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ATP is an extracellular regulator in numerous physiological and pathologic processes. Recently, 7 different subtypes of purinoceptors were identified on either the basolateral or the luminal membrane of pancreatic duct cells. However, the in vivo regulatory role of ATP in pancreatic function has not been established. We investigated the possible regulatory role of endogenous ATP in pancreatic function by measuring ATP concentrations and ATPase activity in pancreatic juice obtained from anesthetized rats and guinea pigs and from human patients undergoing endoscopy. Juice was collected from the main pancreatic duct in rats and guinea pigs under basal conditions or during stimulation with CCK, bombesin, or secretin. In guinea pigs, CCK, bombesin, and secretin did not affect ATP output, although they did stimulate fluid secretion. ATPase activity in the juice was evaluated by measuring the rate of hydrolysis of added ATP. Consistent with the low ATP concentrations in rat pancreatic juice, we found high levels of ATPase activity in this species. This was confirmed by HPLC, which also showed the metabolites of ATP hydrolysis. Ecto-ATPase activity was demonstrated by enzyme histochemistry in both the pancreatic acini and ducts in rats, but it was not detectable in guinea pigs and humans. These differences in ATP levels and ATPase expression may indicate significant species differences in the purinergic regulation of pancreatic secretion.

Elevated levels of extracellular ATP have been observed in many tumors. We have localized NTPDase1/CD39, one of the principal extracellular nucleotide-hydrolyzing enzymes, in normal and cancerous human pancreas. NTPDase/E-ATPDase activity was demonstrated with an enzyme histochemical technique on cryosections of human pancreas. Acinar and duct epithelial cells were devoid of E-ATPDase activity in both normal and transformed tissue. Endothelial cells and smooth muscle around blood vessels and larger ducts showed strong activity. Nerves, connective tissue, and the beta-cells of the islets were also stained. In cancerous tissue this activity was diminished in the smooth muscle around the ducts and was absent from newly formed connective tissue. Immunostaining for CD39 supported these results but revealed the presence of inactive CD39 in the duct epithelial cells. We hypothesize that the significantly diminished activity of NTPDase1 in the tissues surrounding the ducts may be associated with the processes that lead to tumor formation in human pancreas.

Human pancreatic ducts secrete a bicarbonate-rich fluid but our knowledge of the secretory process is based mainly on studies of animal models. Our aim was to determine whether the HCO3- transport mechanisms in a human ductal cell line are similar to those previously identified in guinea-pig pancreatic ducts. Intracellular pH was measured by microfluorometry in Capan-1 cell monolayers grown on permeable filters and loaded with BCECF. Epithelial polarization was assessed by immunolocalization of occludin. Expression of mRNA for key electrolyte transporters and receptors was evaluated by RT-PCR. Capan-1 cells grown on permeable supports formed confluent, polarized monolayers with well developed tight junctions. The recovery of pHi from an acid load, induced by a short NH4+ pulse, was mediated by Na+-dependent transporters located exclusively at the basolateral membrane. One was independent of HCO3- and blocked by EIPA (probably NHE1) while the other was HCO3--dependent and blocked by H2DIDS (probably pNBC1). Changes in pHi following blockade of basolateral HCO3- accumulation confirmed that the cells achieve vectorial HCO3- secretion. Dose-dependent increases in HCO3- secretion were observed in response to stimulation of both secretin and VPAC receptors. ATP and UTP applied to the apical membrane stimulated HCO3- secretion but were inhibitory when applied to the basolateral membrane. HCO3- secretion in guinea-pig ducts and Capan-1 cell monolayers share many common features, suggesting that the latter is an excellent model for studies of human pancreatic HCO3- secretion. Copyright © 2006 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The inhibitory control of pancreatic ductal HCO3- 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 HCO3- secretion by modulating a Cl- -dependent HCO3- 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 HCO3- 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 HCO3- secretion by ∼40% and totally blocked secretin-stimulated secretion. In addition, bisindolylmaleimide I (1 nM to 1 µM), an inhibitor of PKC, relieved the inhibitory effect of SP on secretin-stimulated HCO3- secretion and also reversed the inhibitory effect of PDBu. Western blot analysis revealed that guinea pig pancreatic ducts express the α, βI--, δ-, ε-, η-, θ-, ζ- and μ-isoforms of PKC. In microperfused ducts, luminal H2DIDS (0.5 mM) caused intracellular pH to alkalinize and, like SP, inhibited basal and secretin- stimulated HCO3- secretion. SP did not inhibit secretion further when H2DIDS was present in the lumen, suggesting that SP and H2DIDS both inhibit the activity of an anion exchanger on the luminal membrane of the duct cell. [ABSTRACT FROM AUTHOR]