Pertussis toxin does not prevent alpha adrenergic stimulated breakdown of phosphoinositides or respiration in brown adipocytes

Activation of phosphoinositide turnover and protein kinase C by neurotransmitters that modulate calcium channels in embryonic chick sensory neurons

Gamma aminobutyric acid (GABA) and norepinephrine modulate the excitability of primary chick sensory neurons by decreasing the voltage dependent Ca current. Although previous electrophysiological studies indicate that neurotransmitter modulation of the Ca current in these neurons involves protein kinase C, the biochemical aspects of this mechanism have not been examined directly. We find that both norepinephrine (via a unique alpha receptor subtype) and GABA (via GABAb receptors) linked to pertussis toxin sensitive pathways, stimulate the metabolism of membrane phosphatidylinositol phospholipids in primary chick sensory neurons. In addition, norepinephrine causes the rapid translocation of C kinase activity from cytosolic to membrane associated distribution, consistent with its rapid activation in response to applied neurotransmitter. The pharmacology, pertussis toxin sensitivity and time course of the biochemical changes due to neurotransmitter treatment parallel the effects of these transmitters on calcium current modulation. These biochemical studies confirm the hypothesis that activation of protein kinase C is critically involved in calcium channel modulation in embryonic chick sensory neurons.

Guanine nucleotide binding regulatory proteins: their characteristics and identification

Many biological signals are processed by the binding of chemicals to cell surface receptors. Signals are switched to intracellular language via guanine nucleotide binding regulatory proteins (G-proteins) which are present in all eukaryotic cells. Thus, G-proteins serve as interfaces between receptor-response coupling. Two forms of G-proteins have been reported: conventional G-proteins which are heterotrimeric and consist of alpha, beta, and gamma subunits, and monomeric small molecular weight G-proteins which are generally found as single polypeptides. Recently, high molecular weight G-proteins have also been described. The family of G-proteins contains multiple genes that encode the alpha, beta, or gamma subunits. G-proteins play a pivotal role in excitation-contraction coupling in smooth muscle function and control metabolic and secretory processes. In this review article, we have given a brief overview on the characteristics and methodology for the identification of G-proteins. The heterotrimeric G-proteins are generally identified by Western blotting and ADP-ribosylation with bacterial toxins. The monomeric and high molecular weight G-proteins have been identified by [35S]GTP delta S overlay technique and photoaffinity labeling, respectively. Recently, the use of molecular genetic probes has made it possible to investigate the expression of the message for various G-proteins.

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.

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.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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Muscarinic acetylcholine receptor-mediated phosphoinositide turnover in cultured human retinal pigment epithelium cells

Cultured retinal pigment epithelium cells prepared from post-mortem adult human eyes are shown to contain muscarinic receptors associated with phosphoinositide turnover. Carbachol at a concentration of 100 microM induced a four-fold increase in 3H-inositol phosphates (more than 76% is in the form of 3H-inositol-1-phosphate) accumulation within 45 min in cells prelabelled with 3H-myoinositol and exposed to 5 mM LiCl. The EC50 of carbachol was approx. 70 microM and the saturation concentration was about 1 mM. The carbachol-induced response was blocked by both atropine and pirenzepine, the former being most effective. Pre-exposure of cells to carbachol resulted in desensitization and a drastic reduction in the subsequent carbachol-induced stimulation of 3H-inositol phosphates. The carbachol response could be attenuated by the biologically active phorbol ester, 4 beta-phorbol 12-myristate 13-acetate, and this was nullified by the protein kinase C inhibitor, staurosporine. The biologically inactive phorbol ester, 4 alpha-phorbol 12,13 dideconoate, did not attenuate the carbachol-induced stimulation of 3H-inositol phosphates. Pertussis toxin failed to influence the carbachol receptor-mediated phosphoinositide turnover. These studies provide clear evidence for the occurrence of muscarinic receptors coupled to phosphoinositide hydrolysis on human retinal pigment epithelium cells.

Effects of pertussis toxin on alpha 1-agonist-mediated phosphatidylinositide turnover and myocardial cell hypertrophy in neonatal rat ventricular myocytes

In neonatal rat ventricular myocytes pretreatment with pertussis toxin did not affect 1 microM (-)-norepinephrine stimulation of inositol phosphates or myocardial cell hypertrophy as measured either by protein radiolabelling or by myocardial cell protein content. Thus guanine nucleotide protein(s) ADP-ribosylated by pertussis toxin do not play a role in two alpha 1-adrenoceptor-mediated processes, phosphatidylinositide turnover and induction of myocardial cell hypertrophy.

Alpha 1- and beta-adrenergic regulation of intracellular Ca2+ levels in brown adipocytes

In order to monitor changes in cytosolic Ca2+ levels, brown-fat cells were incubated with the fluorescent Ca2+-indicator fura-2 and the fluorescence intensity ratio followed. The addition of norepinephrine led to a rapid and persistent increase in the cytosolic Ca2+ level, which was dose-dependent with a maximal effect at about 1 microM. The response was diminished in the absence of extracellular Ca2+ and was inhibited more efficiently by phentolamine and prazosin than by propranolol or yohimbine, indicating alpha 1-adrenergic mediation. Accordingly, selective alpha 1-adrenergic stimulation also increased the cytosolic Ca2+ level. However, selective beta-adrenergic stimulation, as well as the adenylate cyclase activator forskolin, were also able to increase the cytosolic Ca2+ level in these cells to a certain extent. It was concluded that the major part of the increase in cytosolic Ca2+ was mediated, as in other cell types, via alpha 1-adrenergic receptors, but that Ca2+ levels were also positively modulated by a cAMP-mediated process. These observations are discussed in relation to known alpha 1/beta synergisms in brown adipose tissue.

Characterization of bradykinin-induced phosphoinositide turnover in neurohybrid NCB-20 cells

Phosphoinositide hydrolysis was studied in neurohybrid NCB-20 cells prelabeled with myo-[3H]inositol. Among nearly 20 neurotransmitters and neuromodulators examined, only bradykinin, carbachol, and histamine significantly increased the accumulation of [3H]inositol monophosphate (IP1) in the presence of lithium. The EC50 of bradykinin was 20 nM and the saturating concentration was approximately 1 microM. The bradykinin response was robust (10-fold) and was potently and selectively blocked by a bradykinin antagonist, B 4881 [D-Arg-(Hyp3, Thi, D-Phe)-bradykinin], with a Ki of 10 nM. This effect of bradykinin appeared to be additive to that mediated by activation of muscarinic cholinergic and histamine H1 receptors. The accumulation induced by bradykinin or carbachol was dependent on the presence of calcium in the incubation medium; less than twofold stimulation was observed in the absence of exogenous calcium. Bradykinin-induced [3H]IP1 accumulation required high concentration of lithium to elicit its maximal stimulation; the concentration of lithium required for half maximal effect was about 13 mM, similar to the value reported previously for carbachol-induced accumulation in the same cell line. In contrast, using related neurohybrid NG108-15 cells, bradykinin-induced [3H]IP1 accumulation was found to require much less lithium. IN the presence of lithium, bradykinin also evoked a transient increase in the production of [3H]-inositol bis- and trisphosphate. Basal and bradykinin-induced phosphoinositide breakdown was inhibited by 4 beta-phorbol 12,13-dibutyrate, but was unaffected by the biologically inactive 4 beta-phorbol. Pretreatment of cells with pertussis toxin induced only about 30% loss of the bradykinin-induced [3H]IP1 accumulation, without affecting basal activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ras-encoded proteins and platelet-derived growth factor on inositol phospholipid turnover in NRK cells

The role of ras-encoded proteins and platelet-derived growth factor (PDGF) in inositol phospholipid metabolism has been studied. PDGF stimulates inositol phospholipid turnover in confluent normal rat kidney (NRK) cells and enhances hydrolysis of phosphatidylinositol monophosphate and phosphatidylinositol bisphosphate in NRK cell membranes in the presence of guanosine 5'-[gamma-thio]triphosphate. The stimulatory effect of PDGF on phosphatidylinositol bisphosphate hydrolysis is not inhibited by pretreatment of NRK cells with pertussis toxin, implying that PDGF-stimulated phospholipase C activity of NRK cells is regulated by a pertussis toxin-insensitive guanine nucleotide-binding protein (G protein) that is different from Gi (inhibitory G protein) or Go (G protein of unknown function). When bacterially made human normal or oncogenic T24 ras protein is added to 32P-labeled NRK cell membranes in the presence of guanosine 5'-[gamma-thio]triphosphate, normal ras protein increases by 3-fold the formation of inositol trisphosphate, whereas T24 ras protein has no significant effect. In addition, normal ras protein and PDGF have additive effects on inositol trisphosphate production. Taken together, these data suggest that normal ras protein stimulates inositol phospholipid turnover in NRK cells by means of a pathway different from the PDGF-regulated one and that oncogenic ras protein is without significant stimulatory effect in this action.

Acetylcholine and cholecystokinin receptors functionally couple by different G-proteins to phospholipase C in pancreatic acinar cells

We have studied the involvement of GTP-binding proteins in the stimulation of phospholipase C from rat pancreatic acinar cells. Pretreatment of permeabilized cells with activated cholera toxin inhibited both cholecystokinin-octapeptide (CCK-OP) and GTP gamma S but not carbachol (CCh)-induced production of inositol trisphosphate. Pertussis toxin had no effect. Neither vasoactive intestinal polypeptide, a stimulator of adenylyl cyclase, nor the cAMP-analogue, 8-bromo cAMP, mimicked the inhibitory effect of cholera toxin on agonist-induced phospholipase C activation. This indicates that inhibition by cholera toxin could not be attributed to a direct interaction of cholera toxin activated Gs with phospholipase C or to an elevation of cAMP. In isolated rat pancreatic plasma membranes cholera toxin ADP-ribosylated a 40 kDa protein, which was inhibited by CCK-OP but not by CCh. We conclude from these data that both CCK- and muscarinic acetylcholine receptors functionally couple to phospholipase C by two different GTP-binding proteins.

Relaxation of bovine mesenteric artery induced by glyceryl trinitrate is attenuated by pertussis toxin

Low concentrations (less than 1 nM) of glyceryl trinitrate (GTN) induced a considerable relaxation of bovine mesenteric arteries (BMA) brought to sustained contraction by the addition of phenylephrine. The concentration-response curve of GTN showed a biphasic pattern with a high and a low affinity component. Pretreatment with pertussis toxin (IAP) attenuated the high affinity relaxant component, but not the low affinity component or the relaxation induced by NaNO2. A polyclonal antibody to IAP counteracted the effect of the toxin on the GTN-response. Low concentrations of GTN increased the cGMP level in BMA via activation of the high affinity pathway. This effect was also inhibited in preparations pretreated with IAP. It is suggested from the present study that GTN induces relaxation of vascular smooth muscle via two separate pathways, the high affinity pathway might involve a receptor-complex interaction with a regulatory protein.

Regulatory GTP-binding proteins: emerging concepts on their role in cell function

The last few years have evidenced a tremendous expansion in our appreciation of the role of regulatory GTP-binding proteins in cellular activation. The availability of cholera and pertussis toxins to detect G proteins as well as methodological advances in the study of cellular function has afforded the opportunity to examine G protein participation in many cellular events. Regulation of adenylyl cyclase and cyclic GMP phosphodiesterase by G proteins has been demonstrated. Phosphatidylinositol-4,5-biphosphate specific phospholipase C activity appears to be subject to G protein control. G proteins regulate inward K+ and Ca2+ channels through a mechanism which may be independent of effects on the above mentioned enzymes. Certainly, the number of G proteins which have been identified from sequencing of complementary DNA affords the potential for G protein involvement in many cellular events. Only three G proteins have however been isolated and functionally characterized, Gs, Gi and transducin. Whether all the functions of these proteins have been identified remains to be seen.

Pertussis toxin stimulates cholecystokinin-induced cyclic AMP formation but is without effect on secretagogue-induced calcium mobilization in exocrine pancreas

The role of a pertussis toxin sensitive GTP-binding protein in mediating between cholecystokinin receptors and phosphatidylinositol 4,5-bisphosphate phosphodiesterase as well as in preventing cholecystokinin from increasing cellular cyclic AMP has been investigated using dispersed acini from rabbit pancreas. Pertussis toxin pretreatment (500 ng/ml, 2 h) did not affect cholecystokinin(octapeptide) (CCK-8)-induced increases in cytosolic free Ca2+ as judged from changes in fluorescence obtained from quin2-loaded acini. Although pretreatment with pertussis toxin was also without effect on resting acinar cell cyclic AMP levels, adenylate cyclase activity was increased, since inhibition of cyclic AMP phosphodiesterase activity by isobutylmethylxanthine (IBMX) resulted in an additional increase in cyclic AMP levels in toxin-treated acini, indicating that acinar cell adenylate cyclase activity is under some tonic inhibitory control by the pertussis toxin-sensitive inhibitory GTP-binding protein (Gi) of the adenylate cyclase system. CCK-8 gave an increase in cyclic AMP levels in both control (1.6-fold) and toxin-treated (2.3-fold) acini, leading to cyclic AMP levels in the toxin-treated acini 2-times as high as those in control acini. In the presence of IBMX, the cyclic AMP response to CCK-8 was again markedly enhanced in acini pretreated with the toxin (3.2- vs. 1.8-fold), resulting in cAMP levels in the toxin-treated acini 3.7-times those in the absence of IBMX, 2.5-times those in control acini in the presence of IBMX and 7.0-times those in control acini in the absence of IBMX. Neither the pretreatment with pertussis toxin, nor the presence of IBMX alone, nor the combination had an effect on basal amylase secretion. However, all three treatments potentiated the stimulatory effect of CCK-8 on amylase secretion and the amount of potentiation was proportional to the cyclic AMP levels reached. Our findings suggest that in the intact pancreatic acinar cell Gi inhibition of the catalytic subunit of the adenylate cyclase may largely be responsible for preventing cholecystokinin from increasing cellular cyclic AMP. They moreover show that cyclic AMP is a modulatory agent in rabbit pancreatic enzyme secretion, not able to stimulate secretion itself, but potentiating effects mediated by the phosphatidylinositol-calcium pathway.

Evidence against a role of a pertussis toxin-sensitive guanine nucleotide-binding protein in the alpha 1-adrenoceptor-mediated positive inotropic effect in the heart

Pertussis toxin, which specifically inactivates guanine nucleotide-binding proteins (N-proteins) involved in the signal transduction in various receptor systems, did not influence the positive inotropic effect of the alpha 1-adrenoceptor agonist phenylephrine in rat isolated left auricles. This indicates that the alpha 1-adrenoceptor-mediated positive inotropic effect does not involve a pertussis toxin-sensitive N-protein.

The role of guanyl nucleotide binding proteins in the formation of inositol phosphates in adrenal glomerulosa cells

A non-hydrolysable GTP analogue enhanced the formation of [3H]inositol polyphosphates in permeabilized adrenal glomerulosa cells. Pertussis toxin, which ADP-ribosylated Ni, failed to influence angiotensin-induced formation of 3H-labelled inositol phosphates and the incorporation of [32F]phosphate into phosphatidylinositol and phosphatidic acid. These results show that Ni is present and a G-protein activates phospholipase C also in glomerulosa cells, however, it is not Ni which couples angiotensin receptors to the enzyme.