B. anthracis
edema toxin (ET) and lethal toxin (LT) are each composed of protective
antigen (PA), necessary for toxin uptake by host cells, and their respective toxic moieties,
edema factor (EF) and lethal factor (LF). Although both toxins likely contribute to
shock during
infection, their mechanisms are unclear. To test whether ET and LT produce arterial relaxation, their effects on
phenylephrine (PE)-stimulated contraction in a Sprague-Dawley rat aortic ring model were measured. Rings were prepared and connected to
pressure transducers. Their viability was confirmed, and peak contraction with 60 mM KCl was determined. Compared with PA pretreatment (control, 60 min), ET pretreatment at concentrations similar to those noted in vivo decreased the mean (±SE) maximum contractile force (MCF; percent peak contraction) in rings generated during stimulation with increasing PE concentrations (96.2 ± 7.0 vs. 57.3 ± 9.1) and increased the estimated PE concentration producing half the MCF (EC50; 10(-7) M, 1.1 ± 0.3 vs. 3.7 ± 0.8, P ≤ 0.002). ET inhibition with PA-directed
monoclonal antibodies, selective EF inhibition with
adefovir, or removal of the ring endothelium inhibited the effects of ET on MCF and EC50 (P ≤ 0.02). Consistent with its
adenyl cyclase activity, ET increased tissue cAMP in endothelium-intact but not endothelium-denuded rings (P < 0.0001 and 0.25, respectively). LT pretreatment, even in high concentrations, did not significantly decrease MCF or increase EC50 (all P > 0.05). In rings precontracted with PE compared with posttreatment with PA (90 min), ET posttreatment produced progressive reductions in contractile force and increases in relaxation in endothelium-intact rings (P < 0.0001) but not endothelium-denuded rings (P = 0.51). Thus, ET may contribute to
shock by producing arterial relaxation.