The mechanism by which bacterial heat-stable
enterotoxins (ST I STA) cause
diarrhea in humans and animals has been linked to the activation of an intestinal membrane-bound
guanylate cyclase.
Guanylin, a recently discovered rat intestinal
peptide, is homologous in structure to ST I and can activate
guanylate cyclase present on the human colonic
carcinoma cell line T84. To directly test the mechanistic association of
guanylate cyclase activation with
diarrhea, we synthesized
guanylin and a
guanylin analog termed N9P10
guanylin and compared their
biological activities with those of a synthetic ST I analog, termed ST Ib(6-18). We report that
guanylin is able to inhibit the binding of a radiolabeled ST I analog to rat intestinal cells but causes
diarrhea in infant mice only at doses at least 4 orders of magnitude higher than that of ST Ib(6-18). In contrast, N9P10
guanylin was enterotoxic in mice at much lower doses than
guanylin but proved to be a weaker inhibitor of radiolabeled ST I than
guanylin in the receptor binding assay. The pattern of
guanylate cyclase activation observed for ST Ib(6-18) and the two
guanylin analogs parallels the results observed in the receptor binding assay rather than those observed in the diarrheal assay. Treatment of
guanylin with
chymotrypsin or lumenal fluid derived from newborn mouse intestines resulted in a rapid loss of binding activity. Together, these results suggest that ST I
enterotoxins may represent a class of long-lived superagonists of
guanylin.