Regulation of changes in cytosolic Ca2+ and Na+ concentrations in rat submandibular gland acini exposed to carbachol and ATP

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.

Role of sodium in mitochondrial membrane depolarization induced by P2X7 receptor activation in submandibular glands

The effect of ATP on mitochondrial membrane depolarization in rat submandibular glands was investigated. Exposure of the cell suspension to high concentrations of ATP induced a sustained depolarization of mitochondrial membrane. This effect was blocked in the presence of magnesium and reproduced by low concentrations of 2',3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP), suggesting the implication of the P2X(7) purinergic receptor. This point was confirmed by comparison of the response to ATP by wild-type and P2X(7) knock-out (P2X(7)R(-/-)) mice. Mitochondria took up calcium after ATP stimulation but the depolarization of the mitochondrial membrane by ATP was not affected by the removal of calcium from the extracellular medium. It was nearly fully suppressed in the absence of sodium and partially blocked by the mitochondrial Na/Ca exchanger inhibitor 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP-37157). Both ATP and monensin increased the uptake of extracellular sodium (as shown by the depolarization of the plasma membrane) but the sodium ionophore did not affect the mitochondrial membrane potential. It is concluded that the activation of P2X(7) receptors depolarizes the mitochondrial membrane. The uptake of extracellular sodium is necessary but not sufficient to induce this response.

A novel protein from Brassica napus has a putative KID domain and responds to low temperature

To identify factors that interact with histone deacetylase (HDAC) in Brassica napus, a yeast two-hybrid library was screened using the Arabidopsis HDA19 as bait. A novel protein, bnKCP1, containing a putative kinase-inducible domain (KID) was found to interact with HDA19. Southern blot analysis indicated that the bnKCP1 gene belongs to a small gene family of at least three members. Northern blot analysis showed bnKCP1 to be strongly expressed in stems, flowers, roots, and immature siliques, but not in leaf blades of seedlings. The accumulation of bnKCP1 transcript in the leaf blades was induced significantly within 4 h of exposure of B. napus seedlings to cold stress, whereas treatment of leaf blades with inomycin, an ionophore of Ca2+, caused a rapid (30 min) but transient induction of bnKCP1 expression. In contrast to that observed in leaf blades, expression of bnKCP1 in the stems was repressed upon cold treatment. In vitro and in vivo protein-binding assays showed that bnKCP1 interacts with HDA19 via the KID domain, and that S188 is critical for bnKCP1-HDA19 interaction. BnKCP1 also exerted modest transactivation of the lacZ reporter gene in yeast through its N-terminal region. These assays suggest that bnKCP1 may function as a transcription factor, which regulates gene expression through interaction with HDA19.

Salivary gland P2 nucleotide receptors

The effects of ATP on salivary glands have been recognized since 1982. Functional and pharmacological studies of the P2 nucleotide receptors that mediate the effects of ATP and other extracellular nucleotides have been supported by the cloning of receptor cDNAs, by the expression of the receptor proteins, and by the identification in salivary gland cells of multiple P2 receptor subtypes. Currently, there is evidence obtained from pharmacological and molecular biology approaches for the expression in salivary gland of two P2X ligand-gated ion channels, P2Z/P2X7 and P2X4, and two P2Y G protein-coupled receptors, P2Y1 and P2Y2. Activation of each of these receptor subtypes increases intracellular Ca2+, a second messenger with a key role in the regulation of salivary gland secretion. Through Ca2+ regulation and other mechanisms, P2 receptors appear to regulate salivary cell volume, ion and protein secretion, and increased permeability to small molecules that may be involved in cytotoxicity. Some localization of the various salivary P2 receptor subtypes to specific cells and membrane subdomains has been reported, along with evidence for the co-expression of multiple P2 receptor subtypes within specific salivary acinar or duct cells. However, additional studies in vivo and with intact organ preparations are required to define clearly the roles the various P2 receptor subtypes play in salivary gland physiology and pathology. Opportunities for eventual utilization of these receptors as pharmacotherapeutic targets in diseases involving salivary gland dysfunction appear promising.

Bromoenol lactone enhances the permeabilization of rat submandibular acinar cells by P2X7 agonists

The permeabilizing effect of P2X(7) agonists was tested in rat submandibular acinar cells using the uptake of ethidium bromide as an index. The uptake of ethidium bromide by acini incubated at 37 degrees C in the presence of 1 mM ATP increased with time and reached after 5 min about 10% of maximal uptake measured in the presence of digitonin. The response to ATP was dose-dependent (half-maximal concentration around 40 microM) and it was decreased when the temperature was lowered to 25 degrees C. Benzoyl-ATP reproduced the response to ATP (half-maximal concentration around 10 microM). UTP or 2-methylthioATP had no effect. The permeabilization in response to ATP was blocked by oxidized ATP and by magnesium and inhibited by Coomassie blue. ATP increased the activity of a calcium-insensitive phospholipase A(2) (iPLA(2)). Bromoenol lactone (BEL) inhibited the iPLA(2) stimulated by ATP but potentiated the uptake of ethidium bromide in response to the purinergic agonist. From these results it is concluded that the activation of P2X(7) receptors permeabilizes rat submandibular acinar cells. The pore-forming activity of the receptor might be negatively regulated by the concomitant activation of the iPLA(2) by the receptor.

Regulation of the Na+-K+(NH4+)-2Cl- cotransporter of rat submandibular glands

A cellular suspension from rat submandibular glands was exposed to different concentrations of NH4Cl, and the variations of the intracellular concentration of calcium ([Ca2+]i) and the intracellular pH (pHi) were measured using fura-2 and 2',7'-bis-(2-carboxy-ethyl)-5(6)-carboxyfluorescein. More than 5 mmol/l NH4Cl significantly increased the [Ca2+]i without affecting the response to 100 micromol/l carbachol. When exposed to 1 and 5 mmol/l NH4Cl, the cells acidified immediately. At 30 mmol/l, NH4Cl first alkalinized the cells and the pHi subsequently dropped. This drop reflects the uptake of NH4+ ions that dissociate to NH3 and H+ in the cytosol. These protons are exchanged for extracellular sodium by the Na+/H+ exchanger because the presence of an inhibitor of the exchanger in the medium increased the acidification induced by 1 mmol/l NH4Cl. Ouabain partly blocked the uptake of NH4+. In the combined presence of ouabain and bumetanide (an inhibitor of the Na+-K+-2Cl- cotransporter), 1 mmol/l NH4Cl alkalinized the cells. The contribution of the Na/K ATPase and the Na+-K+-2Cl- cotransporter in the uptake of NH4+ was independent of the presence of calcium in the medium. Isoproterenol increased the uptake of NH4+ by the cotransporter. Conversely, 1 mmol/l extracellular ATP blocked the basal uptake of NH4+ by the cotransporter. This inhibition was reversed by extracellular magnesium or Coomassie Blue. It was mimicked by benzoyl-ATP but not by CTP, GTP, UTP, ADP, or ADPbetaS. ATP only slightly inhibited the increase of cyclic AMP (-22%) by isoproterenol but fully blocked the stimulation of the cotransporter by the beta-adrenergic agonist. ATP increased the release of 3H-arachidonic acid from prelabeled cells but SK&F 96365, an imidazole-based cytochrome P450 inhibitor, did not affect the inhibition by ATP. It is concluded that the activation of a purinoceptor inhibits the basal and the cyclic AMP-stimulated activity of the Na+-K+-2Cl- cotransporter.

Study on the activation of phospholipases A2 by purinergic agonists in rat submandibular ductal cells

Extracellular ATP and benzoyl-ATP (Bz-ATP) increased the release of [3H]arachidonic acid ([3H]AA) from prelabeled rat submandibular gland (RSMG) ductal cells respectively two- and threefold. Both agonists also increased the release of [3H]AA from acini but at a lower level (+50% and +100% respectively). Carbachol had no significant effect on either cellular population. In ductal cells phorbol myristate acetate, an activator of protein kinase C, slightly increased the basal release of [3H]AA but did not affect the release of [3H]AA in response to ATP. Staurosporine, an inhibitor of protein kinases, inhibited the response to the purines. The removal of calcium from the extracellular medium decreased the response to ATP and Bz-ATP. Only barium could partly substitute for calcium to restore the purinergic response. Zinc inhibited the release of [3H]AA. Permeabilization of the cells with streptolysin O (SLO) activated the calcium-independent phospholipase A2 activity (iPLA2). The iPLA2, not the calcium-dependent PLA2 (cPLA2), released [3H]oleic acid ([3H]OA) from RSMG ductal cells. It is concluded that RSMG ducts have a higher PLA2 activity when compared to acini. This activity is accounted for by iPLA2 and cPLA2. Both enzymes are activated by P2X agonists by a staurosporine-sensitive mechanism. Cells permeabilized with SLO or membranes from Escherichia coli as a substrate are not good models to study the regulation of these enzymes. In intact RSMG ductal cells the two activities can be distinguished by rather specific inhibitors, by different ionic conditions and also by the fatty acid used to label the cells.

Activation by P2X7 agonists of two phospholipases A2 (PLA2) in ductal cells of rat submandibular gland. Coupling of the calcium-independent PLA2 with kallikrein secretion

Isolated ductal cells of rat submandibular gland phospholipid pools were labeled with [3H]arachidonic acid (AA). The tracer was incorporated preferentially to phosphatidylcholine (46% of the lipidic fraction). Extracellular ATP induced the release of [3H]AA to the extracellular medium in a time- and dose-dependent manner (EC50 = 220 microM). Among other agents tested, only 2', 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (Bz-ATP) was able to mimic the effect of ATP (EC50 = 15 microM), without activation of phospholipase C. The purinergic antagonists oxidized ATP, suramin, and Coomassie Blue partly inhibited the response to 1 mM ATP and 100 microM Bz-ATP; the response was also blocked by the addition of Mg2+ or Ni2+. Expression of P2X7 receptor mRNA in these cells was confirmed by reverse transcription-polymerase chain reaction. In the presence of extracellular calcium, the phospholipase A2 inhibitor 2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid (a nonspecific inhibitor), arachidonyl trifluoromethylketone (AACOCF3, an inhibitor of the calcium-dependent cytosolic PLA2 (cPLA2)), and bromoenol lactone (an inhibitor of the calcium-independent PLA2 (iPLA2)) inhibited the release of [3H]AA induced by ATP and Bz-ATP. In the absence of extracellular calcium, the release of [3H]AA in response to the purinergic agonists was still observed; this response was not affected by AACOCF3 and completely blocked by bromoenol lactone. ATP and Bz-ATP stimulated a calcium-independent secretion of kallikrein, which could be blocked by BEL but which was enhanced by AACOCF3. It is concluded that the P2X7 receptor in ductal cells is coupled to kallikrein secretion through a calcium-dependent cPLA2 and a calcium-independent iPLA2.

Differential sensitivity to nickel and SK&F96365 of second messenger-operated and receptor-operated calcium channels in rat submandibular ductal cells

The intracellular concentration of calcium ([Ca2+]i) of rat submandibular ductal cells was measured with the intracellular fluorescent dye Fura-2. Carbachol (100 microM) and ATP (1 mM) both increased the [Ca2+]i. The late response to ATP was blocked by 0.5 mM Ni2+. This concentration of Ni2+ also blocked the increase of the [Ca2+]i and the uptake of manganese and calcium in response to 2'- and 3'-O-(4-benzoylbenzoyl) adenosine 5'-triphosphate (BzATP, 100 microM), a specific agonist of P2X receptors from salivary glands. The increase of the [Ca2+]i in response to 2-methylthioadenosine 5'-triphosphate (2-MeSATP, 100 microM) a specific P2Y agonist in salivary glands or to a muscarinic agonist (carbachol) was not affected by 0.5 mM Ni2+. Only higher concentrations of Ni2+ (in the millimolar range) inhibited the uptake of extracellular calcium in response to carbachol. SK&F96365, a blocker of store-operated calcium channels, inhibited the uptake of extracellular calcium in response to carbachol without affecting the response to BzATP. It is concluded that at low concentrations (below 0.5 mM), Ni2+ inhibits the non-specific cation channel coupled to P2X receptors. The uptake of extracellular calcium by store-operated calcium channels is inhibited by higher concentrations of Ni2+ and by SK&F96365.