Hormone-stimulated polyphosphoinositide breakdown in rat liver plasma membranes. Roles of guanine nucleotides and calcium

Guanosine triphosphate can directly regulate cortisol production by activating Ca(2+)-messenger systems in bovine adrenal fasciculata cells

Adenosine triphosphate (ATP) is known to stimulate cortisol production in vitro, however, the effect of guanosine triphosphate (GTP) on cortisol production is not known. We studied the effect of GTP on cortisol production and investigated the regulation of intracellular signal transduction systems, including the cyclic AMP-dependent and Ca(2+)-messenger systems, in bovine adrenal fasciculata cells. GTP clearly induced cortisol biosynthesis but only to a level less than half the adrenocorticotropic hormone (ACTH)-induced maximum. The binding site for [γ-(35)S]-GTPγS was shown to differ completely from that for ATP and also from those for Gs and Gi, as indicated by the fact that binding was not influenced by pretreatment with cholera toxin and pertussis toxin. GTP significantly increased cytosolic calcium ([Ca(2+)]i) and inositol 1, 4, 5-triphosphate without affecting cyclic AMP formation. GTP-induced cortisol production was suppressed by H-9 and Calphostin C (specific protein kinase C inhibitors) but not by H-8 and KT5720 (specific inhibitors of cyclic AMP-dependent protein kinase), suggesting that GTP activates cortisol biosynthesis possibly via a protein kinase C-dependent pathway. Extracellular calcium may be essential for GTP activity since GTP-induced cortisol production was almost completely suppressed in its absence. In conclusion, it can be postulated that GTP-induced steroid secretion in bovine adrenal fasciculata cells is under paracrine or autocrine control.

Regulation of phosphoinositide and phosphatidylcholine phospholipases by G proteins

Two G proteins that regulate phosphoinositide phospholipase C in liver plasma membranes have been purified to homogeneity in both the heterotrimeric and dissociated forms. The heterotrimers contain a 42 kDa or 43 kDa alpha subunit and a 35 kDa beta subunit. The alpha subunits are not ADP-ribosylated by pertussis toxin and are closely related immunologically to members of the recently identified Gq class of G proteins. The specific phosphoinositide phospholipase C isozyme that responds to the G proteins has been determined to the beta 1 isozyme. GTP analogues stimulate phosphatidylcholine hydrolysis in rat liver plasma membranes. The nucleotide specificity and Mg2+ dependency of the response indicate that it is mediated by a G protein. Phosphatidic acid, diacylglycerol, choline and phosphorylcholine are the products, indicating that both phospholipase D and C activities are involved. Activation of phospholipase D is also indicated by the enhanced production of phosphatidyl-ethanol in the presence of ethanol.

Phospholipase Cbeta serves as a coincidence detector through its Ca2+ dependency for triggering retrograde endocannabinoid signal

Endocannabinoids mediate retrograde signal and modulate transmission efficacy at various central synapses. Although endocannabinoid release is induced by either depolarization or activation of G(q/11)-coupled receptors, it is markedly enhanced by the coincidence of depolarization and receptor activation. Here we report that this coincidence is detected by phospholipase Cbeta1 (PLCbeta1) in hippocampal neurons. By measuring cannabinoid-sensitive synaptic currents, we found that the receptor-driven endocannabinoid release was dependent on physiological levels of intracellular Ca(2+) concentration ([Ca(2+)](i)), and markedly enhanced by depolarization-induced [Ca(2+)](i) elevation. Furthermore, we measured PLC activity in intact neurons by using exogenous TRPC6 channel as a biosensor for the PLC product diacylglycerol and found that the receptor-driven PLC activation exhibited similar [Ca(2+)](i) dependence to that of endocannabinoid release. Neither endocannabinoid release nor PLC activation was induced by receptor activation in PLCbeta1 knockout mice. We therefore conclude that PLCbeta1 serves as a coincidence detector through its Ca(2+) dependency for endocannabinoid release in hippocampal neurons.

Identification of the G protein-activating sequence of the single-transmembrane natriuretic peptide receptor C (NPR-C)

Rat natriuretic peptide clearance receptor (NPR-C) contains four sequences capable of inhibiting adenylyl cyclase. We have undertaken mutational and deletion studies on the intracellular domain of rat NPR-C to determine which of these sequences is functionally relevant. Nine mutant receptors were constructed by deletion of 11 or 28 COOH-terminal residues or by site-directed mutagenesis of basic residues in a 17-amino acid sequence, R(469)RNHQEESNIGKHRELR(485), corresponding to the main active peptide. Substitution of arginine residues (R(469)R(470)) flanking the NH(2) terminus abolished G(i1) and G(i2) and PLC-beta activities and inhibition of adenylyl cyclase. Substitution of one or two basic residues (H(481) and/or R(482) or R(485)) in the COOH-terminal motif (H(481)RELR(485)) greatly decreased or abolished G protein and PLC-beta activities and inhibition of adenylyl cyclase. This implies that sequences NH(2)-terminal to the motif or COOH-terminal to R(470) could not sustain receptor activity in situ, although they exhibited activity when used as synthetic peptides. Deletion of the 11 COOH-terminal residues (E(486) to A(496)) suggested an autoinhibitory function for this sequence. We conclude that the 17-amino acid sequence (R(469) to R(485)) in the middle region of the intracellular domain of NPR-C is both necessary and sufficient for activation of G proteins and effector enzymes.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Affinity purification of pancreastatin receptor-Gq/11 protein complex from rat liver membranes

Pancreastatin, a chromogranin A derived peptide, exerts a glycogenolytic effect on the hepatocyte. This effect is initiated by binding to membrane receptors which are coupled to pertussis toxin insensitive G proteins belonging to the Gq/11 family. We have recently solubilized active pancreastatin receptors from rat liver membranes still functionally coupled to G proteins. Here, we have purified pancreastatin receptors by a two-step procedure. First, pancreastatin receptors with their associated Gq/11 regulatory proteins were purified from liver membranes by lectin absorption chromatography on wheat germ agglutinin immobilized on agarose. A biotinylated rat pancreastatin analog was tested for binding to liver membranes before using it for affinity purification. Unlabeled biotinylated rat pancreastatin competed for 125I-labeled [Tyr0]PST binding to solubilized receptors with a Kd = 0.27 nM, comparable to that of native pancreastatin. The biotinylated analog was immobilized on streptavidin-coated Sepharose beads and used to further affinity purify wheat germ agglutinin eluted receptor material. Specific elution at low pH showed that the receptor protein was purified as an 80-kDa protein in association with a G protein of the q/11 family, as demonstrated by specific immunoblot analysis. The specificity of the receptor band was assessed by chemical cross-linking of the purified material followed by SDS-PAGE and autoradiography. In conclusion, we have purified pancreastatin receptor as a glycoprotein of 80 kDa physically associated with a Gq/11 protein.

G(i-1)/G(i-2)-dependent signaling by single-transmembrane natriuretic peptide clearance receptor

Single-transmembrane natriuretic peptide clearance receptor (NPR-C), which is devoid of a cytoplasmic guanylyl cyclase domain, interacts with pertussis toxin (PTx)-sensitive G proteins to activate endothelial nitric oxide synthase (eNOS) expressed in gastrointestinal smooth muscle cells. We examined the ability of NPR-C to activate other effector enzymes in eNOS-deficient tenia coli smooth muscle cells; these cells expressed NPR-C and NPR-B but not NPR-A. Atrial natriuretic peptide (ANP), the selective NPR-C ligand cANP-(4-23), and vasoactive intestinal peptide (VIP) inhibited (125)I-ANP and (125)I-VIP binding to muscle membranes in a pattern indicating high-affinity binding to NPR-C. Interaction of VIP with NPR-C was confirmed by its ability to inhibit (125)I-ANP binding to membranes of NPR-C-transfected COS-1 cells. In tenia muscle cells, all ligands selectively activated G(i-1) and G(i-2); VIP also activated G(s) via VIP(2) receptors. All ligands stimulated phosphoinositide hydrolysis, which was inhibited by ANP-(1-11), PTx, and antibodies to phospholipase C-beta3 (PLC-beta3) and Gbeta. cANP-(4-23) contracted tenia muscle cells; contraction was blocked by U-73122 and PTx and by antibodies to PLC-beta3 and Gbeta in intact and permeabilized muscle cells, respectively. VIP and ANP contracted muscle cells only after inhibition of cAMP- and cGMP-dependent protein kinases. ANP and cANP-(4-23) inhibited forskolin-stimulated cAMP in a PTx-sensitive fashion. We conclude that NPR-C is coupled to activation of PLC-beta3 via betagamma-subunits of G(i-1) and G(i-2) and to inhibition of adenylyl cyclase via alpha-subunits.

Identification of the G protein-activating domain of the natriuretic peptide clearance receptor (NPR-C)

We have shown recently that the 37-amino acid intracellular domain of the single-transmembrane, natriuretic peptide clearance receptor, NPR-C, which is devoid of kinase and guanylyl cyclase activities, activates selectively Gi1 and Gi2 in gastric and tenia coli smooth muscle. In this study, we have used synthetic peptide fragments of the N-terminal, C-terminal, and middle regions of the cytoplasmic domain of NPR-C to identify the G protein-activating sequence. A 17-amino acid peptide of the middle region (Arg469-Arg485), denoted Peptide 4, which possesses two N-terminal arginine residues and a C-terminal B-B-X-X-B motif (where B and X are basic and non-basic residues, respectively) bound selectively to Gi1 and Gi2, activated phospholipase C-beta3 via the betagamma subunits, inhibited adenylyl cyclase, and induced smooth muscle contraction, in similar fashion to the selective NPR-C ligand, cANP4-23. A similar sequence (Peptide 3), but with a partial C-terminal motif, had minimal activity. Sequences which possessed either the N-terminal basic residues (Peptide 1) or the C-terminal B-B-X-X-B motif (Peptide 2) were inactive. Peptide 2, however, inhibited G protein activation and cellular responses mediated by the stimulatory Peptide 4 and by cANP4-23, suggesting that the B-B-X-X-B motif mediated binding but not activation of G protein, thus causing Peptide 2 to act as a competitive inhibitor of G protein activation.

G protein G alpha q/11 and G alpha i1,2 are activated by pancreastatin receptors in rat liver: studies with GTP-gamma 35S and azido-GTP-alpha-32P

In the liver, pancreastatin exerts a glycogenolytic effect through interaction with specific receptors, followed by activation of phospholipase C and guanylate cyclase. Pancreastatin receptor seems to be coupled to two different G protein systems: a pertussis toxin-insensitive G protein that mediates activation of phospholipase C, and a pertussis toxin sensitive G protein that mediates the cyclic GMP production. The aim of this study was to identify the specific G protein subtypes coupling pancreastatin receptors in rat liver membranes. GTP binding was determined by using gamma-35S-GTP; specific anti-G protein alpha subtype sera were used to block the effect of pancreastatin receptor activation. Activation of G proteins was demonstrated by the incorporation of the photoreactive GTP analogue 8-azido-alpha-32P-GTP into liver membranes and into specific immunoprecipitates of different Galpha subunits from soluble rat liver membranes. Pancreastatin stimulation of rat liver membranes increases the binding of gamma-35S-GTP in a time- and dose-dependent manner. Activation of the soluble receptors still led to the pancreastatin dose-dependent stimulation of gamma-35S-GTP binding. Besides, WGA semipurified receptors also stimulates GTP binding. The binding was inhibited by treatment with anti-Galphaq/11 (85%) and anti-Galphai1,2 (15%) sera, whereas anti-Galphao,i3 serum failed to affect the binding. Finally, pancreastatin stimulates GTP photolabeling of particulate membranes. Moreover, it specifically increased the incorporation of 8-azido-alpha-32P-GTP into Galphaq/11 and Galpha, but not into Galphao,i3 from soluble rat liver membranes. In conclusion, pancreastatin stimulation of rat liver membranes led to the activation of Galphaq/11 and Galphai1,2 proteins. These results suggest that Galphaq/11 and Galphai1,2 may play a functional role in the signaling of pancreastatin receptor by mediating the production of IP3 and cGMP respectively.

Alpha 1B-adrenoceptor interacts with multiple sites of transglutaminase II: characteristics of the interaction in binding and activation

We previously reported that a novel GTP binding protein (G alpha h) is tissue type transglutaminase (TGII) and transmits the alpha 1B-adrenoceptor (AR) signal to phospholipase C (PLC) through its GTPase function. We have also shown that PLC-delta 1 is the effector in TGII-mediated signaling. In this study, interaction sites on TGII for the alpha 1B-AR were identified using a peptide approach and site-directed mutagenesis, including in vivo reconstitution of TGIIs with the alpha 1B-AR and PLC-delta 1. To identify the interaction sites, 11 synthetic peptides covering approximately 132 amino acid residues of the C-terminal domain of TGII were tested. The studies with the peptides revealed that three peptides, L547-I561, R564-D581, and Q633-E646, disrupted formation of an alpha 1-agonist-alpha 1B-AR-TGII complex and blocked alpha 1B-AR-mediated TGase inhibition in a dose-dependent manner, indicating that these peptide regions are involved in recognition and activation of TGII by the alpha 1B-AR. These three regions were further evaluated with full-length TGIIs by constructing and coexpressing each site-directed mutant with the alpha 1B-AR and PLC-delta 1 in COS-1 cells. Supporting the findings with these peptides, these TGII mutants lost 56-82% the receptor binding ability and reduced by 29-68% the level of alpha 1B-AR-mediated IP3 production via PLC-delta 1 as compared to those with wild-type TGII. The results also revealed that the regions of R564-D581 and Q633-E646 were the high-affinity binding sites of TGII for the receptor and critical for the activation of TGII by the receptor. Taken together, the studies demonstrate that multiple regions of TGII interact with the alpha 1B-AR and that the alpha 1B-AR stimulates PLC-delta 1 via TGII.

Suppression of calcium oscillation by tri-n-butyltin chloride in cultured rat hepatocytes

The effects of tri-n-butyltin chloride (TBT), an environmental pollutant, on cytoplasmic free calcium ion concentration ([Ca2+]i) were investigated in primary cultured rat hepatocytes. A high concentration (4.0 microM) of TBT increased resting levels of [Ca2+]i and then induced cell blebs resulting in cell death within 2 h. The increase in [Ca2+]i, but not the cell death, depended on the presence of extracellular Ca2+, suggesting that the increase in [Ca2+]i is not critical for the cytotoxicity of TBT. A low concentration (0.1 microM) of TBT did not have any toxic effect (decrease in ATP content, decrease in viability, and shape change) on cultured hepatocytes and did not change [Ca2+]i. However, the calcium responses induced by phenylephrine, [Arg8]-vasopressin, and ATP were suppressed in the cells pretreated with 0.1 microM TBT for 30 min. The suppression was not observed in the cells pretreated with 0.1 microM TBT for only 1 min. Pretreatment with 0.1 microM TBT for 30 min had no effect on the inositol 1,4,5-triphosphate content or its increase in response to hormonal stimulation. These results suggest that TBT suppresses hormone-induced calcium responses at nontoxic low concentrations.

Pancreastatin activates beta3 isoform of phospholipase C via G(alpha)11 protein stimulation in rat liver membranes

Pancreastatin (PST) receptors have been recently shown to mediate activation of phospholipase C (PLC) in rat liver membranes. There is evidence that the G protein that links pancreastatin receptor with PLC-beta is pertussis toxin-insensitive and belongs to the G(alpha)q family. Here, we have employed blocking antisera to sort out the specific PLC-beta isoform as well as the specific G(alpha) subunit activated by PST receptor in rat liver membranes. The presence of different PLC-beta isoforms was checked by immunoblot analysis. Only PLC-beta4 was not detected, whereas PLC-beta1, beta2 and beta3 were abundant in rat liver membranes. However, only anti-PLC-beta3 serum was able to block the PST receptor response. We also checked the expression of G(alpha)q and Galpha11 in rat liver membranes by immunoblot. Even though both isoforms were present. only anti-Galpha11 serum was able to block the PST receptor response. In order to check the specificity of the blocking antisera, we employed them to block the effect of ADP and thrombin stimulating PLC activity in platelet membranes, a system lacking Galpha11. Anti-G(alpha)q but not anti-Galpha11 sera were able to block the agonist stimulated PLC activity. These data suggest that PST receptor response is mediated by the activation of the beta3 isoform of PLC via Galpha11 protein stimulation in rat liver membranes.

Coexpression of ligand-gated P2X and G protein-coupled P2Y receptors in smooth muscle. Preferential activation of P2Y receptors coupled to phospholipase C (PLC)-beta1 via Galphaq/11 and to PLC-beta3 via Gbetagammai3

P2 receptor subtypes and their signaling mechanisms were characterized in dispersed smooth muscle cells. UTP and ATP stimulated inositol 1,4,5-triphosphate formation, Ca2+ release, and contraction that were abolished by U-73122 and guanosine 5'-O-(3-thio)diphosphate, and partly inhibited (50-60%) by pertussis toxin (PTX). ATP analogs (adenosine 5'-(alpha, beta-methylene)triphosphate, adenosine 5'-(beta, gamma-methylene)triphosphate, and 2-methylthio-ATP) stimulated Ca2+ influx and contraction that were abolished by nifedipine and in Ca2+-free medium. Micromolar concentrations of ATP stimulated both Ca2+ influx and Ca2+ release. ATP and UTP activated Gq/11 and Gi3 in gastric and aortic smooth muscle and heart membranes, Gq/11 and Gi1 and/or Gi2 in liver membranes, and Go and Gi1-3 in brain membranes. Phosphoinositide hydrolysis stimulated by ATP and UTP was mediated concurrently by Galphaq/11-dependent activation of phospholipase (PL) C-beta1 and Gbetagammai3-dependent activation of PLC-beta3. Phosphoinositide hydrolysis was partially inhibited by PTX or by antibodies to Galphaq/11, Gbeta, PLC-beta1, or PLC-beta3, and completely inhibited by the following combinations (PLC-beta1 and PLC-beta3 antibodies; Galphaq/11 and Gbeta antibodies; PLC-beta1 and Gbeta antibodies; PTX with either PLC-beta1 or Galphaq/11 antibody). The pattern of responses implied that P2Y2 receptors in visceral, and probably vascular, smooth muscle are coupled to PLC-beta1 via Galphaq/11 and to PLC-beta3 via Gbetagammai3. These receptors co-exist with ligand-gated P2X1 receptors activated by ATP analogs and high levels of ATP.

Pancreastatin receptor is coupled to a guanosine triphosphate-binding protein of the G(q/11)alpha family in rat liver membranes

Pancreastatin (PST), a recently discovered regulatory peptide derived from chromogranin A, has been shown to have a glycogenolytic effect in the hepatocyte that is mediated by increasing intracellular calcium. Our previous studies on pancreastatin signaling suggested that PST receptor is coupled to some G proteins in the plasma membrane of the hepatocyte. The nature of this interaction was investigated using antisera against G(q/11)alpha by different approaches. Indirect evidence of a pertussis toxin (PT)-insensitive G protein of the family of G(q/11)alpha was obtained by measuring high-affinity guanosine triphosphatase (GTPase) activity in soluble rat liver membranes. PST increased GTPase activity in a dose-dependent manner. This effect was only slightly inhibited by PT pretreatment of the membranes, whereas anti-G(q/11)alpha antisera blocked most of the PST-stimulated GTPase activity. The selective association of the PST receptor with this G protein was further studied by the coelution in wheat germ agglutinin semipurification of the receptor and by immunoprecipitation of the G protein-PST receptor complexes using G-protein-specific antisera. A G protein of the family of G(q/11)alpha was found to be associated with the semipurified PST receptor. Moreover, anti-G(q/11)alpha antisera immunoprecipitated most PST-binding activity (95%), bringing down most of the specific G protein, whereas anti-G(il,2)alpha and -G(o,i3)alpha failed to immunoprecipitate the PST-binding activity. Finally, the coupling of the PST receptor with the effector phospholipase C was disrupted by blocking with G(q/11)alpha antisera, suggesting that a G protein of the family of G(q/11)alpha is a signal mediator from PST receptors to phospholipase C activation in rat liver membranes.

Age-related loss of calcitriol stimulation of phosphoinositide hydrolysis in rat skeletal muscle

We have examined the effects in vitro of calcitriol [1,25(OH)2D3], the hormonal form of vitamin D3, on the breakdown of membrane phosphoinositides in skeletal muscle from young (3 months) and aged (24 months) rats. Calcitriol (10(-9) M) induced a rapid and transient release of IP3/inositol phosphates and diacylglycerol (DAG) from muscle slices/membranes prelabeled with [3H]myo-inositol and [3H]arachidonate, respectively. Inositol phosphate release was maximal at 15 s and then declined. The effects of hormone specificity exhibited as the closely related derivatives of vitamin D3, 25OHD3, 1alphaOHD3 and 24,25(OH)2D3 did not alter muscle inositol phosphate levels. The stimulation of DAG was biphasic, the early phase (15 s) being abolished by neomycin (0.5 mM), an inhibitor of phosphoinositide hydrolysis, similar to IP3 formation and consistent with a role of phospholipase C (PLC) in intracellular signal generation. Neomycin had no effect on the second DAG peak (2 min) induced by calcitriol, suggesting that the late phase of DAG formation is independent from the hydrolysis of phosphoinositides. Higher basal inositol phosphate and DAG levels were detected in muscle from aged rats thereby reducing the effects of the hormone on second messenger generation ( -80 and -60% for IP3 and DAG, respectively). Calcitriol stimulation of PLC was mimicked, in both young and old rats, by GTPgammaS, a non-hydrolyzable analogue of GTP, while GDPbetaS, a G protein inhibitor, suppressed the effect of the hormone. The early effects of calcitriol and GTPgammaS were not additive. Bordetella pertussis toxin abolished by 85% the effects of calcitriol on inositol phosphate release in young rats but was without effect in aged animals. These results demonstrate that calcitriol activates phosphoinositide-PLC in rat skeletal muscle by a mechanism which involves a pertussis-sensitive G protein and that the effects of the hormone are altered with ageing.

Solubilization and molecular characterization of active pancreastatin receptors from rat liver membranes

Pancreastatin receptors were solubilized from rat liver membranes with the nonionic detergent Triton X-100. Binding of a iodinated analog of rat pancreastatin ([125I-Tyr0]pancreastatin) to the soluble fraction was time dependent, saturable, and reversible. Scatchard analysis of binding under equilibrium conditions indicated that the soluble extracts contained a single class of pancreastatin-binding sites, with a binding capacity of 14 fmol/mg protein and a Kd of 0.3 nM. As observed with membrane-bound receptors, binding of [125I]pancreastatin to soluble extracts was inhibited by guanine nucleotides with the following rank order of potency: guanyl-5'-yl-imidodiphosphate > GTP > GDP > GMP, indicating that the soluble receptors are functionally linked to G proteins. Molecular analysis of the soluble pancreastatin receptor by covalent cross-linking to [125I]pancreastatin using disuccinimidyl suberate and further identification on SDS-PAGE indicated a single band of 85,000 Mr. Gel filtration of soluble extracts on Sephacryl S-300 revealed two molecular components with binding abilities (Mr 80,000 and 170,000). The higher molecular mass component was more sensitive to guanine nucleotides, and covalent cross-linking of both components to [125I]pancreastatin and further SDS-PAGE analysis revealed again a single band of 85,000 Mr, suggesting an association of the receptor with a G protein. Moreover, direct evidence that a Gq was present in the same chromatographic fraction was obtained by specific immunodetection. The soluble receptor is a glycoprotein that can be specifically bound to the wheat-germ agglutinin lectin. We conclude that we solubilized active pancreastatin receptors from rat liver membranes, and these results support the conclusion that the liver pancreastatin receptor consists of a 80,000 Mr glycoprotein associated with G proteins.

RGS4 and GAIP are GTPase-activating proteins for Gq alpha and block activation of phospholipase C beta by gamma-thio-GTP-Gq alpha

RGS proteins constitute a newly appreciated and large group of negative regulators of G protein signaling. Four members of the RGS family act as GTPase-activating proteins (GAPs) with apparent specificity for members of the Gi alpha subfamily of G protein subunits. We demonstrate here that two RGS proteins, RGS4 and GAIP, also act as GAPs for Gq alpha, the G alpha protein responsible for activation of phospholipase C beta. Furthermore, these RGS proteins block activation of phospholipase C beta by guanosine 5'-(3-O-thio) triphosphate-Gq alpha. GAP activity does not explain this effect, which apparently results from occlusion of the binding site on G alpha for effector. Inhibitory effects of RGS proteins on G protein-mediated signaling pathways can be demonstrated by simple mixture of RGS4 or GAIP with plasma membranes.

Somatostatin receptor-mediated signaling in smooth muscle. Activation of phospholipase C-beta3 by Gbetagamma and inhibition of adenylyl cyclase by Galphai1 and Galphao

In COS-7 cells, all five cloned somatostatin receptors are coupled via inhibitory G proteins to activation of an unidentified phospholipase C-beta (PLC-beta) isozyme and inhibition of adenylyl cyclase. In the present study, intestinal smooth muscle cells (SMC) that express only one receptor type, sstr3, and possess a full complement of G proteins and PLC-beta isozymes were used to identify the PLC-beta isozyme and the G proteins coupled to it and to adenylyl cyclase. Somatostatin-14 bound with high affinity to intestinal SMC; stimulated D-myo-inositol-1,4,5-trisphosphate (IP3) formation, Ca2+ release, and contraction; and inhibited forskolin-stimulated cAMP formation in a pertussis toxin-sensitive fashion. Somatostatin also stimulated phosphoinositide hydrolysis in plasma membranes. Only those somatostatin analogs that shared a high affinity for sstr3 receptors elicited muscle contraction. IP3 formation, Ca2+ release, and contraction in permeabilized SMC and phosphoinositide hydrolysis in plasma membranes were inhibited (approximately 80%) by pretreatment with antibodies to PLC-beta3 but not other PLC-beta isozymes, and by antibodies to Gbeta but not Galpha. Inhibition of cAMP formation was partially blocked by antibody to Galphai1 or Galphao and additively blocked by a combination of both antibodies. Somatostatin-stimulated [35S]GTPgammaS-Galpha complexes in plasma membranes were bound selectively by Galphai1 and Galphao antibodies. We conclude that in smooth muscle sstr3 is coupled to Gi1 and Go; the alpha subunits of both G proteins mediate inhibition of adenylyl cyclase, while the betagamma subunits mediate activation of PLC-beta3.

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

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

Purinergic agonist and G protein stimulation of phospholipase D in rat liver plasma membranes. Independence from phospholipase C activation

Hormonal regulation of phospholipase D (PLD) was studied in isolated rat liver plasma membranes. Purinergic agents and a submaximal concentration of guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S), a non-hydrolyzable analog of GTP, synergistically stimulate phosphatidylethanol formation, a measure of PLD activity. The rank order of efficacy for stimulation of PLD activity in the presence of 0.2 microM GTP gamma S was beta, gamma-methylene-ATP > adenosine 5'-0-(3-thiotriphosphate) = ATP = ADP = 2-methylthio-ATP > alpha, beta-methylene-ATP = UTP. This pattern of activation does not conform to the series at known P2 receptors. GTP gamma S stimulated PLD activity in a dose-dependent manner, and the GTP gamma S dose-response curve for phosphatidylethanol formation was shifted to the left by an analog of ATP. Activation of PLD by purinergic agents in the presence of GTP gamma S supports the involvement of a purinergic receptor of the P2 class and a GTP-binding protein. Purinergic agents competitively inhibited [35S]adenosine 5'-0-(3-thiotriphosphate) binding to plasma membranes in the rank order adenosine 5'-0'(3-thiotriphosphate) > ATP > alpha,beta-methylene-ATP = UTP >> beta, gamma-methylene-ATP = ADP. Stimulation of phosphoinositide phospholipase C (PI-PLC) by purinergic agents, as measured by release of radioactivity from endogenously myo[3H]inositol-labeled plasma membranes, occurred in the order alpha, beta-methylene-ATP >> 2-methylthio-ATP. Beta, gamma-methylene-ATP had little effect on PI-PLC activity. Different dose-response relationships for agonist-stimulation of PI-PLC and PLD indicate that activation of PI-PLC is not involved in stimulation of PLD in rat liver plasma membranes, and suggest that purinergic activation of PLD occurs via a pathway involving a G protein and a heretofore uncharacterized P2 receptor.

Effects of GTP gamma S on muscarinic receptor-stimulated inositol phospholipid hydrolysis in permeabilized smooth muscle from the small intestine

1. Smooth muscle fragments from the longitudinal layer of the small intestine of the guinea-pig were permeabilized with Staphylococcus aureus alpha toxin (alpha-toxin) and used to investigate the role of G-protein activation in the regulation of muscarinic acetylcholine receptor (AChR)-stimulated inositol phospholipid hydrolysis. 2. The efficiency of alpha-toxin permeabilization was estimated by the release of [3H]-2-deoxyglucose ([3H]-2DG) after prior loading or lactate dehydrogenase (LDH) enzyme release from the smooth muscle fragments. 3. In alpha-toxin-permeabilized smooth muscle, but not in non-permeabilized muscle, GTP gamma S induced time- and concentration-dependent increases in labelled inositol phosphates. Carbachol (CCh) increased labelled inositol phosphates in both permeabilized and non-permeabilized muscle, although the increases were greater in non-permeabilized smooth muscle. The response to 100 microM CCh was severely reduced by 0.5 microM atropine. 4. In permeabilized muscle the effects of GTP gamma S or CCh on inositol phosphate levels were reduced by treatment with pertussis toxin (PTX) and completely inhibited by GDP beta S. 5. GTP gamma S caused a concentration-dependent inhibition of the CCh-induced increases in the levels of labelled inositol phosphates. Dibutyryl cyclic AMP or Sp-cAMPs (adenosine-3',5'-cyclic phosphorothiolate-Sp) reduced the effects of CCh on inositol phosphate levels. 6. The results suggest that muscarinic AChR activation induces inositol phospholipid hydrolysis via more than one G-protein in this smooth muscle and that several mechanisms may contribute to the modulation of both stimulatory and inhibitory responses observed.

A subtype of opioid kappa-receptor is coupled to inhibition of Gi1-mediated phospholipase C activity in the guinea pig cerebellum

PLC activity was stimulated either by 1-100 microM of GTP or by 100-3,000 microM Ca2+ in lysed synaptosomal membranes of the guinea pig cerebellum. The kappa-opioid receptor agonist selectively inhibited the PLC activity stimulated by 100 microM GTP, but not by 100-3,000 microM Ca2+. Pretreatment of membranes with PTX abolished such a kappa-agonist-induced inhibition of PLC activity. The reconstitution of Gi1, but not of Go purified from porcine brains with PTX-treated membranes showed a complete recovery of the kappa-agonist-inhibition of PLC activity. These findings suggest that a novel subtype kappa-receptor mediates inhibition of PLC through inhibiting the intrinsic activity of PTX-substrate G-proteins.

Bacterial stimulators of macrophages

Our current understanding of the interaction between bacteria and macrophages, cells of the immune system that play a major role in the defense against infection, is summarized. Cell-surface structures of Gram-negative and Gram-positive bacteria that account for these interactions are described in detail. Besides surface structures, soluble bacterial molecules, toxins that are derived from pathogenic bacteria, are also shown to modulate macrophage functions. In order to affect macrophage functions, bacterial surface structures have to be recognized by the macrophage and toxins have to be taken up. Subsequently, signal transduction mechanisms are initiated that enable the macrophage to respond to the invading bacteria. To destroy bacteria, macrophages employ many strategies, among which antigen processing and presentation to T cells, phagocytosis, chemotaxis, and different bactericidal mechanisms are considered to be the main weapons.

Enhancement of the positive inotropic effect mediated by alpha 1-adrenoceptors in pertussis toxin-treated rabbit papillary muscles

1. In rabbit papillary muscles, pretreatment with pertussis toxin (PTX) significantly increased the positive inotropic response to isoprenaline and abolished the inhibitory action of carbachol on the isoprenaline response. 2. Phenylephrine in the presence of propranolol produced a positive inotropic effect and prolonged action potential duration through activation of alpha 1-adrenoceptors. Both of the effects of phenylephrine were significantly enhanced by PTX pretreatment. 3. Accumulation of [3H]inositol monophosphate (IP1) in papillary muscles prelabeled with myo-[3H]inositol was increased by phenylephrine in a concentration-dependent manner, which was antagonized by prazosin. Although PTX pretreatment significantly elevated the basal level of [3H]IP1 formation, the phenylephrine-induced increase in [3H]IP1 formation was unaffected. 4. It is concluded that the cardiac responses to alpha 1-adrenoceptor stimulation studied in these experiments are not transduced by a PTX sensitive G protein (Gi). However, the positive inotropic effect and prolongation of action potential duration mediated by alpha 1-adrenoceptor may be negatively regulated by Gi.

Pancreastatin activates pertussis toxin-sensitive guanylate cyclase and pertussis toxin-insensitive phospholipase C in rat liver membranes

We have recently found the calcium dependent glycogenolytic effect of a pancreastatin on rat hepatocytes and the mobilization of intracellular calcium. To further investigate the mechanism of action of pancreastatin on liver we have studied its effect on guanylate cyclase, adenylate cyclase, and phospholipase C, and we have explored the possible involvement of GTP binding proteins by measuring GTPase activity as well as the effect of pertussis toxin treatment of plasma liver membranes on the pancreastatin stimulated GTPase activity and the production of cyclic GMP and myo-inositol 1,4,5-triphosphate. Pancreastatin stimulated GTPase activity of rat liver membranes about 25% over basal. The concentration dependency curve showed that maximal stimulation was achieved at 10(-7)M pancreastatin (EC50 = 3 nM). This stimulation was partially inhibited by treatment of the membranes with pertussis toxin. The effect of pancreastatin on guanylate cyclase and phospholipase C were examined by measuring the production of cyclic GMP and myo-inositol 1,4,5-triphosphate respectively. Pancreastatin increased the basal activity of guanylate cyclase to a maximum of 2.5-fold the unstimulated activity at 30 degrees C, in a time- and dose-dependent manner, reaching the maximal stimulation above control with 10(-7) M pancreastatin at 10 min (EC50 = 0.6 nM). This effect was completely abolished when rat liver membranes had been ADP-ribosylated with pertussis toxin. On the other hand, adenylate cyclase activity was not affected by pancreastatin. Phospholipase C activity of rat liver membranes was rapidly stimulated (within 2-5 min) at 30 degrees C by 10(-7) M pancreastatin, reaching a maximum at 15 min.(ABSTRACT TRUNCATED AT 250 WORDS)

[Arginine]vasopressin hydrolyses phosphoinositides in the medullary thick ascending limb of mouse nephron

NaCl reabsorption across the thick ascending limb of Henle's loop (TAL) is stimulated by several hormones, in particular vasopressin acting through V2 receptors and cyclic AMP production. This study used suspensions of medullary TAL (mTAL) tubules from the mouse nephron to investigate the possibility that, besides activating adenylyl cyclase, vasopressin also stimulates phospholipase C via V1 receptor occupancy. Two different methods, phosphoinositide labelling and inositol trisphosphate (InsP3) radioimmunoassay, were used to show that [arginine]vasopressin (AVP) rapidly stimulated the formation of InsP3, which peaked at 200%-250% of control within the first minute of incubation with 10 nmol/l vasopressin at 37 degrees C, and declined to basal level after 5-10 min. Dose/response curves for InsP3, established at 30 degrees C and 37 degrees C using radioimmunoassay, showed a half-maximal stimulation of InsP3 production at about 1 nmol/l AVP and a maximal response at 10 nmol/l. Similar values were obtained for the response to AVP in terms of cAMP accumulation. InsP3 content in the presence of higher concentrations of AVP (1 mumol/l) was significantly lower (P < 0.001) than in the presence of 10 nmol/l AVP, giving a bell-shaped appearance to the dose/response curve at 37 degrees C but not at 30 degrees C. The V2 receptor agonist, 1-deamino-[8-D-Arg]vasopressin (dAVP) did not stimulate the formation of InsP3, and the V1 receptor antagonist d(CH2)5[Tyr(Me)2]AVP inhibited AVP-induced InsP3 formation, which therefore appeared to be mediated by V1 receptor occupancy. Under the same conditions, AVP also induced the formation of diradylglycerol via V1 receptor activation, with an analogous dose/response curve.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Activation of phospholipase C by heat shock requires GTP analogs and is resistant to pertussis toxin

The heat shock response in mammals consists of a complex array of intracellular reactions initiated by stress, although its regulation is poorly understood. We have investigated the role of transmembrane signal transduction in the response, examining mechanisms involved in the activation of phospholipase C (PLC) by heat shock. In rodent fibroblasts permeabilized with digitonin, heat shock and receptor-mediated PLC activity exhibited a strict GTP analog dependency. This indicates that heat shock-mediated phospholipase activation, in common with receptor mediated stimulation, does not involve direct effects on the phospholipases and suggests the participation of GTP binding (G) proteins in the activation process. When cells were treated with the inhibitor pertussis toxin (PTX), the phospholipases retained their inducibility by heat shock, but became refractory to thrombin treatment, indicating that heat shock may influence PLC activity through a distinct population of G proteins compared to thrombin. The data seem to exclude a role for PTX sensitive G proteins in the production of IP3 after heating and suggest a pathway involving the direct thermal activation of the Gq class of G proteins, which are coupled to the PLC beta 1 isoform.

Contribution of G protein activation to fluoride stimulation of phosphoinositide hydrolysis in human neuroblastoma cells

To examine the possibility that NaF enhances phosphoinositide-specific phospholipase C (PIC) activity in neural tissues by a mechanism independent of a guanine nucleotide binding protein (Gp), we have evaluated the contribution of Gp activation to NaF-stimulated phosphoinositide hydrolysis in human SK-N-SH neuroblastoma cells. Addition of NaF to intact cells resulted in an increase in the release of inositol phosphates (450% of control values; EC50 of approximately 8 mM). Inclusion of U-73122, an aminosteroid inhibitor of guanine nucleotide-regulated PIC activity in these cells, resulted in a dose-dependent inhibition of NaF-stimulated inositol lipid hydrolysis (IC50 of approximately 3.5 microM). When added to digitonin-permeabilized cells, NaF or guanosine-5'-O-thiotriphosphate (GTP gamma S) resulted in a three- and sevenfold enhancement, respectively, of inositol phosphate release. In the combined presence of optimal concentrations of NaF and GTP gamma S, inositol phosphate release was less than additive, indicative of a common site of action. Inclusion of 2-5 mM concentrations of guanosine-5'-O-(2-thiodiphosphate) (GDP beta S) fully blocked phosphoinositide hydrolysis elicited by GTP gamma S, whereas that induced by NaF was partially inhibited (65%). However, preincubation of the cells with GDP beta S resulted in a greater reduction in the ability of NaF to stimulate inositol phosphate release (87% inhibition). Both GTP gamma S and NaF-stimulated inositol phosphate release were inhibited by inclusion of 10 microM U-73122 (54-71%). The presence of either NaF or GTP gamma S also resulted in a marked lowering of the Ca2+ requirement for activation of PIC in permeabilized cells. These results indicate that in SK-N-SH cells, little evidence exists for direct stimulation of PIC by NaF and that the majority of inositol phosphate release that occurs in the presence of NaF can be attributed to activation of Gp.

Inositol 1,4,5-trisphosphate mediates adrenaline activation of K+ conductance in mouse peritoneal macrophages

In mouse peritoneal macrophages, alpha 1-adrenoceptor stimulation evokes a Ca(2+)-dependent K+ current [Io(Adr)][Hara et al. (1991) Pflügers Arch 419:371-379]. The roles of D-myo-inositol 1,4,5-trisphosphate (InsP3) and a GTP-binding protein (G protein) in Io(Adr) were investigated with tight-seal whole-cell recordings and fura-2 fluorescence measurements. Intracellular injection of InsP3 (5-50 microM) evoked transient outward currents [Io(InsP3)] with or without damped oscillations in membrane currents at -40 mV. Dialysis with 0.2 mM guanosine 5'-[3-thio]triphosphate (GTP[gamma S], a poorly hydrolysable GTP analogue) at -40 mV activated oscillatory outward currents or a slowly developing steady current on which such oscillations were superimposed after a delay of 10-90 s. Io(InsP3) and the GTP[gamma S]-induced current [Io(GTP[gamma S])] were accompanied by an increase in conductance. Reversal potentials of both responses closely depended on the extracellular K+ concentration. Fura-2 measurements revealed that Io(InsP3) and Io(GTP[gamma S]) result from a rise in intracellular free Ca2+ concentration ([Ca2+]i). Removal of extracellular Ca2+ did not abolish Io(InsP3) and Io(GTP[gamma S]). Both were blocked by bath-applied charybdotoxin. Intracellular D-myo-inositol 1,3,4,5-tetrakisphosphate (InsP4, 50 microM) did not evoke any responses, whereas D-myo-inositol 2,4,5-trisphosphate [InsP3(2,4,5), 20 microM] elicited an outward current at -40 mV. Io(InsP3) was completely blocked by prior dialysis with the InsP3 receptor antagonist heparin (5 mg/ml). Inclusion of guanosine 5'-[2-thiol] diphosphate (GDP[beta S], 2 mM) or heparin (5 mg/ml) together with GTP[gamma S] in the patch pipette solution completely blocked Io(GTP[gamma S]). These results indicate that intracellular injection of InsP3 or GTP[gamma S] mimic Io(Adr).(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipase D: regulation and functional significance

PLD is a major route for hydrolysis of PC in most tissues, consistent with it playing an important role in signal transduction. The enzyme appears to be activated by a variety of different mechanisms in different tissues, suggesting there might be several different isoforms. Little, however, is known at present about its enzymology and molecular biology. There is little direct evidence to indicate the functional significance of PLD activation but an accumulation of indirect evidence links PLD with prolonged changes in cell function. In particular, two areas where there is strong evidence for a role for PLD are mitogenesis and leukocyte hyperresponsiveness. An important area for future work will be the investigation of how products from the PLD pathway exert these effects. Current evidence suggests an important role for Ca(2+)-independent PKC isoforms and probably also for novel cellular targets for the putative second messenger PA.

Characterization of vasopressin-mediated GSH efflux from Hep G2 cells: significance of protein kinase C

Vasopressin stimulated GSH efflux from Hep G2 cells. The maximal effect was observed at 10nM. Pretreatment with pertussis toxin or cholera toxin for 18 hr increased GSH efflux. Vasopressin-mediated GSH efflux was observed even in the cells pretreated with those compounds. Dibutyryl-cAMP or dibutyryl-cGMP enhanced GSH efflux although an additive effect of vasopressin was not observed. Glucagon and a phorbol ester independently increased GSH efflux while both compounds decreased the effect of vasopressin. Staurosporine, an inhibitor of protein kinase C, inhibited vasopressin-mediated GSH efflux. The effect of vasopressin was observed even in the absence of extracellular Ca2+. Vasopressin stimulates GSH efflux from Hep G2 cells and protein kinase C-dependent pathway may play a significant role in vasopressin-mediated GSH efflux.

Three types of Gi alpha protein of the guinea-pig lung: cDNA cloning and analysis of their tissue distribution

cDNA clones encoding three types of Gi alpha, the alpha subunit of GTP-binding protein (Gi1 alpha, Gi2 alpha, and Gi3 alpha), were isolated from a cDNA library of the guinea-pig lung. Nucleotide sequence analysis revealed a high degree of homology with other mammalian Gi alpha cDNAs. By RNA blot analysis, the expression pattern of Gi1 alpha was more tissue-specific than those of other types of Gi alphas in the guinea-pig tissues examined. While Gi2 alpha and Gi3 alpha mRNAs were ubiquitously expressed in all tissues examined, Gi1 alpha mRNA was mainly expressed in the brain, lung and kidney. These results suggest that each Gi alpha protein may have a different role.

Oleate stimulation of diacylglycerol formation from phosphatidylcholine through effects on phospholipase D and phosphatidate phosphohydrolase

Hydrolysis of exogenous phosphatidylcholine (PtdCho) to 1,2-diacylglycerol by rat liver plasma membranes was stimulated by oleate concentrations as low as 0.1 mM. In the presence of 75 mM ethanol, the fatty acid also enhanced phosphatidylethanol (PtdEtOH) formation from PtdCho. These effects were also observed with linoleate and arachidonate, but not with saturated fatty acids or detergents, and were minimal in microsomes or mitochondria. Release of [3H]choline from exogenous Ptd[3H]Cho was stimulated by oleate, whereas phosphoryl[3H]choline formation was inhibited. Oleate and other unsaturated, but not saturated, fatty acids also stimulated the conversion of exogenous [14C]phosphatidic acid to [14C]diacylglycerol. These data are consistent with stimulatory effects of these fatty acids on both phospholipase D and phosphatidate phosphohydrolase in liver plasma membranes. The stimulatory effect of guanosine 5'-O-[3-thio]triphosphate) (20 microM) on PtdEtOH and diacylglycerol formation from PtdCho was enhanced by low concentrations of oleate. Phospholipase A2 also stimulated PtdEtOH and diacylglycerol formation from exogenous PtdCho. It is proposed that unsaturated fatty acids may play a physiological role in the regulation of diacylglycerol production through activation of phospholipase D and phosphatidate phosphohydrolase.

Histamine activates phosphorylase and inositol phosphate production in guinea pig hepatocytes

In guinea pig hepatocytes, histamine increased phosphorylase activity and inositol phosphate production. Similar effects were obtained with 2-(2-aminoethyl)-thiazole, a histamine H1 receptor agonist, but not with dimaprit or impromidine, H2 receptor agonists. These effects of histamine were dose-dependently inhibited by the H1 antihistamines, (+)-chlorpheniramine and mepyramine (pyrilamine) but not by cimetidine or ranitidine, H2 antagonists. (+)-Chlorpheniramine and mepyramine had similar potencies (apparent Ki values approximately 3 nM) when incubated with the cells for 1 min (phosphorylase a assays) but the former was 15-20-fold more potent than the latter at longer incubation times (apparent Ki values approximately 3-4 nM and 45-90 nM, respectively) indicating that mepyramine is actively metabolized by guinea pig hepatocytes. Histamine increased cytosol calcium approximately 2-fold, an effect also mediated through H1 receptors. The actions of histamine were not affected by in vivo ADP-ribosylation by pertussis toxin. Our data clearly indicate that histamine modulates the metabolism of guinea pig hepatocytes via activation of H1 receptors. These receptors are coupled to the phosphoinositide turnover-calcium mobilization signalling pathway through a pertussis toxin-insensitive process.

Species heterogeneity of hepatic alpha 1-adrenoceptors: alpha 1A-, alpha 1B- and alpha 1C-subtypes

alpha 1-Adrenergic activation stimulated phosphorylase and phosphoinositide turnover in hepatocytes from guinea pigs, rats and rabbits. Chlorethylclonidine inhibited these effects in rat and rabbit cells but not in guinea pig hepatocytes; low concentrations of 5-methyl urapidil blocked the alpha 1 actions in guinea pig and rabbit liver cells, but not in rat hepatocytes. Binding competition experiments also showed high affinity for 5-methyl urapidil in liver membranes from guinea pigs and rabbits and low affinity in those from rats. The data indicated that guinea pig hepatocytes express alpha 1A-, rat hepatocytes alpha 1B- and rabbit hepatocytes alpha 1C- adrenoceptors. This was confirmed by Northern analysis using receptor subtype-selective probes.

Regulation of phosphoinositide hydrolysis in cultured astrocytes by sphingosine and psychosine

The effects of sphingosine and psychosine on phosphoinositide hydrolysis in primary cultured astrocytes were determined. Exposure to sphingosine produced a dose-dependent stimulation of phosphoinositide hydrolysis requiring the presence of external Ca++ for optimal activity. The addition of 10 microM norepinephrine resulted in a stimulation additional to that with sphingosine. The alpha 1-antagonist prazosin completely inhibited norepinephrine-induced phosphoinositide hydrolysis but had no effect on that produced by sphingosine. Psychosine (108 microM), when co-incubated with sphingosine, produced complete inhibition of sphingosine-induced phosphoinositide hydrolysis at all doses of sphingosine tested (33-668 microM). Likewise, psychosine totally inhibited norepinephrine-induced phosphoinositide hydrolysis. The protein kinase C inhibitor staurosporine (1 microM) had no effect on sphingosine-induced phosphoinositide hydrolysis. These findings suggest that lysosphingolipids such as sphingosine and psychosine may play an important role in the regulation of phosphoinositide turnover in astrocytes by a mechanism dependent on extracellular Ca++ and independent of the alpha 1-adrenergic receptor and protein kinase C.

Isolation and characterization of the rat liver AVP receptor using [125I][d(CH2)5'sarcosine7]AVP

1. A vasopressin (AVP) binding protein was purified from rat liver membranes by an improved method using [125I][d(CH2)5'Sarcosine7]AVP, a selective V1 AVP radioligand and a combination of CHAPS solubilization, gel filtration, lectin affinity and FPLC ion exchange chromatography. 2. The purified protein exhibited a maximum binding activity of 2480 pmol/mg protein with a KD of 4.5 nmol/L, which corresponds to a purification of approximately 26,700-fold. The molecular weight of this protein was 70,000 Da. 3. The binding of [125I][d(CH2)5'Sarcosine7]AVP to the solubilized membranes was dependent on the protein concentration, and was inhibited by the unlabelled peptides [d(CH2)5'Sarcosine7]AVP, AVP, and to a lesser degree by peptides with high V2 receptor affinity, such as 1-desamino-D-AVP and [d(CH2)5'D-Ileu2-Ileu4]AVP. 4. In addition, an AVP anti-idiotypic monoclonal antibody bound to both the partially purified and purified lectin affinity AVP binding protein in a concentration-dependent manner. These results indicate that the purified protein displays similar characteristics to the liver membrane-bound AVP V1 receptor.