The studies presented here provide the first detailed analyses of cardiovascular toxicity induced by purified anthrax toxins in a rat model. Both toxins induce significant hypotension. Importantly, LeTx and EdTx appear to produce the hypotension by affecting different parts of the cardiovascular system. Serum levels of toxins were measurable in all tested rats. Lethality and hypotension were associated with doses of toxins that produced peak PA levels in the µg/mL range. This is consistent with the measurements of toxin levels in spore-infected animals 
. Hypotension and death occurred within hr of reaching similar ranges of toxin in the blood of infected rats, rabbits and guinea pigs.
LeTx rapidly induced acute myocardial dysfunction similar to that observed with fulminant myocarditis with significant increases in the LV systolic area, LV diastolic area, and ventricular compliance. Left ventricular dilatation was expected from acute myocardial dysfunction such as fulminant myocarditis. Increased ventricular compliance is implied by the increased Vp. Ogawa reported that Vp was positively correlated with stroke volume, heart rate and left ventricular compliance 
. Since stroke volume and heart rate did not significantly change, then increased ventricular compliance is the likely explanation for increases in Vp. Increased left ventricular compliance may be due to loss of functional intercalated discs that connect the myocardial cells. In contrast, EdTx has an intravascular volume effect. This observation is compatible with “third spacing” or decreased intravascular volume due to rapid fluid shifts out of the blood vessels.
Both observations are consistent with published results of toxin-induced rodent hypotension. Mice intravenously infused with LeTx developed malaise and death by 60–100 hr 
. Corticosteroids and aldosterone were unable to prevent lethality or shock. Histopathology showed hypoxic tissue necrosis in liver, marrow, spleen and heart, pleural and peritoneal fluid and edema. There were elevations of serum erythropoietin and transaminase and hypoalbuminemia, hypofibrinogenemia, thrombocytopenia, elevated prothrombin time and partial thromboplastin time, and disseminated fibrin deposition and hemorrhage. Rats treated with LeTx died after two to twenty-one hr with refractory hypotension, bradycardia, lactic acidosis, and pleural effusions 
. Again, these results are consistent with shock and hypoxic secondary tissue injury. Mice treated with EdTx died with one to three days with hypotensive shock and “third spacing” in the intestinal lumen and other tissues 
. Pathology showed fluid accumulation in intestines, adrenal hemorrhage, lymphocytolysis, osteoblast necrosis, and elevated transaminases and urea nitrogen. These results coincide with our observation of a reduction in preload due to a loss of vascular volume and tone. Additional studies in anthrax toxin-treated rats will be performed in our laboratory to better investigate the cardiac and vascular anatomy.
Previous reports support a critical role for mitogen activated protein kinase (MAPK) signaling in the heart. Tissue culture studies with antisense molecules and knockout mice show cardiomyocyte injury with impaired MEK1/2-ERK1/2 kinase activities 
. Cancer patients treated with a small molecular weight MEK inhibitor develop heart failure 
. Similarly, increases in vascular endothelial cAMP from calcitonin gene-related peptide G-protein activation led to decreased vascular resistance in hepatic, coronary, skin and gastric vessels 
. Thus, the cAMP pathway is important in vascular homeostasis. Additional studies with tissues from anthrax toxin treated animals will be needed to confirm changes in these vital tissue-signaling pathways.
The observed cardiovascular defects induced by anthrax toxins were dependent on the presence of two toxin components (PA+LF or PA+EF). Rats received IV infusion of LF alone survived and showed no symptom of hypotension (data not shown). Previous studies have demonstrated that, in the absence of PA, injection of EF does not cause any symptom in treated animals 
. In addition, only combination of PA with LF or EF, but not LF or EF alone, displays toxicity in macrophage cells 
and suppresses T lymphocyte activation 
Combinations of LeTx and EdTx as occurs in systemic anthrax infections would be predicted to generate more severe and irreversible hypotension due to an inability of either the heart or blood vessels to respond to lesions in the other organ. Preliminary studies with combinations of EdTx and LeTx show worse hypotension and greater lethality 
. Examination of the hemodynamic parameters described in the current study with rats treated with mixtures of LeTx and EdTx will be useful to confirm these predictions.
In summary, these results suggest that the pathophysiology of anthrax shock may be closely linked with circulating toxins and involve both a rapid reduction in left ventricular preload and systolic function. Intervention to rescue animals and patients with anthrax shock will need to address both of these cardiovascular lesions. Rodents and other species provide an excellent system to test new therapeutics for this devastating disease.