Myocardial adenylate cyclase activity in acute murine Chagas' disease

Cyclic AMP decreases the production of NO and CCL2 by macrophages stimulated with Trypanosoma cruzi GPI-mucins

Glycosylphosphatidylinositol-anchored mucin-like glycoproteins (tGPI-mucin) present on the surface of the cellular membrane of Trypanosoma cruzi forms activate toll-like receptors 2 (TLR2) on the surface of immune cells and induce the release of several mediators of inflammation which may be relevant in the context of Chagas disease. Here, we evaluated the ability of tGPI-mucins to activate murine peritoneal macrophages to induce nitric oxide (NO) and monocyte chemoattractant protein-1 (MCP-1/CCL2). We also investigated the ability of compounds which increase or mimic cyclic adenosine monophosphate (AMP) to modulate the production of NO and CCL2. Our data show that elevation of intracellular levels of cyclic AMP prevents the release of NO and CCL2 induced by tGPI-mucins in macrophages. Overall, the release of CCL2 was decreased to a greater extent and at lower concentrations of cyclic AMP-modifying agents than the production of NO. It is suggested that the elevation of cyclic AMP during T. cruzi infection may modify the release of pro-inflammatory mediators and alter significantly the course of T. cruzi infection.

Sjögren autoantibodies modify neonatal cardiac function via M1 muscarinic acetylcholine receptor activation

Isolated congenital heart block may be associated with primary Sjögren syndrome. In this work we describe circulating antibodies in the sera of primary Sjögren syndrome patients that are able to interact with neonatal myocardium by activating muscarinic acetylcholine receptors of M1 subtype. We report on the presence of autoantibodies against the second extracellular loop of human M1 muscarinic acetylcholine receptors in primary Sjögren syndrome mothers whose children have congenital heart block using a synthetic peptide in indirect immunofluorescence technique. Autoantibodies from primary Sjögren syndrome patients gave positive image on neonatal atria but not on adult atria slices. The synthetic M1 peptide selectively abrogated indirect immunofluorescence recognition. The primary Sjögren syndrome-immunoglobulin G also displayed an 'agonist like' activity modifying the intracellular events associated with muscarinic acetylcholine receptor activation. The mechanism appears to occur secondarily to stimulation of phosphoinositides turnover via phospholipase C activation. This, in turn, triggers cascade reactions involving calcium/calmodulin and leads to activation of nitric oxide synthase and soluble guanylate cyclase. All of these effects were selectively blunted by pirenzepine and neutralized by M1 synthetic peptide. These biological effects were not obtained using adult instead of neonatal rat atria and neither occurred with the sera of normal healthy women of childbearing age. It could be concluded that antibodies against neonatal M1 muscarinic acetylcholine receptor may be another serum factor to be considered in the pathophysiology of the development of congenital heart block associated with primary Sjögren syndrome mothers.

Desensitization and sequestration of human m2 muscarinic acetylcholine receptors by autoantibodies from patients with Chagas' disease

Chronic Chagas' disease is associated with pathologic changes of the cardiovascular, digestive, and autonomic nervous system, culminating in autonomic denervation and congestive heart failure. Previously, circulating autoantibodies that activate signaling by cardiac muscarinic acetylcholine receptors (mAChRs) have been described. However, it remains unclear whether the chagasic IgGs directly interact with the m2 mAChRs (predominant cardiac subtype), and, if so, whether chronic exposure of the mAChRs to such activating IgGs would result in receptor desensitization. Here we performed studies with purified and reconstituted hm2 mAChRs and demonstrate that IgGs from chagasic serum immunoprecipitated the mAChRs in a manner similar to an anti-m2 mAChR monoclonal antibody tested in parallel. The chagasic antibodies did not directly interact with the ligand binding site, because the binding of radiolabeled antagonist was unchanged by the addition of the chagasic IgG. In intact cells stably expressing the hm2 mAChR, the chagasic IgGs, but not normal IgGs, mimicked the ability of the agonist acetylcholine to induce two effects associated with agonist-induced receptor desensitization: a decrease in affinity for agonist binding to m2 mAChR and sequestration of the hm2 mAChRs from the cell surface. The results demonstrate that the chagasic IgGs can directly interact with and desensitize m2 mAChRs and provide support for the hypothesis of autoimmune mechanisms having a role in the pathogenesis of Chagas' cardioneuromyopathy.

Differential cholinoceptor subtype-dependent activation of signal transduction pathways in neonatal versus adult rat atria

In this study, we investigated the expression and distribution of muscarinic cholinergic receptors (mAChRs) and the different signaling pathways associated with mAChR activation in atria isolated from adult and neonatal rats. Carbachol stimulation of mAChRs in both neonatal and adult rat atria led to a negative inotropic response with activation of phosphoinositide hydrolysis, an increase in cyclic GMP levels, and a decrease in cyclic AMP production. However, compared with adult atria, neonatal atria showed hypersensitivity in the contractile effect induced by carbachol. Pharmacological analysis with mAChR antagonists indicated that M1 and M2 mAChR subtypes are important mediators of the response to carbachol in neonatal atria. In contrast, in adult atria the effect of the agonist was coupled only to the M2 mAChR subtype. Moreover, an increased number of total mAChRs was labeled in neonatal atrial membranes compared with those of adults. Although a predominant M2 mAChR population is expressed in atria at both stages of development studied, competition binding parameters calculated for carbachol indicated the presence of high-affinity binding sites, with higher affinity in neonates than in adults. These results suggest that the differences observed between neonatal and adult atria in their response to a cholinergic agonist may be related to differential expression of mAChR subtypes and/or changes in functional coupling of mAChR subtypes during development.

A factor from Trypanosoma cruzi induces repetitive cytosolic free Ca2+ transients in isolated primary canine cardiac myocytes

An unusual 120-kDa alkaline peptidase contained in a trypomastigote soluble fraction (TSF) of Trypanosoma cruzi is associated with the induction of repetitive Ca2+ transients and subsequent invasion by the parasite of a number of mammalian cell lines, including tissue culture L6E2 myoblasts (B. A. Burleigh and N. W. Andrews, J. Biol. Chem. 270:5172-5180, 1995; S. N. J. Moreno, J. Silva, A. E. Vercesi, and R. Docampo, J. Exp. Med. 180:1535-1540, 1994; A. Rodríguez, M. G. Rioult, A. Ora, and N. W. Andrews, J. Cell Biol. 129:1263-1273, 1995; I. Tardieux, M. H. Nathanson, and N. W. Andrews, J. Exp. Med. 179:1017-1022, 1994). Using single cell spectrofluorometry and whole-cell patch clamping, we show that TSF produces rapid repetitive cytosolic Ca2+ transients (each associated with cell contraction) in primary cardiac myocytes isolated from dogs. The response of myocytes to TSF was dose dependent in that increasing numbers of cells responded to increasing concentrations of TSF. The TSF-induced Ca2+ transients could be obliterated when TSF was heated or treated with trypsin or the protease inhibitor leupeptin. Aprotinin, pepstatin A, and E-64 did not affect TSF activity. The TSF-induced Ca2+ transients and trypomastigote cell invasion could not be inhibited by alpha (prazosin)- or beta (propanolol)-adrenergic blockers or L-type Ca2+ channel blockers (verapamil, nisoldipine, or cadmium) or by removal of extracellular Ca2+. However, inhibition of pertussis toxin-sensitive G proteins and Ca2+ release from the sarcoplasmic reticulum (with thapsigargin or ryanodine) prevented the TSF-induced Ca2+ transients and cell invasion by trypomastigotes. These data suggested that cardiac myocyte pertussis toxin-sensitive G proteins are associated with the regulation of TSF-induced Ca2+ transients and myocyte invasion by trypomastigotes but are independent of Ca2+ entry into the cytosol via L-type Ca2+ channels. The Ca2+ transients are dependent on release of Ca2+ from sarcoplasmic reticulum Ca2+ stores, but this release is not dependent on extracellular Ca2+ or on the classic model of Ca2+ -induced Ca2+ release in cardiac myocytes. Further, subthreshold depolarizations, together with cell contraction as demonstrated by whole-cell patch clamping, occurred with each Ca2+ transient. However, the depolarizations were of magnitude insufficient to generate an action potential, providing further evidence for a lack of dependence on L-type Ca2+ channels and other voltage-dependent channels (Na+ and K+ channels) in the generation of TSF-induced Ca2+ transients. Our findings suggest that primary canine cardiac myocytes respond to TSF and parasite invasion in ways similar to those of the in vitro cell lines studied to date. Since cardiac myocytes are primary targets for T. cruzi in the vertebrate host, our study indicates that TSF may play a role in the pathogenesis of Chagas' disease in humans.

Lymphocyte G proteins reflect response to treatment in congestive heart failure

Congestive heart failure is associated with chronotropic and inotropic hyporesponsiveness to adrenergic stimulation. A decrease in Gs alpha or an increase in Gi alpha is associated with a decrease in adenylyl cyclase activity. The current study assessed G proteins in response to treatment with direct-acting vasodilators and correlated changes in lymphocyte beta-adrenergic receptor components with changes in hemodynamic variables. Twenty-three patients with severe chronic congestive heart failure (New York Heart Association functional classes III and IV) were studied. Patients were grouped as responders (n = 10) or nonresponders (n = 13) on the basis of clinical assessment of functional status from questionnaires. Therapy was associated with an increase in cardiac index, a decrease in mean arterial pressure, and a decrease in systemic vascular resistance in all patients. Left ventricular filling pressure significantly decreased in responders (26 +/- 2 mm to 13 +/- 3 mm, p < 0.05) but did not change significantly in nonresponders. Similarly, mean right atrial pressure significantly decreased in responders (11 +/- 2 mm Hg to 4 +/- 1 mm Hg, p < 0.05) but did not change in nonresponders. Plasma norepinephrine increased significantly only in nonresponders (679 +/- 100 pg/ml to 1233 +/- 201 pg/ml, p < 0.05). Whereas lymphocyte beta-adrenergic receptor density and Gs did not significantly change, Gi increased after treatment only in the nonresponder group (23 +/- 5 to 51 +/- 11 fmol/mg, p < 0.05). A poor response to direct-acting vasodilators can be distinguished by reactive increases in plasma norepinephrine and lymphocyte Gi in the absence of a decrease in either left- or right-sided filling pressures.

Restoration of the cAMP second messenger pathway enhances cardiac preservation for transplantation in a heterotopic rat model

Current organ preservation strategies subject graft vasculature to severe hypoxia (PO2 approximately 20 Torr), potentially compromising vascular function and limiting successful transplantation. Previous work has shown that cAMP modulates endothelial cell (EC) antithrombogenicity, barrier function, and leukocyte/EC interactions, and that hypoxia suppresses EC cAMP levels. To explore the possible benefits of cAMP analogs/agonists in organ preservation, we used a rat heterotopic cardiac transplant model; dibutyryl cAMP added to preservation solutions was associated with a time- and dose-dependent increase in the duration of cold storage associated with successful graft function. Preservation was also enhanced by 8-bromo-cAMP, the Sp isomer of adenosine 3',5'monophosphorothioate, and types III (indolidan) and IV (rolipram) phosphodiesterase inhibitors. Neither butyrate alone nor 8-bromoadenosine were effective, and the cAMP-dependent protein kinase antagonist Rp isomer of adenosine 3',5'monophosphorothioate prevented preservation enhancement induced by 8-bromo-cAMP. Grafts stored with dibutyryl cAMP demonstrated a 5.5-fold increase in blood flow and a 3.2-fold decreased neutrophil infiltration after transplantation. To explore the role of cAMP in another cell type critical for vascular homeostasis, vascular smooth muscle cells were subjected to hypoxia, causing a time-dependent decline in cAMP levels. Although adenylate cyclase activity was unchanged, diminished oxygen tensions were associated with enhanced phosphodiesterase activity (59 and 30% increase in soluble types III and IV activity, respectively). These data suggest that hypoxia or graft ischemia disrupt vascular homeostasis, at least in part, by perturbing the cAMP second messenger pathway. Supplementation of this pathway provides a new approach for enhancing cardiac preservation, promoting myocardial function, and maintaining vascular homeostatic properties.

Direct coupling of opioid receptors to both stimulatory and inhibitory guanine nucleotide-binding proteins in F-11 neuroblastoma-sensory neuron hybrid cells

Evidence is presented for linkage of opioid receptors directly to the stimulatory G protein (guanine nucleotide-binding protein), Gs, in addition to the generally accepted linkage to the inhibitory and "other" G proteins, gi and Go, in F-11 (neuroblastoma-dorsal root ganglion neuron) hybrid cells. Treatment of intact F-11 cells with cholera toxin decreased specific binding of the opioid agonist [D-Ala2,D-Leu5]enkephalin to F-11 cell membranes by 35%, with the remaining binding retaining high affinity for agonist. Under these conditions cholera toxin influenced the alpha subunit of Gs (Gs alpha) but had no effect on the alpha subunit of Gi/o (Gi/o alpha), based on ADP-ribosylation studies. Pertussis toxin treatment decreased high-affinity opioid agonist binding by about 50%; remaining binding was also of high affinity, even though pertussis toxin had inactivated Gi/o alpha selectively and essentially completely. Simultaneous treatment with both toxins had an additive effect, reducing specific binding by about 80%. While opioid agonists inhibited forskolin-stimulated adenylate cyclase activity of F-11 cells as expected, opioids also stimulated basal adenylate cyclase activity, indicative of interaction with Gs as well as Gi. Cholera toxin treatment attenuated opioid-stimulation of basal adenylate cyclase, whereas pertussis toxin treatment enhanced stimulation. In contrast, inhibition by opioid of forskolin-stimulated activity was attenuated by pertussis toxin but not by cholera toxin. It is concluded that a subset of opioid receptors may be linked directly to Gs and thereby mediate stimulation of adenylate cyclase. This Gs-adenylate cyclase interaction is postulated to be responsible for the novel excitatory electrophysiologic responses to opioids found in our previous studies of sensory neurons and F-11 cells.

Chagas' disease

Chagas' disease, caused by Trypanosoma cruzi, is an important cause of morbidity in many countries in Latin America. The important modes of transmission are by the bite of the reduviid bug and blood transfusion. The organism exists in three morphological forms: trypomastigotes, amastigotes, and epimastigotes. The mechanism of transformation and differentiation is currently being explored, and signal transduction pathways of the parasites may be involved in this process. Parasite adherence to and invasion of host cells is a complex process involving complement, phospholipase, penetrin, neuraminidase, and hemolysin. Two clinical forms of the disease are recognized, acute and chronic. During the acute stage pathological damage is related to the presence of the parasite, whereas in the chronic stage few parasites are found. In recent years the roles of tumor necrosis factor, gamma interferon, and the interleukins in the pathogenesis of this infection have been reported. The common manifestations of chronic cardiomyopathy are arrhythmias and thromboembolic events. Autoimmune, neurogenic, and microvascular factors may be important in the pathogenesis of the cardiomyopathy. The gastrointestinal tract is another important target, and "mega syndromes" are common manifestations. The diagnosis and treatment of this infection are active areas of investigation. New serological and molecular biological techniques have improved the diagnosis of chronic infection. Exacerbations of T. cruzi infection have been reported for patients receiving immuno-suppressive therapy and for those with AIDS.

Trypanosoma cruzi: alteration of cAMP metabolism following infection of human endothelial cells

We have previously reported that Trypanosoma cruzi infection of endothelial cells results in alterations in the metabolism of Ca2+, inositol triphosphate (IP3), and prostacycline (PGI2). In this report, we demonstrate that infection also alters the metabolism of cAMP. Infection of endothelial cells does not significantly alter beta-adrenergic receptor density or affinity, adenylate cyclase activity, and whole-cell cAMP levels. However, incubation of infected endothelial cells with the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) resulted in less than a 60% increase in cell cAMP in contrast to the greater than a 100% increase observed in uninfected endothelial cells under otherwise identical reaction conditions. Infected endothelial cells demonstrated a twofold increase in phosphodiesterase activity when measured directly. Moreover, homogenates prepared from infected endothelial cells previously incubated with isoproterenol for 20 min showed little or no change in PDE activity. In contrast, homogenates prepared from uninfected endothelial cells treated under otherwise identical reaction conditions showed a 5.7-fold increase in PDE activity. In the presence of IBMX, isoproterenol-dependent stimulation of cAMP levels in infected endothelial cells reached a maximum level at 5 min of incubation, and thereafter rapidly declined. In contrast, cAMP levels in uninfected endothelial cells reached a maximum at 2 min of incubation, and thereafter remained elevated throughout the duration of the incubation. Infection-associated changes in isoproterenol dependent stimulation of cAMP accumulation appear to relate, in part, to changes in PDE activity.

An update to silent myocardial ischemia: pathophysiological, diagnostic, and therapeutic approaches

Silent myocardial ischemia (SMI) is a common ischemia process which can be defined as objective evidence of myocardial ischemia without chest pain or other equivalent indications. SMI can occur in totally asymptomatic patients, as well as in patients who have documented coronary artery disease (CAD) and who, on exercise and/or during Holter monitoring, may show ischemic changes. The precise mechanism for pathogenesis and pathophysiology of SMI remains to be clarified. A great deal has been learned about the role of beta-adrenergic receptors, adenylyl cyclase, and guanine nucleotide binding proteins (G proteins) in the myocardial ischemic process of SMI. Moreover, standard exercise test and long-term ECG recordings have proved to be of great value, especially when performed jointly, however, in this field there is still room for expanded knowledge. Pharmacological interventions to date have demonstrated the beneficial effects of beta-adrenergic receptor antagonists and/or calcium antagonists as contributing substantially to reducing both frequency and duration of SMI episodes. However, therapeutic options to improve the prognosis of SMI appear to be limited.

Trypanosoma cruzi: infection of cultured human endothelial cells alters inositol phosphate synthesis

Infection of cultured endothelial cells with Trypanosoma cruzi alters intracellular Ca2+ homeostasis. To help understand the biochemical basis for this phenomenon, we determined the influence of infection on inositol phosphate formation in a broken cell preparation. Inositol phosphates participate in the regulation of cytosolic Ca2+. In uninfected endothelial cells, bradykinin guanosine 5'-O-thiophosphate (GTP tau S), and calcium all stimulated inositol phosphate (IP1), inositol bisphosphate (IP2), and inositol trisphosphate (IP3) formation within 5 sec of incubation. At longer periods of incubation with GTP tau S and bradykinin, formation of IP1 was linear for 30 sec, whereas the rate of IP2 and IP3 generation was maximal at 20 and 5 sec, respectively. Second, infection markedly changed these aspects of inositol phosphate generation. First, unstimulated (basal) levels of IP1 and IP3 were markedly increased over those levels in membranes of uninfected cells. Infection decreased the rate of formation for the three inositol phosphates in response to GTP tau S and bradykinin. Finally, infection diminished the magnitude of inositol phosphate synthesis in response to Ca2+ for IP1, IP2, and IP3, respectively. Studies on G proteins using cholera and pertussis toxin were carried out to determine if the infection-associated changes in inositol phosphate generation could be attributed to functional changes in these regulatory proteins known to participate in the activation of phospholipase C. Infection markedly decreased the magnitude of cholera and pertussis toxin-dependent ADP ribosylation, as compared to control uninfected cells. Incubation of uninfected endothelial cells with cholera and pertussis toxin also decreased the magnitude of cholera and pertussis toxin ADP ribosylation. Despite the similar effects of infection and toxin treatment on subsequent toxin-catalyzed ADP ribosylation, toxin treatment did not influence inositol phosphate generation. Collectively, these results demonstrate an influence of infection on receptor-dependent and -independent synthesis of inositol phosphates, possibly by an action on phospholipase C. The results help to explain the apparent infection-associated increase in basal Ca2+ previously observed and suggest that interference with signal transduction may be a consequence of the presence of the parasite.

Verapamil ameliorates clinical, pathologic and biochemical manifestations of experimental chagasic cardiomyopathy in mice

The influence of long-term verapamil administration on the consequences of Trypanosoma cruzi infection in mice was studied with regard to animal mortality, morbidity, myocardial pathologic features and myocardial beta-adrenergic adenylate cyclase activity. Verapamil administration dramatically decreased the mortality rate from 60% to 6% during the 70 day period of infection. Three clinical stages of infection were evident. In the acute stage (17 days after infection with maximal parasitemia), verapamil treatment not only decreased the incidence of myocardial disease (fibrosis and inflammation), but also protected myocardial beta-adrenergic adenylate cyclase activity. In addition, there was no increase in total body weight, which was regarded as an index of right-sided heart failure. In the subacute stage (30 to 60 days after infection), administration of verapamil continued to decrease myocardial disease and preserve beta-adrenergic adenylate cyclase activity. In addition, verapamil ameliorated the morbidity and mortality associated with this stage of infection. The chronic stage of infection was characterized by a decrease in myocardial disease and in beta-adrenergic adenylate cyclase activity. Thus, independent of the state of infection, long-term verapamil treatment enhanced beta-adrenergic adenylate cyclase activity. In addition, verapamil ameliorated the morbidity associated with infection. Although the relation among these various effects of verapamil in the setting of T. cruzi infection remains to be determined, collectively the results suggested that verapamil administration attenuated the consequences of T. cruzi infection.