We have studied the influence of myocardial
infection with Trypanosoma cruzi on the beta-
adrenergic adenylate cyclase complex in mouse myocardial membranes. The maximal rate of cAMP generation (Vmax) and the concentration of agonist associated with 50% of the maximal activity (apparent Kact) were determined for a series of agents. Six days after
infection, the Vmax for
isoproterenol significantly declines without a change in the apparent Kact. After 21 days of
infection, both the Vmax and apparent Kact for
isoproterenol are reduced. At 6 and 21 days of
infection, the affinity of the beta-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 Vmax (but not the apparent Kact) for
forskolin and Mg2+- 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.