Interactions between second messengers: cyclic AMP and phospholipase A2- and phospholipase C-metabolites

Release of adenosine from human neutrophils stimulated by platelet activating factor, leukotriene B(4) and opsonized zymosan

Isolated human polymorphonuclear leukocytes (PMNL) stimulated by platelet activating factor (PAF), leukotriene B₄ (LTB₄) or opsonized zymosan (OZ) released adenosine measured by thermospray high performance liquid chromatography mass spectrometry in the cell-free supernatants. Stimulation by PAF or LTB₄ resulted in a bellshaped concentration-effect curve; 5 × 10⁻⁷ M PAF, 10⁻⁸ M LTB₄ and 500 μg ml⁻¹ OZ induced peak adenosine release, thus cytotoxic concentrations did not elevate adenosine level in the supernatants. Therefore adenosine release was characteristic of viable cells. As calculated from concentration-effect curves, the rank order of potency for adenosine release was PAF > LTB > OZ. These resuits suggest that adenosine, when bound specifically to membrane receptor sites, may initiate signal transduction, and, in co-operation with other inflammatory mediators, may modulate phagocyte function, e.g. production of chemoluminescence (CL).

Biaryl diacid inhibitors of human s-PLA2 with anti-inflammatory activity

Twenty-four hydrophobic dicarboxylic acids are described which were evaluated as inhibitors of 14 kDa human platelet phospholipase A2 (HP-PLA2). In general, biarylacetic acid derivatives were found to be more active than biaryl acids or biarylpropanoic acids. More potent inhibitors were obtained when hydrophobic groups were attached to the biaryl acid nucleus using an olefin linkage as compared to an ether linkage. Compounds with larger hydrophobic groups were usually more potent inhibitors of HP-PLA2. Five of the compounds disclosed in this report (2, 4, 28, 36b and 36i) were found to possess significant anti-inflammatory activity in a phorbol ester induced mouse ear edema model of chronic inflammation.

Forskolin inhibits 5-hydroxytryptamine-induced phosphoinositide hydrolysis and Ca+2 Mobilisation in canine cultured aorta smooth muscle cells

The effect of forskolin on 5-hydroxytryptamine (5-HT)-induced inositol phosphate (IP) and Ca2+ mobilisation was investigated in canine cultured aorta smooth muscle cells (ASMCs). Pretreatment of ASMCs with forskolin attenuated 5-HT-induced IP accumulation and Ca2+ mobilisation in a time- and concentration-dependent manner. The half-maximal effects (pEC50) of forskolin to attenuate IP and Ca2+ responses to 5-HT occurred at concentrations of 6.28 and 6.64, respectively. Pretreatment of ASMCs with cholera toxin caused a similar inhibition on 5-HT-induced responses. Even after treatment with forskolin for 24 h, the 5-HT-induced responses were still inhibited. The inhibitory effect of forskolin resulted from both a depression of the maximal response and a shift to the right of the concentration-effect curves of 5-HT in these responses. The water-soluble forskolin analogue L-858051 [7-deacetyl-7beta-(gamma-N-methylpiperazino)-butyryl forskolin] significantly inhibited the 5-HT-stimulated IP accumulation. In contrast, the addition of 1,9-dideoxy forskolin, an inactive forskolin analogue, had little effect on IP response. Moreover, SQ-22536 [9-(tetrahydro-2-furanyl)-9-H-purin-6-amine], an inhibitor of adenylate cyclase, and both H-89 [N-(2-aminoethyl)-5-iosquinolinesulphonamide] and HA-1004 [N-(2-guanidinoethyl)-5-iosquinolinesulphonamide], inhibitors of cAMP-dependent protein kinase (PKA), attenuated the ability of forskolin to inhibit the 5-HT-stimulated accumulation of IP in ASMCs. These results indicate that activation of cAMP/PKA might inhibit the 5-HT-stimulated IP accumulation and consequently reduce Ca2+ mobilisation, or inhibit both responses independently.

PLA2 activity regulates Ca2+ storage-dependent cellular proliferation

The objective of this study is to determine the role of arachidonic acid (AA) in cell proliferation by inhibiting AA synthetic enzyme phospholipase A2 (PLA2) and to determine its involvement in the role of the second messenger intracellular calcium (Ca2+). Methods used to determine the effects on proliferation of cell cultures of primary meningioma and astrocytoma U373-MG included treatment with micromolar concentrations of PLA2 inhibitors 4-bromophenacylbromide and quinacrine. Effects of these drugs on proliferation were further investigated by the application of concentrations that inhibit growth by 50% while antagonizing these agents with AA replacement. Free cytosolic Ca2+ was measured with the use of fluorescent dye Fura-2 during PLA2 agonist/antagonist studies. These Ca2+ measurements were performed in the absence of extracellular Ca2+ to identify the contribution of intracellular Ca2+ sources. PLA2 inhibition resulted in decreased growth of cultured astrocytoma and meningioma cells in a dose-dependent manner in the micromolar range. This inhibitory effect was antagonized by the addition of AA. PLA2 inhibition caused an elevation of basal-cytosolic-free [Ca2+] while depleting internal Ca2+ stores. These Ca2+ changes were also antagonized by the addition of AA. In conclusion, these results demonstrate that AA, a PLA2 enzyme product, is involved in regulating the growth rate of these cell types. The PLA2 pathway also regulates the maintenance of the internal Ca2+ stores. Ca2+ is known to be a growth-related intracellular second messenger. These results suggest that the growth regulatory functions of AA are mediated by Ca2+-dependent mechanisms.

Maturational changes in rabbit renal cortical phospholipase A2 activity

Several studies have demonstrated that the neonatal kidney has a markedly attenuated response to parathyroid hormone (PTH); however, the cause for this blunted response is unknown. PTH stimulated cAMP production by 215 +/- 18% in neonatal proximal tubule suspensions compared to a 35 +/- 7% increase in adult proximal tubules. Thus, neonatal proximal tubules have functioning PTH receptors and a greater adenylate cyclase response than the adult segment. In adult proximal tubules, PTH stimulates phospholipase A2 (PLA2) activity and the inhibition of Na,K-ATPase activity by PTH is blocked by inhibitors of PLA2. We examined whether maturational changes in renal cortical activity could play a role in the attenuated response to PTH in the neonatal proximal tubule. Compared to adults, neonates had a lower renal cortical cytosolic PLA2 (cPLA2) activity, assessed as the release of 14C-arachidonic acid (AA) from labeled phosphatidyl choline (0.44 +/- 0.10 vs. 0.74 +/- 0.06% 14C-AA released/min/mg protein, P < 0.05) and microsomal PLA2 activity (0.32 +/- 0.03 vs. 1.20 +/- 0.13% 14C-AA released/min/mg protein, P < 0.001). The protein abundance of cPLA2 was not different between the neonatal and adult renal cortex as assessed by immunoblot assay. Thus, the difference in activities must be due to a difference in regulation of cPLA2. Annexin 1 (lipocortin 1) has been shown to inhibit PLA2 activity by binding to phospholipid substrate. Annexin 1 protein abundance was higher in neonatal than in adult renal cortex (P < 0.001). Thus, the lower activity of PLA2 in the neonatal tubules may be due in part to higher expression of annexin 1. PLA2 activation by PTH, -8-bromo-cAMP and PMA was assessed as 3H-AA release from prelabeled suspensions of neonatal and adult proximal tubules. PTH (10(-7) M), 8-bromo-cAMP (10(-4) M) and PMA (5 x 10(-8) M) significantly increased 3H-AA release from adult tubules (P < 0.05) but had no effect on neonatal tubules (P = NS). Thus, PTH, 8-bromo-cAMP and PMA stimulated PLA2 in adult but not neonatal proximal tubules. In conclusion, the maturational changes in renal cortical PLA2 activity may be a factor in the blunted response of neonatal proximal tubules to PTH.

Regulation of 5-hydroxytryptamine-induced calcium mobilization by cAMP-elevating agents in cultured canine tracheal smooth muscle cells

The effects of increases in cellular adenosine 3'5'-cyclic monophosphate (cAMP) on 5-hydroxytryptamine-(5-HT-) induced generation of inositol phosphates (IPs) and increases in intracellular Ca2+ ([Ca2+]i) were investigated using canine cultured tracheal smooth muscle cells (TSMCs). Cholera toxin and forskolin induced concentration- and time-dependent cAMP formation with half-maximal effects (-logEC50) produced at concentrations of 7.0 +/- 0.5 and 4.9 +/- 0.4 respectively. Pretreatment of TSMCs with either forskolin or dibutyryl cAMP inhibited 5-HT-stimulated responses. Even after treatment for 24h, these agents still inhibited the 5-HT-induced Ca2+ mobilization. The inhibitory effects of these agents produced both depression of the maximal response and a shift to the right of the concentration response curves of 5-HT. The water-soluble forskolin analogue L-858051 [7-deacetyl-7beta-(gamma-N-methylpiperazino)-butyryl forskolin] significantly inhibited the 5-HT-stimulated accumulation of IPs. In contrast, the addition of 1,9-dideoxy forskolin, an inactive forskolin analogue, had little effect on this response. Moreover, SQ-22536 [9-(tetrahydro-2-furanyl)-9-H-purin-6-amine], an inhibitor of adenylate cyclase, and both H-89 [N-(2-aminoethyl)-5-isoquinolinesulphonamide] and HA-1004[N-(2-guanidinoethyl)-5-isoquinolinesulphonamide], inhibitors of cAMP-dependent protein kinase (PKA), attenuated the ability of forskolin to inhibit the 5-HT-stimulated accumulation of IPs. These results suggest that activation of cAMP/PKA was involved in these inhibitory effects of forskolin. The AlF4--induced accumulation of IPs was inhibited by forskolin, suggesting that G protein(s) are directly activated by AlF4-- and uncoupled from phospholipase C by forskolin treatment. These results suggest that activation of cAMP/PKA might inhibit the 5-HT-stimulated phosphoinositide breakdown and consequently reduce the [Ca2+]i increase or inhibit both responses independently.

Age-associated decrease in inositol 1,4,5-trisphosphate and diacylglycerol generation by 1,25(OH)2-vitamin D3 in rat intestine

The hormonal form of vitamin D3, 1,25(OH)2-vitamin D3(1,25[OH]2D3), stimulates the breakdown of membrane phosphoinositides, generating inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) in a variety of cell systems. Several studies suggest that alterations in the receptor-mediated phosphoinositide cascade are involved in the pathophysiology of aging. Therefore, the formation of IP3 and DAG were determined under basal conditions and after stimulation with physiological concentrations of 1,25(OH)2D3 in duodenum from young (3-mo-old) and aged (24-mo-old) rats. The hormone induced a transient and biphasic formation of IP3 and DAG. Values obtained in young rats peaking at 15 s (51% and 42% above basal levels for IP3 and DAG, respectively) and at 3 min (90% and 74% above basal levels for IP3 and DAG, respectively) were significantly decreased in duodenum from senescent animals (IP3: +20% and DAG: +18% above basal level at 15 s; and IP3: +18% and DAG: +29% above basal level at 3 min). The 1,25(OH)2D3-induced generation of DAG in both young and aged duodenum was effectively inhibited in the presence of neomycin, a phospholipase C (PLC) inhibitor, and was dependent on extracellular Ca2+. After the biphasic response, the levels of DAG generated by the hormone (10 min stimulation) remained elevated; the elevation occurred in the absence of IP3 production; and the elevated levels were not abolished by neomycin, implying that phospholipids other than phosphoinositides are the source of DAG. This 1,25(OH)2D3-dependent late phase of DAG generation was also diminished in aged animals. The precise molecular basis and the physiological significance of decreased liberation of IP3 and DAG by 1,25(OH)2D3 in the aged rat duodenum remains to be determined.

The effect of chronic administration of amitriptyline on the effects of subsequent electroconvulsive treatment on responsiveness of alpha 1-and beta-adrenoceptors in the rat cortical slices

Both antidepressant drugs and repeated electroconvulsive shock (ECS) produce adaptive changes in cerebral neurotransmitter systems. As in the clinical practice ECS is used almost always after therapeutical failure of pharmacotherapy, we investigated presently how chronic administration of an antidepressant amitriptyline affects the action of subsequent multiple ECS in rats. Amitriptyline differed from ECS and from other classical antidepressant in producing no beta-downregulation and potentiating the inhibitory effect of protein kinase C activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), on responses of alpha 1-adrenoceptor system to noradrenaline. The action of ECS on alpha 1-adrenoceptor system remained essentially unaffected by previous amitriptyline administration. Its downregulatory effect on responses of beta-adrenoceptor system to noradrenaline, and particularly to isoproterenol, were attenuated by previous drug treatment. The present results suggest that previous chronic administration of antidepressant drugs may alter the effect of subsequent ECS.

Effect of cAMP elevating agents on carbachol-induced phosphoinositide hydrolysis and calcium mobilization in cultured canine tracheal smooth muscle cells

The effects of increases in intracellular adenosine 3',5'-cyclic monophosphate (cAMP) on carbachol-induced generation of inositol phosphates (IPs) and increases in intracellular Ca2+ ([Ca2+]i) were investigated in canine cultured tracheal smooth muscle cells (TSMCs). The cAMP elevating agents, cholera toxin (CTX) and forskolin, induced concentration- and time-dependent cAMP formation with half-maximal effects (-logEC50) at concentrations of 7.6 +/- 1.3 g/ml and 4.8 +/- 0.9 M, respectively. Forskolin caused a concentration-dependent inhibition of carbachol-induced increase in [Ca2+]i with half-maximal inhibition (-logEC50) at 5.2 +/- 0.7 M. Pretreatment of TSMCs with either CTX (10 micrograms/ml, 4 h), forskolin (10-100 microM, 30 min), or dibutyryl cAMP (1 mM, 30 min) inhibited carbachol-stimulated Ca2+ mobilization and IPs accumulation. The inhibitory effects of these agents produced both depression of the maximal response and a shift to the right of the concentration-response curve of carbachol without changing the EC50 values. After treatment with forskolin for 24 h, carbachol-induced IPs accumulation and Ca2+ mobilization were close to those of control group. SQ-22536 [9-(tetrahydro-2-furanyl)-9H-purin-6-amine, 10 microM], an inhibitor of adenylate cyclase, and HA-1004 [N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride, 50 microM], an inhibitor of cAMP-dependent protein kinase (PKA), attenuated the ability of forskolin to inhibit carbachol-induced IPs accumulation. Moreover, the inactive analogue of forskolin, 1,9-dideoxy forskolin, did not inhibit these responses evoked by carbachol, suggesting that activation of cAMP/PKA was involved in these inhibitory effects of forskolin. The KD and Bmax values of the muscarinic receptor (mAChR) for [3H]-N-methyl scopolamine binding were not significantly changed by forskolin treatment for 30 min and 24 h, suggesting that the inhibitory effect of forskolin is distal to the mAChR. The locus of this inhibition was further investigated by examining the effect of forskolin treatment on AIF4(-)-stimulated IPs accumulation in canine TSMCs. The AIF4(-)-induced response was inhibited by forskolin, supporting the notion that G protein(s) are directly activated by AIF4- and uncoupled to phospholipase C by forskolin treatment. We conclude that cAMP elevating agents inhibit carbachol-stimulated generation of IPs and Ca2+ mobilization in canine cultured TSMCs. Since generation of IPs and increases in [Ca2+]i are very early events in the activation of mAChRs, attenuation of these events by cAMP elevating agents might well contribute to the inhibitory effect of cAMP on tracheal smooth muscle formation.

PAF stimulates cAMP formation in P388D1 macrophage-like cells via the formation and secretion of prostaglandin E2 in an autocrine fashion

The role of cAMP in the formation of prostaglandin E2 (PGE2) was investigated in bacterial lipopolysaccharide (LPS)-primed P388D1 macrophage-like cells stimulated with platelet activating factor (PAF). cAMP levels and PGE2 secretion were correlated with stimulation by PAF or ionomycin. Indomethacin inhibited cAMP formation induced by PAF, but not PGE2-stimulated cAMP production. Inositol (1,4,5)-trisphosphate levels were strongly reduced by exogenous PGE2 and increased by H-89, an inhibitor of PKA. However, exogenous PGE2 did not affect PAF-stimulated PGE2 formation. These results suggest that cAMP levels in P388D1 cells are regulated by PGE2 in an autocrine fashion. Evidence is presented that this feedback mechanism regulates inositol (1,4,5)-triphosphate levels in these cells, while PGE2 formation is not affected.

Possible involvement of arachidonic acid and eicosanoids in metamorphic events in Hydractinia echinata (Coelenterata; Hydrozoa)

Upon induction of metamorphosis, larvae of the marine hydroid Hydractinia echinata release [14C]-arachidonic acid from previously labeled endogenous sources. The lipoxygenase inhibitors nordihydroguaiaretic acid and 5,8,11,14-eicosatetraynoic acid inhibited metamorphosis induced by Cs+ and 1,2-sn-dioctanoylglycerol, whereas the inhibitors of cyclooxygenase, indomethacin, and acetylsalicylic acid were ineffective, suggesting a role for lipoxygenase metabolites of arachidonic acid in induction of metamorphosis. Lipoxygenase products in Hydractinia echinata were isolated and identified by gas chromatography/mass spectrometry. 8- and 12-HETE were the most abundant metabolites. In cytosolic fractions from larvae activity of an arachidonic acid metabolizing enzyme, presumably a lipoxygenase, was found. The metabolic product was identified by 1H-NMR and chiral phase HPLC as 8(R)-HETE. Its production was strongly inhibited by NDGA, but not by indomethacin.

The role of cyclic-3',5'-adenosine monophosphate in prostaglandin-mediated inhibition of basic calcium phosphate crystal-induced mitogenesis and collagenase induction in cultured human fibroblasts

Synovial fluid basic calcium phosphate (BCP) crystals are associated with severe destructive arthropathies characterised by synovial proliferation and non-inflammatory degradation of intra-articular collagenous structures. BCP crystals stimulate fibroblast and chondrocyte mitogenesis, metalloprotease secretion and prostaglandin production. As a tissue protective effect of prostaglandins has been suggested, we recently studied the effect of PGE1 on BCP crystal-induced mitogenesis and collagenase mRNA accumulation in human fibroblasts (HF). We demonstrated a dose-dependent inhibition of BCP crystal-induced mitogenesis and collagenase mRNA accumulation. The mechanism of PGE1 inhibition of BCP crystal-induced mitogenesis and collagenase mRNA accumulation was therefore explored. PGE1 (100 ng/ml) increased HF intracellular cAMP 40-fold over control. BCP alone caused no such change but inhibited the PGE1-induced increase in intracellular cAMP by at least 60%. The PGE1-induced increase in intracellular cAMP was also blocked by the adenyl cyclase inhibitor, 2',5'-dideoxyadenosine (ddA) (10 microM) and ddA reversed the PGE1-mediated inhibition of BCP crystal-induced mitogenesis. Dibutyryl cAMP also inhibited BCP crystal-induced mitogenesis in a concentration-dependent manner. Agents which increase intracellular cAMP levels such as the adenyl cyclase activator forskolin and the phosphodiesterase, inhibitor 3-isobutyl-1-methylxanthine (IBMX) mimicked the effect of PGE1 on HF collagenase mRNA levels. PGE1 inhibits the biologic effects of BCP crystals through the cAMP signal transduction pathway and such inhibition may have significant therapeutic implications.

Forskolin and phorbol myristate acetate inhibit intracellular Ca2+ mobilization induced by amitriptyline and bradykinin in rat frontocortical neurons

Regulations of the increase in intracellular Ca2+ concentration ([Ca2+]i) and inositol 1,4,5-trisphosphate (IP3) production by increasing intracellular cyclic AMP (cAMP) levels or activating protein kinase C (PKC) were studied in rat frontocortical cultured neurons. Amitriptyline (AMI; 1 mM), a tricyclic antidepressant, and bradykinin (BK; 1 microM) stimulated IP3 production and caused transient [Ca2+]i increases. Pretreatment with forskolin (100 microM, 15 min) decreased the AMI- and BK-induced [Ca2+]i increases by 33 and 48%, respectively. However, this treatment had no effect on the AMI- and BK-induced IP3 productions. Dibutyryl-cAMP (2 mM, 15 min) also decreased the AMI- and BK-induced [Ca2+]i increases by 23 and 47%, respectively. H-8 (30 microM), an inhibitor of protein kinase A (PKA), attenuated the ability of forskolin to inhibit the AMI- and BK-induced [Ca2+]i increases, suggesting that the activation of cAMP/PKA was involved in these inhibitory effects of forskolin. On the other hand, forskolin treatment had no effect on 20 mM caffeine-, 10 microM glutamate-, or 50 mM K(+)-induced [Ca2+]i increases. Pretreatment with phorbol 12-myristate 13-acetate (PMA; 100 nM, 90 min) decreased both the AMI-induced [Ca2+]i increases and the IP3 production by 31 and 25%, respectively. H-7 (200 microM), an inhibitor of PKC, inhibited the ability of PMA to attenuate the [Ca2+]i increases. PMA also inhibited the BK-induced IP3 production and the [Ca2+]i increases.(ABSTRACT TRUNCATED AT 250 WORDS)

Second messengers in neuronal signaling

The past decade has witnessed an enormous increase in our knowledge of the variety and complexity of intracellular signaling events that follow receptor binding on the cell surface. This overview emphasizes the phosphoinositidase C-mediated dual messenger pathway in brain and in brain-derived cells, with special reference to possible significance for research on the dementias.

Effects of an inhibitor of adenylate cyclase on acrosome reaction induced by protein kinase C activators

To determine whether the adenylate cyclase and the protein kinase C pathways act independently to modulate the human sperm acrosome reaction, we studied the effects of 2'-O-methyladenosine (adenylate cyclase inhibitor) on acrosome reactions induced by protein kinase C activators (phorbol diesters and synthetic diacylglycerols) or an adenylate cyclase stimulator (forskolin:FR). Fifty aliquots of capacitated spermatozoa were divided into 5 groups (A, B, C, D, and E), each containing 10 samples. One control aliquot (CN) and five experimental aliquots (EX1, EX2, EX3, EX4, and EX5) were prepared from each sample. Phorbol 12-myristate, 13-acetate (PMA, 10 mumol/L), 4 beta-phorbol 12,13-didecanoate (PDD, 0.1 mumol/L), 1-oleoyl-2-acetyl-sn-glycerol (OAG, 50 mumol/L), 1,2-dioctanoyl-sn-glycerol (DOG, 50 mumol/L), or FR (10 mumol/L) was added to each of the experimental aliquots in groups A, B, C, D, and E, respectively. Increasing concentrations of 2'-O-methyladenosine were added to aliquots EX2, EX3, EX4, and EX5. After an incubation period of 25 min at 37 degrees C, it was found that the percentage of acrosome-reacted spermatozoa (%ARS) was significantly and dose-dependently decreased by 2'-O-methyladenosine concentrations of 1 mM or more. Within each group, the %ARS was significantly higher in EX1 aliquots than in CN aliquots. The reduction of acrosome reactions induced by protein kinase C activators by the adenylate cyclase inhibitor suggests that the protein kinase C pathway interacts with the adenylate cyclase pathway to modulate the human sperm acrosome reaction.

'Cross-talk' between phospholipase C and adenylyl cyclase involves regulation of G-protein levels in GH3 rat pituitary cells

We have investigated the possibility that adenylyl cyclase (AC) activity and membrane protein levels of the alpha-subunits of the stimulatory and inhibitory G-proteins of AC (Gs alpha and G(i)-2 alpha) in cultured prolactin-producing rat pituitary adenoma cells (GH3 cells) are modulated by phospholipase C (PLC)-generated second messengers. Pretreatment of cells (6-48 h) with ionomycin (1 microM) or 1-oleoyl-2-acetylglycerol (OAG; 1 microM) showed that ionomycin regulated Gs alpha levels in a time-dependent, biphasic manner; a two-fold increase followed a 40% initial reduction, while OAG lowered Gs alpha levels by more than 50% at all time-points. G(i)-2 alpha levels remained unchanged by both pretreatments. OAG, but not ionomycin, increased basal AC activity without increasing enzyme protein levels. Alterations in AC responsiveness to peptide hormones (e.g. thyroliberin and vasoactive intestinal peptide) correlated to membrane Gs protein alpha-subunit content. These results demonstrate the involvement of G-protein translation regulation as one mechanism of 'cross-talk' between the PLC- and AC-dependent signalling pathways.

Participation of prostaglandin E2 in dopamine D2 receptor-dependent potentiation of arachidonic acid release

Stimulation of dopamine D2 receptors potentiates Ca2+ ionophore- or ATP-induced arachidonic acid (AA) release in D2 receptor cDNA-transfected Chinese hamster ovary (CHO) cells [CHO(D2)]. By using a combination of chromatographic, biochemical, and radioimmunochemical techniques, we show here that prostaglandin (PG) E2 is a major product of AA metabolism in CHO(D2) cells stimulated with the Ca2+ ionophore A23187. Formation of this PG was markedly increased by the concomitant application of quinpirole, a D2 receptor agonist. In addition, PGE2 enhanced D2-dependent amplification of AA release, either when it was added (EC50 = 100 nM) or when it was produced endogenously, as shown by experiments carried out with the cyclooxygenase inhibitor indomethacin. The results suggest that PGE2 may participate in D2 receptor-mediated potentiation of AA release in CHO(D2) cells. They also support a functional role for this PG in the modulation of dopaminergic transmission in areas of the CNS, such as amygdala and hypothalamus, where high levels of both PGE2 and dopamine D2 receptors are found.

The effect of a cAMP analogue on Ca2+ ionophore-, antigen- and agonist-induced inositol phosphate release in rat basophilic leukaemia (RBL-1) cells

The effect of the stable cAMP analogue 8-Br-cAMP on leukotriene D4 (LTD4)-, 5'-N-ethyl-carboxamidoadenosine (NECA)-, antigen- and Ca2+ ionophore-induced inositol phosphate (IP) production was studied in RBL-1 cells. The cAMP analogue significantly inhibited LTD4- and antigen induced-IP production, thus supporting the hypothesis of a negative interaction between cAMP and phosphoinositide breakdown in blood cells. Ionophore-induced IP release, which was blocked by a 5-lipoxygenase inhibitor and by a LT-receptor antagonist, and therefore is probably mediated by LTs, was also inhibited by 8-Br-cAMP. NECA-induced IP release was not significantly inhibited by the cyclic nucleotide, thus showing that the effect described herein is not a general action on receptor-activated phospholipase C. 8-Br-cAMP did, however, inhibit GTP gamma S-induced IP release in permeabilised RBL-1 cells, thus suggesting that the inhibition does not occur at the receptor level but might be due, at least in part, to an effect on some receptor-coupled G proteins.