Signaling mechanisms underlying muscarinic receptor-mediated increase in contraction rate in cultured heart cells

We have investigated the mechanisms by which stimulation of cardiac muscarinic receptors result in paradoxical stimulatory effects on cardiac function, using cultured neonatal rat ventricular myocytes as a model system. Application of low concentrations of carbachol (CCh) (EC50 = 35 nM) produced an atropine-sensitive decrease in spontaneous contraction rate, while, in cells pretreated with pertussis toxin, higher concentrations of CCh (EC50 = 26 microM) elicited an atropine-sensitive increase in contraction rate. Oxotremorine, an m2 muscarinic acetylcholine receptor (mAChR) agonist, mimicked the negative but not the positive chronotropic response to CCh. Reverse transcription followed by polymerase chain reaction carried out on mRNA obtained from single cells indicated that ventricular myocytes express mRNA for the m1, m2, and, possibly, m4 mAChRs. The presence of m1 and m2 mAChR protein on the surface membranes of the cultured ventricular myocytes was confirmed by immunofluorescence. The CCh-induced positive chronotropic response was significantly inhibited by fluorescein-tagged antisense oligonucleotides directed against the m1, but not the m2 and m4, mAChR subtypes. The response was also inhibited by antisense oligonucleotides against Gqalpha protein. Finally, inhibition of CCh-induced phosphoinositide hydrolysis with 500 microM neomycin or 5 microM U73122 completely abolished the CCh-induced positive chronotropic response. These results are consistent with the stimulatory effects of mAChR activation on the rate of contractions in cultured ventricular myocytes being mediated through the m1 mAChR coupled through Gq to phospholipase C-induced phosphoinositide hydrolysis.

A1 adenosine receptor-mediated Ins(1,4,5)P3 generation in allergic rabbit airway smooth muscle

The signal transduction pathway for A1 adenosine receptor in airway smooth muscle from allergic rabbits was studied by investigating the effect of the selective A1 adenosine-receptor agonist N6-cyclopentyladenosine (CPA) on tissue levels of inositol 1,4, 5-trisphosphate [Ins(1,4,5)P3] measured by protein binding assay. CPA caused a rapid, transient, and concentration-dependent elevation of Ins(1,4,5)P3 in airways from allergic rabbits. The agonist also produced a concentration-dependent contraction of the airway preparations from these animals. Both the Ins(1,4,5)P3 and contractile responses generated by CPA were attenuated by the phospholipase C (PLC) inhibitor U-73122, indicating the coupling of these responses to PLC. The CPA-induced Ins(1,4,5)P3 production observed in the allergic rabbit tissues was also inhibited by the adenosine-receptor antagonist 8-( p-sulfophenyl)-theophylline, suggesting that the effect was mediated by A1 adenosine receptors. On the other hand, the A2 adenosine-receptor agonist CGS-21680 was ineffective in altering the tissue concentration of Ins(1,4,5)P3, indicating that A2 adenosine receptors may not be involved in the activation of PLC in the allergic rabbit airway smooth muscle. In this preparation, the Gi-Go inhibitor pertussis toxin (PTX) attenuated the CPA-induced Ins(1,4,5)P3 accumulation, providing evidence that the generation of Ins(1,4,5)P3 by A1 adenosine-receptor stimulation is coupled to a PTX-sensitive G protein(s). The results suggest that activation of A1 adenosine receptors in allergic rabbit airway smooth muscle causes the production of Ins(1,4,5)P3 via a PTX-sensitive G protein-coupled PLC, and this signaling mechanism may be involved, at least in part, in the generation of contractile responses. It is hypothesized that this process may contribute to adenosine-induced bronchoconstriction in allergic asthma.

Role of group I metabotropic glutamate receptors in traumatic spinal cord white matter injury

Metabotropic glutamate receptors (mGluRs) participate in glutamate neural transmission, but their role in the pathophysiology of spinal cord injury (SCI) has not been explored. Accordingly, we examined the role of group I mGluRs, which are linked to phospholipase C, in mediating SCI using an in vitro model. A dorsal column segment was isolated from the spinal cord of adult rats, maintained in vitro, and injured by compression for 15 sec with a clip having a 2 g closing force. Under control conditions after SCI, the compound action potential (CAP) amplitude was reduced to 69.1 +/- 5.4% of baseline. Blockade of group I mGluR receptors with MCPG, 4CPG, or AIDA resulted in improved recovery of CAP amplitude (82.2 +/- 2.0%, 86.2 +/- 3.9%, and 86.0 +/- 2.5% of baseline, respectively). The group I/II agonist trans-ACPD and selective group I agonist DHPG exacerbated the posttraumatic reduction of CAP amplitude. The phospholipase C inhibitor U-73122 improved recovery of CAP amplitude after traumatic spinal cord axonal injury. Western blotting and immunocytochemistry demonstrated the presence of mGluR1alpha-immunopositive astrocytes and the absence of mGluR5 in spinal cord white matter. These studies are consistent with the hypothesis that activation of group I mGluR receptors after SCI exacerbates posttraumatic axonal injury through a phospholipase C dependent mechanism. The presence of mGluR1alpha labeling on astrocytes suggests a role for these cells in the pathophysiology of SCI. Additional studies in vivo, are required to further clarify the role of mGluRs in acute traumatic SCI.

Sphingosylphosphorylcholine stimulates mitogen-activated protein kinase via a Ca2+-dependent pathway

In cultured porcine aortic smooth muscle cells, sphingosylphosphorylcholine (SPC), ATP, or bradykinin (BK) induced a rapid dose-dependent increase in the cytosolic Ca2+ concentration ([Ca2+]i) and also stimulated inositol 1,4,5-trisphosphate (IP3) generation. Pretreatment of cells with pertussis toxin blocked the SPC-induced IP3 generation and [Ca2+]i increase but had no effect on the action of ATP or BK. In addition, SPC stimulated the mitogen-activated protein kinase (MAPK) and increased DNA synthesis, whereas neither ATP nor BK produced such effects. Both the SPC-induced MAPK activation and DNA synthesis were pertussis toxin sensitive. SPC-induced MAPK activation was blocked by treatment of cells with the phospholipase C inhibitor, U-73122, or the intracellular Ca2+-ATPase inhibitor, thapsigargin, but not by removal of extracellular Ca2+. Lysophosphatidic acid induced cellular responses similar to SPC in a pertussis toxin-sensitive manner in terms of [Ca2+]i increase, IP3 generation, MAPK activation, and DNA synthesis. Platelet-derived growth factor (PDGF) also induced a [Ca2+]i increase, MAPK activation, and DNA synthesis in the same cells; however, the PDGF-induced MAPK activation was not sensitive to pertussis toxin and changes in [Ca2+]i. SPC-induced MAPK activation was inhibited by pretreatment of cells with staurosporine, W-7, or calmidazolium. Our results suggest that, in porcine aortic smooth muscle cells, MAPK is not activated by the increase in [Ca2+]i unless a pertussis toxin-sensitive G protein is simultaneously stimulated, indicating the role of Ca2+ in pertussis toxin-sensitive G protein-mediated MAPK activation.

In vivo signal transduction of tetrodotoxin-sensitive nociceptive responses by substance P given into the planta of the mouse hind limb

1. We developed a simple and sensitive peripheral analgesic test in mice. 2. Substance P (SP) given into the planta (i.pl.) of the mouse hind limb produced a flexor response. The flexor response was dependent on SP doses (0.1-100 pmol, i.pl.). When SP (10 pmol) was given every 5 min, there were stable flexor responses. These nociceptive responses were completely abolished by CP-96,345, a neurokinin 1 receptor antagonist. 3. SP-induced responses were also blocked by several signal transduction-related compounds, such as tetrodotoxin, EGTA, and U73122, a selective phospholipase C inhibitor. 4. These findings suggest that SP depolarizes peripheral nerve endings, possibly through inositol trisphosphate (Ins P3)-gated Ca2+ influx, followed by induction of action potentials in the peripheral axons of primary afferent neurons.

Induction of Ca2+-calmodulin signaling by hard-surface contact primes Colletotrichum gloeosporioides conidia to germinate and form appressoria

Hard-surface contact primes the conidia of Colletotrichum gloeosporioides to respond to plant surface waxes and a fruit-ripening hormone, ethylene, to germinate and form the appressoria required for infection of the host. Our efforts to elucidate the molecular events in the early phase of the hard-surface contact found that EGTA (5 mM) and U73122 (16 nM), an inhibitor of phospholipase C, inhibited (50%) germination and appressorium formation. Measurements of calmodulin (CaM) transcripts with a CaM cDNA we cloned from C. gloeosporioides showed that CaM was induced by hard-surface contact maximally at 2 h and then declined; ethephon enhanced this induction. The CaM antagonist, compound 48/80, completely inhibited conidial germination and appressorium formation at a concentration of 3 microM, implying that CaM is involved in this process. A putative CaM kinase (CaMK) cDNA of C. gloeosporioides was cloned with transcripts from hard-surface-treated conidia. A selective inhibitor of CaMK, KN93 (20 microM), inhibited (50%) germination and appressorium formation, blocked melanization, and caused the formation of abnormal appressoria. Scytalone, an intermediate in melanin synthesis, reversed the inhibition of melanization but did not restore appressorium formation. The phosphorylation of 18- and 43-kDa proteins induced by hard-surface contact and ethephon was inhibited by the treatment with KN93. These results strongly suggest that hard-surface contact induces Ca2+-calmodulin signaling that primes the conidia to respond to host signals by germination and differentiation into appressoria.

Phospholipase C inhibitors attenuate arrhythmias induced by kappa-receptor stimulation in the isolated rat heart

To determine whether the phospholipase C (PLC)/inositol 1,4,5 trisphosphate (IP3)/Ca2+ pathway mediates cardiac arrhythmias induced by kappa-opioid receptor stimulation, the effects of U50,488H, a selective kappa-opioid receptor agonist, on cardiac rhythm in a isolated perfused rat heart, intracellular calcium ([Ca2+]i) in a single ventricular myocyte and IP3 production in myocytes were studied in the presence and absence of PLC inhibitors. U50,488H, the effects of which had been shown to be abolished by a selective kappa-receptor antagonist, nor-binaltorphimine, induced arrhythmias dose-dependently and increased both [Ca2+]i and IP3-production in the heart. More importantly, the effects of U50,488H were blocked by PLC inhibitors, neomycin and streptomycin. To further confirm the selectivity of action of the PLC inhibitor, the effects of another PLC inhibitor U73122 and its inactive structural analog, U73343, on cardiac rhythm in the isolated perfused rat heart were compared. The former did, while the latter did not, block the arrhythmogenic effect of U50,488H. We also determined whether the effects of kappa-receptor stimulation involves a pertussis toxin (PTX)-sensitive G-protein. We found that pretreatment with PTX at 4 microg/l for 10 min, a treatment shown to affect PTX sensitive G-protein-mediated functions, attenuated significantly the U50,488H-induced arrhythmias. The present study provides evidence that kappa-receptor stimulation-induced cardiac arrhythmias involves, at least partly, the PLC/IP3/Ca2+ pathway as well as a PTX sensitive G-protein.

P2Y and P2U receptors differentially release intracellular Ca2+ via the phospholipase c/inositol 1,4,5-triphosphate pathway in astrocytes from the dorsal spinal cord

In astrocytes, raising intracellular Ca2+ concentration is a principal mechanism for transducing extracellular signals following activation of cell-surface receptors. Receptors that may be activated by purine nucleotides, P2 receptors, are known to be expressed by astrocytes from dorsal spinal cord; these astrocytes express two distinct subtypes of P2 receptor, P2Y and P2U. A main goal of the present study was to determine the intracellular signalling pathways mediating the Ca2+ responses produced by stimulating these receptors. Experiments were done using cultured astrocytes from rat dorsal spinal cord. Ca2+ responses were evoked by 2-methylthio-ATP or UTP, nucleotides previously shown to selectively activate P2Y and P2U receptors, respectively, in these cells. P2Y- and P2U-evoked Ca2+ responses were found not to depend upon extracellular Ca2+ and were blocked by thapsigargin, a Ca2+-ATPase inhibitor known to deplete inositol 1,4,5-triphosphate-sensitive Ca2+ stores. Intracellular application of the inositol 1,4,5-triphosphate-sensitive receptor antagonist, heparin, or of the G-protein inhibitor guanosine 5'-O-(2-thiodiphosphate), blocked the P2Y- and P2U-evoked Ca2+ responses. Moreover, the responses were prevented by the phospholipase C inhibitor, U-73122, but were unaffected by the inactive analogue, U-73343. These results indicate that P2Y and P2U receptors on dorsal spinal astrocytes are linked via G-protein coupling to release of intracellular Ca2+ via the phospholipase C/inositol 1,4,5-triphosphate pathway. When we assessed the releasable pools of intracellular Ca2+, by repeated agonist applications in zero extracellular Ca2+, we found that the pool accessed by activating P2U receptors was only a subpool of that accessed by activating P2Y receptors. This implies that there are separable inositol 1,4,5-triphosphate-releasable pools of Ca2+ in dorsal spinal astrocytes and that these may be differentially released by activating distinct metabotropic P2 receptors. This differential release of Ca2+ may be important for physiological as well as pathophysiological events occurring within the spinal cord.

P2 receptor-mediated signal transduction in Ehrlich ascites tumor cells

The mechanisms, by which the P2 receptor agonists adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) evoke an increase in the free cytosolic calcium concentration ([Ca2+]i) and in intracellular pH (pHi), have been investigated in Ehrlich ascites tumor cells. The increase in [Ca2+]i evoked by ATP or UTP is abolished after depletion of intracellular Ca2+ stores with thapsigargin in Ca2+-free medium, and is inhibited by U73122, an inhibitor of phospholipase C (PLC), indicating that the increase in [Ca2+]i is primarily due to release from intracellular, Ins(1,4,5)P3-sensitive Ca2+ stores. ATP also activates a capacitative Ca2+-entry pathway. ATP as well as UTP evokes a biphasic change in pHi, consisting of an initial acidification followed by alkalinization. Suramin and 4,4'-diisothiocyano-2,2'-stilbene-disulfonic acid (DIDS) inhibit the biphasic change in pHi, apparently by acting as antagonists at P2 receptors. The alkalinization evoked by the P2 receptor agonists is found to be due to activation of a 5'-(N-ethyl-N-isopropyl)amiloride (EIPA)-sensitive Na+/H+ exchanger. ATP and UTP elicit rapid cell shrinkage, presumably due to activation of Ca2+ sensitive K+ and Cl- efflux pathways. Preventing cell shrinkage, either by incubating the cells at high extracellular K+ concentration, or by adding the K+-channel blocker, charybdotoxin, does not affect the increase in [Ca2+]i, but abolishes the activation of the Na+/H+ exchanger, indicating that activation of the Na+/H+ exchanger is secondary to the Ca2+-induced cell shrinkage.

The phospholipase C inhibitor U73122 increases cytosolic calcium in MDCK cells by activating calcium influx and releasing stored calcium

The effects of the phospholipase C (PLC) inhibitor U73122 on intracellular calcium levels ([Ca2+]i) were studied in MDCK cells. U73122 elevated [Ca2+]i dose-dependently. Ca2+ influx contributed to 75% of 20 microM U73122-induced Ca2+ signals. U73122 pretreatment abolished the [Ca2+]i transients evoked by ATP and bradykinin, suggesting that U73122 inhibited PLC. The Ca2+ signals among individual cells varied considerably. The internal Ca2+ source for the U73122 response was the endoplasmic reticulum (ER) since the response was abolished by thapsigargin. The depletion of the ER Ca2+ store triggered a La3+-sensitive capacitative Ca2+ entry. Independently of the internal release and capacitative Ca2 entry, U73122 directly evoked Ca2+ influx through a La3+-insensitive pathway. The U73122 response was augmented by pretreatment of carbonylcyanide m-chlorophynylhydrozone (CCCP), but not by Na+ removal, implicating that mitochondria contributed significantly in buffering the Ca2+ signal, and that efflux via Na+/Ca2+ exchange was insignificant.

Bradykinin causes contraction in rat uterus through the same signal pathway as oxytocin

The signal pathway for bradykinin-induced contraction of the uterine smooth muscle was investigated by comparing the effect of blocking agents on bradykinin and oxytocin induced contractions of the isolated rat uterus in organ bath. The phospholipase C inhibitor U-73,122 abolished the effect of both bradykinin and oxytocin. Inhibition of non-voltage-dependent Ca2+ influx by SK & F 96,365 reduced the contraction induced by both agonists to about 20% of control. The tissues failed to contract when they were exposed to bradykinin or oxytocin in Ca(2+)-free Krebs-Henseleit buffer with 2 mM EDTA. Both bradykinin and oxytocin induced further contraction when the tissues were partially depolarized and partially contracted by 30 mM KCl. These observations suggest that bradykinin, like oxytocin, activates phospholipase C which generates IP3 with a subsequent release of Ca2+ from intracellular stores followed by store-operated Ca2+ influx. Thus, membrane potential independent steps appear to be important in bradykinin-induced contraction in the rat uterus.

Molecular mechanism(s) of action of norepinephrine on the expression of the angiotensinogen gene in opossum kidney cells

BACKGROUND

Norepinephrine (NE) is the major endogenous neurotransmitter of the renal sympathetic nerves interacting with both the alpha- and beta-adrenoceptors in the renal proximal tubules. We have previously reported that isoproterenol and iodoclonidine stimulate the expression of the angiotensinogen (ANG) gene in opossum kidney (OK) proximal tubular cells via the beta1-adrenoceptor and alpha2-adrenoceptor, respectively. We hypothesized that NE may interact with the beta- and/or alpha2-adrenoceptors to stimulate the expression of the ANG gene in OK cells.

METHODS

The fusion genes containing the various lengths of the 5'-flanking regulatory sequence of the rat ANG gene fused with a human growth hormone (hGH) gene as a reporter were stably transfected into the OK cells. The stimulatory effect of NE on the expression of the fusion genes was evaluated by the amount of immunoreactive hGH (IR-hGH) secreted into the culture medium.

RESULTS

The addition of NE stimulated the expression of the fusion gene, pOGH (ANG N-1498/+18) in a dose-dependent manner. The stimulatory effect of NE was inhibited in the presence of propranolol, atenolol, Rp-cAMP, yohimbine, staurosporine, H-7 and U73122 but not in the presence of ICI 118,551 and prazosin. The addition of a combination of isoproterenol and iodoclonidine synergistically stimulated the expression of pOGH (ANG N-1498/+18) as compared to the addition of isoproterenol and iodoclonidine alone. Furthermore, the addition of NE, forskolin, 8-Br-cAMP or phorbol 12-myristate (PMA) stimulated the expression of pOGH (ANG N-806/-779/-53/+18), a fusion gene containing the putative cAMP responsive element (CRE, ANG N-806/-779) upstream of the ANG promoter (ANG N-53/+ 18) in OK 95 cells, but had no effect on the expression of fusion genes containing the mutant of the CRE.

CONCLUSION

These studies demonstrate that the stimulatory effect of NE on the expression of the ANG gene in OK cells may be mediated via both the beta1- and alpha2-adrenoceptors and via the CRE (ANG N-806/-779) in the 5'flanking region of rat ANG gene.

PKA induces Ca2+ release and enhances ciliary beat frequency in a Ca2+-dependent and -independent manner

The intent of this work was to evaluate the role of cAMP in regulation of ciliary activity in frog mucociliary epithelium and to examine the possibility of cross talk between the cAMP- and Ca2+-dependent pathways in that regulation. Forskolin and dibutyryl cAMP induced strong transient intracellular Ca2+ concentration ([Ca2+]i) elevation and strong ciliary beat frequency enhancement with prolonged stabilization at an elevated plateau. The response was not affected by reduction of extracellular Ca2+ concentration. The elevation in [Ca2+]i was canceled by pretreatment with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, thapsigargin, and a phospholipase C inhibitor, U-73122. Under those experimental conditions, forskolin raised the beat frequency to a moderately elevated plateau, whereas the initial strong rise in frequency was completely abolished. All effects were canceled by H-89, a selective protein kinase A (PKA) inhibitor. The results suggest a dual role for PKA in ciliary regulation. PKA releases Ca2+ from intracellular stores, strongly activating ciliary beating, and, concurrently, produces moderate prolonged enhancement of the beat frequency by a Ca2+-independent mechanism.

Postsynaptic phospholipase C activity is required for the induction of homosynaptic long-term depression in rat hippocampus

The necessity for phospholipase C (PLC), the enzyme which produces the second messenger molecules inositol trisphosphate (IP3) and diacylglycerol, for the induction of long-term depression (LTD) was tested at Schaffer collateral-CA1 synapses in hippocampal slices in vitro. We report here that bath application of a selective cell-permeant PLC inhibitor, U-73122 (10 microM), does block the induction of LTD. In contrast, neither the inactive analog U-73343 (10 microM), nor application of U-73122 during the maintenance phase of LTD, impaired expression of LTD. Furthermore, postsynaptic infusion of U-73122 (100 microM) into single CA1 pyramidal neurons also prevented the induction of LTD. Since mGluR5 is the only metabotropic glutamate receptor subtype coupled to inositide turnover in field CA1, we conclude that the postsynaptic calcium store necessary for the induction of homosynaptic LTD is gated by IP3, through activation of mGluR5 coupled to phospholipase C.

Adrenergic regulation of calcium-activated potassium current in cultured rabbit pigmented ciliary epithelial cells

1. The effects of adrenergic agonists on K+ currents were studied in cultured rabbit pigmented ciliary epithelial (PCE) cells. 2. Outward K+ current (IK) was reduced by tetraethylammonium chloride, the Ca2+-activated K+ (K(Ca)) channel blocker iberiotoxin (IbTX), or Ca2+-free external Ringer solution. The calcium ionophore ionomycin increased an IbTX-sensitive IK in PCE cells. 3. The adrenergic agonists adrenaline and phenylephrine increased IK in PCE cells. The induced current was blocked by IbTX and the alpha1-antagonist prazosin, suggesting that adrenergic agonists activate IK(Ca) via alpha1-adrenoreceptors. 4. Internal dialysis of D-myo-inositol 1,4, 5-trisphosphate (IP3) increased IK, whilst pre-incubation of PCE cells with thapsigargin or the phospholipase C (PLC) inhibitor U-73122 reduced phenylephrine-induced increases in IK(Ca). Adrenergic increases in IK(Ca) were mediated by a pertussis toxin-insensitive G protein. 5. These results demonstrate that IK(Ca) channels in rabbit PCE cells are coupled to alpha1-adrenergic receptors and a PLC/IP3 signalling pathway. Activation of these channels may modulate fluid secretion by the ciliary epithelium.

Role of calcium in adaptive cytoprotection and cell injury induced by deoxycholate in human gastric cells

We have developed an in vitro model of adaptive cytoprotection induced by deoxycholate (DC) in human gastric cells and have shown that pretreatment with a low concentration of DC (mild irritant, 50 microM) significantly attenuates injury induced by a damaging concentration of DC (250 microM). This study was undertaken to assess the effect of the mild irritant on changes in intracellular Ca2+ and to determine if these perturbations account for its protective action. Protection conferred by the mild irritant was lost when any of its effects on intracellular Ca2+ were prevented: internal Ca2+ store release via phospholipase C and inositol 1,4, 5-trisphosphate sustained Ca2+ influx through store-operated Ca2+ channels or eventual Ca2+ efflux. We also investigated the relationship between Ca2+ accumulation and cellular injury induced by damaging concentrations of DC. In cells exposed to high concentrations of DC, sustained Ca2+ accumulation as a result of extracellular Ca2+ influx, but not transient changes in intracellular Ca2+ content, appeared to precede and induce cellular injury. We propose that the mild irritant disrupts normal Ca2+ homeostasis and that this perturbation elicits a cellular response (involving active Ca2+ efflux) that subsequently provides a protective action by limiting the magnitude of intracellular Ca2+ accumulation.

Different signaling pathway between sphingosine-1-phosphate and lysophosphatidic acid in Xenopus oocytes: functional coupling of the sphingosine-1-phosphate receptor to PLC-xbeta in Xenopus oocytes

In Xenopus oocytes, both sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) activate Ca2+-dependent oscillatory Cl- currents by acting through membrane-bound receptors. External application of 50 microM S1P elicited a long-lasting oscillatory current that continued over 30 min from the beginning of oscillation, with 300 nA (n = 11) as a usual maximum peak of current, whereas 1-microM LPA treatment showed only transiently oscillating but more vigorous current responses, with 2,800 nA (n = 18) as a maximum peak amplitude. Both phospholipid-induced Ca2+-dependent Cl- currents were observed in the absence of extracellular Ca2+, were blocked by intracellular injection of the Ca2+ chelator, EGTA, and could not be elicited by treatment with thapsigargin, an inhibitor of endoplasmic reticulum (ER) Ca2+ ATPase. Intracellular Ca2+ release appeared to be from inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store, because Cl- currents were blocked by heparin injection. Pretreatment with the aminosteroid, U-73122, an inhibitor of G protein-mediated phospholipase C (PLC) activation, to oocytes inhibited the current responses evoked both by S1P and LPA. However, when they were injected with 10 ng of antisense oligonucleotide (AS-ODN) against Xenopus phospholipase C (PLC-xbeta), oocytes could not respond to S1P application, whereas they responded normally to LPA, indicating that the S1P signaling pathway goes through PLC-xbeta, whereas LPA signaling goes through another unknown PLC. To determine the types of G proteins involved, we introduced AS-ODNs against four types of G-protein alpha subunits that were identified in Xenopus laevis; G(q)alpha, G11alpha, G0alpha, and G(i1)alpha. Among AS-ODNs against the G alphas tested, AS-G(q)alpha and AS-G(i1)alpha to S1P and AS-G(q)alpha and AS-G11alpha to LPA specifically reduced current responses, respectively, to about 20-30% of controls. These results demonstrate that LPA and S1P, although they have similar structural features, release intracellular Ca2+ from the IP3-sensitive pool, use different components in their signal transduction pathways in Xenopus oocytes.

Real-time optical monitoring of ligand-mediated internalization of alpha1b-adrenoceptor with green fluorescent protein

The study of G protein-coupled receptor signal transduction and behavior in living cells is technically difficult because of a lack of useful biological reagents. We show here that a fully functional alphalb-adrenoceptor tagged with the green fluorescent protein (alphalbAR/GFP) can be used to determine the molecular mechanism of intemalization of alphalbAR/ GFP in living cells. In mouse alphaT3 cells, alpha1bAR/GFP demonstrates strong, diffuse fluorescence along the plasma membrane when observed by confocal laser scanning microscope. The fluorescent receptor binds agonist and antagonist and stimulates phosphatidylinositol/Ca2+ signaling in a similar fashion to the wild receptor. In addition, alpha1bAR/ GFP can be internalized within minutes when exposed to agonist, and the subcellular redistribution of this receptor can be determined by measurement of endogenous fluorescence. The phospholipase C inhibitor U73,122, the protein kinase C activator PMA, and inhibitor staurosporine, and the Ca2+-ATPase inhibitor thapsigargin were used to examine the mechanism of agonist-promoted alphalbAR/GFP redistribution. Agonist-promoted internalization of alphalbAR/GFP was closely linked to phospholipase C activation and was dependent on protein kinase C activation, but was independent of the increase in intracellular free Ca2+ concentration. This study demonstrated that real-time optical monitoring of the subcellular localization of alphalbAR (as well as other G protein-coupled receptors) in living cells is feasible, and that this may provide a valuable system for further study of the biochemical mechanism(s) of agonist-induced receptor endocytosis.

Calcium signalling through nucleotide receptor P2Y2 in cultured human vascular endothelium

Microfluorometric measurements in Fura-2-loaded single cultured human vascular endothelial cells were used to characterize the intracellular calcium [Ca2+]i responses triggered by extracellular application of adenosine 5'-triphosphate (ATP) and other nucleotides. Application of ATP or uridine 5'-triphosphate (UTP) gave rise to dose-dependent elevations of [Ca2+]i in all the cells tested. At saturating concentrations of agonist, the [Ca2+]i response was biphasic, with an early peak and a sustained plateau. Unlike peak responses, the sustained Ca2+ plateau was sensitive to removal of Ca2+ from the external medium. Mn2+ quenching revealed the presence of Ca2+ influx during the agonist-induced calcium plateau. The agonist-evoked calcium plateau was inhibited in a dose-dependent manner by the Cl-channel blocker NPPB, by the divalent cation Ni2+ and by the imidazole antimycotic econazole. Previously, these compounds have been shown to block store-operated Ca2+ entry. The two phases of the agonist-evoked [Ca2+]i response were blocked by the specific phospholipase C inhibitor U-73122 and by intracellular injection of low molecular weight heparin, suggesting the involvement of IP3-sensitive intracellular Ca2+ stores. The pharmacological profile of the response, using different nucleotides and analogues, ATP = UTP > ADP = UDP, and no responses to P2X1 and P2Y1 agonists, suggested the involvement of P2Y2 receptors. The expression of mRNA for the P2Y2 receptor was detected by RT-PCR analysis. These results indicate that P2Y2 receptors linked to intracellular Ca2+ mobilization are present in human vascular endothelial cells. The initial [Ca2+]i mobilization is followed by a phase of elevated [Ca2+]i influx.

Vesicle exocytosis stimulated by alpha-latrotoxin is mediated by latrophilin and requires both external and stored Ca2+

alpha-Latrotoxin (LTX) stimulates massive neurotransmitter release by two mechanisms: Ca2+-dependent and -independent. Our studies on norepinephrine secretion from nerve terminals now reveal the different molecular basis of these two actions. The Ca2+-dependent LTX-evoked vesicle exocytosis (abolished by botulinum neurotoxins) is 10-fold more sensitive to external Ca2+ than secretion triggered by depolarization or A23187; it does not, however, depend on the cation entry into terminals but requires intracellular Ca2+ and is blocked by drugs depleting Ca2+ stores and by inhibitors of phospholipase C (PLC). These data, together with binding studies, prove that latrophilin, which is linked to G proteins and inositol polyphosphate production, is the major functional LTX receptor. The Ca2+-independent LTX-stimulated release is not inhibited by botulinum neurotoxins or drugs interfering with Ca2+ metabolism and occurs via pores in the presynaptic membrane, large enough to allow efflux of neurotransmitters and other small molecules from the cytoplasm. Our results unite previously contradictory data about the toxin's effects and suggest that LTX-stimulated exocytosis depends upon the co-operative action of external and intracellular Ca2+ involving G proteins and PLC, whereas the Ca2+-independent release is largely non-vesicular.

Interleukin-3 and Flt3-ligand induce adhesion of Baf3/Flt3 precursor B-lymphoid cells to fibronectin via activation of VLA-4 and VLA-5

Adhesion of hematopoietic cells to extracellular matrix components is important for blood cell development. However, little is known regarding the potential influence of IL-3 on this process for precursor B cells and Flt3-ligand has not yet been implicated in induction of adhesion of any blood cell types to extracellular matrix components. Therefore, we examined the characteristics of cytokine-induced cell adhesion to fibronectin (FN), using as a model the murine precursor B cell line, Baf3, a factor-dependent cell line requiring IL-3 for both growth and survival. Since factor-dependent hematopoietic cell lines expressing Flt3 receptor are extremely rare, we also studied Baf3/Flt3, a subline of Baf3 transduced with the Flt3 receptor gene. IL-3 induced adhesion of Baf3 and Baf3/Flt3 cells to FN, while Flt3-ligand induced adhesion of Baf3/Flt3 cells only. Whereas both Baf3 and Baf3/Flt3 cells expressed VLA-4 and -5 integrins as FN receptors, expression levels of VLA-4 and -5 were not affected by IL-3 or Flt3-ligand treatment. However, blocking experiments using anti-integrin antibodies showed that cytokine-induced adhesion of cells depended on both VLA-4 and -5 suggesting that IL-3 and Flt3-ligand activated these integrins. PI-3 kinase inhibitor wortmannin, PKC inhibitor H-7, or PKA inhibitor HA1004 did not suppress adhesion induced by IL-3 or Flt3-ligand; in contrast, PLC inhibitor U-73122 did suppress adhesion, suggesting the possibility that PLC, but not PI-3 kinase, PKC, or PKA, may be involved in this process. Since it is known that IL-3 and Flt3-ligand receptors are expressed on precursor B cells, and these receptors are downregulated during B cell maturation of primary cells, the induction of precursor B cell adhesion to FN by IL-3 and Flt3-ligand may contribute a mechanism by which precursor B cells adhere to bone marrow stroma, thereby influencing their development.

Magnolol induces cytosolic-free Ca2+ elevation in rat neutrophils primarily via inositol trisphosphate signalling pathway

In the present study, we describe the role of inositol trisphosphate in the signalling pathway that leads to the elevation of cytosolic-free Ca2+ in rat neutrophils stimulated with magnolol, a compound isolated from the cortex of Magnolia officinalis. Magnolol increased [Ca2+]i, by stimulating Ca2+ release from internal stores and Ca2+ influx across the plasma membrane, in a concentration-dependent manner. Ni2+ and [6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H -pyrrole-2,5-dione (U73122), but not pertussis toxin, inhibited the magnolol-induced Ca2+ influx. Measurement of cellular levels of inositol trisphosphate showed a clear increase upon exposure to magnolol. U73122 but not ryanodine suppressed the Ca2+ release from internal stores caused by magnolol. Pretreatment of cells with formyl-Met-Leu-Phe (fMLP) or cyclopiazonic acid greatly reduced the [Ca2+]i changes caused by the subsequent addition of magnolol. Collectively, these findings suggest that a pertussis toxin-insensitive inositol trisphosphate signalling pathway is involved in the magnolol-induced [Ca2+]i elevation in rat neutrophils.

U73122 blocked the cGMP-induced calcium release in sea urchin eggs

U73122, a phospholipase C inhibitor, dose dependently blocks the cGMP-induced Ca2+ release in sea urchin eggs and homogenates. U73122 inhibition was prevented by cotreatment with dithiothreitol (DTT), but DTT is ineffective when eggs or homogenates were pretreated with U73122. U73122 action is different from the other sulfhydryl reagents, thimerosal and N-ethylmaleimide, which cause Ca2+ release in egg homogenates at high concentration, but at lower concentration have no significant effect on cGMP-induced Ca2+ release. Histone, a reported NAD glycohydrolase (NADase) activator, was found to induce Ca2+ release in egg homogenates via the same pathway as the cGMP response, since histone-induced Ca2+ release is blocked by Rp-8-pCPT-cGMPS, a cGMP-dependent protein kinase (PKG) inhibitor, and nicotinamide, a NADase inhibitor. Histone-induced Ca2+ release is similarly blocked by U73122. The aminosteroid U73122 does not inhibit cADPR-induced Ca2+ release, which is significantly reduced by PKG inhibitors. Furthermore, U73122 has no significant effect on phorbol 12-myristate 13-acetate induced-cytoplasmic alkalinization in intact eggs, which depends on protein kinase C activity. These results suggest that U73122 does not act as a general serine-threonine protein kinase inhibitor, and the aminosteroid inhibition of the cGMP-induced Ca2+ release may interfere with ADP ribosyl cyclase activity.

Pituitary adenylate cyclase activating polypeptide induces multiple signaling pathways in rat peritoneal mast cells

Pituitary adenylate cyclase activating polypeptide (PACAP) is a high-affinity ligand for at least two types of G-protein coupled receptors, the PACAP type 1 and type 2 receptor. In this study it is demonstrated that the C-terminal PACAP-fragment PACAP(6-27) stimulates serotonin release from rat peritoneal mast cells with higher potency (EC50: 0.2 vs. 2.0 microM) than the PACAP receptor ligand PACAP(1-27). PACAP-induced degranulation of rat peritoneal mast cells was abolished by pertussis toxin and by benzalkonium chloride (IC50: 9.1 microg/ml) indicating the involvement of heterotrimeric G-proteins of the Gi-type. The PACAP effect was also reduced by inhibitors of the phosphatidylinositol specific phospholipase C ((U73122), IC50: 4 microM; (ET-18-O-CH3), IC50: 18 microM), by D609, a specific inhibitor of the phosphatidylcholine specific phospholipase C (IC50: 41 microM), by the protein kinase C-inhibitor staurosporine (IC50: 0.6 microM) and by the lipoxygenase inhibitor nordihydroguaiaretic acid (NGDA) but not by indomethacin. It is concluded that PACAP peptides stimulate secretion in rat peritoneal mast cells in a PACAP receptor-independent manner, probably via direct activation of heterotrimeric G-proteins of the Gi-type; these G-proteins may lead to a sequential activation of different signaling cascades (see above), which may converge at the level of one or more staurosporine-sensitive protein kinase.

Adenosine receptor mediated stimulation of intracellular calcium in acutely isolated astrocytes

The characteristics of adenosine receptors found in glial fibrillary acid protein (GFAP)-positive astrocytes acutely isolated from the cerebral cortices of 4- to 12-day old rats were examined by evaluating the effects of adenosine and its analogues on intracellular calcium levels. First, these effects were compared with those seen in primary astrocytic cultures, and it was found that acutely isolated astrocytes showed much greater sensitivity to adenosine than their cultured counterparts. Then, the adenosine evoked calcium responses in acutely isolated cells were evaluated under various conditions. The responses to adenosine were not inhibited by papaverine, an uptake blocker, or by removal of extracellular calcium. U73122, a phospholipase C inhibitor, was able to completely inhibit the adenosine response. The receptor inhibitor 3-isobutyl-1-methylxanthine inhibited the calcium response to adenosine, providing evidence that the response is not coupled to the xanthine-insensitive A3 receptor. The stimulatory action of NECA, a non-selective analogue, was blocked neither by the A2A-selective receptor antagonist 8-(3-chlorostyryl) caffeine nor by the A1-selective receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. The A2B receptor antagonist alloxazine, however, was able to completely inhibit the increase in intracellular calcium produced by NECA. Taken together, these data suggest that the adenosine-evoked calcium response in acutely isolated astrocytes is coupled to the A2B receptor.