Myocardial adenylate cyclase activity in acute murine Chagas' disease

We have studied the influence of myocardial infection with Trypanosoma cruzi on the β-adrenergic adenylate cyclase complex in mouse myocardial membranes. The maximal rate of cAMP generation (V(max)) and the concentration of agonist associated with 50% of the maximal activity (apparent K(act)) were determined for a series of agents. Six days after infection, the V(max) for isoproterenol significantly declines without a change in the apparent K(act). After 21 days of infection, both the V(max) and apparent K(act) for isoproterenol are reduced. At 6 and 21 days of infection, the affinity of the β-receptor for [¹²⁵I]iodocyanopindolol declines from 0.84 to 3.6 and 3 nM, respectively, while the receptor density increases with the duration of infection from 33 to 57 and 82 fmol/mg protein, respectively. The V(max) (but not the apparent K(act)) for forskolin and Mg ²⁺-and Mn²⁺-associated activities declines also after 21 days. Another adenylate cyclase activity, which was stimulated by the nonhydrolyzable guanine nucleotide Gpp(NH)p, declines in relation to the duration of infection. Inhibitors of adenylate cyclase activity were also studied. Inhibition of adenylate cyclase activity by adenosine and by Gpp(NH)p (in the presence of forskolin) declines after 21 days of infection. The results suggested that the coupling proteins Ns and Ni, which mediate stimulatory or inhibitory control of receptors to adenylate cyclase activity, might be altered by infection. As monitored by cholera toxin- and pertussis toxin-dependent ADP ribosylation of their respective substrates, which include Ns and Ni proteins, respectively, there are declines in the availability of both substrates as a result of T. cruzi infection. For infected membranes, the addition of NADP enhances the magnitude of cholera toxin-dependent ADP ribosylation and renders the magnitude of pertussis toxin-dependent ADP ribosylation equal to that observed in uninfected membranes. The results support the hypothesis that infection with T. cruzi results in profound generalized alterations of the adenylate cyclase complex at several different sites.