The intestinal hormone guanylin and bacterial heat-stable enterotoxins (STs) are members of a peptide family that activates intestinal membrane guanylate cyclase. Two different peptides that activate the human intestinal T84 cell guanylate cyclase have been purified from urine and intestinal mucosa of opossums (Didelphis virginiana). The highly acidic peptide, QEDCELCINVACTGC, was named uroguanylin because it was isolated from urine and shares 53% identity with guanylin. A second peptide, SHTCEICAFAA-CAGC, was purified from urine and intestinal mucosa. This alanine-rich peptide was 47% identical to uroguanylin and 73% identical to human guanylin, suggesting that it may be an opossum homologue of guanylin. Synthetic uroguanylin-(2-15) (i.e., EDCELCINVACTGC) was 10-fold more potent than synthetic rat guanylin, but both peptides were less potent than Escherchia coli ST in the T84 cell cGMP bioassay. Uroguanylin-(2-15) and guanylin inhibited 12'I-ST binding to T84 cell receptors in competitive radioligand binding assays. Transepi-

Guanylin and uroguanylin are intestinal peptides that stimulate chloride secretion by activating a common set of receptor-guanylate cyclase signaling molecules located on the mucosal surface of enterocytes. High mucosal acidity, similar to the pH occurring within the f luid microclimate domain at the mucosal surface of the intestine, markedly enhances the cGMP accumulation responses of T84 human intestinal cells to uroguanylin. In contrast, a mucosal acidity of pH 5.0 renders guanylin essentially inactive. T84 cells were used as a model epithelium to further explore the concept that mucosal acidity imposes agonist selectivity for activation of the intestinal receptors for uroguanylin and guanylin, thus providing a rationale for the evolution of these related peptides. At an acidic mucosal pH of 5.0, uroguanylin is 100-fold more potent than guanylin, but at an alkaline pH of 8.0 guanylin is more potent than uroguanylin in stimulating intracellular cGMP accumulation and transepithelial chloride secretion. The relative affinities of uroguanylin and guanylin for binding to receptors on the mucosal surface of T84 cells is inf luenced dramatically by mucosal acidity, which explains the strong pH dependency of the cGMP and chloride secretion responses to these peptides. The guanylin-binding affinities for peptide-receptor interaction were reduced by 100-fold at pH 5 versus pH 8, whereas the affinities of uroguanylin for these receptors were increased 10-fold by acidic pH conditions. Deletion of the N-terminal acidic amino acids in uroguanylin demonstrated that these residues are responsible for the increase in binding affinities that are observed for uroguanylin at acidic pH. We conclude that guanylin and uroguanylin evolved distinctly different structures, which enables both peptides to regulate, in a pHdependent fashion, the activity of receptors that control intestinal salt and water transport via cGMP.Guanylin and uroguanylin are structurally related peptides that were isolated from intestinal mucosa and urine (1-5). A receptor for guanylin and uroguanylin that has been identified at the molecular level is a transmembrane form of guanylate cyclase, termed GC-C (6). This membrane protein was originally discovered as an intestinal receptor for the heat-stable toxin (ST) peptides, which are secreted intraluminally by enteric bacteria that cause traveler's diarrhea (7). Bacterial ST peptides are related in primary structure to uroguanylin and guanylin, thus acting as molecular mimics of the enteric peptide hormones (reviewed in refs. 8 and 9). Membrane receptor-guanylate cyclases are found on the luminal surface of enterocytes throughout the small and large intestine and in other epithelia (10-13). Binding of peptide agonists to an extracellular domain of the receptor activates the intracellular catalytic domain producing the second messenger cGMP within target enterocytes (1-6). Intracellular cGMP stimulates transepithelial chloride secretion by regulating the phosphorylation state and chloride channel activit...

Heat-stable enterotoxins activate guanylate cyclase, whereas heat-labile enterotoxins stimulate adenylate cyclase. Both classes of toxins cause secretory diarrhea at least in part by stimulating Cl- secretion in the intestine. The mechanism for regulation of Cl- secretion by guanosine 3',5'-cyclic monophosphate (cGMP) was investigated using cultured T84 intestinal cells as a model for intestinal crypt cells. Escherichia coli heat-stable enterotoxin (ST) markedly stimulated cGMP production in T84 cells. Cl- secretion across T84 cell monolayers cultured on permeable filters was stimulated by E. coli ST, cholera toxin, or 8-BrcAMP, but 8-BrcGMP was ineffective. cGMP analogues that are known to be potent and specific activators of cGMP-dependent protein kinase (cG-kinase) also had little effect on 36Cl- uptake by T84 cells cultured in plastic dishes. E. coli ST, forskolin, cholera toxin, or membrane-permeant cAMP analogues markedly increased 36Cl- uptake into T84 cells. The general protein kinase inhibitor, staurosporine, inhibited the stimulation of Cl- permeability elicited by E. coli ST, vasoactive intestinal peptide (VIP), or 8-BrcAMP. DEAE-Sephacel chromatography revealed a predominant type II isoform of cAMP-dependent protein kinase (cA-kinase) in T84 cells, whereas little or no cytosolic cG-kinase activity was found. Treatment of T84 cells with E. coli ST or VIP resulted in an increase in the cA-kinase activity ratio (-cAMP/+cAMP) if the cytosolic enzyme was assayed at reduced temperature (on ice).(ABSTRACT TRUNCATED AT 250 WORDS)

Pathogenic strains of enteric bacteria secrete small heat-stable toxins (STs) that activate membrane guanylyl cyclase receptors found in the intestine. The intestinal peptide agonists, guanylin and uroguanylin, are structurally related to STs. Receptors for ¹²⁵I-ST were found throughout the entire length of the intestinal tract of all the birds examined. These receptors were restricted to intestinal epithelial cells covering villi and forming intestinal glands and were not observed in other strata of the gut wall. The most intense labeling of receptors by ^!25I-ST occurred in the region of the microvillus border of individual enterocytes. There appeared to be a decrease in receptor density distally along the length of the small intestine, although labeling of receptors by ^l25I-ST was observed throughout the small intestine and colon. Cellular cGMP accumulation responses to Escherichia coli ST and rat guanylin in the domestic turkey and duck were greater in the proximal small intestine compared to the distal small intestine or colon. Brush border membranes (BBM) isolated from the mucosa of proximal small intestine of turkeys exhibited agonist-stimulated guanylyl cyclase activity. The rank order potency for enzyme activation was E. coli ST > uroguanylin > guanylin. Competitive radioligand binding assays using ¹²⁵I-ST and turkey intestine BBM revealed a similar rank order affinity for the receptors that was exemplified by the K_d values of ST 2.5 nM, uroguanylin 80 nM and guanylin 2.6 µM. It may be concluded that functional receptors for the endogenous peptides, guanylin and uroguanylin, occur in the apical membranes of enterocytes throughout the avian intestine. The receptor-guanylyl cyclase(s) of proximal small intestine were preferentially activated by uroguanylin relative to guanylin, but both endogenous peptides were less potent than their molecular mimic, E. coli ST.