Escherichia coli enterotoxin (STa) binds to receptors, stimulates guanyl cyclase, and impairs absorption in rat colon.

To determine the contribution of the colon in Escherichia coli heat-stable enterotoxin-mediated diarrheal disease, toxin binding, guanyl cyclase activation, and toxin-induced water flux in the rat colon and ileum were compared. Scatchard analysis suggested a single class of heat-stable enterotoxin receptors with an affinity constant of binding of 10(9) L/mol in both colonocytes and ileocytes; however, the number of toxin receptors per cell was 3.5-fold greater in coloncytes than ileocytes (8.32 +/- 1.33 x 10(5) vs. 2.33 +/- 0.28 x 10(5) receptors per cell; P = 0.02). Heat-stable enterotoxin stimulated guanyl cyclase activation in an identical dose-dependent manner in proximal colonic and ileal membranes, with similar sensitivity and maximum response. Heat-stable enterotoxin also inhibited net water flux to a similar degree in both colon and ileum (-47.8 vs. -48.4 microL.cm-1.h-1, respectively) at a dose of 8 nmol/L. At this dose in the colon, because of a higher baseline of absorption, absorption continued, but at a diminished level. At this dose in the ileum, heat-stable enterotoxin induced net secretion. These data are consistent with the concept that heat-stable enterotoxin-induced diarrheal disease results from a decreased absorptive capacity in the colon in the face of increased small intestinal fluid secretion.