulated pancreatic secretion during local anesthesia of the duodenal mucosa. The effects on this reduction in secretin-stimulated secretion of cholinergic or adrenergic blocking agents (alone or in combination) and of truncal vagotomy, were studied in conscious dog with gastric and pancreatic fistulae. For each agent and for secretin alone in normal and vagotomized dogs, a comparison was made of pancreatic secretion with and without lignocaine anesthesia of the duodenal mucosa. Lignocaine reduced pancreatic secretion with secretin alone, and stimulated it during infusion of atropine. The changes in both protein bicarbonate secretion were blocked by pentolinium and by phenoxybenzamine whereas propranolol mainly blocked the effects on bicarbonate output. The effect of truncal vagotomy resembled that of atropine. These results suggest the existence of two enteropancreatic reflex mechanisms; an excitory cholinergic vagal reflex and an inhibitory, atropine-resistant non-vagal reflex. Both are blocked by pentolinium (a ganglion blocker) and by phenoxy-benzamine, suggesting the involvement of alpha-adrenergic receptors probably also at the level of the ganglion cell. Beta-adrenergic receptors are also involved in the regulation of bicarbonate and fluid secretion.

Vanadate stimulates rat pancreatic enzyme secretion through the release of calcium from an intracellular store

Orthovanadate accelerates 45Ca efflux and enzyme secretion from the rat pancreas incubated in either control (2.5 mM Ca) or nominally Ca-free buffers. Secretion induced by vanadate does not appear to be mediated by changes in either adenylate cyclase or sodium pump activity. Instead, vanadate appears to act at an intracellular site to cause the release of calcium from the same pool mobilised by acetylcholine. Vanadate action is not inhibited by DIDS. The effect of pH on vanadate action may be accounted for by changes in the distribution of the vanadates. Vanadyl sulphate inhibits secretion evoked by acetylcholine. This suggests that intracellular reduction of vanadate (+5 oxidation state) to the +4 oxidation state may account for an inhibitory component observed during stimulation with vanadate.

Distribution of elements in the pancreatic exocrine cells determined by electron probe X-ray microanalysis

We measured the intracellular electrolytes of acinar cells by making electron probe X-ray microanalysis of hydrated and dehydrated sections of freshly frozen dog pancreas. The concentrations of electrolytes in the cytoplasm were: Na 4.8 +/- 2.1, K 132 +/- 15, Cl 14 +/- 4.7, P 165 +/- 36, S 19 +/- 2.8 and in zymogen granules: Na 6 +/- 5, K 60 +/- 16, Cl 31 +/- 20, P 36 +/- 8, S 172 +/- 25, Ca 7 +/- 5 (mean +/- S.D. mmol/kg wet weight). The cytoplasm, which is rich in rough endoplasmic reticulum, had low Na and high K concentrations, as compared with levels in the acinar cells of other exocrine glands such as the submandibular gland, the bulk of which is occupied by secretory granules. Though the representative feature of secretory granules was a high S content, occasionally low S peaks of spectra from secretory granules were obtained. These findings may reflect the content of mature zymogen granules and immature condensing vacuoles. Pilocarpine stimulation increased cytoplasmic Na, Cl and Ca and decreased K levels in the pancreatic acinar cells. This indicates that secretory stimulation increases the permeability of the cell membrane to Na, Cl and K ions and that there is a simultaneous Ca release from the intracellular Ca stores such as zymogen granules and endoplasmic reticulum, and/or Ca influx from the extracellular space.

Anion secretion by the isolated rabbit pancreas

The isolated rabbit pancreas secretes a fluid containing chloride and bicarbonate in about equal concentrations. Replacement of bicarbonate by acetate, phosphate or isethionate, replacement of Na+ by Li+ and addition of ouabain to the bathing medium of the pancreas inhibit the secretion of fluid, chloride and bicarbonate in a similar fashion and by maximally 100%. Replacement of chloride by isethionate inhibits fluid secretion by maximally 50%, chloride secretion by 90% and bicarbonate secretion by 20%. It is concluded that fluid secretion is based on a Na+-gradient-dependent bicarbonate influx or proton efflux in the ductular cell, and that the secretion of chloride is secondary to that of bicarbonate.

Exocrine and endocrine secretion from isolated perfused rat pancreas with islet cell tumors induced by streptozotocin and nicotinamide

Pancreatic exocrine and endocrine function in the rat with islet cell tumors induced by streptozotocin and nicotinamide was studied in the in vitro isolated perfused pancreas. The tumor-bearing pancreas secreted significant amounts of insulin even at 2.8 mM glucose stimulation. Further, insulin response to 8.3 mM glucose stimulation was greater in the tumor-bearing pancreas than in the control. Not only endocrine, but also exocrine, disorders were found in the rat pancreas bearing islet cell tumors. In contrast to the increased response of insulin, amylase output in response to 0.1 ng/ml cerulein was significantly lower in the tumor-bearing than in the control pancreas, although there was no difference in pancreatic juice flows from both groups. These results suggest that enzyme secretory function of the pancreas with islet cell tumors may be suppressed in the presence of some interrelationship between the exocrine and endocrine portion of the pancreas with islet cell tumors.

Effect of ouabain and furosemide on pepsinogen secretion by gastric glands in vitro

Gastric glands were isolated from rabbit stomach and pepsinogen secretion was measured after stimulation with isoproterenol, forskolin, 8-bromo cyclic adenosine monophosphate (8-bromo cAMP), cholecystokinin octapeptide (CCK-OP), carbachol, and hyperosmolar medium. The responses to these stimuli in medium containing 143 mM Na+ and 5.4 mM K+ (normal medium) were compared with responses to the same stimuli in media containing either 0 Na+ and 5.4 mM K+, or 143 mM Na+ and O K+. In addition, the effects of ouabain and furosemide on secretion elicited by these stimuli were determined. Medium containing 0 Na+ inhibited all stimuli. Medium containing 0 K+ inhibited the action of 8-bromo cAMP and stimuli postulated to be mediated by cAMP. Ouabain inhibited the same stimuli as O K+ medium, and, in addition, inhibited the response to hyperosmolar medium. However, ouabain enhanced the response to CCK-OP. Furosemide inhibited the response to hyperosmolar medium but had no effect on the action of any secretagogue employed. Intraglandular [Na+] increased and [K+] decreased after exposure to K+-free medium or ouabain. cAMP content of the glands was assayed after stimulation with both isoproterenol and hyperosmolar medium. Isoproterenol and hyperosmolar medium significantly increased cAMP levels. The results are discussed in relation to possible involvement of ion transport or intracellular ion concentration in the secretory process.

Electrophysiological studies on the cultured cells obtained from transplantable pancreatic carcinoma in Syrian golden hamsters

A pancreatic ductal carcinoma was established as a transplantable tumor line in an inbred strain of Syrian golden hamsters. Intracellular recordings of membrane potentials and input resistance were made from cultured cells obtained from the transplanted tumors using indwelling glass microelectrode. The mean value of the resting membrane potential was -46.5 +/- 1.8 mV (S.E.) (n = 13), while the mean resting input resistance was 21.2 +/- 4.3 M omega (S.E.) (N = 13). Dibutyryl cyclic AMP (2 X 10(-3)M) caused a marked hyperpolarization of about 30 mV accompanied by a reduction of input resistance. The transplantable tumor and its cultured cell line developed in this study have demonstrated their effectiveness as a reliable experimental model for use in pancreatic cancer research.

Effect of acarbose on exocrine and endocrine pancreatic function in the rat

Exocrine and endocrine pancreatic function were studied simultaneously in the isolated perfused pancreas from rats fed a normal or an acarbose-containing diet (150 mg/100 g food) for 20 days. Body weight gain of acarbose-treated rats was slightly lower than that of control rats, despite a larger food intake. Basal and caerulein-stimulated flow rates of pancreatic juice from acarbose-treated rat pancreases were similar to those from controls, suggesting that the treated rat pancreas has normal sensitivity and responsiveness to caerulein. On the other hand, amylase output in response to caerulein was significantly decreased in acarbose-treated rat pancreases, though basal output was normal. The addition of acarbose to the diet for 20 days had no effect on the speed of the insulin response to glucose and caerulein, but the magnitude of insulin secretion to glucose stimulation was reduced by 40% and the caerulein-induced additional output of insulin by 30% in the treated group as compared with the control group. The present investigation has demonstrated that inhibition of key enzymes for carbohydrate digestion decreases not only the secretory responsiveness of amylase from acinar cells to caerulein stimulation but also the sensitivity of the insulin-secretory mechanism of pancreatic B cells to glucose and non-glucose stimulation.

Ionophore A23187 can mimick the changes in membrane permeability that occur during acetylcholine-stimulation of pancreatic acinar secretion

Acetylcholine (ACh) released from vagal terminals increases the permeability of the pancreatic acinar membrane to Na+ and Ca2+ ions. In this report, we compare the induced changes in intracellular Na+ and Ca2+ electrode potentials (ENa and ECa) due to ACh-stimulation of acini with those observed during stimulation with the calcium ionophore, A23187, which mimicks the action of ACh on pancreatic secretion. Stimulation with ACh concentrations varying from 10(-8) to 10(-5) M and with A23187 concentrations of 10(-6) and 10(-5) M caused parallel increases in cytosolic Ca2+ and Na+ ([Ca]i, [Na]i). The magnitude of the increases in [Ca]i and [Na]i due to A23187-stimulation further indicate that when presented with a calcium challenge the acinar cells continue to regulate [Ca]i close to physiological levels and suggest that the observed increases in ionized calcium could reflect much larger increases in complexed Ca2+. ACh-stimulation following removal of either extracellular Na+ or Ca2+ ions, eliminated the intracellular increases found when the removed ions is present, but did not affect the increases usually found with the other ion. The independence of the permeability changes to either the presence of Ca2+ or Na+ indicates the ACh-induced currents carried by Na+ and Ca2+ are also independent. The selective translocation of Na+ and Ca2+ during acetylcholine-stimulation in a manner analogous to the changes observed when ionophore A23187 was used as stimulus, indicates the ability of the activated acinar membrane to function as an ionophore.

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 effects of manganese, cobalt and calcium on amylase secretion and calcium homeostasis in rat pancreas

1. Mn(2+) evoked an atropine-resistant secretion of amylase from the isolated pancreas of the young rat. The lowest effective concentration of Mn(2+) was 10(-3)m. The response to 10(-2)m-Mn(2+) was biphasic, an initial peak being followed by a slow sustained rise in amylase output. The maximal effect of 10(-2)m-Mn(2+) was to double the basal rate of amylase secretion after 70 min incubation.2. Co(2+) (10(-2)m) also stimulated amylase secretion. The maximal rate, about three times the basal value, was attained after 20 min incubation. Atropine partially inhibited this effect.3. Ca(2+) (10(-2)m) evoked an atropine-resistant amylase secretion similar in both magnitude and time course to the sustained phase observed with 10(-2)m-Mn(2+).4. Mn(2+) (10(-4)-10(-2)m) also increased the rate of (45)Ca efflux from the gland. Maximal efflux rates were attained after 30 min incubation and thereafter declined to basal values. A small increase was also observed with 10(-2)m-Co(2+), but not with 10(-2)m-Ca(2+). The effect of Co(2+) was almost completely abolished by atropine.5. Reducing the extracellular Ca(2+) concentration from 2.5 x 10(-3) to 10(-5)m did not reduce amylase secretion in response to 10(-2)m-Mn(2+), but secretion was abolished in a Ca(2+)-free medium containing EGTA. The increase in (45)Ca efflux rate evoked by Mn(2+) was inversely related to the extracellular Ca(2+) concentration.6. Mn(2+) (10(-2)m) increased the concentration of cyclic 3',5'-guanosine monophosphate (cyclic GMP) within the pancreas. Also, Mn(2+) accumulated within the cellular pool of the gland. The time course of both these effects was similar to the time course of (45)Ca efflux.7. Mn(2+) displaced Ca(2+) bound to isolated pancreatic microsomal membranes. The cation-binding sites on these membranes probably have a higher affinity for Mn(2+) than Ca(2+).8. We conclude that Mn(2+) stimulates enzyme secretion by displacing membrane-bound Ca(2+), the resulting increase in cytosolic Ca(2+) concentration activating the secretory mechanism.9. Mn(2+) partially inhibited amylase secretion stimulated by optimal doses of either acetylcholine (ACh) or caerulein. Maximal inhibition (about 60%) occurred with 10(-3)m-Mn(2+) (i.e. the lowest concentration required to stimulate secretion in the absence of secretagogues). Decreasing the extracellular Ca(2+) concentration reduced the inhibitory effect of Mn(2+).10. When glands were exposed to ACh and Mn(2+) simultaneously, the time required for inhibitory effects to develop was inversely related to the dose of ACh and the concentration of Mn(2+).11. Mn(2+) did not alter the acceleration of (45)Ca efflux evoked by ACh or by caerulein in a medium containing 2.5 x 10(-3)m-Ca(2+). However, under conditions of Ca(2+) deprivation ACh-stimulated (45)Ca efflux was greatly enhanced.12. Mn(2+) reduced the total amount of Ca(2+) accumulated into the cellular pool of the pancreas after 60 min incubation, but had no effect on the initial, rapid phase of Ca(2+) uptake.13. The effects of Mn(2+) on the relationship between ACh dose, amylase release and the extracellular Ca(2+) concentration suggest that the inhibitory actions of Mn(2+) cannot be explained by a simple, competitive interaction with the stimulant or with extracellular Ca(2+). However, the time course of inhibition is consistent with a requirement for Mn(2+) to accumulate within the acinar cells.14. Mn(2+) partially inhibited amylase secretion stimulated by hyperosmolarity and also increased the (45)Ca efflux rate under these conditions.15. Our results are not consistent with Mn(2+) exerting its inhibitory effect on secretagogue-stimulated enzyme secretion solely by blocking Ca(2+) influx from the extracellular space. We conclude that inhibition probably depends on the ability of Mn(2+) to displace Ca(2+) from binding sites involved in secretion, presumably coupled with a reduced ability of Mn(2+) to replace Ca(2+) in the secretory process.

The effect of extracellular calcium deprivation on amylase secretion and 45Ca efflux from rat pancreas

1. The role of extracellular Ca(2+) in pancreatic enzyme secretion and (45)Ca efflux evoked by acetylcholine (ACh) and caerulein has been assessed in the incubated, uncinate pancreas of young rats.2. In a medium containing 2.5 x 10(-3)m-Ca(2+), the maximal rates of amylase secretion evoked by optimal doses of each secretagogue were similar. However, the time courses of amylase release during prolonged stimulation of the gland were different.3. The time course of amylase secretion in response to an optimal dose of ACh (10(-5)m) was characterized by an initial rapid increase followed by a slow sustained rise. For caerulein (10(-8)m), an initial rapid rise was followed either by a plateau or slight decline in the rate of amylase secretion.4. Both secretagogues produced similar increases in the rate coefficient of (45)Ca efflux from the gland.5. With supra-optimal doses of the secretagogues, amylase secretion, but not the rate coefficient of (45)Ca efflux, was depressed.6. Reducing the extracellular Ca(2+) concentration did not have a marked effect on basal amylase secretion but inhibited the action of both secretagogues. When the Ca(2+) concentration was 10(-6)m or lower, these inhibitory effects were irreversible. Amylase secretion stimulated by ACh was more sensitive to extracellular Ca(2+) deprivation than that stimulated by caerulein, the concentration required for half-maximal secretion being about 9-fold greater for ACh.7. Decreasing the extracellular Ca(2+) concentration increased both the basal and stimulated rate coefficients of (45)Ca efflux.8. Our results support the hypothesis that pancreatic enzyme secretagogues act by releasing bound Ca(2+) from sites within the acinar cell. Furthermore, they suggest that the site utilized by ACh is more amenable to depletion, via changes in extracellular Ca(2+) concentration, than that employed by caerulein.

Role of calcium in exocrine pancreas secretion. VII. Effect of sodium on enzyme secretion and calcium metabolism in rabbit pancreatic fragments and acinar cells

1. The role of sodium in the pancreatic stimulus-secretion coupling has been studied. 2. Reduction of the extracellular sodium concentration or addition of ouabain to the medium inhibits the stimulation of enzyme secretion by carbachol. 3. Incubation in low sodium medium or in the presence of ouabain increases the exchangeable calcium content, but not the total calcium content of acinar cells. 4. Depending on preincubation time and the substrate replacing sodium in the low sodium medium, the carbachol-induced Ca45(2)+ efflux may be blocked, but it is not blocked by addition of ouabain to a normal Na+ medium. 5. Stimulation of enzyme secretion by the calcium ionophore A23187 in the presence of external calcium is inhibited in low sodium as well as in ouabain containing media. 6. These findings suggest that reduction of the Na+ gradient across the plasma membrane blocks the coupling between intracellular calcium release and exocytosis, while under certain conditions it also blocks the coupling between hormone-receptor interactions and intracellular calcium release.

Cholinergic secretory and inhibitory mechanisms in canine pancreatic secretion

Dose-response relationships for pancreatic stimulants and the interactions by atropine were studied in conscious gastric and duodenal fistula dogs. Secretin, caerulein, and bethanechol, the two latter against background secretin, induced similar maximal secretions of water and bicarbonate, and maximal protein outputs with the two latter were not different. Actions of atropine differed according to type and dose of stimulant, dose of atropine, and secretory variable studied. In the dose interval of 10-40 microg x kg(-1) x h(-1), atropine suppressed the secretin-stimulated water and bicarbonate, but these or lower doses enhanced the response to submaximal caerulein. The secretion after 200 microg x kg(-1) x h(-1) or less bethanechol was unaffected by atropine, but the suppressed response to higher bethanechol doses was reversed and enhanced. These findings are compatible with the presence of one ductal secretory process sensitive to cholinergic influence in three ways: one secretory, partly atropine-sensitive, required for submaximal secretin's optimal action; one secretory, stimulated by bethanechol, and atropine-resistant; and one inhibitory, atropine-sensitive, triggered by caerulein (and high doses of bethanechol). Atropine at low doses inhibited the protein output by bethanechol but enhanced the submaximal caerulein response, which again indicates the presence of an inhibitory atropine-sensitive cholinergic principle. It is proposed that pancreatic polypeptide may be the mediator of this inhibition and that vasoactive intestinal polypeptide could be the mediator of the atropine-resistant cholinergic stimulation of water and bicarbonate secretion.

Mouse pancreatic acinar cells: the anion selectivity of the acetylcholine-opened chloride pathway

1. Anion replacement experiments were performed on superfused in vitro mouse pancreatic tissue and the effects on the electrical response of acinar cells to ACh investigated. 2. Electrical measurements were made with two micro-electrodes inserted into electrically coupled cells. ACh was applied by microionophoresis. Potential recordings were taken before, during and after changeover from the control superfusion fluid, containing Cl-, to one containing the substituted anion. 3. From the results obtained the tested anions were classified into three groups: I, Cl(-)-like anions: Br-, I- and NO3-, causing either no change or a negative displacement of the ACh null-potential, compared to that measured in the control Cl(-)-containing solution, and only small changes in the resting and stimulated electrical properties of the acinar cell, II, ions less permeable than Cl-: isethionate, acetate, sulphate and hippurate, showing a positive displacement of the ACh null-potential and a similar or increased resting cell input resistance, and III, methylsulphate and benzenesulphonate, causing a negatively displaced ACh null-potential but showing changes in the resting electrical properties of the acinar cells characteristic of anions in group II. 4. The ACh null-potential sequence, in order of decreasing negativity, was NO3- greater than or equal to benzenesulphonate greater than or equal to I- greater than or equal to methylsulphate greater than Br- greater than or equal to Cl- greater than isethionate greater than acetate greater than or equal to sulphate greater than hippurate. 5. Experiments involving the use of bicarbonate demonstrated that it does not contribute significantly to the value of the ACh null-potential. 6. The sequence of the anions in group I were compared to the Eisenman series I, suggesting that the ACh-opened Cl- pathway comprises a large hydrated ion channel bearing a lining of weak positive charges. 7. A quantitative relationship was sought between the ACh null-potential and extracellular Cl-. It was found that a tenfold reduction in the extracellular concentration resulted in a 15 mV positive shift of the null-potential.

Effect of carbachol on radiosodium uptake by dispersed pancreatic acinar cells

The effects of carbachol on uptake of 22Na by enzymatically dispersed rat pancreatic acinar cells were determined. Carbachol caused a slight but significant increase in uptake of 22Na by the cells in the presence of absence of ouabain (10(-3) M). A maximal response was obtained with 10(-6) M carbachol. The effects of carbachol were blocked by 10(-5) M atropine. Caerulein (10(-7) M) also stimulated 22Na uptake, while epinephrine (10(-4) M) and substance P (10(-7) M) did not. Carbachol did not stimulate 22Na uptake in the absence of extracellular Ca, although Ca omission significantly elevated basal 22Na uptake. The divalent cationophore A-23187 caused Ca-dependent 22Na uptake at 20 microM concentration but not at 0.3 microM. These results, when considered with earlier reports by others, suggest that muscarinic receptor activation leads to an increase in permeability of the acinar cell membrane to Na, and that Ca may be second messenger for this effect.

Potentiation of cholecystokinin-induced exocrine secretion by both exogenous and endogenous insulin in isolated and perfused rat pancreata

Using an isolated perfused rat pancreas preparation, the interrelationship between the endocrine and exocrine portions of the pancreas were studied. Addition of exogenous rat insulin (1-20 mU/ml) to the perfusing solution potentiated the action of cholecystokinin (CCK) (1 mU/ml) to increase both pancreatic juice flow and the release of the enzyme, amylase. Raising the glucose concentration in the perfusing solution from 2.5 to 17.5 mM both increased endogenous insulin release and potentiated the CCK-induced exocrine secretory response. Two lines of evidence indicated that this effect of glucose on the exocrine pancreas was mediated by endogenous insulin release. First, the addition of comparable amounts of xylose or galactose to the perfusion medium neither released insulin nor potentiated the CCK-induced response. Second, epinephrine blocked the effect of high glucose on both insulin release and potentiation of CCK action. Epinephrine alone did not affect the action of CCK. The magnitude of the exocrine response induced by high glucose was comparable to that of 2.5 mU/ml exogenous insulin. It seems possible that pancreatic acinar cells can be exposed to insulin levels of this magnitude in situ.

Effect of acetylcholine stimulation on calcium and sodium uptake by the isolated rat pancreas

1. The role of Ca2+ and Na+ ions in 'stimulus-secretion' coupling in the isolated uncinate pancreas of 4-week-old rats has been examined using radioisotope tracer techniques. The amount of 45Ca2+ and 22Na+ (mumole/g wet wt.) taken up by unstimulated glands was compared to that of glands in which amylase release was stimulated by ACh(10(-5) M) at various incubation times. 2. The amount of 45Ca taken up by the glands within 60 min of incubation was not found to be increased by the presence of ACh(10(-5) M). In fact, during short incubation periods the 45Ca uptake was significantly less in the stimulated glands than the unstimulated glands. 3. Presence of ACh(10(-5) M) did cause a significant initial increase in 22Na uptake lasting up to 20 min from onset of stimulation. 4. These results indicate that the rise in intracellular Ca2+ concentration which is suggested by an increase in 45Ca efflux during the action of pancreatic secretagogues, is not a consequence of increased 45Ca uptake by the pancreas; but they indicate that an initial action of ACh could be to elicit an increase in Na+ influx and that Na+ ions are likely to be involved in the action of ACh on pancreatic acinar cells.

Rat pancreas adenylate cyclase. VI. Role of the enzyme in secretin stimulated enzyme secretion

1. The responsiveness of adenylate cyclase and enzyme secretin for secretin and the C-terminal octapeptide of pancreozymin has been investigated in particulate fractions of the pancreas of five different species. 2. The adenylate cyclase is sensitive to the C-terminal octapeptide of pancreozymin in all species investigated. 3. The enzyme is much more sensitive to secretin in rat and cat than in mouse and rabbit, whereas with guinea pig intermediate values are obtained. 4. The enzyme secretion is stimulated by secretin in pancreatic fragments of rat and cat, but not in those of mouse and rabbit. 5. These results suggest that in species where secretin stimulated enzyme secretion, it does so by stimulating the adenylate cyclase system.