Role of luminal ATP in regulating electrogenic Na(+) absorption in guinea pig distal colon

Extracellular ATP hydrolysis in Caco-2 human intestinal cell line

Extracellular nucleotides and nucleosides activate signaling pathways that play major roles in the physiology and pathophysiology of the gastrointestinal tract. Ectonucleotidases hydrolyze extracellular nucleotides and thus regulate ligand exposure to purinergic receptors. In this study, we investigated the expression, localization and activities of ectonucleotidases using Caco-2 cells, a model of human intestinal epithelial cells. In addition, by studying ATP release and the rates of extracellular ATP (eATP) hydrolysis, we analyzed the contribution of these processes to the regulation of eATP in these cells. Results show that Caco-2 cells regulate the metabolism of eATP and by-products by ecto-nucleoside triphosphate diphosphohydrolase-1 and -2, a neutral ecto-phosphatase and ecto-5'-nucleotidase. All these ectoenzymes were kinetically characterized using intact cells, and their presence confirmed by denatured and native gels, western blot and cytoimmunofluorescence techniques. In addition, regulation of eATP was studied by monitoring the dynamic balance between intracellular ATP release and ectoATPase activity. Following mechanical and hypotonic stimuli, Caco-2 cells triggered a strong but transient release of intracellular ATP, with almost no energy cost, leading to a steep increase of eATP concentration, which was later reduced by ectoATPase activity. A data-driven algorithm allowed quantifying and predicting the rates of ATP release and ATP consumption contributing to the dynamic accumulation of ATP at the cell surface.

NTPDase1 and -2 are expressed by distinct cellular compartments in the mouse colon and differentially impact colonic physiology and function after DSS colitis

ATP is both an important mediator of physiological gut functions such as motility and epithelial function, and a key danger signal that mediates cell death and tissue damage. The actions of extracellular ATP are regulated through the catalytic functions extracellular nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), -2, -3, and -8, which ultimately generate nucleosides. Ectonucleotidases have distinct cellular associations, but the specific locations and functional roles of individual NTPDases in the intestine are still poorly understood. Here, we tested the hypothesis that differential and cell-selective regulation of purine hydrolysis by NTPDase1 and -2 plays important roles in gut physiology and disease. We studied Entpd1 and Entpd2 null mice in health and following colitis driven by 2% dextran sulfate sodium (DSS) administration using functional readouts of gut motility, epithelial barrier function, and neuromuscular communication. NTPDase1 is expressed by immune cells, and the ablation of Entpd1 altered glial numbers in the myenteric plexus. NTPDase2 is expressed by enteric glia, and the ablation of Entpd2 altered myenteric neuron numbers. Mice lacking either NTPDase1 or -2 exhibited decreased inhibitory neuromuscular transmission and altered components of inhibitory junction potentials. Ablation of Entpd2 increased gut permeability following inflammation. In conclusion, the location- and context-dependent extracellular nucleotide phosphohydrolysis by NTPDase1 and -2 substantially impacts gut function in health and disease.NEW & NOTEWORTHY Purines are important mediators of gastrointestinal physiology and pathophysiology. Nucleoside triphosphate diphosphohydrolases (NTPDases) regulate extracellular purines, but the roles of specific NTPDases in gut functions are poorly understood. Here, we used Entpd1- and Entpd2-deficient mice to show that the differential and cell-selective regulation of purine hydrolysis by NTPDase1 and -2 plays important roles in barrier function, gut motility, and neuromuscular communication in health and disease.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

P2Y4-mediated regulation of Na+ absorption in the Reissner's membrane of the cochlea

The epithelial cells of Reissner's membrane (RM) are capable of transporting Na(+) out of endolymph via epithelial Na(+) channel (ENaC). However, much remains to be known as to mechanism of regulation of Na(+) absorption in RM. We investigated P2Y signaling as a possible regulatory mechanism of ENaC in gerbil RM using voltage-sensitive vibrating probe technique and immunohistochemistry. Results showed that UTP induced partial inhibition of the amiloride-sensitive short-circuit current but did not change short-circuit current when applied in the presence of amiloride. The inhibitory effect of UTP was not completely reversible in minutes. The response to UTP was inhibited by reactive blue-2 and 2',3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate but not by suramin or pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid, which indicates this P2Y receptor as the P2Y(4) subtype. The phospholipase C (PLC) inhibitors 1-[6[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione and 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine markedly inhibited the effect of UTP on ENaC. In contrast, neither modulation of protein kinase C nor application of 2-aminoehoxydiphenyl borate affected P2Y(4)-mediated inhibition of ENaC. Immunoreactive staining for P2Y(4) was observed in the RM, apical membrane of stria vascularis, spiral ligament, and organ of Corti, including outer hair cell, inner hair cell, outer pillar cell, Deiters' cell, and Hensen cell. These results suggest that the physiological role of P2Y(4) receptor in RM is likely to regulate Na(+) homeostasis in the endolymph. The acute inhibition of ENaC activity by activation of P2Y(4) receptor is possibly mediated by decrease of phosphatidylinositol 4,5-biphosphate in the plasma membrane through PLC activation.

Basal release of ATP: an autocrine-paracrine mechanism for cell regulation

Cells release adenosine triphosphate (ATP), which activates plasma membrane-localized P2X and P2Y receptors and thereby modulates cellular function in an autocrine or paracrine manner. Release of ATP and the subsequent activation of P2 receptors help establish the basal level of activation (sometimes termed "the set point") for signal transduction pathways and regulate a wide array of responses that include tissue blood flow, ion transport, cell volume regulation, neuronal signaling, and host-pathogen interactions. Basal release and autocrine or paracrine responses to ATP are multifunctional, evolutionarily conserved, and provide an economical means for the modulation of cell, tissue, and organismal biology.

ENaC, renal sodium excretion and extracellular ATP

Sodium balance determines the extracellular fluid volume and sets arterial blood pressure (BP). Chronically raised BP (hypertension) represents a major health risk in Western societies. The relationship between BP and renal sodium excretion (the pressure/natriuresis relationship) represents the key element in defining the BP homeostatic set point. The renin-angiotensin-aldosterone system (RAAS) makes major adjustments to the rates of renal sodium secretion, but this system works slowly over a period of hours to days. More rapid adjustments can be made by the sympathetic nervous system, although the kidney can function well without sympathetic nerves. Attention has now focussed on regulatory mechanisms within the kidney, including extracellular nucleotides and the P2 receptor system. Here, we discuss how extracellular ATP can control renal sodium excretion by altering the activity of epithelial sodium channels (ENaC) present in the apical membrane of principal cells. There remains considerable controversy over the molecular targets for released ATP, although the P2Y(2) receptor has received much attention. We review the available data and reflect on our own findings in which ATP-activated P2Y and P2X receptors make adjustments to ENaC activity and therefore sodium excretion.

Adrenergic activation of electrogenic K+ secretion in guinea pig distal colonic epithelium: desensitization via the Y2-neuropeptide receptor

Adrenergic activation of electrogenic K+ secretion in isolated mucosa from guinea pig distal colon was desensitized by peptide-YY (PYY). Addition of PYY or neuropeptide-Y (NPY) to the bathing solution of mucosae in Ussing chambers suppressed the short-circuit current (Isc) corresponding to electrogenic Cl- secretion, whether stimulated by epinephrine (epi), prostaglandin-E2 (PGE2), or carbachol (CCh). Neither peptide markedly inhibited the large transient component of synergistic secretion (PGE2 + CCh). Sustained Cl- secretory Isc was inhibited approximately 65% by PYY or NPY, with IC50s of 4.1 +/- 0.9 nM and 9.4 +/- 3.8 nM, respectively. This inhibition was eliminated by BIIE0246, an antagonist of the Y2-neuropeptide receptor (Y2-NpR), but not by Y1-NpR antagonist BVD10. Adrenergic sensitivity for activation of K+ secretion in the presence of Y2-NpR blockade by BIIE0246 was (EC50s) 2.9 +/- 1.2 nM for epi and 13.3 +/- 1.0 nM for norepinephrine, approximately fourfold greater than in the presence of PYY. Expression of mRNA for both Y1-NpR and Y2-NpR was indicated by RT-PCR of RNA from colonic mucosa, and protein expression was indicated by immunoblot. Immunoreactivity (ir) for Y1-NpR and Y2-NpR was distinct in basolateral membranes of columnar epithelial cells in the crypts of Lieberkühn as well as intercrypt surface epithelium. Adrenergic nerves in proximity with crypts were detected by ir for dopamine-beta-hydroxylase, and a portion of these nerves also contained NPY(ir). BIIE0246 addition increased secretagog-activated Isc, consistent with in vitro release of either PYY or NPY. Thus PYY and NPY were able to suppress Cl- secretory capacity and desensitize the adrenergic K+ secretory response, providing a direct inhibitory counterbalance against secretory activation.

Purinergic regulation of the epithelial Na+ channel

1. The epithelial Na(+) channel (ENaC) is a major conductive pathway that transports Na(+) across the apical membrane of the distal nephron, the respiratory tract, the distal colon and the ducts of exocrine glands. The ENaC is regulated by hormonal and humoral factors, including extracellular nucleotides that are available from the epithelial cells themselves. 2. Extracellular nucleotides, via the P2Y2 receptors (P2Y2Rs) at the basolateral and apical membrane of the epithelia, trigger signalling systems that inhibit the activity of the ENaC and activate Ca(2+) -dependent Cl(-) secretion. 3. Recent data from our laboratory suggest that stimulation of the P2Y2Rs at the basolateral membrane inhibits ENaC activity by a signalling mechanism that involves G beta gamma subunits freed from a pertussis toxin (PTX)-sensitive G-protein and phospholipase C (PLC) beta 4. A similar signalling mechanism is also partially responsible for inhibition of the ENaC during activation of apical P2Y2Rs. 4. Stimulation of apical P2Y2Rs also activates an additional signalling mechanism that inhibits the ENaC and involves the activated Galpha subunit of a PTX-insensitive G-protein and activation of an unidentified PLC. The effect of this PTX-insensitive system requires the activity of the basolateral Na(+)/K(+)/2Cl(-) cotransporter.

TRPV3, a thermosensitive channel is expressed in mouse distal colon epithelium

The thermo-transient receptor potential (thermoTRP) subfamily is composed of channels that are important in nociception and thermo-sensing. Here, we show a selective expression of TRPV3 channel in the distal colon throughout the gastrointestinal tract. Expression analyses clearly revealed that TRPV3 mRNA and proteins were expressed in the superficial epithelial cells of the distal colon, but not in those of the stomach, duodenum or proximal colon. In a subset of primary epithelial cells cultured from the distal colon, carvacrol, an agonist for TRPV3, elevated cytosolic Ca(2+)concentration in a concentration-dependent manner. This response was inhibited by ruthenium red, a TRPV channel antagonist. Organotypic culture supported that the carvacrol-responsive cells were present in superficial epithelial cells. Moreover, application of carvacrol evoked ATP release in primary colonic epithelial cells. We conclude that TRPV3 is present in absorptive cells in the distal colon and may be involved in a variety of cellular functions.

Distal colonic Na(+) absorption inhibited by luminal P2Y(2) receptors

Luminal P2 receptors are ubiquitously expressed in transporting epithelia. In steroid-sensitive epithelia (e.g., lung, distal nephron) epithelial Na(+) channel (ENaC)-mediated Na(+) absorption is inhibited via luminal P2 receptors. In distal mouse colon, we have identified that both, a luminal P2Y(2) and a luminal P2Y(4) receptor, stimulate K(+) secretion. In this study, we investigate the effect of luminal adenosine triphosphate/uridine triphosphate (ATP/UTP) on electrogenic Na(+) absorption in distal colonic mucosa of mice treated on a low Na(+) diet for more than 2 weeks. Transepithelial electrical parameters were recorded in an Ussing chamber. Baseline parameters: transepithelial voltage (V (te)): -13.7 +/- 1.9 mV (lumen negative), transepithelial resistance (R (te)): 24.1 +/- 1.8 Omega cm(2), equivalent short circuit current (I (sc)): -563.9 +/- 63.8 microA/cm(2) (n = 21). Amiloride completely inhibited I (sc) to -0.5 +/- 8.5 microA/cm(2). Luminal ATP induced a slowly on-setting and persistent inhibition of the amiloride-sensitive I (sc) by 160.7 +/- 29.7 microA/cm(2) (n = 12, NMRI mice). Luminal ATP and UTP were almost equipotent with IC(50) values of 10 microM and 3 microM respectively. In P2Y(2) knock-out (KO) mice, the effect of luminal UTP on amiloride-sensitve Na(+) absorption was absent. In contrast, in P2Y(4) KO mice the inhibitory effect of luminal UTP on Na(+) absorption remained present. Semiquantitative polymerase chain reaction did not indicate regulation of the P2Y receptors under low Na(+) diet, but it revealed a pronounced axial expression of both receptors with highest abundance in surface epithelia. Thus, luminal P2Y(2) and P2Y(4) receptors and ENaC channels co-localize in surface epithelium. Intriguingly, only the stimulation of the P2Y(2) receptor mediates inhibition of electrogenic Na(+) absorption.

The role of epithelial P2Y2 and P2Y4 receptors in the regulation of intestinal chloride secretion

UTP-induced chloride secretion by the intestinal mucosa mounted in Ussing chambers was assessed by measurement of the short-circuit current (I(sc)) in the presence of phloridzin in the case of jejunum or amiloride in the case of colon to eliminate any contribution of electrogenic Na(+) movement to the net ionic transport. Since we have previously demonstrated the absence of chloride-secretory response to apical UTP in the jejunum from P2Y(4)-null mice, in the present study we studied the response to basolateral UTP in the jejunum and to either apical or basolateral UTP in the colon, in both P2Y(2)- and P2Y(4)-deficient mice. In the jejunum, the chloride-secretory response to basolateral UTP was partially reduced in both P2Y(2)- (40%) and P2Y(4)- (60%) null mice. In the colon, both apical or basolateral UTP increased the I(sc). That response was abolished in a chloride-free medium. The colonic chloride-secretory response to either basolateral or apical UTP was abolished in P2Y(4)-deficient mice, but not significantly affected in P2Y(2)-deficient mice. The chloride-secretory response to forskolin was potentiated by prior basolateral addition of UTP and this potentiation was abolished in P2Y(4)-null mice. The jejunum of mice homozygous for the DeltaF508 mutation of cystic fibrosis transmembrane conductance regulator was responsive to UTP, but the magnitude of that response was smaller than in the wild-type littermates. In conclusion, the P2Y(4) receptor fully mediates the chloride-secretory response to UTP in both small and large intestines, except at the basolateral side of the jejunum, where both P2Y(2) and P2Y(4) receptors are involved.

Cholinergic inhibition of electrogenic sodium absorption in the guinea pig distal colon

Submucosal cholinergic and noncholinergic neurons in intestines have been shown to be involved in regulating epithelial transport functions, particularly stimulating Cl(-) secretion. This study investigates the role of submucosal cholinergic neurons in regulating electrogenic Na(+) absorption in distal colon. Amiloride-sensitive short-circuit current (I(sc)) and (22)Na(+) flux were measured in mucosal and mucosal-submucosal preparations mounted in Ussing chambers. In the mucosal preparation, carbachol (CCh) added to the serosal side inhibited amiloride-sensitive I(sc) and amiloride-sensitive (22)Na(+) absorption. The inhibitory effect of CCh was observed at approximately 0.1 microM, and maximum inhibition of approximately 70% was attained at approximately 30 microM (IC(50) = approximately 1 microM). CCh-induced inhibition of amiloride-sensitive I(sc) was almost totally abolished by 10 microM atropine. Treatment of the tissue with ionomycin markedly reduced amiloride-sensitive I(sc), but a subsequent addition of CCh further decreased it. Also, CCh still had an inhibitory effect, although significantly attenuated, after the tissue had been incubated with a low-Ca(2+) solution containing ionomycin and BAPTA-AM. Applying electrical field stimulation to submucosal neurons in the mucosal-submucosal preparation resulted in inhibition of amiloride-sensitive I(sc), approximately 33% of this inhibition being atropine sensitive. Physostigmine inhibited amiloride-sensitive I(sc), this effect being abolished by atropine. In conclusion, submucosal cholinergic and noncholinergic neurons were involved in inhibiting electrogenic Na(+) absorption in colon. This inhibition by cholinergic neurons was mediated by muscarinic receptor activation.