Pertussis toxin-catalyzed ADP-ribosylation: effects on the coupling of inhibitory receptors to the adenylate cyclase system

The quantitative analysis of the mechanism involved in pertussis toxin-mediated cell clustering and its implications in the in vitro quality control of diphtheria tetanus and whole cell pertussis vaccines

Some of the adverse side-effects such as leukocytosis, hyperinsulinemia, hypoglycemia and sensitization to histamine, caused by diphtheria, tetanus and whole cell pertussis (DTwP) vaccines are related to the presence of non-inactivated pertussis toxin (PTx) residues (NiPTxR). The CHO cell clustering assay is an in vitro assay to measure NiPTxR in DTwP vaccines based on the ability of active PTx to cause cellular clustering. To study the biochemical mechanism involved in the clustering effect in CHO cells induced by PTx and by two DTwP vaccines, the levels of total cyclic cAMP were measured and compared to those obtained after treatment with cholera toxin (CTx) able to induce CHO cells elongation instead of cell clustering. Our results showed an increment of cAMP levels by CTx and total cell elongation in CHO cells. However, changes in cAMP levels were not associated with the total clustering induced by PTx or by DTwP vaccines. The high correlation seen between the levels of NiPTxR in the DTwP vaccines determined by the in vivo lethal histamine sensitization (HIST) assay and the in vitro CHO cell clustering assay indicated that the latter could be a suitable alternative test to HIST assay for the toxicological approval and release of batches of DTwP vaccines in their final formulation for human use in accordance with the application of the 3R's principle.

Beta-3 Adrenoceptor Signaling Pathways in Urothelial and Smooth Muscle Cells in the Presence of Succinate

Succinate, an intermediate metabolite of the Krebs cycle, can alter the metabolomics response to certain drugs and controls an array of molecular responses in the urothelium through activation of its receptor, G-protein coupled receptor 91 (GPR91). Mirabegron, a β3-adrenergic receptor (β3-AR) agonist used to treat overactive bladder syndrome (OAB), increases intracellular cAMP in the detrusor smooth muscle cells (SMC), leading to relaxation. We have previously shown that succinate inhibits forskolin-stimulated cAMP production in urothelium. To determine whether succinate interferes with mirabegron-mediated bladder relaxation, we examined their individual and synergistic effect in urothelial-cell and SMC signaling. We first confirmed β3-AR involvement in the mirabegron response by quantifying receptor abundance by immunoblotting in cultured urothelial cells and SMC and cellular localization by immunohistochemistry in rat bladder tissue. Mirabegron increased cAMP levels in SMC but not in urothelial cells, an increase that was inhibited by succinate, suggesting that it impairs cAMP-mediated bladder relaxation by mirabegron. Succinate and mirabegron increased inducible nitric oxide synthesis and nitric oxide secretion only in urothelial cells, suggesting that its release can indirectly induces SMC relaxation. Succinate exposure decreased the expression of β3-AR protein in whole bladder in vivo and in SMC in vitro, indicating that this metabolite may lead to impaired pharmacodynamics of the bladder. Together, our results demonstrate that increased levels of succinate in settings of metabolic stress (e.g., the metabolic syndrome) may lead to impaired mirabegron and β3-AR interaction, inhibition of cAMP production, and ultimately requiring mirabegron dose adjustment for its treatment of OAB related to these conditions.

When Escherichia coli doesn't fit the mold: A pertussis-like toxin with altered specificity

Bacterial toxins introduce protein modifications such as ADP-ribosylation to manipulate host cell signaling and physiology. Several general mechanisms for toxin function have been established, but the extent to which previously uncharacterized toxins utilize these mechanisms is unknown. A study of an Escherichia coli pertussis-like toxin demonstrates that this protein acts on a known toxin substrate but displays distinct and dual chemoselectivity, suggesting this E. coli pertussis-like toxin may serve as a unique tool to study G-protein signaling in eukaryotic cells.

Virus stimulation of human mast cells results in the recruitment of CD56⁺ T cells by a mechanism dependent on CCR5 ligands

The trafficking of effector cells to sites of infection is crucial for antiviral responses. However, the mechanisms of recruitment of the interferon-γ-producing and cytotoxic CD56(+) T cells are poorly understood. Human mast cells are sentinel cells found in the skin and airway and produce selected proinflammatory mediators in response to multiple pathogen-associated signals. The role of human mast cell-derived chemokines in T-cell recruitment to virus infection was examined. Supernatants from primary human cord blood-derived mast cells (CBMCs) infected with mammalian reovirus were examined for chemokine production and utilized in chemotaxis assays. Virus-infected CBMCs produced several chemokines, including CCL3, CCL4, and CCL5. Supernatants from reovirus-infected CBMCs selectively induced the chemotaxis of CD8(+) T cells (10±1%) and CD3(+)CD56(+) T cells (19±5%). CD56(+) T-cell migration was inhibited by pertussis toxin (65±9%) and met-RANTES (56±7%), a CCR1/CCR5 antagonist. CD56(+) T cells expressed CCR5, but little CCR1. The depletion of CCL3, CCL4, and CCL5 from reovirus-infected CBMC supernatants significantly (41±10%) inhibited CD56(+) T-cell chemotaxis. This study demonstrates a novel role for mast cells and CCR5 in CD56(+) T-cell trafficking and suggests that human mast cells enhance immunity to viruses through the selective recruitment of cytotoxic effector cells to virus infection sites. These findings could be exploited to enhance local T-cell responses in chronic viral infection and malignancies at mast cell-rich sites.

G(i/o) protein-dependent and -independent actions of Pertussis Toxin (PTX)

Pertussis toxin (PTX) is a typical A-B toxin. The A-protomer (S1 subunit) exhibits ADP-ribosyltransferase activity. The B-oligomer consists of four subunits (S2 to S5) and binds extracellular molecules that allow the toxin to enter the cells. The A-protomer ADP-ribosylates the α subunits of heterotrimeric G_i/o proteins, resulting in the receptors being uncoupled from the G_i/o proteins. The B-oligomer binds proteins expressed on the cell surface, such as Toll-like receptor 4, and activates an intracellular signal transduction cascade. Thus, PTX modifies cellular responses by at least two different signaling pathways; ADP-ribosylation of the Gα_i/o proteins by the A-protomer (G_i/o protein-dependent action) and the interaction of the B-oligomer with cell surface proteins (G_i/o protein-independent action).

Effect of pertussis toxin and herbimycin A on proteinase-activated receptor 2-mediated cyclooxygenase 2 expression in Helicobacter pylori-infected gastric epithelial AGS cells

Helicobacter pylori (H. pylori) is an important risk factor for chronic gastritis, peptic ulcer, and gastric cancer. Proteinase-activated receptor 2 (PAR2), subgroup of G-protein coupled receptor family, is highly expressed in gastric cancer, and chronic expression of cyclooxygenase-2 (COX-2) plays an important role in H. pylori-associated gastric carcinogenesis and inflammation. We previously demonstrated that H. pylori induced the expression of PAR2 and COX-2 in gastric epithelial cells. Present study aims to investigate whether COX-2 expression induced by H. pylori in Korean isolates is mediated by PAR2 via activation of G(i) protein and Src kinase in gastric epithelial AGS cells. Results showed that H. pylori-induced COX-2 expression was inhibited in the cells transfected with antisense oligonucleotide for PAR2 or treated with Gi protein blocker pertussis toxin, Src kinase inhibitor herbimycin A and soybean trypsin inbitor, indicating that COX-2 expression is mediated by PAR2 through activation of Gi protein and Src kinase in gastric epithelial cells infected with H. pylori in Korean isolates. Thus, targeting the activation of PAR2 may be beneficial for prevention or treatment of gastric inflammation and carcinogenesis associated with H. pylori infection.

Attenuated Bordetella pertussis vaccine candidate BPZE1 promotes human dendritic cell CCL21-induced migration and drives a Th1/Th17 response

New vaccines against pertussis are needed to evoke full protection and long-lasting immunological memory starting from the first administration in neonates--the major target of the life-threatening pertussis infection. A novel live attenuated Bordetella pertussis vaccine strain, BPZE1, has been developed by eliminating or detoxifying three important B. pertussis virulence factors: pertussis toxin, dermonecrotic toxin, and tracheal cytotoxin. We used a human preclinical ex vivo model based on monocyte-derived dendritic cells (MDDCs) to evaluate BPZE1 immunogenicity. We studied the effects of BPZE1 on MDDC functions, focusing on the impact of Bordetella-primed dendritic cells in the regulation of Th and suppressor T cells (Ts). BPZE1 is able to activate human MDDCs and to promote the production of a broad spectrum of proinflammatory and regulatory cytokines. Moreover, conversely to its parental wild-type counterpart BPSM, BPZE1-primed MDDCs very efficiently migrate in vitro in response to the lymphatic chemokine CCL21, due to the inactivation of pertussis toxin enzymatic activity. BPZE1-primed MDDCs drove a mixed Th1/Th17 polarization and also induced functional Ts. Experiments performed in a Transwell system showed that cell contact rather than the production of soluble factors was required for suppression activity. Overall, our findings support the potential of BPZE1 as a novel live attenuated pertussis vaccine, as BPZE1-challenged dendritic cells might migrate from the site of infection to the lymph nodes, prime Th cells, mount an adaptive immune response, and orchestrate Th1/Th17 and Ts responses.

Signaling pathways of isoproterenol-induced ERK1/2 phosphorylation in primary cultures of astrocytes are concentration-dependent

Stimulation of β-adrenoceptors activates the canonical adenylate cyclase pathway (via G(s) protein) but can also evoke phosphorylation of extracellular-regulated kinases 1 and 2 (ERK(1/2) ) via G(s)/G(i) switching or β-arrestin-mediated recruitment of Src. In primary cultures of mouse astrocytes, activation of the former of these pathways required micromolar concentrations of the β(1)/β(2) -adrenergic agonist isoproterenol, that acted on β(1)-adrenoceptors, whereas the latter was activated already by nanomolar concentrations, acting on β(2) receptors. Protein kinase A activity was required for G(s)/G(i) switching, which was followed by Ca(2+) release from intracellular stores and G(iα)- and metalloproteinase-dependent transactivation of the epidermal growth factor receptor (EGFR; at its Y1173 phophorylation site), via its receptor-tyrosine kinase, β-arrestin 1/2 recruitment, and MAPK/ERK kinase-dependent ERK(1/2) phosphorylation. ERK(1/2) phosphorylation by Src activation depended on β-arrestin 2, but not β-arrestin 1, was accompanied by Src/EGFR co-precipitation and phosphorylation of the EGFR at the Src-phosphorylated Y845 site and the Y1045 autophosphorylation site; it was independent of transactivation but dependent on MAPK/ERK kinase activity, suggesting EGFR phosphorylation independently of the receptor-tyrosine kinase or activation of Ras or Raf directly from Src. Most astrocytic consequences of activating either pathway (or both) are unknown, but morphological differentiation and increase in glial fibrillary acidic protein in response to dibutyryl cAMP-mediated increase in cAMP depend on G(s)/G(i) switching and transactivation.

Pertussis toxin signals through the TCR to initiate cross-desensitization of the chemokine receptor CXCR4

Pertussis toxin (PTx) has been shown to exert a variety of effects on immune cells independent of its ability to ADP-ribosylate G proteins. Of these effects, the binding subunit of PTx (PTxB) has been shown to block signaling via the chemokine receptor CCR5, but the mechanism involved in this process is unknown. Here, we show that PTxB causes desensitization of a related chemokine receptor, CXCR4, and explore the mechanism by which this occurs. CXCR4 is the receptor for the chemokine stromal cell-derived factor 1alpha (SDF-1alpha) and elicits a number of biological effects, including stimulation of T cell migration. PTxB treatment causes a decrease in CXCR4 surface expression, inhibits G protein-associated signaling, and blocks SDF-1alpha-mediated chemotaxis. We show that PTxB mediates these effects by activating the TCR signaling network, as the effects are dependent on TCR and ZAP70 expression. Additionally, the activation of the TCR with anti-CD3 mAb elicits a similar set of effects on CXCR4 activity, supporting the idea that TCR signaling leads to cross-desensitization of CXCR4. The inhibition of CXCR4 by PTxB is rapid and transient; however, the catalytic activity of PTx prevents CXCR4 signaling in the long term. Thus, the effects of PTx holotoxin on CXCR4 signaling can be divided into two phases: short term by the B subunit, and long term by the catalytic subunit. These data suggest that TCR crosstalk with CXCR4 is likely a normal cellular process that leads to cross-desensitization, which is exploited by the B subunit of PTx.

Molecular mechanisms of the cytotoxicity of ADP-ribosylating toxins

Bacterial pathogens utilize toxins to modify or kill host cells. The bacterial ADP-ribosyltransferases are a family of protein toxins that covalently transfer the ADP-ribose portion of NAD to host proteins. Each bacterial ADP-ribosyltransferase toxin modifies a specific host protein(s) that yields a unique pathology. These toxins possess the capacity to enter a host cell or to use a bacterial Type III apparatus for delivery into the host cell. Advances in our understanding of bacterial toxin action parallel the development of biophysical and structural biology as well as our understanding of the mammalian cell. Bacterial toxins have been utilized as vaccines, as tools to dissect host cell physiology, and more recently for the development of novel therapies to treat human disease.

Melatonin at pharmacological doses enhances human osteoblastic differentiation in vitro and promotes mouse cortical bone formation in vivo

Melatonin is known to regulate a variety of physiological processes including control of circadian rhythms, regulation of seasonal reproductive function, regulation of body temperature, and so forth. Accumulating evidence from in vitro and in vivo experiments using rodent and chicken has also suggested that melatonin may have an influence on skeletal growth and bone formation. However, little is known about the effects of melatonin on human osteoblasts, which thus remains to be elucidated. This study was performed to determine whether melatonin could affect the proliferation and differentiation of human osteoblasts in vitro and to demonstrate the possibility that melatonin could be applied as a pharmaceutical agent to shorten the treatment period of bone fracture, various osteotomies, and bone distraction. Reverse transcription-polymerase chain reaction and Western blot analysis showed that human osteoblasts expressed melatonin 1a receptor and that its expression levels decreased gradually with the age of the hosts. Melatonin stimulated the proliferation and alkaline phosphatase activity of human osteoblasts in a dose-dependent manner at the pharmacological concentrations. Melatonin also promotes gene expression of type I collagen, osteopontin, bone sialoprotein, and osteocalcin in a dose-dependent manner, and stimulated the mineralized matrix formation in vitro. Moreover, intraperitoneal administration of melatonin to mice increased the volume of newly formed cortical bone of femora. These results demonstrated that melatonin directly accelerated the differentiation of osteoblasts of human as well as rodent and chicken and also suggested that melatonin could be applied as a pharmaceutical agent to promote bone regeneration.

Age Increases Expression and Receptor-Mediated Activation of Gαi in Human Atria

Recently, we demonstrated that beta2AR and several other Galphas-coupled receptors in human atria also couple to Galphai, a G protein that inhibits adenylyl cyclase (AC). The present study was undertaken to determine whether age increases expression of Galphai in human atrium, and more specifically whether it results in an increase in receptor-mediated activation of Galphai. Right atrial appendages were obtained from 14 mature adult (40-55 years) and 14 elderly (71-79 years) patients undergoing cardiac surgery. Immunoblotting of atrial membranes indicates that elderly atria have 82 +/- 18% more Galphai2 than atria from mature adults (P < 0.002); this increase in Galphai with age is confirmed by pertussis toxin-catalyzed ADP-ribosylation as well as by photoaffinity labeling with [32P]azidoanilido-GTP. We also find that receptor-mediated activation of Galphai is greater in elderly atria and that both basal and receptor-mediated AC activities decrease in elderly atria. These decreases in AC activity can be reversed by disabling Galphai with pertussis toxin, indicating that the age-dependent increases in Galphai expression and activation have functional consequences. Because beta2ARs in human atria mediate contractility through cAMP-mediated phosphorylation of phospholamban, we conclude that an age-induced increase in Galphai may have a role in depressing cardiac function in aged human atria.

Age increases cardiac Galpha(i2) expression, resulting in enhanced coupling to G protein-coupled receptors

Cardiac G protein-coupled receptors that function through stimulatory G protein Galpha(s), such as beta(1)- and beta(2)-adrenergic receptors (beta(1)ARs and beta(2)ARs), play a key role in cardiac contractility. Recent data indicate that several Galpha(s)-coupled receptors in heart also activate Galpha(i), including beta(2)ARs (but not beta(1)ARs). Coupling of cardiac beta(2)ARs to Galpha(i) inhibits adenylyl cyclase and opposes beta(1)AR-mediated apoptosis. Dual coupling of beta(2)AR to both Galpha(s) and Galpha(i) is likely to alter beta(2)AR function in disease, such as congestive heart failure in which Galpha(i) levels are increased. Indeed, heart failure is characterized by reduced responsiveness of betaARs. Cardiac betaAR-responsiveness is also decreased with aging. However, whether age increases cardiac Galpha(i) has been controversial, with some studies reporting an increase and others reporting no change. The present study examines Galpha(i) in left ventricular membranes from young and old Fisher 344 rats by employing a comprehensive battery of biochemical assays. Immunoblotting reveals significant increases with age in left ventricular Galpha(i2), but no changes in Galpha(i3), Galpha(o), Galpha(s), Gbeta(1), or Gbeta(2). Aging also increases ADP-ribosylation of pertussis toxin-sensitive G proteins. Consistent with these results, basal as well as receptor-mediated incorporation of photoaffinity label [(32)P]azidoanilido-GTP indicates higher amounts of Galpha(i2) in older left ventricular membranes. Moreover, both basal and receptor-mediated adenylyl cyclase activities are lower in left ventricular membranes from older rats, and disabling of Galpha(i) with pertussis toxin increases both basal and receptor-stimulated adenylyl cyclase activity. Finally, age produces small but significant increases in muscarinic potency for the inhibition of both beta(1)AR- and beta(2)AR-stimulated adenylyl cyclase activity. The present study establishes that Galpha(i2) increases with age and provides data indicating that this increase dampens adenylyl cyclase activity.

Beta(2)-adrenergic and several other G protein-coupled receptors in human atrial membranes activate both G(s) and G(i)

Cardiac G protein-coupled receptors that couple to Galpha(s) and stimulate cAMP formation (eg, beta-adrenergic, histamine, serotonin, and glucagon receptors) play a key role in cardiac inotropy. Recent studies in rodent cardiac myocytes and transfected cells have revealed that one of these receptors, the beta(2)-adrenergic receptor (AR), also couples to the inhibitory G protein Galpha(i) (activation of which inhibits cAMP formation). If beta(2)ARs could be shown to couple to Galpha(i) in the human heart, it would have important ramifications, because levels of Galpha(i) increase with age and in failing human heart. Therefore, we investigated whether beta(2)ARs in the human heart activate Galpha(i). By photoaffinity labeling human atrial membranes with [(32)P]azidoanilido-GTP, followed by immunoprecipitation with antibodies specific for Galpha(i), we found that Galpha(i) is activated by stimulation of beta(2)ARs but not of beta(1)ARs. In addition, we found that other Galpha(s)-coupled receptors also couple to Galpha(i), including histamine, serotonin, and glucagon. When coupling of these receptors to Galpha(i) is disrupted by pertussis toxin, their ability to stimulate adenylyl cyclase is enhanced. These data provide the first evidence that beta(2)AR and many other Galpha(s)-coupled receptors in human atrium also couple to Galpha(i) and that abolishing the coupling of these receptors to Galpha(i) increases the receptor-mediated adenylyl cyclase activity.

Involvement of the carboxyl terminus of the third intracellular loop of the cannabinoid CB1 receptor in constitutive activation of Gs

The human cannabinoid receptor CB1 functionally couples primarily to Gi-, but also to Gs-mediated pathways to modulate intracellular cyclic AMP (cAMP) levels. To probe the features of the receptor that may be involved in promoting interactions with one G protein type over another, we generated the L341A/A342L mutant CB1 receptor. The double mutation involved the swap in position of two adjacent residues in the carboxyl-terminal segment of the third intracellular loop of CB1. This resulted in partial constitutive activation of the receptor and an agonist-independent enhancement in cAMP levels. Characterization following treatment with either pertussis or cholera toxin indicated that the constitutive activity is selective for a Gs- and not a Gi-mediated pathway. Treatment with the CB1-specific inverse agonist SR141716A inhibited the basal accumulation of cAMP in the presence of pertussis toxin, establishing that the effect is CB1 mediated. The binding of the agonist CP-55,940 to the L341A/A342L receptor was not markedly different from that for the wild-type receptor despite the constitutive Gs activity. This may reflect a preference of this ligand for an activated receptor state associated with the Gi coupling form and underscores the potential for developing therapeutics that selectively activate one pathway over another.

Calcitonin gene-related peptide modulates acid-mediated regulation of somatostatin and gastrin release from rat antrum

BACKGROUND & AIMS

Acid has been shown to stimulate calcitonin gene-related peptide (CGRP) release from peripheral sensory afferent nerve endings in the stomach. The aim of this study was to determine whether endogenous CGRP was involved, by a neurocrine mechanism, in acid-mediated stimulation of somatostatin and inhibition of gastrin release.

METHODS

A two-compartment sleeve of antral mucosal/submucosal tissue was perfused to determine sensory nerve and endocrine cell responses to luminal acid. CGRP receptor antagonist, CGRP8-37, was used to inhibit the actions of endogenously released CGRP.

RESULTS

Perfusion of the antral sleeve lumen with media of increasing hydrogen ion concentration caused pH-dependent increases in CGRP and somatostatin release and decrease in gastrin release. CGRP8-37 inhibited significantly basal somatostatin (-36%) and stimulated basal gastrin (+65%) release (P < 0.02). Furthermore, CGRP8-37 administration prevented luminal acid-mediated inhibition of gastrin release and stimulation of somatostatin release. These results indicate that CGRP8-37 prevented acid-mediated feedback inhibition of gastrin release and acid-induced feedforward somatostatin release.

CONCLUSION

These results suggest that CGRP plays an important role in the response of antral D and G cells to luminal acid and that local effector action of endogenous CGRP participates in regulation of antral regulatory peptide secretion.

Embryonic stem cells lacking a functional inhibitory G-protein subunit (alpha i2) produced by gene targeting of both alleles

The alpha i2 subunit of the inhibitory heterotrimeric guanine nucleotide-binding proteins is highly conserved in mammals and is expressed in all cell types, but its exact function is not yet defined. We have investigated the role of this protein by producing embryonic stem (ES) cells lacking a functional alpha i2 gene. These alpha i2-null cell lines regulate adenylyl cyclase and grow and differentiate in vitro the same as wild-type ES cells. Homologous recombination was used to sequentially inactivate both copies of the alpha i2 gene. The first allele was inactivated by insertion of a neomycin-resistance gene. We modified the hygromycin B-resistance gene for improved expression in ES cells and used this gene to inactivate the remaining normal allele. The techniques used should be generally applicable to other genes whether or not they are expressed in ES cells.

Infection of L6E9 myoblasts with Trypanosoma cruzi alters adenylate cyclase activity and guanine nucleotide binding proteins

We studied the consequences of infection of L6E9 myoblasts with T. cruzi on the adenylate cyclase complex to test the hypothesis that infection alters the functional properties of the guanine nucleotide regulatory proteins, Ns and Ni. Stimulating activities of adenylate cyclase due to isoproterenol, isoproterenol plus Gpp(NH)p, or forskolin (activities mediated by Ns) are not altered by infection. However, inhibitory activities mediated by Ni [Gpp(NH)p, acetylcholine, and adenosine inhibition of forskolin-dependent adenylate cyclase activity] are compromised by infection. The reduction in adenosine's inhibition of forskolin-dependent adenylate cyclase activity is seen throughout the effective concentration range of adenosine. Pertussis toxin does not change basal or stimulated adenylate cyclase activity in infected cells compared with normal uninfected cells, nor does it alter the inhibiting action of adenosine. To evaluate the coupling proteins (Ns and Ni) involved in the stimulation and inhibition of adenylate cyclase more directly, cholera- and pertussis-toxin-dependent ADP ribosylation studies were performed. The incorporation of [32P]ADP ribose in the presence (specific) or absence (nonspecific) of the toxins was markedly decreased in membranes prepared from infected cells. However, in membranes prepared from infected or uninfected cells previously treated with pertussis toxin, there was a significant reduction in specific pertussis-toxin dependent ADP ribosylation. The infection-associated diminution in toxin-dependent ADP ribosylation complements the impaired inhibition of adenylate cyclase data. Collectively, the data further substantiate an infection-associated alteration in the adenylate cyclase complex, probably at the level of the guanine nucleotide binding proteins.

Pertussis toxin inhibits negative inotropic and negative chronotropic muscarinic cholinergic effects on the heart

We injected rats with pertussis toxin, known to cause ADP ribosylation of the Gi regulatory protein of the adenylate cyclase complex and of another closely related GTP binding protein in the heart, and after 7 days we examined several effects of muscarinic activation on the heart. The negative chronotropic effect of carbamoylcholine on spontaneously beating perfused hearts was conspicuously diminished. While 10(-5) mol/l carbamoylcholine invariably produced heart arrest in control rats, the heart rate did not decrease by more than 20% in the toxin-treated rats even when the concentration of carbamoylcholine was raised to 10(-2) mol/l. The negative inotropic effect of carbamoylcholine examined on electrically paced ventricles perfused with isoproterenol was reduced, while the maximum positive inotropic effect of isoproterenol was substantially increased after toxin treatment. The inhibitory action of carbamoylcholine on the isoproterenol-stimulated accumulation of cyclic AMP in the heart auricles was attenuated. The weakening by pertussis toxin of the negative inotropic effect of carbamoylcholine is probably mainly due to the ADP ribosylation of the Gi regulatory protein and the subsequent loss of influence of muscarinic receptors on adenylate cyclase. The blockade of the negative chronotropic action of carbamoylcholine by pertussis toxin strongly indicates, together with other recently published evidence, that the Gi or another closely related GTP binding protein in the cardiac pacemaker cells is involved in the coupling of muscarinic receptors to the K+ channels.

Role of the A subunit of pertussis toxin in alteration of Chinese hamster ovary cell morphology

The mechanism by which pertussis toxin induces morphological changes in Chinese hamster ovary cells was studied to determine whether the resulting clustered growth pattern is due to toxin-catalyzed ADP-ribosylation of a cellular substrate. While pertussis toxin was extremely potent in inducing morphological changes in Chinese hamster ovary cells, preparations of isolated A subunit or B oligomer exhibited greatly reduced activity. The clustered growth response of these cells correlated with ADP-ribosylation of a 41-kilodalton cellular substrate for the toxin in that the toxin concentration and time of exposure to the toxin required for ADP-ribosylation were the same as those needed for alterations in cellular morphology. Moreover, pertussis toxin modified by either chemical or photolytic methods exhibited similar decreases in the ability to ADP-ribosylate the cellular substrate and alter cell morphology. These results suggest that clustering of Chinese hamster ovary cells is due to toxin-catalyzed ADP-ribosylation of a 41-kilodalton substrate. Therefore, alteration in Chinese hamster ovary cell morphology can be used as a measure of toxin activity. This assay should prove to be a useful tool in the development and evaluation of new pertussis vaccines.

Further probes into the molecular sites of damage to cerebral adenylate cyclase following postischemic reperfusion

A variety of pharmacological agents were used as experimental probes to determine with greater precision the site(s) of damage to cerebral adenylate cyclase as a consequence of postischemic reperfusion in the gerbil. A paradigm of 60-min bilateral ischemia followed by 40-min reperfusion results in a decreased sensitivity of the catalytic site of adenylate cyclase to Mn2+. Likewise, the GTP-transducer site (guanine nucleotide regulatory or G protein) revealed depressed responses to GTP in the absence or presence of norepinephrine, dopamine agonists, substance P, yohimbine, and cholera and pertussis toxins. Moreover, a crude preparation of GTPase disclosed that damage elicited by postischemic reperfusion was directed to the higher-affinity form of this enzyme, which is associated with the overall function of the guanine nucleotide regulatory protein. Injury to adenylate cyclase was unrelated either to the ability of adrenergic ligands to bind to associated receptor sites or to the capacity of the brain to generate visual evoked potentials in response to visual stimuli.