Purinergic signalling in the pancreas in health and disease

Role of vesicular nucleotide transporter VNUT (SLC17A9) in release of ATP from AR42J cells and mouse pancreatic acinar cells

ATP is released from cells in response to various stimuli. Our previous studies on pancreas indicated that pancreatic acini could be major stores of secreted ATP. In the present study, our aim was to establish the role of the vesicular nucleotide transporter (VNUT), SLC17A9, in storage and release of ATP. Freshly prepared acini from mice and AR42J rat acinar cells were used in this study. We illustrate that in AR42J cells, quinacrine (an ATP store marker) and Bodipy ATP (a fluorescent ATP analog) co-localized with VNUT-mCherry to vesicles/granules. Furthermore, in acini and AR42J cells, a marker of the zymogen granule membranes, Rab3D, and VNUT co-localized. Dexamethasone treatment of AR42J cells promoted formation of acinar structures, paralleled by increased amylase and VNUT expression, and increased ATP release in response to cholinergic stimulation. Mechanical stimulus (pressure) and cell swelling also induced ATP release, but this was not influenced by dexamethasone, most likely indicating different non-zymogen-related release mechanism. In conclusion, we propose that VNUT-dependent ATP release pathway is associated with agonist-induced secretion process and downstream purinergic signalling in pancreatic ducts.

ATP metabolism in skeletal muscle arterioles

The purpose of this study was to investigate the metabolism of Adenosine triphosphate (ATP) in skeletal muscle resistance arterioles and to determine whether this metabolism is altered during the rapid growth phase of the rat. We attempted to quantify ATP metabolism in gastrocnemius first‐order arterioles from 8‐, 10‐, and 12‐week‐old rats. We measured ATP metabolism using an ATPase/GTPase assay with whole vessel segments as well as using a real‐time adenosine biosensor following electric field stimulation. Our first method of measuring ATP metabolism allowed us to measure the amount of free phosphate produced with ATP as a substrate. When ecto‐nucleotidase activity was inhibited by ARL67156, pyridoxal phosphate‐6‐azophenly‐2′, 4′‐disulfonic acid (PPADS), or suramin prior to adding ATP, we found that the rate of phosphate production was significantly reduced by 27%, 21%, and 22%, respectively (P < 0.05). Our second method of measuring ATP metabolism allowed us to measure the amount of adenosine produced following electric field stimulation of the arteriole with and without nucleotidase inhibitors. Surprisingly, we found that adenosine overflow was not attenuated by nucleotidase inhibitors. We concluded that ecto‐phosphodieterase/phyrophophatase (E‐NPP), ecto‐diadenosine polyphosphatase (ApnA), NTPDase1 and 2, and E5NT may be present on the gastrocnemius 1A arteriole and do play a role in ATP metabolism. Between the ages of 8 weeks and 12 weeks, however, overall ATP metabolism may not change.

The purpose of this study was to investigate the metabolism of Adenosine triphosphate (ATP) in skeletal muscle resistance arterioles and whether it is altered during the rapid growth phase of the rat. We concluded that ecto‐phosphodieterase/phyrophophatase (E‐NPP), ecto‐diadenosine polyphosphatase (ApnA), NTPDase1 and 2, and E5NT may be present on the gastrocnemius 1A arteriole and do play a role in ATP metabolism. Between the ages of 8 weeks and 12 weeks, however, overall ATP metabolism may not change.

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.

Investigation of the functional expression of purine and pyrimidine receptors in porcine isolated pancreatic arteries

Receptors for purines and pyrimidines are expressed throughout the cardiovascular system. This study investigated their functional expression in porcine isolated pancreatic arteries. Pancreatic arteries (endothelium intact or denuded) were prepared for isometric tension recording and preconstricted with U46619, a thromboxane A(2) mimetic; adenosine-5'-diphosphate (ADP), uridine-5'-triphosphate (UTP) and MRS2768, a selective P2Y(2) agonist, were applied cumulatively, while adenosine-5'-triphosphate (ATP) and αβ-methylene-ATP (αβ-meATP) response curves were generated from single concentrations per tissue segment. Antagonists/enzyme inhibitors were applied prior to U46619 addition. ATP, αβ-meATP, UTP and MRS2768 induced vasoconstriction, with a potency order of αβ-meATP > MRS2768 > ATP ≥ UTP. Contractions to ATP and αβ-meATP were blocked by NF449, a selective P2X(1) receptor antagonist. The contraction induced by ATP, but not UTP, was followed by vasorelaxation. Endothelium removal and DUP 697, a cyclooxygenase-2 inhibitor, had no significant effect on contraction to ATP but attenuated that to UTP, indicating actions at distinct receptors. MRS2578, a selective P2Y(6) receptor antagonist, had no effect on contractions to UTP. ADP induced endothelium-dependent vasorelaxation which was inhibited by MRS2179, a selective P2Y(1) receptor antagonist, or SCH58261, a selective adenosine A(2A) receptor antagonist. The contractions to ATP and αβ-meATP were attributed to actions at P2X(1) receptors on the vascular smooth muscle, whereas it was shown for the first time that UTP induced an endothelium-dependent vasoconstriction which may involve P2Y(2) and/or P2Y(4) receptors. The relaxation induced by ADP is mediated by P2Y(1) and A(2A) adenosine receptors. Porcine pancreatic arteries appear to lack vasorelaxant P2Y(2) and P2Y(4) receptors.

Diminished adenosine A1 receptor expression in pancreatic α-cells may contribute to the pathology of type 1 diabetes

Prediabetic NOD mice exhibit hyperglucagonemia, possibly due to an intrinsic α-cell defect. Here, we show that the expression of a potential glucagon inhibitor, the adenosine A1 receptor (Adora1), is gradually diminished in α-cells of NOD mice, autoantibody-positive (AA(+)) and overtly type 1 diabetic (T1D) patients during the progression of disease. We demonstrated that islet inflammation was associated with loss of Adora1 expression through the alternative splicing of Adora1. Expression of the spliced variant (Adora1-Var) was upregulated in the pancreas of 12-week-old NOD versus age-matched NOD.B10 (non-diabetes-susceptible) control mice and was detected in the pancreas of AA(+) patients but not in control subjects or overtly diabetic patients, suggesting that inflammation drives the splicing of Adora1. We subsequently demonstrated that Adora1-Var expression was upregulated in the islets of NOD.B10 mice after exposure to inflammatory cytokines and in the pancreas of NOD.SCID mice after adoptive transfer of activated autologous splenocytes. Adora1-Var encodes a dominant-negative N-terminal truncated isoform of Adora1. The splicing of Adora1 and loss of Adora1 expression on α-cells may explain the hyperglucagonemia observed in prediabetic NOD mice and may contribute to the pathogenesis of human T1D and NOD disease.

Purinergic signalling and diabetes

The pancreas is an organ with a central role in nutrient breakdown, nutrient sensing and release of hormones regulating whole body nutrient homeostasis. In diabetes mellitus, the balance is broken-cells can be starving in the midst of plenty. There are indications that the incidence of diabetes type 1 and 2, and possibly pancreatogenic diabetes, is rising globally. Events leading to insulin secretion and action are complex, but there is emerging evidence that intracellular nucleotides and nucleotides are not only important as intracellular energy molecules but also as extracellular signalling molecules in purinergic signalling cascades. This signalling takes place at the level of the pancreas, where the close apposition of various cells-endocrine, exocrine, stromal and immune cells-contributes to the integrated function. Following an introduction to diabetes, the pancreas and purinergic signalling, we will focus on the role of purinergic signalling and its changes associated with diabetes in the pancreas and selected tissues/organ systems affected by hyperglycaemia and other stress molecules of diabetes. Since this is the first review of this kind, a comprehensive historical angle is taken, and common and divergent roles of receptors for nucleotides and nucleosides in different organ systems will be given. This integrated picture will aid our understanding of the challenges of the potential and currently used drugs targeted to specific organ/cells or disorders associated with diabetes.

The P2X7 receptor supports both life and death in fibrogenic pancreatic stellate cells

The pancreatic stellate cells (PSCs) have complex roles in pancreas, including tissue repair and fibrosis. PSCs surround ATP releasing exocrine cells, but little is known about purinergic receptors and their function in PSCs. Our aim was to resolve whether PSCs express the multifunctional P2X7 receptor and elucidate how it regulates PSC viability. The number of PSCs isolated from wild type (WT) mice was 50% higher than those from the Pfizer P2X7 receptor knock out (KO) mice. The P2X7 receptor protein and mRNA of all known isoforms were expressed in WT PSCs, while KO PSCs only expressed truncated versions of the receptor. In culture, the proliferation rate of the KO PSCs was significantly lower. Inclusion of apyrase reduced the proliferation rate in both WT and KO PSCs, indicating importance of endogenous ATP. Exogenous ATP had a two-sided effect. Proliferation of both WT and KO cells was stimulated with ATP in a concentration-dependent manner with a maximum effect at 100 µM. At high ATP concentration (5 mM), WT PSCs, but not the KO PSCs died. The intracellular Ca²⁺ signals and proliferation rate induced by micromolar ATP concentrations were inhibited by the allosteric P2X7 receptor inhibitor az10606120. The P2X7 receptor-pore inhibitor A438079 partially prevented cell death induced by millimolar ATP concentrations. This study shows that ATP and P2X7 receptors are important regulators of PSC proliferation and death, and therefore might be potential targets for treatments of pancreatic fibrosis and cancer.

Potentiation of sulfonylurea action by an EPAC-selective cAMP analog in INS-1 cells: comparison of tolbutamide and gliclazide and a potential role for EPAC activation of a 2-APB-sensitive Ca2+ influx

Tolbutamide and gliclazide block the K(ATP) channel K(ir)6.2/Sur1, causing membrane depolarization and stimulating insulin secretion in pancreatic beta cells. We examined the ability of the EPAC-selective cAMP analog 8-pCPT-2'-O-Me-cAMP-AM to potentiate the action of these drugs and the mechanism that might account for it. Insulin secretion stimulated by both 200 μM tolbutamide and 20 μM gliclazide, concentrations that had equivalent effects on membrane potential, was inhibited by thapsigargin (1 μM) or the L-type Ca(2+) channel blocker nicardipine (2 μM) and was potentiated by 8-pCPT-2'-O-Me-cAMP-AM at concentrations ≥2 μM in INS-1 cells. Ca(2+) transients stimulated by either tolbutamide or gliclazide were inhibited by thapsigargin or nicardipine and were significantly potentiated by 8-pCPT-2'-O-Me-cAMP-AM at 5 μM but not 1 μM. Both tolbutamide and gliclazide stimulated phospholipase C activity; however, only gliclazide did so independently of its activity at K(ATP) channels, and this activity was partially inhibited by pertussis toxin. 8-pCPT-2'-O-Me-cAMP-AM alone (5 μM) did not stimulate insulin secretion, but did increase intracellular Ca(2+) concentration significantly, and this activity was inhibited by 25 μM 2-aminoethoxydiphenylborate (2-APB) or the removal of extracellular Ca(2+). 8-pCPT-2'-O-Me-cAMP-AM potentiation of insulin secretion stimulated by tolbutamide was markedly inhibited by 2-APB (25 μM) and enhanced by the PKC inhibitor bisindolylmaleimide I (1 μM). Our data demonstrate that the actions of both tolbutamide and gliclazide are strongly potentiated by 8-pCPT-2'-O-Me-cAMP-AM, that gliclazide can stimulate phospholipase C activity via a partially pertussis toxin-sensitive mechanism, and that 8-pCPT-2'-O-Me-cAMP-AM potentiation of tolbutamide action may involve activation of a 2-APB-sensitive Ca(2+) influx.