Lysates of the protozoan parasite Entamoeba histolytica altered active electrolyte transport when present on the serosal surface of rabbit ileum and rat colon. The lysate-induced effects on electrolyte transport were similar to those caused by serotonin, and were blocked by bufotenine, an analog known to inhibit the action of serotonin. The transport effects were partially inhibited by antibody to serotonin. The amebic lysates were shown to contain serotonin by radioimmunoassay, high-performance liquid chromatography, and thin-layer chromatography. These results suggest that the serotonin present in Entamoeba histolytica may be important in the diarrhea seen in amebiasis.

Shiga toxins (Stx) produced by enterohemorrhagic E. coli (EHEC) cause life threatening aspects of foodborne diseases. The most common complication, hemolytic uremic syndrome, results from renal damage by Stx after the toxin crosses the intestinal epithelial barrier and enters the bloodstream. However, the mechanisms of Stx1 transport across the intestinal epithelium are not well studied. Here we show that sequestration of G‐actin by Latrunculin B (LatB) increases Stx1 transport across the intestinal epithelial T‐84 cells ten times (n=6, p<0.05). Neither Cytochalasin D nor Jasplakinolide affect toxin transcytosis. Stx1 transcytosis modulated by LatB was transcellular and not through the tight junctions, because tannic acid inhibited this Stx1 transcytosis. These data indicate that Stx1 transcellular transcytosis might be regulated in vivo by actin polymerizing‐depolymerizing proteins. Immunoblotting of cecal tissue from rabbits infected with Stx1 producing EHEC showed changes in expression of two actin monomer binding proteins, profilin and cofilin, compared to animals infected with similar bacteria without Stx1 phage. Profilin was downregulated, while cofilin phosphorylation was increased in Stx1‐containing rabbits. We conclude that Stx1 transcellular transcytosis is significantly facilitated by actin turnover. The role of cofilin and profilin in Stx1 transcytosis is under investigation.