guanylyl-imidodiphosphate and Chagas-Cardiomyopathy

Acute Trypanosoma cruzi infection is commonly associated with disorders of impulse conduction and muscle contraction in heart. In order to determine the extent to which receptor function changed in response to infection, infected neonatal rat cardiac myocytes in culture were compared with matched controls with regard to chronotropic response and Ca2+ mobilization following the application of adrenergic agonists. At 7-9 days in culture (5-7 days postinfection), spontaneous beat rates of control myocytes were four times as rapid as those in infected cells. Control cells responded to 10(-5) M isoproterenol (ISO) and 10(-6) M norepinephrine (NE) with increases in beat rate of 34 and 40%, respectively. Effects of ISO on infected cells were similar, and adenylate cyclase activity was similar in control and infected cells when measured in the presence of ISO alone or in combination with Gpp(NH)p. NE produced a more marked chronotropic response in infected cultures and altered Ca2+ mobilization. NE treatment increased Ca2+ levels in control cardiac myocytes from 51.8 +/- 4.4 to 113 +/- 16 nM (in 0 Ca2+ medium) and from 85.2 +/- 6.8 to 131.3 +/- 24.5 nM (1 mM external Ca2+). In infected cardiac myocytes, NE increased Ca2+ from 116.8 +/- 17 to 164.7 +/- 9.6 nM (in 0 Ca2+ medium) and from 132.2 +/- 13.2 to 162.5 +/- 0.3 nM (1 mM Ca2+ medium). Thus, basal and alpha-adrenergic-stimulated Ca2+ levels were higher in infected than uninfected myocytes regardless of the extracellular Ca2+ levels, although the fractional increase in infected myocytes was significantly lower than that in controls (1.4- and 1.2-fold vs 2.2- and 1.5-fold). Therefore, both chronotropic and Ca(2+)-mobilization responses to the alpha-adrenergic agonist NE are altered in T. cruzi-infected cardiac myocytes; the chronotropic response of similarly infected cells to the beta-adrenergic agonist ISO was not affected. These data indicating that T. cruzi infection may be associated with a dissociation in responses to these agonists suggest a possible mechanism to explain, in part, the cardiac dysfunction characteristic of Chagas' disease.

Topics: Animals; Animals, Newborn; Calcium; Cells, Cultured; Chagas Cardiomyopathy; Guanylyl Imidodiphosphate; Heart; Heart Rate; Hydrogen-Ion Concentration; Image Processing, Computer-Assisted; Isoproterenol; Myocardial Contraction; Myocardium; Norepinephrine; Prazosin; Rats; Receptors, Adrenergic, alpha; Receptors, Adrenergic, beta; Signal Transduction; Temperature; Time Factors; Trypanosoma cruzi

Infection of beagles with an opossum-derived strain of Trypanosoma cruzi (Tc-O) results in features of early and chronic chagasic cardiomyopathy, that is, increases in PR interval, atrioventricular block, and frequent ventricular premature contractions, ventricular tachycardia, and decreased left ventricular ejection fraction. These signs are not observed in animals infected with a canine strain of T. cruzi (Tc-D). To understand the biochemical basis for these early cardiac effects, we examined the beta-adrenergic adenylate cyclase complex in myocardial membranes prepared from animals infected with either of the two strains. In animals infected with Tc-O (symptomatic), the maximum velocity (Vmax) decreased and concentration of agonist resulting in 50% of Vmax (Kact) increased for isoproterenol-dependent adenylate cyclase activity; in animals infected with Tc-D (asymptomatic), Vmax and Kact for isoproterenol were unchanged from control, uninfected animals. beta-Receptor density decreased by 20% in symptomatic animals with no change in affinity, whereas no differences were observed between uninfected and infected asymptomatic animals. A complex pattern of changes was apparent in the guanine nucleotide binding protein, Gs, in the setting of infection. Alterations in cholera toxin-dependent ADP-ribosylation patterns as well as immunochemical detection with anti-G alpha s antisera suggested a change in the biochemical nature of the Gs species and not necessarily a physical loss of this protein. Reconstitution of adenylate cyclase activity in cyc- membranes demonstrated a decrease in hormone-sensitive Gs activity in membranes prepared from symptomatic animals without a change in activity demonstrable in the presence of Gpp(NH)p. Collectively, the results suggest that the depression in beta-adrenergic adenylate cyclase activity associated with symptomatic infection of beagles with T. cruzi occurs primarily as a result of changes in the Gs protein complex, most likely resulting in an uncoupling of the beta-adrenergic receptor from the Gs protein.

Topics: Adenosine Diphosphate Ribose; Adenylyl Cyclases; Animals; Binding, Competitive; Chagas Cardiomyopathy; Cholera Toxin; Dogs; Electrocardiography; GTP-Binding Proteins; Guanylyl Imidodiphosphate; Immune Sera; Iodocyanopindolol; Isoproterenol; Myocardium; Pindolol; Receptors, Adrenergic, beta; Trypanosoma; Trypanosoma cruzi