Reciprocal effects of mucosal PH on cgmp and CI secretion responses of intestinal cells to uroguanylin and guanylin

Uroguanylin and guanylin are intestinal hormones that function in an intrinsic pathway to stimulate receptor-guanylate cyclase activity in enterocytes. Why these two peptides are produced together with their apical membrane receptor-guanylate cyclase effector molecules in epithelia lining the intestine is not understood. We used T84 intestinal cells as a model to investigate the functional responses to guanylin, uroguanyiin and bacterial heat-stable enterotoxin (ST) under the variable pH conditions that exist in vivo at the apical surface of enterocytes. The potency of guanylin was 10-fold greater under alkaline conditions of apical extracellular pH (7.8-8.0) than at acidic pH (5.0-5.5) for stimulating cGMP accumulation and transepithelial chloride secretion. In contrast, uroguanyiin was 10-fold more potent at acidic pH than at alkaline pH for stimulating cGMP production and chloride secretion. £. co/i ST was more potent than guanylin and uroguanyiin at either acidic or alkaline pH Radioligand binding assays revealed that the relative affinities of these peptides for receptors on T84 cells at pH 5 was ST > uroguanyiin > guanylin, but at pH 8 was ST > guanylin > uroguanyiin, which agreed with the rank-order potencies of these agonists for eliciting functional responses at acidic versus alkaline pH. We propose that the primary structures of uroguanyiin and guanylin have evolved to provide the intestine with highly potent and effective hormones for the regulation of guanylate cyclase activity and chloride secretion over the widely variable microclimate pH found at the apical surface of enterocytes. In comparison, the primary structures of the bacterial STs that cause Travellers diarrhea contributes to their "superagonist" properties for stimulating guanylate cyclase activity and chloride transport under the acidic to alkaline conditions that commonly occur in the intestinal lumen.