Calcium release from separate receptor-specific intracellular stores induced by histamine and ATP in a hamster cell line

Arachidonic acid mediates non-capacitative calcium entry evoked by CB1-cannabinoid receptor activation in DDT1 MF-2 smooth muscle cells

Cannabinoid CB1-receptor stimulation in DDT1 MF-2 smooth muscle cells induces a rise in [Ca2+]i, which is dependent on extracellular Ca2+ and modulated by thapsigargin-sensitive stores, suggesting capacitative Ca2+ entry (CCE), and by MAP kinase. Non-capacitative Ca2+ entry (NCCE) stimulated by arachidonic acid (AA) partly mediates histamine H1-receptor-evoked increases in [Ca2+]i in DDT1 MF-2 cells. In the current study, both Ca2+ entry mechanisms and a possible link between MAP kinase activation and increasing [Ca2+]i were investigated. In the whole-cell patch clamp configuration, the CB-receptor agonist CP 55, 940 evoked a transient, Ca2+-dependent K+ current, which was not blocked by the inhibitors of CCE, 2-APB, and SKF 96365. AA, but not its metabolites, evoked a transient outward current and inhibited the response to CP 55,940 in a concentration-dependent manner. CP 55,940 induced a concentration-dependent release of AA, which was inhibited by the CB1 antagonist SR 141716. The non-selective Ca2+ channel blockers La3+ and Gd3+ inhibited the CP 55,940-induced current at concentrations that had no effect on thapsigargin-evoked CCE. La3+ also inhibited the AA-induced current. CP 55,940-induced AA release was abolished by Gd3+ and by phospholipase A2 inhibition using quinacrine; this compound also inhibited the outward current. The CP 55,940-induced AA release was strongly reduced by the MAP kinase inhibitor PD 98059. The data suggest that in DDT1 MF-2 cells, AA is an integral component of the CB1 receptor signaling pathway, upstream of NCCE and, via PLA2, downstream of MAP kinase.

The antiepileptic drug levetiracetam decreases the inositol 1,4,5-trisphosphate-dependent [Ca2+]I increase induced by ATP and bradykinin in PC12 cells

The present study explores the hypothesis that the new anti-epileptic drug levetiracetam (LEV) could interfere with the inositol 1,4,5-trisphosphate (IP(3))-dependent release of intracellular Ca(2+) initiated by G(q)-coupled receptor activation, a process that plays a role in triggering and maintaining seizures. We assessed the effect of LEV on the amplitude of [Ca(2+)](i) response to bradykinin (BK) and ATP in single Fura-2/acetoxymethyl ester-loaded PC12 rat pheochromocytoma cells, which express very high levels of LEV binding sites. LEV dose-dependently reduced the [Ca(2+)](i) increase, elicited either by 1 microM BK or by 100 microM ATP (IC(50), 0.39 +/- 0.01 microM for BK and 0.20 +/- 0.01 microM for ATP; Hill coefficients, 1.33 +/- 0.04 for BK and 1.38 +/- 0.06 for ATP). Interestingly, although the discharge of ryanodine stores by a process of calcium-induced calcium release also took place as part of the [Ca(2+)](i) response to BK, LEV inhibitory effect was mainly exerted on the IP(3)-dependent stores. In fact, the drug was still effective after the pharmacological blockade of ryanodine receptors. Furthermore, LEV did not affect Ca(2+) stored in the intracellular deposits since it did not reduce the amplitude of [Ca(2+)](i) response either to thapsigargin or to ionomycin. In conclusion, LEV inhibits Ca(2+) release from the IP(3)-sensitive stores without reducing Ca(2+) storage into these deposits. Because of the relevant implications of IP(3)-dependent Ca(2+) release in neuron excitability and epileptogenesis, this novel effect of LEV could provide a useful insight into the mechanisms underlying its antiepileptic properties.

Synergistic effects of adenosine A1 and P2Y receptor stimulation on calcium mobilization and PKC translocation in DDT1 MF-2 cells

1. The effect of adenosine analogues and of nucleotides, alone or in combination, on intracellular calcium, accumulation of inositol (1,4,5) trisphosphate (InsP3), and on activation of protein kinase C (PKC) was studied in DDT1 MF2 cells derived from a Syrian hamster myosarcoma. These cells were found to express mRNA for A1 and some as yet unidentified P2Y receptor(s). 2. Activation of either receptor type stimulated the production of InsP3 and raised intracellular calcium in DDT1 MF2 cells. Similarly, the A1 selective agonist N6-cyclopentyladenosine (CPA) increased PKC-dependent phosphorylation of the substrate MBP(4-14) and induced a PKC translocation to the plasma membrane as determined using [3H]-phorbol dibutyrate (PDBu) binding in DDT1 MF-2 cells. However, neither adenosine nor CPA induced a significant translocation of transiently transfected gamma-PKC-GFP from the cytosol to the cell membrane. In contrast to adenosine analogues, ATP and UTP also caused a rapid but transient translocation of gamma-PKC-GFP and activation of PKC. 3. Doses of the A1 agonist CPA and of ATP or UTP per se caused barely detectable increases in intracellular Ca2+ but when combined, they caused an almost maximal stimulation. Similarly, adenosine (0.6 microM) and UTP (or ATP, 2.5 microM), which per se caused no detectable translocation of either gamma- or epsilon-PKC-GFP, caused when combined a very clear-cut translocation of both PKC subforms, albeit with different time courses. These results show that simultaneous activation of P2Y and adenosine A1 receptors synergistically increases Ca2+ transients and translocation of PKC in DDT1 MF-2 cells. Since adenosine is rapidly formed by breakdown of extracellular ATP, such interactions may be biologically important.

Adenosine triphosphate activates mitogen-activated protein kinase in human granulosa-luteal cells

ATP has been shown to activate the phospholipase C/diacylglycerol/protein kinase C (PKC) pathway. However, little is known about the downstream signaling events. The present study was designed to examine the effect of ATP on activation of the mitogen-activated protein kinase (MAPK) signaling pathway and its physiological role in human granulosa-luteal cells. Western blot analysis, using a monoclonal antibody that detected the phosphorylated forms of extracellular signal-regulated kinase-1 and -2 (p42(mapk) and p44 (mapk), respectively), demonstrated that ATP activated MAPK in a dose- and time-dependent manner. Treatment of the cells with suramin (a P2 purinoceptor antagonist), neomycin (a phospholipase C inhibitor), staurosporin (a PKC inhibitor), or PD98059 (an MAPK/ERK kinase inhibitor) significantly attenuated the ATP-induced activation of MAPK. In contrast, ATP-induced MAPK activation was not significantly affected by pertussis toxin (a G(i) inhibitor). To examine the role of G(s) protein, the intracellular cAMP level was determined after treatment with ATP or hCG. No significant elevation of intracellular cAMP was noted after ATP treatment. To determine the role of MAPK in steroidogenesis, human granulosa-luteal cells were treated with ATP, hCG, or ATP plus hCG in the presence or absence of PD98059. RIA revealed that ATP alone did not significantly affect the basal progesterone concentration. However, hCG-induced progesterone production was reduced by ATP treatment. PD98059 reversed the inhibitory effect of ATP on hCG-induced progesterone production. To our knowledge, this is the first demonstration of ATP-induced activation of the MAPK signaling pathway in the human ovary. These results support the idea that the MAPK signaling pathway is involved in mediating ATP actions in the human ovary.

Signal transduction of cannabinoid CB1 receptors in a smooth muscle cell line

1. The effects of cannabinoid (CB) receptor stimulation on membrane currents in single cells from the Syrian hamster vas deferens cell line DDT1MF-2 were investigated using the whole cell patch clamp technique. 2. The CB receptor agonist CP55,940 evoked a concentration-dependent transient outward current. The selective CB1 receptor ligand SR141716 (1 microM), but not the selective CB2 receptor ligand SR144528 (1 microM), inhibited the outward current. Pertussis toxin (100 ng ml-1 for 20 h) completely abolished the outward current. 3. Western blotting with an antibody against the rat (r)CB1 receptor showed a band characteristic for the CB1 receptor around 63 kDa in DDT1MF-2 cells. 4. The reversal potential for the outward current measured using a voltage ramp protocol was -84 +/- 5 mV. The current was inhibited by the Ca2+-dependent K+ channel blockers iberiotoxin (10 nM) and charybdotoxin (10 nM). 5. Removal of Ca2+ from the bathing solution, or the addition of 0.1 mM Cd2+ completely abolished the outward current evoked by 10 microM CP55,940. 6. The sarcoplasmic Ca2+ pump inhibitor thapsigargin reduced the outward current evoked by 10 microM CP55,940 in a concentration-dependent manner. 7. The mitogen-activating protein kinase (MAP kinase) inhibitor PD98059, but not the phospholipase C inhibitor U73122, inhibited the outward current evoked by 10 microM CP55,940. 8. The adenylyl cyclase inhibitor SQ22,536 (100 microM) and 8-Br-cyclic AMP (10 microM) significantly reduced the outward current evoked by 10 microM CP55,940. 9. Our data suggest that CB1 receptor stimulation in DDT1MF-2 cells leads to activation of a large conductance Ca2+-dependent K+ channel through a Gi/Go protein-mediated rise in [Ca2+]i, for which both inhibition of adenylyl cyclase and activation of MAP kinase are required. In addition, the cannabinoid-induced increase in [Ca2+]i is likely to arise from capacitive Ca2+ entry.

Delta9-tetrahydrocannabinol activates [Ca2+]i increases partly sensitive to capacitative store refilling

Delta9-tetrahydrocannabinol induces [Ca2+]i increases in DDT1MF-2 smooth muscle cells. Both Ca2+ entry and release from intracellular Ca2+ stores were concentration dependently activated. The Ca2+ entry component contributed most to the increases in [Ca2+]i. Stimulation with delta9-tetrahydrocannabinol after functional downregulation of intracellular Ca2+ stores by longterm thapsigargin treatment, still induced a major Ca2+ entry and a minor Ca2+ release component. Thapsigargin sensitive influx and release were selectively inhibited by the cannabinoid CB1 receptor antagonist SR141716A. No effects on [Ca2+]i were obtained after stimulation with the CB2 receptor agonist palmitoylethanolamide. This study is the first demonstration of (1) Ca2+ release from thapsigargin sensitive intracellular stores and capacitative Ca2+ entry via CB1 receptor stimulation and of (2) an additional delta9-tetrahydrocannabinol induced thapsigargin insensitive component, mainly representing Ca2+ influx which is neither mediated by CB1 nor CB2 receptor stimulation.

A large-conductance K+ channel that is inhibited by the cytoskeleton in the smooth muscle cell line DDT1 MF-2

1. DDT1 MF-2 smooth muscle cells responded to the bath application of histamine or ATP with an increase in the cytosolic Ca2+ concentration ([Ca2+]c) and the whole-cell K+ current, IK(BA). 2. In cell-attached patches, histamine (100 microM) activated currents through a 200 pS K+ channel ('BKA' channel). In the absence of agonists, the BKA channel was activated by excision of the patch. Both histamine and patch excision increased the channel activity (NPo; where N is the number of channels per patch and Po is the open probability) by reducing the long closures between the bursts of openings. 3. In inside-out patches, the BKA channel had a conductance of 201 +/- 4 pS (symmetrical solutions of 150 mM KCl, 2 mM MgCl2 and 2 mM EGTA). Replacement of K+ in the patch electrode by Na+, Li+ or Cs+ prevented the flow of inward currents and reduced the outward K+ conductance to 113 pS. 4. NPo was insensitive to changes in [Ca2+]c from 10 nM to 1 microM. NPo was also not modified either by cytosolic Na+, ATP, GTP, GTP gamma S, dithiothreitol or TEA (10 mM) or by extracellular 4-aminopyridine (5 mM), glibenclamide (20 microM) or TEA (10 mM). The BKA channel was blocked by 5 mM intracellular BaCl2 or by 10 nM extracellular iberiotoxin. 5. In cell-attached patches, BKA channel activity could be induced by 1 microM cytochalasin B, applied either through the patch pipette or in the bath solution. The effects of cytochalasin B, of patch excision, or of histamine on NPo were not additive but saturative. 6. Whole DDT1 MF-2 cells had resting potentials of -10 mV, dominated by the chloride conductance; the resting potential changed to -82 mV when the K+ conductance was increased by cytochalasin B or by histamine. The effects of cytochalasin B and histamine on IK(BA) were not additive but saturative. 7. We discuss the hypothesis that the interaction between the cytoskeleton and the BKA channel promotes the long channel closures; depolymerization of F-actin may constitute a mechanism by which the agonists histamine or ATP disinhibit BKA channel activity.

The effect of the PKC inhibitor GF109203X on the release of Ca2+ from internal stores and Ca2+ entry in DDT1 MF-2 cells

1. The effects of the specific protein kinase C (PKC) inhibitor, GF109203X, were measured on the cytoplasmic Ca2+ concentration ([Ca2+]i), and on histamine H1 receptor- and thapsigargin-mediated increases in [Ca2+]i in DDT1 MF-2 smooth muscle cells. 2. After pretreatment of cells with GF109203X (5 microM, 45 min), the histamine (100 microM)-induced initial rise in [Ca2+]i, representing Ca2+ mobilization from internal stores, was inhibited (by 59 +/- 7%). The slowly declining phase of the histamine induced Ca2+ response, reflecting Ca2+ entry, was enhanced (83 +/- 26%) in the presence of the PKC inhibitor. 3. The histamine induced release of Ca2+ from internal stores, measured after blocking Ca2+ entry with LaCl3 was inhibited by GF109203X in a concentration-dependent manner (IC50: 3.1 +/- 1.1 microM). 4. Histamine-induced formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) was not changed in the presence of GF109203X. 5. The PKC activating phorbol ester, phorbol 12-myristate 13-acetate (PMA, 1 microM), strongly reduced histamine-induced Ins(1,4,5)P3 formation (58 +/- 16%). This effect was reversed by GF109203X (5 microM). Furthermore, PMA diminished histamine evoked Ca2+ release (50 +/- 6%) and blocked Ca2+ entry completely. 6. The rise in [Ca2+]i caused by blocking endoplasmic reticulum Ca2(+)-ATPase with thapsigargin (1 microM), was strongly reduced (57 +/- 3%) after pretreatment of cells with GF109203X. Downregulation of PKC by long-term pretreatment of cells with PMA (1 microM, 48 h) did not abolish this effect of GF109203X (48 +/- 3% inhibition). 7. In permeabilized DDT, MF-2 cells preloaded with 45Ca2+ in the presence of GF109203X, the amount of 45Ca2+ released by Ins(1,4,5)P3 (10 microM) was markedly reduced (42 +/- 9%). GF109203X did not release Ca2+ itself and did not impair Ins(1,4,5)P3 receptor function. 8. Uptake of 45Ca2+ by intact cells, representing Ca2+ entry, was enhanced by GF109203X (65 +/- 11%), by histamine (24 +/- 6%) and also by thapsigargin (121 +/- 10%). The GF109203X- and the thapsigargin-induced uptake of 45Ca2+ were not additive. 9. These data suggest that GF109203X reduces the filling-state of intracellular Ins(1,4,5)P3 sensitive Ca2+ stores by inhibiting the Ca2+ uptake into these stores, thereby promoting store-dependent (capacitive) Ca2+ entry.

Plasma membrane Ca2+ pumping plays a prominent role in adenosine A1 receptor mediated changes in [Ca2+]i in DDT1 MF-2 cells

Adenosine A1 receptor mediated formation of inosito 1,4,5-trisphosphate (Ins(1,4,5)P3) and accumulation of cytoplasmic Ca2+ ([Ca2+]i) were investigated in DDT1 MF-2 smooth muscle cells. A strong reduction of the adenosine and N6-cyclopentyladenosine (CPA) induced rise in [Ca2+]i was observed after blocking Ca2+ entry across the plasma membrane with LaCl3. This effect of LaCl3 was not observed in the absence of extracellular Ca2+; it was not caused by reduced Ins(1,4,5)P3 formation or changed Ins(1,4,5)P3 induced Ca2+ release, or influenced by temperature. The inhibition of the CPA induced increase in [Ca2+]i by LaCl3 was strongly counteracted in the presence of ortho-vanadate, an inhibitor of plasma membrane Ca2+ ATPase. Ortho-vanadate might also reduce protein tyrosine-phosphate phosphatase activity involved in tyrosine kinase mediated phospholipase C (PLC) activation. However, ortho-vanadate and tyrphostin 25, a tyrosine kinase inhibitor, did not affect the CPA induced formation of Ins(1,4,5)P3. Taken together, these results show a strong contribution of Ca2+ pumping across the plasma membrane to the regulation of [Ca2+]i mediated by adenosine A1 receptors. Na+/Ca2+ exchange only played a minor role in the initial phase of CPA induced Ca2+ metabolism as measured in low Na+ containing solution. The mechanism by which adenosine A1 receptors activate plasma membrane Ca2+ ATPase pumps does not include direct stimulation of pumps, but most likely involves an indirect pathway activated by a rapid increase in [Ca2+]i.

Neomycin inhibits histamine and thapsigargin mediated Ca2+ entry in DDT1 MF-2 cells independent of phospholipase C activation

The histamine H1 receptor mediated increase in cytoplasmic Ca2+ ([Ca2+]i) was measured in the presence of the known phospholipase C (PLC) inhibitor, neomycin. Neomycin (1 mM) inhibited the histamine (100 microM) induced rise in [Ca2+]i to the same extent as observed after blocking Ca2+ entry with LaCl3. Likewise, the increase in [Ca2+]i after re-addition of CaCl2 (2 mM) to extracellular Ca2+ deprived and histamine pretreated cells was strongly reduced by neomycin. However, neomycin did not inhibit the histamine induced formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) or the release of Ca2+ from internal stores. These results show that neomycin blocks histamine induced Ca2+ entry independent of phospholipase C activation. Inhibition of intracellular store Ca(2+)-ATPase by thapsigargin (1 microM), elicited an increase in [Ca2+]i due to a leakage from the stores, subsequently followed by store-dependent Ca2+ entry. Thapsigargin induced Ca2+ entry was also completely blocked by neomycin. These results indicate that neomycin inhibits histamine and thapsigargin induced Ca2+ entry. This inhibition is most likely exerted at the level of plasma membrane Ca2+ channels.

Different mechanisms of Ca2(+)-handling following nicotinic acetylcholine receptor stimulation, P2U-purinoceptor stimulation and K(+)-induced depolarization in C2C12 myotubes

1. The increase in intracellular CA2+ on nicotinic acetylcholine receptor (nAChR) stimulation, P2U-purinoceptor stimulation and K(+)-induced depolarization was investigated in mouse C2C12 myotubes by use of fura-2 fluorescence to characterize the intracellular organisation of Ca2+ releasing stores and Ca(2+)-entry process. 2. Stimulation of nAChRs with carbachol induced a rapid rise in internal Ca2+ (EC50 = 0.85 +/- 0.09 microM), followed by a sustained phase. The Ca2+ response evoked by carbachol (10 microM) was completely blocked by the nAChR antagonist, pancuronium (3 microM), but was not affected by the muscarinic antagonist, atropine (3 microM), or under conditions when Ca2+ entry was blocked by La3+ (50 microM) or diltiazem (10 microM). Addition of pancuronium (3 microM) during the sustained phase of the carbachol-evoked response did not affect this phase. 3. Stimulation of P2U purinoceptors with ATP (1 mM) induced a somewhat higher biphasic Ca2+ response (EC50 of the rapid phase: 8.72 +/- 0.08 microM) than with carbachol. Pretreatment with La3+ abolished the sustained phase of the ATP-induced Ca2+ response, while the response was unaffected by diltiazem or pancuronium. 4. Stimulation of the cells with high K+ (60 mM), producing the same depolarization as with carbachol (10 microM), induced a rapid monophasic Ca2+ response, insensitive to diltiazem, pancuronium or La3+. 5. Under Ca(2+)-free conditions, the sustained phase of the carbachol- and ATP-evoked responses were abolished. Pre-emptying of depolarization-sensitive stores by high K+ under Ca(2+)-free conditions did not affect the carbachol- or ATP-evoked Ca2+ mobilization and vice versa. Preincubation of the cells with ATP in the absence of extracellular Ca2+ decreased the amplitude of the subsequent carbachol-induced Ca2+ response to 11%, while in the reverse procedure the ATP-induced response was decreased to 65%. Ca2+ mobilization evoked by simultaneous addition of optimal concentrations of carbachol and ATP was increased compared to levels obtained with either agonist. 6. Preincubation with high K+ under normal conditions abolished the sustained phase of the ATP-evoked Ca2+ response. The carbachol response consisted only of the sustained phase in the presence of high K+. 7. The carbachol-induced Ca2+ response was completely abolished under low Na+/Ca(2+)-free conditions, while under low Na+ conditions only a sustained Ca2+ response was observed. The ATP- and K(+)-induced responses were changed compared to Ca(2+)-free conditions. 8. ATP (300 microM) induced the formation of Ins(1,4,5)P3 under Ca(2+)-free conditions with a comparable time course to that found for the rise in internal Ca2+. In contrast to ATP, carbachol (10 microM) did not affect Ins(1,4,5)P3 levels under Ca(2+)-free conditions. 9. It is concluded that the Ca2+ release from discrete stores of C2C12 myotubes is induced by stimulation of nAChRs, P2U-purinoceptors and by high K+. Only the P2U-purinoceptor and nAChR activated stores show considerable overlap in releasable Ca2+. Sustained Ca(2+)-entry is activated by stimulation of nAChRs and P2U-purinoceptors via separate ion-channels, which are different from the skeletal muscle nAChR-coupled cation-channel.

Neuropeptide Y2-type receptor-mediated activation of large-conductance Ca(2+)-sensitive K+ channels in a human neuroblastoma cell line

We have proposed recently that a pertussistoxin-insensitive Ca2+ influx stimulated by Y2-type receptor activation in CHP-234 human neuroblastoma cells underlies increases in intracellular free Ca2+ concentration ([Ca2+]i) induced by neuropeptide Y (NPY), which were strictly dependent on extracellular Ca2+ and independent of internal Ca2+ stores. We describe here the actions of NPY in these same cells, using the activity of Ca(2+)-activated K+ channels as an indicator of [Ca2+]i. The elementary slope conductance of these channels was 110 +/- 3 pS (with an asymmetrical K+ gradient), their activity was greatly increased by application of ionomycin, and they were reversibly blocked by 1 mM tetraethylammonium (TEA) and 100 nM charybdotoxin. Application of 100 nM NPY, in the presence but not in the absence of extracellular Ca2+, increased the channel open probability. ATP applied in the absence of external Ca2+ caused rises both in channel open probability and [Ca2+]i. Inositol trisphosphate production was stimulated by ATP but not by NPY. In outside-out patches, NPY increased channel open probability, indicating that NPY-associated Ca2+ influx does not require all the intracellular machinery present in intact cells. Channel activation by NPY was unaffected by the replacement of guanosine 5'-triphosphate (GTP) by (guanosine 5'-O-(2-thiodiphosphate) (GDP[ beta S]), a non-hydrolysable GDP analogue, in the pipette internal solution, consistent with the lack of involvement of G-proteins in the coupling of Y2-type receptors to Ca2+ influx in CHP-234 cells.

Histamine and ATP mobilize calcium by activation of H1 and P2u receptors in human lens epithelial cells

1. Cytoplasmic calcium concentration ([Ca2+]1) of single superfused tissue-cultured human lens epithelial cells (HLEC) was monitored using the fluorescent dye fura-2; the resting values were low and stable for several hours ([Ca2+]i = 96 +/- 20 nM; mean +/- S.D., n = 16). 2. Continuous superfusion with either ATP or histamine (0.1-10 microM) produced regular oscillations in [Ca2+]i that could be maintained for a short time in the absence of external calcium. 3. Short (30 s) pulses of histamine (0.1-100 microM) induced a transient rise in [Ca2+]i, the time course of which was insensitive to the removal of external calcium. The rate of rise and the amplitude of the response were very sensitive to agonist concentration, whereas the rate of recovery was relatively constant. 4. The responses to long pulses of histamine (> 100 s) consisted of an initial transient followed by a maintained [Ca2+]i which returned to baseline on removal of external calcium. 5. The kinetics of the responses to short and long pulses of ATP (0.1-100 microM) were very similar to those of histamine and showed a similar sensitivity to the presence or absence of external calcium. 6. The histamine responses were abolished by triprolidine (1 microM), but unaffected by ranitidine (1 microM), indicating that an Hi receptor subtype is activated by histamine. 7. The ATP responses were reversibly inhibited by suramin and the potency sequence for a range of agonists was ATP = UTP = ATP gamma S > ADP = GTP >> AMP = adenosine, indicating that activation of a P2u receptor subtype was responsible for the increase in [Ca2+]i. 8. Both histamine and ATP responses were abolished by thapsigargin (100 nM), confirming that calcium release from intracellular stores was responsible for the initial peak of the response. Application of either agonist during the plateau phase of the thapsigargin response often led to a marked, but reversible, decline in [Ca2+]i, indicating the presence of a further, normally hidden, calcium regulatory factor associated with the presence of the agonist. 9. Maximal concentrations of either histamine or ATP totally emptied the calcium store as a subsequent application of the other agonist (or thapsigargin), in the absence of external calcium, failed to induce a further increase in the calcium signal.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

The role of inositol 1,3,4,5-tetrakisphosphate in internal Ca2+ mobilization following histamine H1 receptor stimulation in DDT1 MF-2 cells

Receptor-activated formation of inositol phosphates results in mobilization of intracellular stored Ca2+ in a variety of cells, including vas deferens derived DDT1 MF-2 cells. Stimulation of the histamine H1 receptor on these cells caused a pronounced formation of inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) with respect to that of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3). In this study, the role of inositol phosphates, in particular Ins(1,3,4,5)P4 on the internal Ca(2+)-releasing process was investigated in permeabilized and histamine-stimulated intact DDT1 MF-2 cells. In permeabilized cells. Ins(1,4,5)P3 induced a concentration-dependent release of intracellular stored Ca2+. Addition of Ins(1,3,4,5)P4 did not cause Ca2+ mobilization, but its presence enhanced the amount of Ca2+ released by Ins(1,4,5)P3, thereby increasing the total Ca(2+)-releasing capacity. The effect of both inositol phosphates was inhibited by heparin, known to block Ins(1,4,5)P3-sensitive receptors. Thus, the additional amount of Ca2+ released by Ins(1,3,4,5)P4 is mediated, either via Ins(1,4,5)P3-sensitive Ca2+ channels, or via different heparin-sensitive Ca2+ channels activated by both Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Histamine H1 receptor stimulation in intact cells induced a Ca(2+)-dependent K+ current, representing Ca2+ release from internal stores if receptor-activated Ca2+ entry from the extracellular space was prevented under Ca(2+)-free conditions or in the presence of La3+. This transmembrane current was abolished in the presence of intracellularly applied heparin. Depletion of Ins(1,4,5)P3-sensitive Ca2+ stores by internal application of Ins(1,4,5)P3 reduced the histamine evoked K+ current to some extent if the contribution of external Ca2+ was excluded.(ABSTRACT TRUNCATED AT 250 WORDS)

Arachidonic acid is functioning as a second messenger in activating the Ca2+ entry process on H1-histaminoceptor stimulation in DDT1 MF-2 cells

This study was carried out to identify the cellular component activating the histamine-stimulated Ca2+ entry in vas-deferens-derived DDT1 MF-2 cells. H1-histaminoceptor stimulation resulted in a rise in intracellular Ca2+ concentration, caused by Ca2+ release from inositol phosphate-sensitive Ca2+ stores and Ca2+ entry from the extracellular space, accompanied by a transient Ca(2+)-activated outward K+ current. The histamine-evoked K+ current was still observed after preventing inositol phosphate-induced Ca2+ mobilization by intracellularly applied heparin. This current was activated by Ca2+ entry from the extracellular space, because it was abolished in the presence of the Ca(2+)-channel blocker La3+ or under Ca(2+)-free conditions. H1-histaminoceptor-activated Ca2+ entry was also observed in the presence of intracellularly applied Ins(1,4,5)P3 and Ins(1,3,4,5)P4, depleting their respective Ca2+ stores and pre-activating the inositol phosphate-regulated Ca2+ entry. Thus the ability of histamine to activate Ca2+ entry independently of Ca2+ mobilization and the formation of inositol phosphates suggests that another component is involved to initiate the Ca(2+)-entry process. It was observed that H1-histaminoceptor stimulation resulted in a pronounced release of arachidonic acid (AA) in DDT1 MF-2 cells. Exogenously applied AA induced a concentration-dependent increase in internal Ca2+ due to activation of Ca2+ entry from the extracellular space. Slow inactivation of the AA-sensitive Ca2+ channels is suggested by the slow decline in Ca2+ entry. In accord, the histamine-induced Ca2+ entry was not observed with AA-pre-activated Ca2+ channels. Inhibition of the lipoxygenase and cyclo-oxygenase pathway did not affect the AA-induced Ca2+ and the concomitant K+ current were decreased in the presence of AA and caused by Ca2+ mobilization from internal stores. Blocking this internal Ca2+ release by heparin, in the presence of AA, resulted in abolition of the histamine-induced Ca(2+)-regulated K+ current. These observations show that AA, released on H1-histaminoceptor stimulation in DDT1 MF-2 cells, is functioning as a second messenger to activate plasma-membrane Ca2+ channels promoting Ca2+ entry from the extracellular space.

Regulation of histamine- and UTP-induced increases in Ins(1,4,5)P3, Ins (1,3,4,5)P4 and Ca2+ by cyclic AMP in DDT1 MF-2 cells

1. Stimulation of P2U-purinoceptors with UTP or histamine H1-receptors with histamine gave rise to the formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) in DDT1 MF-2 smooth muscle cells. 2. Stimulation of P2U-purinoceptors or histamine H1-receptors caused an increase in cytoplasmic Ca2+, consisting of an initial peak, representing the release of Ca2+ from internal stores and a sustained phase representing Ca2+ influx. 3. The P2U-purinoceptor-mediated Ca(2+)-entry mechanism was more sensitive to UTP than Ca(2+)-mobilization (EC50: 3.3 microM +/- 0.4 microM vs 55.1 microM +/- 9.2 microM), in contrast to these processes activated by histamine H1-receptors (EC50: 5.8 microM +/- 0.6 microM vs 3.1 microM +/- 0.5 microM). 4. Pre-stimulation of cells with several adenosine 3':5'-cyclic monophosphate (cyclic AMP) elevating agents, reduced the histamine H1-receptor-mediated formation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Forskolin completely inhibited Ins(1,4,5)P3 formation (IC50: 158 +/- 24 nM) whereas Ins(1,3,4,5)P4 formation was inhibited by only 45% (IC50: 173 +/- 16 nM). The P2U-purinoceptor-mediated production of these inositol phosphates was not affected by cyclic AMP. 5. Forskolin and isoprenaline reduced the histamine-induced increase in cytoplasmic Ca2+, as measured in Ca2+ containing medium and in nominally Ca(2+)-free medium but did not change the UTP-induced increase in cytoplasmic Ca2+. 6. These results clearly demonstrate that cyclic AMP differentially regulates components of the histamine induced phospholipase C signal transduction pathway. Furthermore, cyclic AMP does not affect the phospholipase C pathway activated by stimulation of P2U-purinoceptors in DDT1 MF-2 cells.

The phospholipase C activating P2U purinoceptor also inhibits cyclicAMP formation in DDT1 MF-2 smooth muscle cells

The P2U purinoceptor mediated effect on cellular cAMP was investigated in DDT1 MF-2 smooth muscle cells. Stimulation of these receptors by ATP or UTP caused a pronounced decrease of about 50% in cellular cAMP levels in forskolin or isoprenaline pretreated cells. This action of the nucleotides was concentration dependent with an IC50 of 9.4 +/- 0.2 microM and 29.0 +/- 0.5 microM for UTP and ATP, respectively and was inhibited by the P2-purinoceptor antagonist suramin. The cAMP level appeared to be modified by intracellular Ca2+, represented by an initial decline in cAMP. Neither inactivation of protein kinase C by staurosporine nor elevated cytoplasmic Ca2+ concentrations interfered with the sustained decrease in cAMP levels induced by ATP or UTP, showing that this effect is not mediated via the phospholipase C pathway known to be activated after P2U purinoceptor stimulation in DDT1 MF-2 cells. Pertussis toxin inhibited the action of these nucleotides on the cellular cAMP level. It can be concluded that the P2U purinoceptor in DDT1 MF-2 cells is coupled to different G-proteins, activating phospholipase C and inhibiting adenylyl cyclase activity.

Activation of the phospholipase C pathway by ATP is mediated exclusively through nucleotide type P2-purinoceptors in C2C12 myotubes

1. The presence of a nucleotide receptor and a discrete ATP-sensitive receptor on C2C12 myotubes has been shown by electrophysiological experiments. In this study, the ATP-sensitive receptors of C2C12 myotubes were further characterized by measuring the formation of inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) and internal Ca2+. 2. The nucleotides ATP and UTP caused a concentration-dependent increase in Ins(1,4,5)P3 content with comparable time courses (EC50: ATP 33 +/- 2 microM, UTP 80 +/- 4 microM). ADP was less effective in increasing Ins(1,4,5)P3 content of the cells, while selective agonists for P1-, P2X- and P2Y-purinoceptors, adenosine, alpha,beta-methylene ATP and 2-methylthio ATP, appeared to be ineffective. 3. Under Ca(2+)-free conditions, the basal level of Ins(1,4,5)P3 was lower than in the presence of Ca2+, and the ATP- and UTP-induced formation of Ins(1,4,5)P3 was diminished. 4. The Ins(1,4,5)P3 formation induced by optimal ATP and UTP concentrations was not additive. ATP- and UTP-induced Ins(1,4,5)P3 formation showed cross-desensitization, whereas cross-desensitization was absent in responses elicited by one of the nucleotides and bradykinin. 5. The change in Ins(1,4,5)P3 content induced by effective nucleotides was inhibited by suramin. Schild plots for suramin inhibition of Ins(1,4,5)P3 formation in ATP- and UTP-stimulated myotubes showed slopes greater than unity (1.63 +/- 0.09 and 1.37 +/- 0.11, respectively). Apparent pA2 values were 4.50 +/- 0.48 and 4.41 +/- 0.63 for ATP and UTP, respectively. 6. Stimulation of the cells with ATP or UTP induced a rapid increase in intracellular Ca2+, followed by a slow decline to basal levels. Ca2+ responses reached lower maximal values and did not show the slow phase in the absence of extracellular Ca2+. The ATP and UTP-evoked increase in intracellular Ca2+ was not additive and showed cross-desensitization. Cross-desensitization was absent in myotubes stimulated with one of the nucleotides and bradykinin.7. These results show that ATP- and UTP-induced formation of Ins(1,4,5)P3, Ca2+ release from internal stores and Ca2+-influx from the extracellular space are mediated exclusively via the nucleotide type P2-purinoceptor in mouse C2C12 myotubes.

Characterization of P2-purinoceptor mediated cyclic AMP formation in mouse C2C12 myotubes

1. The formation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and inositol(1,4,5)trisphosphate (Ins(1,4,5)P3), induced by ATP and other nucleotides was investigated in mouse C2C12 myotubes. 2. ATP (100 microM) and ATP gamma S (100 microM) caused a sustained increase in cyclic AMP content of the cells, reaching a maximum after 10 min. The cyclic AMP content reached a maximum in the presence of 100 microM ATP, followed by a decline at higher ATP concentrations. ATP-induced cyclic AMP formation was inhibited by the P2-purinoceptor antagonist, suramin. 3. Myotubes hydrolysed ATP to ADP at a rate of 9.7 +/- 1.0 nmol mg-1 protein min-1. However, further hydrolysis of ADP to AMP and adenosine was negligible. 4. The cyclic AMP formation induced by ADP (10 microM-1 mM) showed similar characteristics to that induced by ATP, but a less pronounced decline was observed than with ATP. ADP-induced cyclic AMP formation was blocked by suramin, while cyclic AMP formation elicited by adenosine (10 microM-1 mM) was insensitive to suramin. 5. The ATP analogue, alpha,beta-methylene-ATP also induced a suramin-sensitive cyclic AMP formation, while 2-methylthio-ATP and the pyrimidine, UTP, did not affect cyclic AMP levels. 6. Stimulation of the myotubes with ATP or UTP (10 microM-1 mM) caused a concentration-dependent increase in the Ins(1,4,5)P3 content of the cells. ADP (100 microM-1 mM) was less effective. Adenosine did not affect Ins(1,4,5)P3 levels. 7. Incubation of the cells with UTP (30 microM- 1 mM) inhibited the ATP- and ADP-induced cyclic AMP formation, suggesting that stimulation of the 'nucleotide' type P2-receptor inhibits P2-purinoceptor mediated cyclic AMP formation in C2C12 myotubes. In contrast, UTP (30 microM-I mM) enhanced adenosine-induced cyclic AMP formation.8. Adenosine-sensitive P1-purinoceptors activating cyclic AMP formation were found in C2C12 myotubes.Further, a novel P2-purinoceptor is postulated, sensitive to ATP, ADP and ATPgammaS, which also activates the formation of cyclic AMP in C2C12 myotubes.

Action of histamine on single smooth muscle cells dispersed from the rabbit pulmonary artery

1. The effects of histamine, noradrenaline and caffeine were studied in freshly dispersed smooth muscle cells from the rabbit pulmonary artery using the perforated patch technique. 2. In potassium-containing solutions at a holding potential of -50 mV all three agents evoked net inward and outward currents. At 0 mV only an outward current was observed and this response was not produced in potassium-free conditions. 3. In K(+)-free solutions the reversal potential (Er) of the inward current was dependent upon the transmembrane anion gradient but not on the cation gradient. Thus all three agents produced an increase in potassium and chloride conductance. 4. Bath-applied histamine evoked repetitive potassium and chloride currents in many cells. Bath-applied noradrenaline and caffeine, to a lesser extent, also caused repetitive currents but the brief ionophoretic application of noradrenaline never produced oscillations in membrane current. 5. Histamine reduced or abolished the amplitude of spontaneous transient potassium and chloride currents. 6. Histamine-induced currents were blocked by caffeine (10 mM) but could be recorded in Ca(2+)-free bathing solutions. 7. It is concluded that in the rabbit pulmonary artery histamine evokes single and oscillatory membrane potassium and chloride currents which are mediated by the release of calcium from intracellular caffeine-sensitive stores.