Homologous desensitization of substance-P-induced inositol polyphosphate formation in rat parotid acinar cells

A Chitosan-Based Liposome Formulation Enhances the In Vitro Wound Healing Efficacy of Substance P Neuropeptide

Currently, there is considerable interest in developing innovative biodegradable nanoformulations for controlled administration of therapeutic proteins and peptides. Substance P (SP) is a neuropeptide of 11 amino acids that belongs to the tachykinins family and it plays an important role in wound healing. However, SP is easily degradable in vivo and has a very short half-life, so the use of chitosan-based nanocarriers could enhance its pharmaceutical properties. In light of the above, the aim of this work was to produce and characterize chitosan-coated liposomes loaded with SP (SP-CH-LP) as novel biomaterials with potential application in mucosal wound healing. The loaded system’s biophysical properties were characterized by dynamic light scattering with non-invasive back scattering (DLS-NIBS), mixed mode measurements and phase analysis light scattering (M3-PALS) and high performance liquid chromatography with ultraviolet/visible light detection (HPLC-UV/VIS). Then, the efficacy of the obtained nanoformulations was examined via proof-of-principle experiments using in vitro cell assays. These assays showed an increment on cell motility and proliferation after treatment with free and encapsulated neuropeptides. Additionally, the effect of SP on wound healing was enhanced by the entrapment on CH-LP. Overall, the amenability of chitosan-based nanomaterials to encapsulate peptides and proteins constitutes a promising approach towards potential novel therapies to treat difficult wounds.

Characteristics of neurokinin A-induced salivary fluid secretion in perfused rat submandibular gland

Tachykinins such as neurokinin A (NKA) and substance P have been demonstrated to induce salivary fluid secretion in vivo. However, characteristics of salivary fluid secretion induced by tachykinins in salivary glands have not been well elucidated. In this study, the effects of the tachykinin NKA on salivary fluid secretion were investigated in isolated, perfused rat submandibular gland. NKA provoked salivary fluid secretion, which consisted of transient and sustained phases, in a dose-dependent manner. In fura-2-loaded dispersed cells of the rat submandibular gland, the doses of NKA in which induced salivary fluid secretion caused an increase in intracellular Ca(2+) concentration. When Ca(2+) was removed from the perfusate to examine the effect of Ca(2+) mobilization on NKA-induced fluid secretion, only the transient salivary fluid secretion occurred. When the gland was perfused with the Ca(2+)-free perfusate containing the intracellular Ca(2+) chelator BAPTA-AM, NKA failed to induce salivary fluid secretion. NKA also induced an increase in oxygen consumption, but which was reduced by the removal of Ca(2+) from perfusate. Salivary fluid is secreted via transcellular and paracellular pathways in acinar cells of salivary glands. To examine the contribution of paracellular pathway to NKA-induced salivary fluid secretion, the glands were perfused with a perfusate containing Lucifer yellow (LY), a cellular impermeable substance, and then were stimulated with NKA, which provoked secretion of LY in the saliva. These results suggest that the NKA-induced salivary fluid secretion is Ca(2+)-dependent and that the paracellular pathway contributes to the secretion.

Involvement of phospholipase D in the cAMP-regulated exocytosis of rat parotid acinar cells

The activation of beta-adrenergic receptors in rat parotid acinar cells causes intracellular cAMP elevation and appreciably stimulates the exocytotic release of amylase into saliva. The activation of Ca(2+)-mobilizing receptors also induces some exocytosis. We investigated the role of phospholipase D (PLD) in regulated exocytosis in rat parotid acinar cells. A transphosphatidylation assay detected GTPgammaS (a nonhydrolyzable analogue of GTP)-dependent PLD activity in lysates of rat parotid acinar cells, suggesting that PLD is activated by small molecular mass GTP-binding proteins. The PLD inhibitor, neomycin, suppressed cAMP-dependent exocytosis in saponin-permeabilized cells. Signaling downstream of PLD was disrupted by 1-butanol due to conversion of the PLD reaction product (phosphatidic acid) to phosphatidylbutanol. The stimulation of exocytosis by isoproterenol as well as by a Ca(2+)-mobilizing agonist (methacholine) was inhibited by 1-butanol. These results suggest that PLD is important for regulated exocytosis in rat parotid acinar cells.

4-Bromophenacyl bromide induces Ca2+ influx in human gingival fibroblasts

4-Bromophenacyl bromide (BPB) is generally used as a phospholipase A(2) (PLA2) inhibitor. In the present study, we demonstrate that BPB induces Ca2+ influx in human gingival fibroblasts. In fura-2-loaded human gingival fibroblasts, BPB evoked a transient increase in intracellular Ca2+ concentration ([Ca2+]i) in a dose-dependent manner. The BPB-induced Ca2+ mobilization was also shown in a single fluo-3-loaded-fibroblast. The BPB-induced increase in [Ca2+]i was completely abolished by the elimination of the external Ca2+. Ca2+ influx induced by the Ca2+-mobilizing agonist histamine was markedly enhanced in the presence of BPB. These suggest that the BPB-induced Ca2+ mobilization is due to the influx of extracellular Ca2+. However, it is unlikely that the effect of BPB is dependent on the inhibition of PLA2 activity, because other PLA2 inhibitors, such as AACOCF3, quinacrine dihydrochloride and manoalide, failed to induce Ca2+ mobilization. Chemical compounds similar to BPB, but which have no -CH2-Br at position 1 in the benzene ring failed to evoke Ca2+ mobilization, indicating that the position of -CH2--Br in BPB is important for causing the Ca2+ influx.

The N-terminal domain of substance P is required for complete homologous desensitization but not phosphorylation of the rat neurokinin-1 receptor

The agonist activity of substance P (SP) is a function of the C-terminal domain of the peptide. A C-terminal SP fragment (SP(6-11)) and analog (septide) and neurokinin A (NKA; a related tachykinin with a divergent N-terminal amino acid sequence) were found to be full neurokinin-1 receptor (NK-1R) agonists, but were not able to desensitize the receptor maximally as much as SP. Substance P caused 95.6 +/- 0.9% maximal desensitization of the NK-1R whereas SP(6-11), septide, and NKA(only)caused 74 +/- 3.5, 50.6 +/- 8, and 71.5 +/- 4.4% maximal desensitization, respectively (mean +/- SEM; P < 0.001 vs SP). When a series of SP C-terminal fragment peptides were tested for their NK-1R desensitizing activity, it was found that SP(5-11)and SP(6-11)caused significantly less maximal NK-1R desensitization than SP. SP N-terminal fragment peptides had no effect on the ability of SP(6-11)to compete with(3)H-SP binding, generate an IP(3)response, or cause NK-1R desensitization when tested with or without SP(6-11). SP, SP(6-11), septide, and NKA all maximally stimulated 8-9-fold increases in NK-1R phosphorylation. When attached to the C-terminal domain of SP responsible for NK-1R binding and agonism, the N-terminus of SP is responsible for 25-50% of homologous desensitization and this may occur via a mechanism other than NK-1R phosphorylation.

Lack of a C-terminal tail in the mammalian gonadotropin-releasing hormone receptor confers resistance to agonist-dependent phosphorylation and rapid desensitization

The mammalian gonadotropin-releasing hormone receptor (GnRH-R) is, at present, the only G-protein-coupled receptor that activates phospholipase C and lacks a C-terminal tail. We have previously demonstrated that this unique structural feature is associated with resistance to rapid desensitization of phosphoinositide signaling in COS-7 and HEK-293 cells (Heding, A., Vrecl, M., Bogerd, J., McGregor, A., Sellar, R., Taylor, P. L., and Eidne, K. A. (1998) J. Biol. Chem. 273, 11472-11477). Using receptors tagged with a nonapeptide of the influenza hemagglutinin protein to enable immunoprecipitation, we now demonstrate that the mammalian GnRH-R is not phosphorylated in an agonist-dependent manner. In contrast, the mammalian thyrotropin-releasing hormone receptor and the African catfish GnRH-R, both of which have a C-terminal tail, are phosphorylated in response to agonist challenge. Furthermore, chimeras of the mammalian GnRH-R with the C-terminal tail of either the mammalian thyrotropin-releasing hormone receptor or the catfish GnRH-R are also phosphorylated in an agonist-dependent manner. Only those receptors having C-terminal tails showed desensitization of phosphoinositide responses within 5-10 min of agonist challenge. We also show that the internalization of all these receptors when expressed transiently in COS-7 cells is similar. This dissociates receptor internalization from rapid desensitization and demonstrates that the lack of a C-terminal tail in the mammalian GnRH-R results in an inability of the receptor to undergo agonist-dependent phosphorylation and that this results directly in a resistance to rapid desensitization.

Real-time visualization of the cellular redistribution of G protein-coupled receptor kinase 2 and beta-arrestin 2 during homologous desensitization of the substance P receptor

The substance P receptor (SPR) is a G protein-coupled receptor (GPCR) that plays a key role in pain regulation. The SPR desensitizes in the continued presence of agonist, presumably via mechanisms that implicate G protein-coupled receptor kinases (GRKs) and beta-arrestins. The temporal relationship of these proposed biochemical events has never been established for any GPCR other than rhodopsin beyond the resolution provided by biochemical assays. We investigate the real-time activation and desensitization of the human SPR in live HEK293 cells using green fluorescent protein conjugates of protein kinase C, GRK2, and beta-arrestin 2. The translocation of protein kinase C betaII-green fluorescent protein to and from the plasma membrane in response to substance P indicates that the human SPR becomes activated within seconds of agonist exposure, and the response desensitizes within 30 s. This desensitization process coincides with a redistribution of GRK2 from the cytosol to the plasma membrane, followed by a robust redistribution of beta-arrestin 2 and a profound change in cell morphology that occurs after 1 min of SPR stimulation. These data establish a role for GRKs and beta-arrestins in homologous desensitization of the SPR and provide the first visual and temporal resolution of the sequence of events underlying homologous desensitization of a GPCR in living cells.

Translocation of Arf1 to the secretory granules in rat parotid acinar cells

We investigated the interaction of ADP-ribosylation factor (Arf) with the secretory granules in rat parotid acinar cells. The 20. 5-kDa small-molecular-mass GTP-binding protein in the cytosolic fraction of rat parotid acinar cells was identified as ADP-ribosylation factor1 by using a pan-Arf monoclonal antibody and isotype-specific polyclonal antibodies for Arf proteins 1, 3, 5, and 6. Incubation of the cytosolic fraction with isolated secretory granule membranes in the presence of GTPgammaS resulted in the translocation of Arf1 from the cytosolic fraction to the secretory granule membranes. The translocation was not observed in the presence of GDPbetaS in place of GTPgammaS, indicating that the process is GTP-dependent. The immunoelectron microscopy experiment confirmed Arf1 is translocated to the secretory granules. A prior treatment of the granule membranes with trypsin inhibited the translocation of Arf1 at 2 mM Mg2+, but had no effect in the absence of Mg2+ (condition of spontaneous conversion of Arf-GDP to Arf-GTP). Thus, the trypsin-sensitive nucleotide exchange activity for Arf1 is probably associated with the secretory granule membranes. These results demonstrate Arf1 translocates to the secretory granules in rat parotid acinar cells.

cGMP production is coupled to Ca(2+)-dependent nitric oxide generation in rabbit parotid acinar cells

We investigated the mechanism of guanosine 3',5'-monophosphate (cGMP) production in rabbit parotid acinar cells. Methacholine, a muscarinic cholinergic agonist, stimulated cGMP production in a dose-dependent manner but not isoproterenol, a beta-adrenergic receptor stimulant. Methacholine-stimulated cGMP production has been suggested to be coupled to Ca2+ mobilization, because intracellular Ca2+ elevating reagents, such as thapsigargin and the Ca2+ ionophore A23187, mimicked the effect of methacholine. The cGMP production induced by Ca2+ mobilization has also been suggested to be coupled to nitric oxide (NO) generation because the effects of methacholine, thapsigargin and A23187 on cGMP production were blocked by NG-nitro-L-arginine methyl ester (L-NAME), a specific inhibitor of nitric oxide synthase (NOS), and hemoglobin, a scavenger of nitric oxide (NO). Sodium nitroprusside (SNP), a NO donor, stimulated cGMP production. Furthermore, methacholine stimulated NO generation, and NOS activity in the cytosolic fraction in rabbit parotid acinar cells was exclusively dependent on Ca2+. These findings suggest that cGMP production induced by the activation of muscarinic cholinergic receptors is coupled to NO generation via Ca2+ mobilization.

Ranakinin, a naturally occurring tachykinin, stimulates phospholipase C activity in the frog adrenal gland

We have previously shown that the frog adrenal gland is innervated by a dense network of fibers containing ranakinin, one of the endogenous tachykinins in the amphibian Rana ridibunda, and we have found that ranakinin stimulates in vitro corticosteroid secretion by frog adrenal tissue. To elucidate the mechanism of action of ranakinin on the frog adrenal gland, we investigated the effect of ranakinin on cAMP formation and polyphosphoinositide metabolism. Incubation of frog adrenal explants with various tachykinins, including ranakinin, substance P, neurokinin A, or neurokinin B, did not produce any significant modification of cAMP concentrations. In contrast, ranakinin induced a time- and dose-dependent stimulation of inositol phosphate formation with a concomitant decrease in membrane polyphosphoinositides. Pretreatment of the tissue slices with the phospholipase C inhibitor U-73122 or with pertussis toxin completely abolished the stimulatory effect of ranakinin on inositol phosphate formation. Prolonged administration of U-73122 to perifused frog adrenal explants markedly attenuated the ranakinin-evoked stimulation of corticosterone and aldosterone secretion. Taken together, these data indicate that in the frog adrenal gland, ranakinin has no effect on the adenylyl cyclase system, but enhances polyphosphoinositide hydrolysis. The stimulatory action of ranakinin on inositol phosphate formation and corticosteroid secretion is mediated through activation of a phospholipase C positively coupled to a pertussis toxin-sensitive G protein.

Human substance P receptor expressed in Sf9 cells couples with multiple endogenous G proteins

To identify the G proteins involved in the function of human substance P receptor (hSPR), the receptor was expressed in Sf9 cells using the baculovirus expression system. Maximal hSPR expression was up to 65 pmol/mg membrane protein. The following data indicated that hSPR in Sf9 membranes is coupled to endogenous G proteins: 1) binding of agonist radioligand [125I]BHSP to the receptor was sensitive to guanine nucleotides; and 2) stimulation of the receptor increased [35S]GTPgammaS binding. The hSPR-associated G proteins were identified by photoaffinity labeling with [alpha-32P]-azidoanilido GTP ([alpha-32P]AAGTP), followed by immunoprecipitation of the labeled G proteins with antibodies specific for various Galpha-subunits. These experiments showed that stimulation of hSPR in Sf9 membranes activated multiple endogenous G proteins including Galpha(o), Galpha(q/11), and Galpha(s). While hSPR's ability to associate with Gq/11 is well-documented, the present study provides the first evidence of hSPR's potential to activate Galpha(o) and Galpha(s).

The tachykinin NK1 receptor. Part I: ligands and mechanisms of cellular activation

The tachykinin NK1 receptor is widely expressed in the mammalian central and peripheral nervous system. Powerful pharmacological tools (agonists and antagonists) are now available to elucidate the physiological role of NK1 receptors at these levels, as well as to understand their role in diseases and establish the possible therapeutic usefulness of NK1 receptor antagonists for treatment of human diseases. The structure-activity studies that have led to the development of potent peptide and non-peptide ligands for the tachykinin NK1 receptor are here reviewed. Among the peptide agonists and antagonists, linear and cyclic sequences have been developed. The non peptide antagonists belong to different chemical classes, i.e. steroids, perhydroisoindolones, quinuclidines, piperidines and tryptophane derivatives. The first non peptide antagonists for NK1 receptors have been obtained by random screening of chemical compounds large collections. The resulting leads were optimized with 'classic' structure activity approaches, aiming at identifying 'common' motifs for interaction with the receptor by ligands of different chemical classes. The results derived from the recent application of molecular biology techniques were useful to drive the design of new ligands toward a precise structural definition of ligand-receptor bi-molecular interactions. Studies on mutant receptors have established that the sites of interaction of peptide agonists and non peptide antagonists with the tachykinin NK1 receptor are largely non overlapping. Moreover, data obtained from mutagenesis of the NK1 receptor further indicate that some amino acid residues in the NK1 receptor sequence are critical for determining the binding affinity of some but not all ligands. Therefore, different antagonists discovered from random screening may not possess common points of interaction or common structural and conformational characteristics for their interaction with the tachykinin NK1 receptor. The tachykinin NK1 receptor couples with G-proteins to determine its biological effects in target cells. Several G-proteins both sensitive (Go, Gi) and insensitive (Gq, G11) to pertussis toxin can mediate the action of NK1 receptors. Moreover, several second messanger signalling systems (elevation of intracellular calcium, stimulation of phosphoinositol turnover, arachidonic acid mobilization, cAMP accumulation) have to be activated following NK1 receptor signalling. Also a direct modulation of certain ion channels at membrane level has been proposed. The NK1 receptor undergoes prompt and significant tachyphylaxis upon exposure to the agonist: this has been shown to be linked with receptor internalization which also occurs physiologically when the NK1 receptor is stimulated by endogenous tachykinins.

Role of the cytoskeleton in calcium signaling in NIH 3T3 cells. An intact cytoskeleton is required for agonist-induced [Ca2+]i signaling, but not for capacitative calcium entry

Treatment of NIH 3T3 cells with cytochalasin D (10 microM, 1 h at 37 degrees C) disrupted the actin cytoskeleton and changed the cells from a planar, extended morphology, to a rounded shape. Calcium mobilization by ATP or by platelet-derived growth factor was abolished, while the ability of thapsigargin (2 microM) to empty calcium stores and activate calcium influx was unaffected. Similar experiments with nocodazole to depolymerize the tubulin network yielded identical results. Platelet-derived growth factor induced an increase in inositol phosphates, and this increase was undiminished in the presence of cytochalasin D. Therefore, the blockade of agonist responses by this drug does not result from decreased phospholipase C. Injection of inositol 1,4,5-trisphosphate (IP3) released calcium to the same extent in control and cytochalasin D-treated cells. Confocal microscopic studies revealed a significant rearrangement of the endoplasmic reticulum after cytochalasin D treatment. Thus, disruption of the cytoskeleton blocks agonist-elicited [Ca2+]i mobilization, but this effect does not result from a lower calcium storage capacity, impaired function of the IP3 receptor, or diminished phospholipase C activity. We suggest that cytoskeletal disruption alters the spatial relationship between phospholipase C and IP3 receptors, impairing phospholipase C-dependent calcium signaling. Capacitative calcium entry was not altered under these conditions, indicating that the coupling between depletion of intracellular calcium stores and calcium entry does not depend on a precise structural relationship between intracellular stores and plasma membrane calcium channels.

A substance P (neurokinin-1) receptor mutant carboxyl-terminally truncated to resemble a naturally occurring receptor isoform displays enhanced responsiveness and resistance to desensitization

Two isoforms of the substance P (SP) receptor, differing in the length of the cytoplasmic carboxyl-terminus by approximately 8 kDa, have been detected previously in rat salivary glands and other tissues. The binding and functional properties of these two isoforms have been investigated using full-length (407 amino acids) and carboxyl-terminally truncated (324 amino acids) rat SP receptors transfected stably into Chinese hamster ovary cells. Both the full-length and the truncated receptor bound radiolabeled SP with a similar Kd ( approximately 0.1 nM). The average number of high affinity SP binding sites per cell was 1.0 x 10(5) and 0.3 x 10(5) for the full-length and the truncated SP receptor, respectively. In both cell lines, SP induced a rapid but transient increase in cytosolic calcium concentration ([Ca2+]i), which consisted of the release of Ca2+ from intracellular stores and the influx of extracellular Ca2+. Both components are dependent on phospholipase C activation. Although the full-length and the truncated receptor utilize the same calcium pathways, they differ in their EC50 values (0.28 nM for the full-length; 0.07 nM for the truncated). These differences in responsiveness may be related to the observed differences in receptor desensitization. The truncated receptor, in contrast to the full-length receptor, does not undergo rapid and long-lasting desensitization. Cells possessing the short isoform of the SP receptor would thus be expected to exhibit a prolonged responsiveness.

Cyclic AMP-induced desensitization of G-protein-regulated phospholipase C in turkey erythrocyte membranes

The interaction of the cyclic AMP and inositol lipid signalling systems was studied in turkey erythrocytes. Elevation of intracellular cyclic AMP concentrations by pretreatment of the cells with forskolin or 8-Br-cAMP resulted in a marked decrease in responsiveness of phospholipase C to G-protein activators in membranes prepared from treated cells. Decreases in responsiveness occurred with a t1/2 of approximately 5 min and were reversible after transfer of desensitized cells to drug-free medium. Pretreatment of the cells with forskolin inhibited inositol phosphate formation in a concentration-dependent manner and addition of the phosphodiesterase inhibitor IBMX 93-isobutyl-1-methylxanthine) during pretreatment increased the capacity of forskolin to desensitize phospholipase C activity. IBMX also produced a similar potentiation of forskolin-stimulated accumulation of cyclic AMP in turkey erythrocytes. Isoproterenol pretreatment of the cells induced, like forskolin, partial inhibition of inositol phosphate generation in response to G-protein activators and to P2y purinoceptor and beta-adrenoceptor agonists. The capacity of isoproterenol to induce desensitization of phospholipase C activity also was increased by the presence of IBMX during pretreatment of the cells. H8 (N-[2-(methylamino)ethyl]-5-isoquinoline-sulfonamide), an inhibitor of cyclic AMP-regulated protein kinase, completely prevented forskolin-induced desensitization but only partially blocked isoproterenol-induced desensitization. These results indicate that the cyclic AMP signalling cascade has a major inhibitory influence on receptor- and G-protein-activated inositol lipid signaling.

Vesicle-associated membrane protein 2 is essential for cAMP-regulated exocytosis in rat parotid acinar cells. The inhibition of cAMP-dependent amylase release by botulinum neurotoxin B

Amylase exocytosis of the parotid gland is mediated by intracellular cAMP. To investigate whether cAMP-dependent secretion has a mechanism similar to that of regulated neuroexocytosis, we examined the expression of synaptosome-associated proteins. In rat parotid acinar cells, we found 25 (p25) and 18 kDa (p18) proteins reacted with antibodies against Rab3A and vesicle-associated membrane protein 2 (VAMP-2), respectively. On the other hand, syntaxin 1 and SNAP-25, which interact with VAMP-2 at synapses, were undetectable. Rab3A-like p25 and VAMP-2-like p18 were also expressed in other exocrine acinar cells. The latter was localized at secretory granule membranes, and the former was detected in secretory granule and cytosolic fractions. The antibody against VAMP-2 used in this study did not react with cellubrevin, and p18 was cleaved with botulinum neurotoxin B. Thus, we identified p18 as VAMP-2. Botulinum neurotoxin B inhibited the cAMP-induced amylase release from streptolysin O-permeabilized acinar cells. Therefore, VAMP-2 is required for cAMP-regulated amylase release in rat parotid acinar cells. This is the first report that VAMP-2 is involved in regulated exocytosis that is independent of Ca2+.

Histamine H1 receptor-stimulated Ca2+ signaling pathway in human periodontal ligament cells

We studied histamine-induced Ca2+ mobilization in human periodontal ligament (HPDL) cells. Histamine induced a transient rise in intracellular Ca2+ ([Ca2+]i) and maintained a sustained phase in the presence of extracellular Ca2+. In the absence of extracellular Ca2+, the transient peak was slightly reduced and the sustained phase was decreased to the basal level. The initial rise in [Ca2+]i was attributed to two components: intracellular Ca2+ release and Ca2+ influx, whereas the sustained phase was due to Ca2+ influx. After depletion of intracellular Ca2+ stores with thapsigargin, a known Ca(2+)-ATPase inhibitor, histamine-induced increase in [Ca2+]i was significantly reduced, suggesting histamine induces Ca2+ release from inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]- and thapsigargin-sensitive Ca2+ stores. Histamine-induced peak in [Ca2+]i was increased dose-dependently in the presence and absence of extracellular Ca2+. The histamine-mediated response in [Ca2+]i was specifically attenuated by chlorpheniramine (H1 antagonist) but not by cimetidine (H2 antagonist), clearly indicating that activation of H1 receptor mediates histamine-induced Ca2+ mobilization. We next examined the effect of histamine on inositol phosphates formation. Histamine stimulated the formation of inositol phosphates which changed time-dependently. In particular, the formation of Ins(1,4,5)P3 was increased significantly for 10 s. The histamine-induced Ca2+ mobilization caused an increase of prostaglandin E2 (PGE2) release which was reduced in excluding extracellular Ca2+. These results indicate that activation of histamine H1 receptor induces the accumulation of Ins(1,4,5)P3 and the following transient increase in [Ca2+]i, and elicits the release of PGE2 which may be coupled with Ca2+ influx.

Characterization and regulation of neurokinin1 receptors in primary cultures of rat neonatal spinal neurons

Neurokinin1 receptors are the primary target of substance P released from neurons during neural transmission, yet little is known regarding the regulation of neurokinin1 receptors on neurons. 125I-Bolton-Hunter-substance P was used in the present studies to determine whether primary cultures of rat neonatal spinal cord express neurokinin1 receptors and, therefore, can be used as a model to explore regulation of neuronal neurokinin1 receptors. 125I-Bolton-Hunter-substance P bound to a single site in neuron-enriched cultures with an affinity of 256 nM, which is comparable to its high affinity binding on adult rat spinal cord. Treatment of neuron-enriched cultures with 10 nM substance P resulted in a 47% decrease in 125I-Bolton-Hunter-substance P binding at 24 h which was due to a decrease in affinity of the receptor. However, at 48 h after treatment with substance P, 125I-Bolton-Hunter-substance P binding increased by 44%. The increase in binding was due to a two-fold increase in receptor density. These changes occurred in the presence of 0.5 microM tetrodotoxin, decreasing the likelihood that the changes in binding were secondary to the release of transmitters by substance P. Furthermore, substance P1-7, a metabolite of substance P that has putative physiological effects, did not alter 125I-Bolton-Hunter-substance P binding. These data provide evidence that neuronal neurokinin1 receptors are under homologous regulation in primary cultures of neonatal rat spinal cord and suggest that a similar mechanism occurs in vivo. Furthermore, the data suggest that a decrease in affinity of the neurokinin1 receptor may contribute, in part, to tachyphylaxis to substance P.

Tachykinins alter inositol phosphate formation, but not cyclic AMP levels, in primary cultures of neonatal rat spinal neurons through activation of neurokinin receptors

The naturally occurring tachykinins, substance P, neurokinin A and neurokinin B, induce the formation of inositol phosphates or cAMP in a variety of tissues but their effects on neurons have not been resolved. We used primary cultures of neonatal rat spinal cord to determine whether neurokinin receptors mediate changes in these second messengers in spinal neurons. We found that substance P, neurokinin A and neurokinin B induced the formation of inositol phosphates in a concentration-dependent manner with similar potencies (EC50S: 3.6, 5.7 and 21.3 nM, respectively), but at concentrations tested (0.1-1.0 microM) these peptides had no effect on cAMP levels. All three tachykinins induced the formation of inositol phosphates predominately by activation of neurokinin1 receptors. CP-96,345 and WIN 51,708, neurokinin1 receptor antagonists, attenuated the response to substance P, neurokinin A and neurokinin B. GR 103,537, a neurokinin2 receptor antagonist, had no effect on the responses induced by any of the tachykinins. Furthermore, the selective neurokinin1 receptor agonist, GR-73632, induced the formation of inositol phosphates in a concentration-dependent manner, whereas the selective neurokinin2 receptor agonist, GR-64349, generated inositol phosphates only at the highest concentration tested (10 microM). Senktide, a neurokinin3 receptor agonist, did not induce the formation of inositol phosphates at any of the concentrations tested (0.01-10 microM). Inositol phosphate formation appeared to be due to a direct effect of the tachykinins on neuronal neurokinin1 receptors. These results suggest that biological responses in spinal neurons following activation of neurokinin1 receptors are mediated mainly by the hydrolysis of phosphoinositol 4,5-bisphosphate to form inositol 1,4,5-trisphosphate and diacylglycerol. It remains to be determined which of these second messengers mediates the increased neuronal excitability and depolarization that occurs in response to substance P.

Further studies on the mechanism of action of substance P in rat brain, involving selective phosphatidylinositol hydrolysis

We have suggested that substance P, in cerebral cortex, causes a phosphatidylinositol (PI) breakdown by a dual mechanism suggesting the involvement of either phospholipase A2 or phospholipase C. We have presently characterized further these effects. Substance P (65 pM) provoked an increase in lysoPI concomitant with a decrease in PI level. This finding confirms the involvement of phospholipase A2 activation. To study the involvement of phospholipase C in the action of higher doses (0.65 microM) of the peptide, we used pulse-chase experiments (where phospholipid depletion was monitored) and short-term 32P-labeled slices (where phospholipid synthesis was studied). Substance P evoked an acceleration of both hydrolysis and resynthesis of PI as early as 15 s. A prolonged exposure (30 min) resulted in stimulation of PI hydrolysis without subsequent resynthesis. The peptide did not cause any effect on inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate. These alterations in PI metabolism take place simultaneously with a generation of diacylglycerol which showed two maxima at both indicated times.

Central cardiovascular and behavioral effects of carboxy- and amino-terminal fragments of substance P in conscious rats

The central cardiovascular and behavioral effects of carboxy- (SP 5-11, SP 6-11, SP 7-11, SP 8-11) and amino- (SP 1-7, SP 1-9) terminal substance P (SP) fragments were compared with those of SP 1-11 in conscious rats. In addition, the ability of these SP-fragments to induce desensitization of the central NK1 receptor was investigated. SP 1-11 (50 pmol) injected i.c.v. induced an increase in mean arterial blood pressure (MAP), heart rate (HR) and a typical behavioral response consisting of face washing (FW), hindquarter grooming (HQG) and wet-dog shakes (WDS). The cardiovascular and behavioral responses to equimolar doses of SP 5-11 and SP 6-11 were similar to those of SP 1-11, however, only SP 5-11 induced exactly the same behavioral pattern as SP 1-11. SP 6-11 was more potent in inducing FW and WDS than SP 1-11 or SP 5-11. The carboxy-terminal SP-fragments, SP 7-11 and SP 8-11, and the amino-terminal SP-fragments, SP 1-7, SP 1-9, did not elicit any significant cardiovascular or behavioral responses. Pretreatment with SP 1-11 reduced the cardiovascular and behavioral responses to subsequent injections of SP 1-11. Of all SP-fragments tested, only SP 5-11 was able to attenuate the cardiovascular and behavioral responses to SP 1-11. Our results demonstrate that SP 6-11 represents the shortest carboxy-terminal amino acid sequence, that after i.c.v. injection, elicits the same cardiovascular response as SP 1-11, but fails to desensitize the NK1 receptor. The carboxy-terminal fragment, SP 5-11, is the shortest amino acid sequence which produces the same pattern of central cardiovascular and behavioral responses as SP 1-11 and also retains the ability to desensitize the NK1 receptor like SP 1-11.

The dephosphorylation of 22-kDa phosphoprotein by type 2B protein phosphatase in rat parotid acinar cells

In saponin-permeabilized rat parotid acinar cells, cyclic AMP and 3-isobutyl-1-methylxanthine stimulated the phosphorylation of three particulate proteins with molecular masses of 34, 26 and 22 kDa. The particulate fractions containing 22-kDa phosphoprotein were isolated from the cells labelled with [gamma-32P]ATP and used to study the dephosphorylation of the 22-kDa phosphoprotein. When the labelled fractions were incubated at 30 degrees C in the presence of 0.3 mM CaCl2 and 10 micrograms calmodulin, dephosphorylation of the 22-kDa phosphoprotein was evoked. Further addition of the type 2B phosphatase (Ca2+/calmodulin-stimulated protein phosphatase purified from bovine brain) resulted in a remarkable dephosphorylation of the 22-kDa phosphoprotein. Western immunoblotting showed that type 2B protein phosphatase exists in rat parotid acinar cells. These results suggest that type 2B protein phosphatase in those cells is involved in the dephosphorylation of the 22-kDa phosphoprotein.

Carbachol, substance P, and phorbol ester promote the tyrosine phosphorylation of protein kinase C delta in salivary gland epithelial cells

The initiation of saliva formation by parotid acinar cells, which comprise the majority of cells in this salivary gland, is initiated by the release of neurotransmitters (acetylcholine, substance P) from parasympathetic nerves. In response to substance P and the muscarinic agonist carbachol, two ligands that activate phospholipase C-linked receptors, which stimulate fluid secretion, PKC delta was phosphorylated on tyrosine residues. The maximal agonist-dependent tyrosine phosphorylation occurred within seconds of the addition of either agonist and then returned rapidly to a smaller increased level. Phorbol ester also caused a rapid increase in tyrosine phosphorylation, which reached a maximal level 5 min after the addition of phorbol 12-myristate 13-acetate. The increase in tyrosine phosphorylation of PKC delta was blocked by tyrosine kinase inhibitors genistein and staurosporine. Ionophore-mediated elevation of [Ca2+]i or activation of the beta-adrenergic receptor, epidermal growth factor receptor, or insulin receptor did not promote the tyrosine phosphorylation of PKC delta. These results indicate that tyrosine phosphorylation plays a role in early signal transduction events promoted by the activation of muscarinic and substance P receptors and suggests that the tyrosine phosphorylation of PKC delta has a role in the activation of fluid secretion by neurotransmitters binding to phospholipase C-linked receptors.

Heterologous desensitization of bombesin- and vasopressin-stimulated phospholipase D activity in Swiss 3T3 fibroblasts

Bombesin- and vasopressin-stimulated phospholipase D (PLD) activities are rapidly desensitized in 3T3 cells, in addition both agonists are subject to heterologous desensitization. Binding studies showed that homologous desensitization was partly a result of loss of cell surface receptors, whilst heterologous desensitization was independent of receptor changes. Pretreatment with either agonist reduced subsequent GTP gamma S-stimulated PLD activity by 50% whereas a pretreatment with GTP gamma S did not attenuate the response, suggesting that the G-protein or downstream effector systems were affected by receptor activation resulting in desensitization. The desensitization of receptor-stimulated PLD activation provides support for the phospholipase functioning in a key signalling pathway.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Possible involvement of adenylylation in the modification of a 26 kDa protein in rat parotid acinar cells

1. Adenylylation, a posttranslational modification of proteins, was investigated in saponin-permeabilized acinar cells of the rat parotid gland. 2. When cells were incubated with [2,8-3H]ATP, several proteins, including a 26 kDa protein in the particulate fraction, were labeled. 3. Upon incubation of cells with [alpha-32P]ATP in the presence of cAMP and 3-isobutyl-1-methylxanthine, 32P-labeling of the 26 kDa protein was observed. 4. After treatment with snake venom phosphodiesterase, [32P]AMP was released from the 26 kDa protein. Such release was not observed when cells were labeled with [gamma-32P]ATP. 5. The 32P-labeling pattern of proteins with [alpha-32P]ATP was clearly different from that with [adenylate-32P]NAD+. 6. The results suggest that the 26 kDa protein is one of the adenylylation substrates in rat parotid acinar cells.

Modulatory effect of 4 beta-phorbol 12-myristate 13-acetate (PMA) on carbachol-induced Ca2+ mobilization in rat parotid acinar cells

Treatment of rat parotid acinar cells with 4 beta-phorbol 12-myristate 13-acetate (PMA) significantly inhibited an increase in cytosolic free Ca2+ concentration ([Ca2+]i) induced by carbachol (CCh), a muscarinic agonist. The CCh-induced increase in [Ca2+]i was also inhibited by another active phorbol ester, 4 beta-phorbol 12,13-dibutyrate, but not by 4 alpha-phorbol 12,13-didecanoate, which does not activate protein kinase C. The treatment with PMA had no effect on increases in [Ca2+]i evoked by ionomycin and thapsigargin, which do not stimulate phosphoinositide hydrolysis. In contrast, an increase in [Ca2+]i induced by NaF, a direct activator of GTP-binding proteins, was delayed in the presence of PMA. The formation of inositol phosphates in response to CCh was suppressed significantly by PMA treatment. In radioligand binding assays, PMA did not directly interfere with the specific binding of [3H]quinuclidinyl benzilate ([3H]QNB), a muscarinic antagonist, to plasma membranes. Furthermore, the [3H]QNB binding to plasma membranes prepared from the PMA-pretreated cells was not different from that to the control membranes. These results indicate that PMA attenuated the CCh-induced increase in [Ca2+]i through inhibition of phosphoinositide hydrolysis. Activation of protein kinase C may play a role in negative-feedback control of the muscarinic pathway in rat parotid acinar cells.

Substance P and neurokinin A induced desensitization to cardiovascular and behavioral effects: evidence for the involvement of different tachykinin receptors

Desensitization and cross-desensitization to the cardiovascular and behavioral effects elicited by intracerebroventricular (i.c.v.) substance P (SP) and neurokinin A (NKA) injections were examined in conscious, freely moving rats. The cardiovascular responses to equimolar doses of both peptides were identical, however, the pattern of the behavioral responses differed. Relative to SP, NKA was weaker in eliciting hindquarter grooming but more effective in eliciting wet dog shakes. SP pretreatment (50 pmol) desensitized the cardiovascular and behavioral responses to both, subsequent injections of SP (50 pmol) as well as of NKA (50 or 500 pmol) injected 30 or 60 min after SP, indicating cross-desensitization. NKA pretreatment (50 pmol) partly reduced the cardiovascular but not the behavioral responses to subsequent equimolar doses of NKA. The cardiovascular responses to SP (50 pmol) were reduced only 30 min but not 60 min after pretreatment with a 10 times higher dose of NKA (500 pmol). Of all behavioral manifestations to i.c.v. SP, only hindquarter grooming was attenuated by pretreatment with either dose of NKA. The equal potency of SP and NKA in eliciting the cardiovascular effects but different pattern of behavioral responses to these peptides suggest an involvement of different types of tachykinin receptors in mediating the central effects of the two peptides. The fact that NKA induced cross-desensitization selectively to one type of behavioral manifestations elicited by SP, indicates the existence of two subtypes of SP (NK1) receptors in the rat brain.

Desensitization of cell signalling mediated by phosphoinositidase C

The waning of responses to cell-surface receptor activation during persistent stimulation with agonists (desensitization) is a feature common to many forms of transmembrane signalling. However, information is scarce regarding the regulatory processes that modulate the extensive group of receptors linked via phosphoinositidase C to the production of inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. This situation is now beginning to change. Recent data indicate (1) that very rapid desensitization, possibly associated with receptor phosphorylation, regulates receptors linked to phosphoinositidase C, (2) that different receptors are desensitized to varying extents, (3) that receptor internalization can mediate desensitization at later times and (4) that signalling can be regulated at additional sites downstream of phosphoinositidase C. As Richard Wojcikiewicz and colleagues discuss here, these diverse regulatory events provide the means by which the breakdown of phosphoinositides and cellular responsiveness to their products are controlled during cell stimulation.

Tachykinin receptors: a radioligand binding perspective

The last decade has witnessed major breakthroughs in the study of tachykinin receptors. The currently described NK-1, NK-2, and NK-3 receptors have been sequenced and cloned from various mammalian sources. A far greater variety of tachykinin analogues are now available for use as selective agonists and antagonists. Importantly, potent nonpeptide antagonists highly selective for the NK-1 and NK-2 receptors have been developed recently. These improved tools for tachykinin receptor characterization have enabled us to describe at least three distinct receptor types. Furthermore, novel antagonists have yielded radioligand binding and functional data strongly favoring the existence of putative subtypes of NK-1 and especially NK-2 receptors. Whether these subtypes are species variants or true within-species subtypes awaits further evidence. As yet undiscovered mammalian tachykinins, or bioactive fragments, may have superior potency at a specific receptor class. The common C terminus of tachykinins permits varying degrees of interaction at essentially all tachykinin receptors. Although the exact physiological significance of this inherent capacity for receptor "cross talk" remains unknown, one implication is for multiple endogenous ligands at a single receptor. For example, NP gamma and NPK appear to be the preferred agonists and binding competitors at some NK-2 receptors, previously thought of as exclusively "NKA-preferring." Current evidence suggests that tachykinin coexistence and expression of multiple receptors may also occur with postulated NK-2 and NK-1 receptor subtypes. Other "tachykinin" receptors may recognize preprotachykinins and the N terminus of SP. In light of these recent developments, the convenient working hypothesis of three endogenous ligands (SP, NKA, and NKB) for three basic receptor types (NK-1, NK-2, and NK-3) may be too simplistic and in need of amendment as future developments occur (Burcher et al., 1991b). In retrospect, the 1980s contributed greatly to our understanding of the structure, function, and regulation of tachykinins and their various receptors. The development of improved, receptor subtype-selective antagonists and radioligands, in addition to recent advances in molecular biological techniques, may lead to a more conclusive pharmacological and biochemical characterization of tachykinin receptors. The 1990s may prove to be the decade of application, where a better understanding of the roles played by endogenous tachykinins (at various receptor subtypes) under pathophysiological conditions will no doubt hasten the realization of clinically useful therapeutic agents.

G protein beta gamma-subunits inhibit purified adenylate cyclase independent of the activation by Ca2+ and calmodulin

Adenylate cyclase purified by affinity chromatography was activated about 2.5-fold in a Ca(2+)- and calmodulin-dependent fashion. G protein beta gamma-subunits, an inhibitor in the receptor-mediated inhibition of adenylate cyclase, inhibited the purified cyclase by more than 80%. The extent of beta gamma-induced inhibition was not affected by the activation with Ca2+ and calmodulin. Moreover, the prior addition of the beta gamma-subunits to the cyclase did not prevent the subsequent activation of the enzyme by Ca2+ and calmodulin. We conclude that the beta gamma-subunits inhibit adenylate cyclase activity in a calmodulin-independent mode.

Evidence that substance-P receptors do not exist in mouse parotid and submandibular acinar cells

Substance P (SP) had no effect on cytosolic Ca2+ concentration ([Ca2+]i) and inositol phosphate formation in mouse parotid and submandibular acinar cells, but induced a rapid increase in [Ca2+]i and production of inositol phosphates in rat acinar cells. In addition, SP did not stimulate amylase release from mouse parotid acini, but SP-induced amylase release was seen in rats. These results indicate that the mouse lacks SP receptors on parotid and submandibular acinar cells.

Substance P receptor desensitization in the dorsal horn: possible involvement of receptor-G protein complexes

The repeated administration of a high dose of substance P (SP) onto the spinal cord has been shown to attenuate behavioral responses to an intense heat (tail-flick) or noxious mechanical stimulus (paw pressure). Studies performed to investigate the action of spinal SP have suggested that changes in behavioral responses involve endogenous opiate or neurokinin systems. This study was performed to investigate whether the binding characteristics of SP receptors in the dorsal horn are altered following successive administration of SP. Two populations of [3H]-SP binding sites were distinguished on the basis of their binding affinity. Gpp(NH)p, a stable analogue of GTP, decreased the size and affinity of the high affinity binding component selectively labelled with [125I]-Bolton Hunter-SP. Repeated intrathecal administration of SP (15 micrograms) which reduced behaviors also reduced the number and affinity of high affinity binding sites. Thus, attenuated behaviors in response to repeated administration of SP are paralleled by an alteration of SP binding in the dorsal horn. The altered agonist affinity seen under desensitizing conditions raises the possibility that SP receptor desensitization involves an uncoupling of receptor-G protein complexes.

Vasoconstrictor and dilator responses to neurokinin A in isolated guinea pig heart

Effects of neurokinin A (NKA) and substance P (SP) on coronary resistance vessels were studied in isolated guinea pig hearts perfused with isotonic buffer containing 20 mM KCl. Injections of NKA and SP caused dose-dependent reductions in perfusion pressure with ED50 values of 14.0 and 0.326 pmol, respectively. Blockade of nitric oxide synthesis or removal of the endothelium inhibited vasodilator responses to neurokinins. Infusions of NKA or SP caused tachyphylaxis and cross-desensitization to the other neurokinin but not to acetylcholine. Injections of 2.5 nmol NKA increased perfusion pressure by 31 +/- 8% when given after tachyphylaxis developed to infused SP (2.5 nmol/100 microliters/min). It was concluded that 1) neurokinins cause an endothelium-dependent relaxation of coronary resistance vessels by stimulating NK-1 receptors on endothelial cells, and 2) desensitization of the receptor mediating vasodilation unmasks a vasoconstrictor response to NKA.

Autoradiographic analysis of 125I-substance P binding in rat spinal cord following chronic constriction injury of the sciatic nerve

Using receptor binding and autoradiographic techniques, changes in Bolton-Hunter labeled 125I-substance P (125I-BH-SP) binding were determined in laminae I/II, V and X of rat lumbar spinal cord after chronic constriction injury (CCI) of the sciatic nerve. When compared to the sham-operated side of the control group, SP binding significantly increased ipsilateral to the CCI in laminae I/II at 5, 10 and 20 days after injury and in lamina V at 5 days after injury. Scatchard analysis was performed on the 125I-BH-SP binding to the NK1 receptor in laminae I/II of rats 5 days after generation of the CCI. A significant decrease in the Kd of 125I-BH-SP binding was observed in laminae I/II ipsilateral to CCI when compared with the control side (ipsilateral to sham surgery). There was no significant change in the Bmax in laminae I/II ipsilateral to CCI. The changes in 125I-BH-SP binding in the rat spinal cord that occurred after CCI were found in areas of the spinal cord that receive terminations of nociceptive primary afferent fibers. The increased affinity of the NK1 binding site that we report could result in an increase in SP receptor activation in laminae I/II. Such central changes in SP binding may contribute to the neuropathic pain syndrome observed in rats with the CCI.

Isoproterenol-stimulated labelling of particulate proteins by using [adenylate-32P]NAD+ independent on a cAMP-dependent protein kinase in parotid acinar cells

When saponin-permeabilized rat parotid acinar cells were incubated with [adenylate-32P]NAD+, labelling of proteins (33, 27 and 23 kDa) in particulate fractions of the cells was stimulated by isoproterenol. The effect of isoproterenol was completely blocked by a beta-antagonist. Both forskolin or cAMP mimicked the effect of isoproterenol on the labelling. However, an inhibitor of cAMPdPK failed to induce complete inhibition of the effects of isoproterenol, forskolin and cAMP. When the labelled proteins were treated with snake venom phosphodiesterase, neither [32P]5'-AMP nor [32P]phosphoribosyladenosine was released. These results suggest that covalent modification of proteins with NAD+, which is distinct from ADP-ribosylation and cAMPdPK-dependent phosphorylation, is coupled to beta-receptor-cAMP signalling system in rat parotid acinar cells.

Sphingosine stimulates calcium mobilization in rat parotid acinar cells

In fura-2-loaded parotid acinar cells, 50-200 microM sphingosine induced an increase in cytosolic Ca2+ ([Ca2+]i). When extracellular Ca2+ was chelated by EGTA, 50 microM sphingosine failed to increase [Ca2+]i, but 100 or 200 microM sphingosine induced a slight and transient increase in [Ca2+]i. The addition of LaCl3 to the medium resulted in the same effect as chelation of extracellular Ca2+. When cells were incubated in low Ca2+ medium containing sphingosine, and extracellular Ca2+ was subsequently added, a rapid increase in [Ca2+]i depending on the concentration of sphingosine was shown. In low Ca2+ medium, a slight increase in [Ca2+]i induced by high concentrations of sphingosine was not shown after the transient increase in [Ca2+]i elicited by methacholine. Inhibitors of protein kinase C, H-7 and K252a, did not mimic the effect of sphingosine on [Ca2+]i. These results suggest that sphingosine stimulates Ca(2+)-influx and further stimulates the release of Ca2+ from agonist-sensitive intracellular pools by a mechanism that is independent of protein kinase C.

Protein kinase C-dependent diacylglycerol formation is mediated via Ca2+/calmodulin in parotid cells

The kinetics of carbachol-induced sn-1,2-diacylglycerol (DAG) formation and the underlying mechanism(s) involved in parotid acinar cells were investigated. Supramaximal concentrations of carbachol for amylase secretion (10 microM) caused a transient rise in DAG levels at 10 s. In contrast, this rapid rise was not elicited by 1 microM carbachol, which is the maximally effective concentration for amylase secretion. Carbachol (10 microM) also increased DAG levels linearly up to 20 min, which were sustained for up to a further 10 min. DAG formation stimulated by 1 microM carbachol was biphasic; the first peak was observed after 5 min and the second after 20 min. DAG formation induced by 0.01-0.1 microM carbachol was concentration-dependent and monophasic, peaking at 5 min. The second peak evoked by carbachol was partly inhibited by Ca2+ deprivation from the extracellular space, whereas the first peak was not. Similar results were obtained in experiments using Ca2+ antagonists such as verapamil and LaCl3. The protein kinase C inhibitors, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) and staurosporine, and a calmodulin antagonist, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), significantly inhibited the second DAG peak produced by 1 microM carbachol, but did not alter the first peak. The degree of inhibition of the second peak by these antagonists was comparable. Furthermore, the inhibitory effect of staurosporine and W-7 was concentration-dependent. The A23187-induced accumulation of DAG also was abolished by both staurosporine and W-7. These data indicate that a protein kinase C-dependent mechanism(s) is involved in mediating the second DAG accumulation peak induced by 1 microM carbachol and is mainly regulated by the Ca(2+)-calmodulin complex.

Relationship between the calcium-mobilizing action of inositol 1,4,5-trisphosphate in permeable AR4-2J cells and the estimated levels of inositol 1,4,5-trisphosphate in intact AR4-2J cells

Various experimental strategies were employed in an effort to explain the previously reported [Horstman, Takemura & Putney (1988) J. Biol. Chem. 263, 15297-15303] paradoxically high levels of inositol 1,4,5-trisphosphate [(1,4,5)IP3] in resting and substance-P-stimulated AR4-2J cells. The concentration-effect curves for substance-P-induced [3H](1,4,5)IP3 formation in [3H]inositol-labelled cells and substance-P-induced increase in intracellular [Ca2+] were essentially superimposable, suggesting that formation of (1,4,5)IP3 is limiting for cellular Ca2+ mobilization. In electrically permeabilized AR4-2J cells, (1,4,5)IP3 and other inositol polyphosphates stimulated Ca2+ release with potencies similar to those reported for other cell types, including the parent pancreatic acinar cell. Compartmentalization of basal (1,4,5)IP3 was suggested by the fact that this material was stable in the presence of antimycin A, although this toxin completely blocked agonist stimulation of phospholipase C. However, subcellular fractionation as well as permeabilization of the cells with Staphylococcus aureus alpha-toxin failed to provide evidence for binding or sequestration of [3H](1,4,5)IP3 in AR4-2J cells. The density of (1,4,5)IP3 receptors in AR4-2J cells was not sufficiently large to impose non-linearity in the relationship between (1,4,5)IP3 concentration and (1,4,5)IP3-induced Ca2+ release. Thus the apparent high concentrations of (1,4,5)IP3 in resting and stimulated AR4-2J cells are not indicative of atypically low sensitivity or high concentration of (1,4,5)IP3 receptors, nor is there evidence for compartmentalization of (1,4,5)IP3 outside of the cytoplasm in these cells. It is possible that soluble factors in the cytoplasm of AR4-2J cells regulate the free concentration of (1,4,5)IP3 or the sensitivity of receptors to (1,4,5)IP3.

Sphingosine increases inositol trisphosphate in rat parotid acinar cells by a mechanism that is independent of protein kinase C but dependent on extracellular calcium

In rat parotid acinar cells prelabelled with [3H]-inositol, sphingosine stimulated the accumulation of [3H]-inositol polyphosphates. When the cells were exposed to sphingosine, [3H]-inositol trisphosphate (InsP3) was accumulated in a time- and dose-dependent manner. When the extracellular Ca2+ was chelated by 1 mM EGTA, the effect of sphingosine on InsP3 accumulation was completely inhibited. Ionophores, A23187 and ionomycin, had no significant effect on InsP3 accumulation. An inhibitor of protein kinase C, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), failed to stimulate InsP3 accumulation. In the homogenate of parotid acinar cells, InsP3 3-kinase and 5-phosphomonoesterase activities were not affected by sphingosine. These results suggest that sphingosine activates phosphoinositide turnover by a mechanism dependent upon extracellular Ca2+, but different from that of an ionophore, and independent of protein kinase C.

Diaphragmatic fatigue is associated with decreased inositol 1,4,5-trisphosphate formation

The ability of the rat diaphragm to produce inositol 1,4,5-trisphosphate (InsP3) in response to a maximal contractile stimulus was determine in vitro in both fatigued and nonfatigued diaphragms. InsP3 was produced during a maximal contraction of the diaphragm. After inducing fatigue, there was a significant reduction in the production of InsP3 compared with that in nonfatigued muscle. The maximal force generated by the diaphragm was also decreased after fatigue. A significant positive linear correlation was found between the force developed by the diaphragm and the amount of InsP3 liberated.

Does beta-adrenoceptor activation stimulate Ca2+ mobilization and inositol trisphosphate formation in parotid acinar cells?

The effects of the beta-adrenoceptor agonist, isoprenaline, on Ca2+ mobilization and inositol phosphate formation in parotid acinar cells were examined. Isoprenaline (2 microM) failed to increase cytosolic [Ca2+] in acinar cells, as measured by Fura-2 fluorescence, even in the presence of a phosphodiesterase inhibitor. Likewise, neither the 8-bromo nor the dibutyryl derivatives of cAMP (both at 2 mM concentration) increased [Ca2+]i. However, in confirmation of results previously published, a higher concentration of isoprenaline (200 microM) increased cytosolic [Ca2+]i of rat parotid acinar cells, from 104 +/- 4 nM to 151 +/- 18 nM. The increase in [Ca2+]i in response to isoprenaline, while transient in the absence of extracellular Ca2+, was sustained in Ca2(+)-containing medium. This isoprenaline-stimulated Ca2+ signal was more potently antagonized by phentolamine than by propranolol, suggesting that the higher concentration of isoprenaline activated alpha-adrenoceptors. Furthermore, the Ca2+ signal generated in response to the alpha-adrenoceptor agonist, phenylephrine, also was blocked by the same concentrations of propranolol necessary to block the effects of isoprenaline, suggesting that propranolol may block alpha-adrenoceptors under certain experimental conditions. The high concentration of (-)isoprenaline (200 microM) also increased inositol (1,4,5) trisphosphate and inositol (1,3,4) trisphosphate formation 45% within 30 s. Analogous to the increase in intracellular Ca2+, the formation of inositol phosphates stimulated by isoprenaline was more potently antagonized by the alpha-adrenoceptor antagonist, phentolamine, than by the beta-adrenoceptor antagonist, propranolol, again suggesting that isoprenaline interacts with alpha-adrenoceptors on parotid cells. Thus, the effects of isoprenaline on [Ca2+]i do not appear to be mediated by cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)