Elevated cAMP in intestinal epithelial cells during experimental cholera and salmonellosis

Cholera and salmonellosis are two diarrheal diseases in which intestinal tissue cyclic adenosine monophosphate (cAMP) concentrations are elevated. Investigations of each experimental disease were initiated to identify the specific intestinal cells containing the elevated cAMP. Epithelial cells were eluted from the mucosa of infected and control intestinal loops of adult rabbits, after which the cAMP content of the epithelial cell fractions and the lamina propria cells was extracted and assayed. The identity of the epithelial cells (in the villus tip-to-crypt cell gradient) was monitored by measuring their intracellular alkaline phosphatase activity, while scanning electron microscopy was used to visualize the effects of infection and cell elution techniques. Clearly, in both experimental cholera and salmonellosis, elevated cAMP levels were associated with crypt epithelial cells. Villus tip epithelial cells from either infection tended to contain less cAMP than those of noninfected control tissue. In Salmonella-infected loops, it was apparent that cAMP was also elevated in lamina propria cell fractions. Lamina propria cells from V. cholerae-infected intestinal loops contained only basal levels of cAMP. In vitro exposure of isolated intestinal cells from normal rabbit intestine to a cell-free lysate of Salmonella resulted in elevation of cAMP in the epithelial cells and lamina propria cells. We conclude that in experimental cholera and salmonellosis, significant elevation of the cAMP levels occurred in intestinal crypt cells, consistent with an enterotoxin-mediated mechanism. In Salmonella-infected loops, it was unclear if the increased concentration of cAMP in lamina propria cells was generated by enterotoxin released from the invasive salmonellae or by prostaglandins formed during the inflammatory response to the bacteria, or by both mechanisms.