Effect of vasoactive intestinal peptide, carbachol and other agonists on the membrane voltage of pancreatic duct cells

Pantoprazole and riluzole target H+/K+-ATPases and pH-sensitive K+ channels in pancreatic cancer cells

Pancreatic ductal adenocarcinoma (PDAC) remains the most lethal cancer type. PDAC is characterized by fibrotic, hypoxic, and presumably acidic tumor microenvironment (TME). Acidic TME is an important player in tumor development, progression, aggressiveness, and chemoresistance. The dysregulation of ductal ion transporters/channels might contribute to extracellular pH (pHe) acidification and PDAC progression. Our aim was to test whether H+/K+-ATPases and pH-sensitive K+ channels contribute to these processes and could be targeted by clinically approved drugs. We used human pancreatic cancer cells adapted to various pHe conditions and grown in monolayers and spheroids. First, we created cells expressing pHoran4 at the outer plasma membrane and showed that pantoprazole, the H+/K+-ATPase inhibitor, alkalinized pHe. Second, we used FluoVolt to monitor the membrane voltage (Vm) and showed that riluzole hyperpolarized Vm, most likely by opening of pH-sensitive K+ channels such as TREK-1. Third, we show that pantoprazole and riluzole inhibited cell proliferation and viability of monolayers and spheroids of cancer cells adapted to various pHe conditions. Most importantly, combination of the two drugs had significantly larger inhibitory effects on PDAC cell survival. We propose that co-targeting H+/K+-ATPases and pH-sensitive K+ channels by re-purposing of pantoprazole and riluzole could provide novel acidosis-targeted therapies of PDAC.

Acid-base transport in pancreas-new challenges

Along the gastrointestinal tract a number of epithelia contribute with acid or basic secretions in order to aid digestive processes. The stomach and pancreas are the most extreme examples of acid (H(+)) and base (HCO(-) 3) transporters, respectively. Nevertheless, they share the same challenges of transporting acid and bases across epithelia and effectively regulating their intracellular pH. In this review, we will make use of comparative physiology to enlighten the cellular mechanisms of pancreatic HCO(-) 3 and fluid secretion, which is still challenging physiologists. Some of the novel transporters to consider in pancreas are the proton pumps (H(+)-K(+)-ATPases), as well as the calcium-activated K(+) and Cl(-) channels, such as KCa3.1 and TMEM16A/ANO1. Local regulators, such as purinergic signaling, fine-tune, and coordinate pancreatic secretion. Lastly, we speculate whether dys-regulation of acid-base transport contributes to pancreatic diseases including cystic fibrosis, pancreatitis, and cancer.

The cystic fibrosis of exocrine pancreas

The cystic fibrosis transmembrane conductance regulator (CFTR) protein is highly expressed in the pancreatic duct epithelia and permits anions and water to enter the ductal lumen. This results in an increased volume of alkaline fluid allowing the highly concentrated proteins secreted by the acinar cells to remain in a soluble state. This work will expound on the pathophysiology and pathology caused by the malfunctioning CFTR protein with special reference to ion transport and acid-base abnormalities both in humans and animal models. We will also discuss the relationship between cystic fibrosis (CF) and pancreatitis, and outline present and potential therapeutic approaches in CF treatment relevant to the pancreas.

Ion transport in human pancreatic duct epithelium, Capan-1 cells, is regulated by secretin, VIP, acetylcholine, and purinergic receptors

OBJECTIVES

The objective of the study was to establish a solid model of polarized epithelium for human pancreatic ducts, where electrical parameters could be measured as indicators of ion transport. Further, we aimed to determine functional expression of several receptors, in particular, purinergic receptors, and determine their effects on ion transport.

METHODS

Human adenocarcinoma cell line Capan-1 cells were grown on permeable supports and set in Ussing chambers for electrophysiological recordings. Transepithelial voltage (Vte), resistance, and short-circuit currents (Isc) were measured in response to agonists.

RESULTS

Secretin, vasoactive intestinal peptide (VIP), acetylcholine, forskolin, ionomycin, adenosine 5'-triphosphate (ATP), uridine 5'-triphosphate (UTP), 3'-O-(4-benzoyl)benzoyl ATP, and adenosine induced lumen negative Vte and Isc. These changes were consistent with anion secretion, as verified in forskolin-stimulated preparations. Extracellular nucleotides, ATP, and UTP, applied from luminal and basolateral sides, caused largest responses: Vte increased up to -5 mV, Isc increased to 20 to 30 μA/cm, and resistance decreased by up to 200 Ω·cm.

CONCLUSIONS

Transepithelial transport in human pancreatic duct epithelium, Capan-1 cells, is regulated by secretin, VIP, acetylcholine, adenosine, and purinergic P2 receptors; and this human model has a good potential for studies of physiology and pathophysiology of pancreatic duct ion transport.

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.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

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.

Protein kinase C mediates the inhibitory effect of substance P on HCO3- secretion from guinea pig pancreatic ducts

The inhibitory control of pancreatic ductal HCO(3)(-) secretion may be physiologically important in terms of limiting the hydrostatic pressure developed within the ducts and in terms of switching off pancreatic secretion after a meal. Substance P (SP) inhibits secretin-stimulated HCO(3)(-) secretion by modulating a Cl(-)-dependent HCO(3)(-) efflux step at the apical membrane of the duct cell (Hegyi P, Gray MA, and Argent BE. Am J Physiol Cell Physiol 285: C268-C276, 2003). In the present study, we have shown that SP is present in periductal nerves within the guinea pig pancreas, that PKC mediates the effect of SP, and that SP inhibits an anion exchanger on the luminal membrane of the duct cell. Secretin (10 nM) stimulated HCO(3)(-) secretion by sealed, nonperfused, ducts about threefold, and this effect was totally inhibited by SP (20 nM). Phorbol 12,13-dibutyrate (PDBu; 100 nM), an activator of PKC, reduced basal HCO(3)(-) secretion by approximately 40% and totally blocked secretin-stimulated secretion. In addition, bisindolylmaleimide I (1 nM to 1 microM), an inhibitor of PKC, relieved the inhibitory effect of SP on secretin-stimulated HCO(3)(-) secretion and also reversed the inhibitory effect of PDBu. Western blot analysis revealed that guinea pig pancreatic ducts express the alpha-, beta(I)-, delta-, epsilon-, eta-, theta-, zeta-, and mu-isoforms of PKC. In microperfused ducts, luminal H(2)DIDS (0.5 mM) caused intracellular pH to alkalinize and, like SP, inhibited basal and secretin-stimulated HCO(3)(-) secretion. SP did not inhibit secretion further when H(2)DIDS was present in the lumen, suggesting that SP and H(2)DIDS both inhibit the activity of an anion exchanger on the luminal membrane of the duct cell.

Ca2+-activated Cl- channels can substitute for CFTR in stimulation of pancreatic duct bicarbonate secretion

This study addresses the mechanisms by which a defect in CFTR impairs pancreatic duct bicarbonate secretion in cystic fibrosis. We used control (PANC-1) and CFTR-deficient (CFPAC-1; DeltaF508 mutation) cell lines and measured HCO3- extrusion by the rate of recovery of intracellular pH after an alkaline load and recorded whole cell membrane currents using patch clamp techniques. 1) In PANC-1 cells, cAMP causes parallel activation of Cl- channels and of HCO3- extrusion by DIDS-sensitive and Na+-independent Cl-/HCO3- exchange, both effects being inhibited by Cl- channel blockers NPPB and glibenclamide. 2) In CFPAC-1 cells, cAMP fails to stimulate Cl-/HCO3- exchange and Cl- channels, except after promoting surface expression of DeltaF508-CFTR by glycerol treatment. Instead, raising intracellular Ca2+ concentration to 1 micromol/l or stimulating purinergic receptors with ATP (10 and 100 micromol/l) leads to parallel activation of Cl- channels and HCO3- extrusion. 3) K+ channel function is required for coupling cAMP- and Ca2+-dependent Cl- channel activation to effective stimulation of Cl-/HCO3- exchange in control and CF cells, respectively. It is concluded that stimulation of pancreatic duct bicarbonate secretion via Cl-/HCO3- exchange is directly correlated to activation of apical membrane Cl- channels. Reduced bicarbonate secretion in cystic fibrosis results from defective cAMP-activated Cl- channels. This defect is partially compensated for by an increased sensitivity of CF cells to purinergic stimulation and by alternative activation of Ca2+-dependent Cl- channels, mechanisms of interest with respect to possible treatment of cystic fibrosis and of related chronic pancreatic diseases.

How cystic fibrosis affects pancreatic ductal bicarbonate secretion

Pancreatic bicarbonate secretion is impaired in patients with cystic fibrosis. This article reviews recent advances in bicarbonate dependent transporters in pancreatic duct cells and discusses their regulation in cystic fibrosis.

Oral administration of a synthetic trypsin inhibitor increases pancreatic duct function in CCK-A receptor-deficient rats

The effects of oral administration of a synthetic trypsin inhibitor on bicarbonate secretion were examined in cholecystokinin A (CCK-A) receptor-deficient (OLETF) rats and compared with Wistar rats. Rats were fed chow containing 0.1% trypsin inhibitor for 7 days. Rats were prepared with cannulae draining bile and pancreatic juice separately and with duodenal and extrajugular vein cannulae after 3-day trypsin inhibitor ingestion. Then the animals were maintained in Bollman cages, and the experiments were conducted 4 days after surgery. After 1.5 h of basal secretion with bile-pancreatic juice return, bile-pancreatic juice was diverted for 2 h. The responses of bicarbonate secretion to bile-pancreatic juice diversion were significantly enhanced in rats treated with trypsin inhibitor compared with those given a control diet, whereas responses of fluid and protein secretion were not affected in OLETF rats. The response of protein secretion, but not those of fluid or bicarbonate secretion, was enhanced in Wistar rats by treatment with trypsin inhibitor. Carbonic anhydrase II gene expression was increased by 7-day treatment with trypsin inhibitor only in OLETF rats, and not in Wistar rats.

Purinoceptors evoke different electrophysiological responses in pancreatic ducts. P2Y inhibits K(+) conductance, and P2X stimulates cation conductance

In epithelia, extracellular nucleotides are often associated with regulation of ion transporters, especially Cl(-) channels. In this study, we investigated which purinoceptors are present in native pancreatic ducts and how they regulate ion transport. We applied whole-cell patch-clamp recordings, intracellular Ca(2+) and pH measurements, and reverse transcription-polymerase chain reaction (RT-PCR) analysis. The data show two types of purinoceptors and cellular responses. UTP and ATP produced large Ca(2+) transients, a decrease in intracellular pH, 8-10-mV depolarization of the membrane voltage, and a decrease in the whole-cell conductance. The membrane effects were due to closure of K(+) channels, as confirmed by dependence on extracellular K(+). UTP/ATP effects could be associated with P2Y(2) purinoceptors, and RT-PCR revealed mRNAs for P2Y(2) and P2Y(4) receptors. On the other hand, 2', 3'-O-4-benzoylbenzoyl-ATP induced Ca(2+) influx and approximately 20-mV depolarization of the membrane voltage with a concomitant increase in the whole-cell conductance. These effects were dependent on extracellular Na(+), not Cl(-), indicating opening of cation channels associated with P2X(7) purinoceptors. RT-PCR showed mRNAs for P2X(7) and P2X(4) receptors. In microperfused ducts, luminal (but not basolateral) ATP caused large depolarizations of membrane voltages recorded with microelectrodes, consistent with luminal localization of P2X(7) receptors. Thus, P2Y(2) (and possibly P2Y(4)) purinoceptors inhibit K(+) channels and may not support secretion in native ducts. P2X(7) (and possibly P2X(4)) receptors are associated with cation channels and may contribute to regulation of secretion.

beta-Adrenergic regulation of ion transport in pancreatic ducts: patch-clamp study of isolated rat pancreatic ducts

BACKGROUND & AIMS

In the intact pancreas, bicarbonate secretion is thought to be controlled by a number of regulators, including adrenergic agonists. The aim of this study was to investigate the effects of adrenergic agonists on pancreatic ducts, which are the site of bicarbonate secretion.

METHODS

Small intralobular ducts were isolated from rat pancreas and studied in vitro by the whole-cell patch clamp technique. Cell membrane voltages and currents were indicators of cellular ion transport. In some ducts, intracellular Ca2+ activity was measured by fluorescence optical methods.

RESULTS

Unstimulated duct cells had a membrane voltage (Vm) of about -50 mV. Isoproterenol had a concentration-dependent effect on Vm; at 10(-7) mol/L, it depolarized Vm by 20-25 mV and the cell conductance increased by 100 nanosiemens. These effects were a result of opening of luminal Cl- channels. Phenylephrine had much smaller effects. At comparable concentrations, it depolarized Vm by a few millivolts. Neither agonist had significant effects on intracellular Ca2+.

CONCLUSIONS

This study provides the first direct evidence that adrenergic stimulation, namely, that of beta-adrenoceptors, controls ion transport in pancreatic ducts. Similar to secretin, isoproterenol stimulation leads to opening of luminal Cl- channels, and HCO3- enters the lumen in exchange for Cl-.

Tachykinins in the gut. Part II. Roles in neural excitation, secretion and inflammation

The preprotachykinin-A gene-derived peptides substance (substance P; SP) and neurokinin (NK) A are expressed in intrinsic enteric neurons, which supply all layers of the gut, and extrinsic primary afferent nerve fibers, which innervate primarily the arterial vascular system. The actions of tachykinins on the digestive effector systems are mediated by three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Within the enteric nervous system, SP and NKA are likely to mediate, or comediate, slow synaptic transmission and to modulate neuronal excitability via stimulation of NK3 and NK1 receptors. In the intestinal mucosa, tachykinins cause net secretion of fluid and electrolytes, and it appears as if SP and NKA play a messenger role in intramural secretory reflex pathways. Secretory processes in the salivary glands and pancreas are likewise influenced by tachykinins. The gastrointestinal arterial system may be dilated or constricted by tachykinins, whereas constriction and an increase in the vascular permeability are the only effects seen in the venous system. Various gastrointestinal disorders are associated with distinct changes in the tachykinin system, and there is increasing evidence that tachykinins participate in the hypersecretory, vascular and immunological disturbances associated with infection and inflammatory bowel disease. In a therapeutic perspective, it would seem conceivable that tachykinin antagonists could be exploited as antidiarrheal, antiinflammatory and antinociceptive drugs.

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.

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.

The effect of secretagogues on ion conductances of in vitro perfused, isolated rabbit colonic crypts

Several secretagogues were used in this study, including those which enhance intracellular cyclic adenosine monophosphate (cAMP) production, as well as others which elevate intracellular Ca2+ activity and are known to increase Cl- secretion in the intact colon and in colonic carcinoma cell lines. They were examined with respect to their effects on electrophysiological properties in isolated rabbit distal colonic crypts. Crypts were dissected manually and perfused in vitro. Transepithelial voltage (Vte), transepithelial resistance (Rte), membrane voltage across the basolateral membrane (Vbl), and fractional basolateral membrane resistance (FRbl), were estimated. Basolateral prostaglandin E2 (PGE2, > or = 0.1 mumol/l), vasoactive intestinal peptide (VIP, 1 nmol/l) and adenosine (0.1 mmol/l) induced an initial depolarisation and a secondary partial repolarisation of Vbl. In the case of adenosine, the initial depolarization of Vbl was by 31 +/- 2 mV (n = 47). Rte fell significantly from 16.4 +/- 3.6 to 14.2 +/- 3.7 omega.cm2 (n = 6), and FRbl increased significantly from 0.11 +/- 0.02 to 0.51 +/- 0.10 (n = 6). In the second phase the repolarisation of Vbl amounted 11 +/- 2 mV (n = 47) and a steady-state (Vbl) of -51 +/- 2 mV (n = 47) was reached. Rte fell further and significantly to a steady-state value of 12.4 +/- 3.8 omega.cm2 (n = 6) and FRbl fell significantly to 0.42 +/- 0.13 (n = 6). In 30% of the experiments, a transient hyperpolarisation of Vbl by 8 +/- 2 mV (n = 14) was seen during wash out of adenosine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of ATP, carbachol and other agonists on intracellular calcium activity and membrane voltage of pancreatic ducts

The pancreatic duct has been regarded as a typical cAMP-regulated epithelium, and our knowledge about its Ca2+ homeostasis is limited. Hence, we studied the regulation of intracellular calcium, [Ca2+]i, in perfused rat pancreatic ducts using the Ca(2+)-sensitive probe fura-2. In some experiments we also measured the basolateral membrane voltage, Vbl, of individual cells. The resting basal [Ca2+]i was relatively high, corresponding to 263 +/- 28 nmol/l, and it decreased rapidly to 106 +/- 28 nmol/l after removal of Ca2+ from the bathing medium (n = 31). Carbachol increased [Ca2+]i in a concentration-dependent manner. At 10 mumol/l the fura-2 fluorescence ratio increased by 0.49 +/- 0.06 (n = 24), corresponding to an increase in [Ca2+]i by 111 +/- 15 nmol/l (n = 17). ATP, added to the basolateral side at 0.1 mmol/l and 1 mmol/l, increased the fluorescence ratio by 0.67 +/- 0.06 and 1.01 +/- 14 (n = 46; 12), corresponding to a [Ca2+]i increase of 136 +/- 22 nmol/l and 294 +/- 73 nmol/l respectively (n = 15; 10). Microelectrode measurements showed that ATP (0.1 mmol/l) hyperpolarized Vbl from -62 +/- 3 mV to -70 +/- 3 mV, an effect which was in some cases only transient (n = 7). This effect of ATP was different from that of carbachol, which depolarized Vbl. Applied together with secretin, ATP delayed the secretin-induced depolarization and prolonged the initial hyperpolarization of Vbl (n = 4). Several other putative agonists of pancreatic HCO3- secretion were also tested for their effects on [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)