Regulation of intestinal uroguanylin/guanylin receptor-mediated responses by mucosal acidity

Isoflurane-induced acidosis depresses basal and PGE(2)-stimulated duodenal bicarbonate secretion in mice

When running in vivo experiments, it is imperative to keep arterial blood pressure and acid-base parameters within the normal physiological range. The aim of this investigation was to explore the consequences of anesthesia-induced acidosis on basal and PGE(2)-stimulated duodenal bicarbonate secretion. Mice (strain C57bl/6J) were kept anesthetized by a spontaneous inhalation of isoflurane. Mean arterial blood pressure (MAP), arterial acid-base balance, and duodenal mucosal bicarbonate secretion (DMBS) were studied. Two intra-arterial fluid support strategies were used: a standard Ringer solution and an isotonic Na(2)CO(3) solution. Duodenal single perfusion was used, and DMBS was assessed by back titration of the effluent. PGE(2) was used to stimulate DMBS. In Ringer solution-infused mice, isoflurane-induced acidosis became worse with time. The blood pH was 7.15-7.21 and the base excess was about -8 mM at the end of experiments. The continuous infusion of Na(2)CO(3) solution completely compensated for the acidosis. The blood pH was 7.36-7.37 and base excess was about 1 mM at the end of the experiment. Basal and PGE(2)-stimulated DMBS were markedly greater in animals treated with Na(2)CO(3) solution than in those treated with Ringer solution. MAP was slightly higher after Na(2)CO(3) solution infusion than after Ringer solution infusion. We concluded that isoflurane-induced acidosis markedly depresses basal and PGE(2)-stimulated DMBS as well as the responsiveness to PGE(2), effects prevented by a continuous infusion of Na(2)CO(3). When performing in vivo experiments in isoflurane-anesthetized mice, it is recommended to supplement with a Na(2)CO(3) infusion to maintain a normal acid-base balance.

Identification of two functional guanylin receptors in eel: multiple hormone-receptor system for osmoregulation in fish intestine and kidney

Guanylyl cyclase C (GC-C) is a single transmembrane receptor for a family of intestinal hormones, guanylins. In the eel, we previously identified three guanylins, whose gene expression was enhanced in the intestine after transfer from fresh water to seawater. However, only limited information is available about the structure and function of their receptor(s). In the present study, we cloned full-length cDNAs encoding two isoforms of GC-C, named GC-C1 and GC-C2, from eel intestine. The predicted GC-C proteins consisted of extracellular ligand-binding domain, membrane-spanning domain, kinase-like domain and cyclase catalytic domain, in which GC-C-specific sequences were largely conserved. Phylogenetic analyses showed that the cloned membrane GCs are grouped with the GC-C of other vertebrates but not with GC-A and GC-B. However, eel GC-Cs appear to have undergone unique structural evolution compared with other GC-Cs. The three eel guanylins (guanylin, uroguanylin and renoguanylin), but not eel atrial natriuretic peptide, stimulated cGMP production dose-dependently in COS cells expressing either of the cloned cDNAs, providing functional support for assignment as eel guanylin receptors. The potency order for cGMP production was uroguanylin > guanylin > or = renoguanylin for GC-C1; guanylin > or = renoguanylin > uroguanylin for GC-C2. The distinctive ligand selectivity was consistent with the low homology (53%) of the extracellular domain of the two GC-Cs compared with that observed for other domains (74-90%). Both GC-C genes were expressed in the alimentary tract (esophagus, stomach and intestine) and kidney, and their expression was higher in the intestine of seawater-adapted eels than that of freshwater eels just as observed with the guanylin genes. However, the expression of the receptor genes was unchanged for 24h after transfer of eels from fresh water to seawater or vice versa, showing slower response of the receptors to salinity changes than their ligands. Collectively, the multiple guanylin-GC-C system may be involved as a paracrine factor in seawater adaptation at the intestine and kidney of the eel.

Effect of uroguanylin on potassium and bicarbonate transport in rat renal tubules

The effect of uroguanylin (UGN) on K+and H+secretion in the renal tubules of the rat kidney was studied using in vivo stationary microperfusion. For the study of K+secretion, a tubule was punctured to inject a column of FDC-green-colored Ringer's solution with 0.5 mmol KCl/L ± 10−6mol UGN/L, and oil was used to block fluid flow. K+activity and transepithelial potential differences (PD) were measured with double microelectrodes (K+ion-selective resin vs. reference) in the distal tubules of the same nephron. During perfusion, K+activity rose exponentially, from 0.5 mmol/L to stationary concentration, allowing for the calculation of K+secretion (JK). JKincreased from 0.63 ± 0.06 nmol·cm–2·s–1in the control group to 0.85 ± 0.06 in the UGN group (p < 0.01). PD was –51.0 ± 5.3 mV in the control group and –50.3 ± 4.98 mV in the UGN group. In the presence of 10−7mol iberiotoxin/L, the UGN effect was abolished: JKwas 0.37 ± 0.038 nmol·cm–2·s–1in the absence of, and 0.38 ± 0.025 in the presence of, UGN, indicating its action on maxi-K channels. In another series of experiments, renal tubule acidification was studied, using a similar method: proximal and distal tubules were perfused with solutions containing 25 mmol NaHCO3/L. Acidification half-time was increased both in proximal and distal segments and, as a consequence, bicarbonate reabsorption decreased in the presence of UGN (in proximal tubules, from 2.40 ± 0.26 to 1.56 ± 0.21 nmol·cm–2·s–1). When the Na+/H+exchanger was inhibited by 10−4mol hexamethylene amiloride (HMA)/L, the control and UGN groups were not significantly different. In the late distal tubule, after HMA, UGN significantly reduced JHCO3–, indicating an effect of UGN on H+-ATPase. These data show that UGN stimulated JK+by acting on maxi-K channels, and decreased JHCO3–by acting on NHE3 in proximal and H+-ATPase in distal tubules.

Cellular effects of guanylin and uroguanylin

Ingestion of a salty meal induces secretion of guanylin (GN) and uroguanylin (UGN) into the intestinal lumen, where they inhibit Na+ absorption and induce Cl-, HCO3-, and water secretion. Simultaneously, these hormones stimulate renal electrolyte excretion by inducing natriuresis, kaliuresis, and diuresis. GN and UGN therefore participate in the prevention of hypernatremia and hypervolemia after salty meals. The signaling pathway of GN and UGN in the intestine is well known. They activate enterocytes via guanylate cyclase C (GC-C), which leads to cGMP-dependent inhibition of Na+/H+ exchange and activation of the cystic fibrosis transmembrane regulator. In GC-C-deficient mice, GN and UGN still produce renal natriuresis, kaliuresis, and diuresis, suggesting different signaling pathways in the kidney compared with the intestine. Signaling pathways for GN and UGN in the kidney differ along the various nephron segments. In proximal tubule cells, a cGMP- and GC-C-dependent signaling was demonstrated for both peptides. In addition, UGN activates a pertussis toxin-sensitive G-protein-coupled receptor. A similar dual signaling pathway is also known for atrial natriuretic peptide. Recently, a cGMP-independent signaling pathway for GN and UGN was also shown in principal cells of the human and mouse cortical collecting duct. Because GN and UGN activate different signaling pathways in specific organs and even within the kidney, this review focuses on more recent findings on cellular effects and signaling mechanisms of these peptides and their pathophysiologic implications in the intestine and the kidney.

Guanylin and uroguanylin induce natriuresis in mice lacking guanylyl cyclase-C receptor

BACKGROUND

Guanylin (GN) and uroguanylin (UGN) are intestinally derived peptide hormones that are similar in structure and activity to the diarrhea-causing Escherichia coli heat-stable enterotoxins (STa). These secretagogues have been shown to affect fluid, Na+, K+, and Cl- transport in both the intestine and kidney, presumably by intracellular cyclic guanosine monophosphate (cGMP)-dependent signal transduction. However, the in vivo consequences of GN, UGN, and STa on renal function and their mechanism of action have yet to be rigorously tested.

METHODS

We hypothesized that intravenous administration of GN, UGN, or STa would cause an increase in natriuresis in wild-type mice via cGMP and guanylyl cyclase-C (GC-C, Gucy2c), the only known receptor for these peptide-hormones, and that the peptide-induced natriuresis would be blunted in genetically altered mice devoid of GC-C receptors (GC-C(-/-) null).

RESULTS

In wild-type mice using a modified renal clearance model, GN, UGN, and STa elicited significant natriuresis, kaliuresis, and diuresis as well as increased urinary cGMP levels in a time- and dose-dependent fashion. Absolute and fractional urinary sodium excretion levels were greatest approximately 40 minutes following a bolus infusion with pharmacologic doses of these peptides. Unexpectedly, GC-C(-/-) null mice also responded to the GN peptides similarly to that observed in wild-type mice. Glomerular filtration rate (GFR), blood pressure, and plasma cGMP in the mice (wild-type or GC-C(-/-) null) did not significantly vary between the vehicle- and peptide-treatment groups. The effects of UGN may also influence long-term renal function due to down-regulation of the Na+/K+ ATPase gamma-subunit and the Cl- channel ClC-K2 by 60% and 75%, respectively, as assessed by differential display polymerase chain reaction (PCR) (DD-PCR) and Northern blot analysis of kidney mRNA from mice treated with UGN.

CONCLUSION

GN, UGN, and STa act on the mouse kidney, in part, through a cGMP-dependent, GC-C-independent mechanism, causing significant natriuresis by renal tubular processes. UGN may have further long-term effects on the kidney by altering the expression of such transport-associated proteins as Na+/K+ ATPase and ClC-K2.

Regulation of uterine function: a biochemical conundrum in the regulation of smooth muscle relaxation

Premature birth accounts for the majority of fetal morbidity and mortality in the developed world and is disproportionately represented in some populations, such as African Americans in the United States. The costs associated with prematurity are staggering in both monetary and human terms. Present therapeutic approaches for the treatment of labor leading to preterm delivery are inadequate and our understanding of the regulation of myometrial smooth muscle contraction-relaxation is incomplete. The ability of nitric oxide to relax smooth muscle has led to an interest in employing nitric oxide-donors in the treatment of preterm labor. Fundamental differences exist, however, in the regulation of uterine smooth muscle relaxation and that of other smooth muscles and constitute a conundrum in our understanding. We review the evidence that nitric oxide-mediated relaxation of myometrial smooth muscle, unlike vascular or gastrointestinal smooth muscle, is independent of global elevation of cyclic guanosine 5'-monophosphate. Applying our current understanding of microdomain signaling and taking clues from genomic studies of pregnancy, we offer a framework in which to view the apparent conundrum and suggest testable hypotheses of uterine relaxation signaling that can explain the mechanistic distinctions. We propose that understanding these mechanistic distinctions in myometrium will reveal molecular targets that are unique and thus may be explored as therapeutic targets in the development of new uterine smooth muscle-specific tocolytics.

Guanosine 3',5'-cyclic monophosphate: a tapeworm-secreted signal molecule communicating with the rat host's small intestine

Tapeworms alter the physiological environment of the host's small intestinal lumen by contracting the intestinal smooth muscle, thereby slowing the transit of intestinal contents. We hypothesize that parasite-to-host molecular signaling is responsible for the specific patterns of small intestinal smooth muscle contraction observed both during tapeworm infection and after the infusion of tapeworm-secreted molecules into the intestinal lumen of unanesthetized rats. Of the tapeworm-secreted compounds tested, only lumenal infusion of guanosine 3',5'-cyclic monophosphate (cGMP) induced contractile patterns that mimic those observed during tapeworm infection. The response to cGMP occurred in a concentration-dependent fashion. Our study clearly demonstrates that cGMP can serve as an extracellular signal molecule regulating small intestinal motility mechanisms in vivo.

A novel role for uroguanylin in the regulation of sodium balance

Uroguanylin is a peptide hormone that regulates sodium excretion by the kidney when excess NaCl is consumed. A new study demonstrates that mice deficient in uroguanylin have blunted urinary sodium excretion responses to oral sodium loads in addition to elevated blood pressure (see related article beginning on page 1244). A physiological role for uroguanylin is discussed, linking the intestine and kidney in an endocrine axis for the maintenance of sodium balance.

Sudden infant death syndrome: is it a transepithelial transport disorder?

The first limiting factor of dietary zinc deficiency has been described as a loss of the protective role of zinc against auto-oxidation of membrane sulfhydryl (SH) compounds. It has now been established that the prohormones (nutriuretic peptides) of the intestinal guanylin family are activated extracellularly by conversion of cysteines in the peptide to disulfide bridges. The induction of uroguanylin mRNA is elevated in intestinal zinc deficiency and nutriuretic peptides regulate epithelial transport of salt and water. Nitric oxide (NO) is also a modulator of salt and water transport. The constitutive forms of nitric oxide synthase (cNOS) in neurons and endothelial cells are calcium-dependent. The inducible form of nitric oxide synthase (iNOS) is activated by bacterial entero-toxins and damaged mucosa with NO penetrating the cell and acting directly on guanylate cyclase. The activated receptor-guanylate cyclases initiating the intracellular cycle 3'-5' guanasine monophosphate (cyclic GMP) cascade in target cells results in a flux of chloride and water into the intestinal lumen. Most of the actions of NO are mediated by activation of cyclic GMP. High-altitude pulmonary edema (HAPE) is associated with a defect in transepithelial water transport. It is suggested that dietary zinc, by modulating thiol oxidation to disulfides in guanylin prohormones to active hormones, is associated with salt and water secretion such that the overworked heart in hypoxemia increases the production and release of natriuretic peptides to activate guanylate cyclase receptors in target tissue in sudden infant death syndrome.

Guanylin, uroguanylin, and heat-stable euterotoxin activate guanylate cyclase C and/or a pertussis toxin-sensitive G protein in human proximal tubule cells

Membrane guanylate cyclase C (GC-C) is the receptor for guanylin, uroguanylin, and heat-stable enterotoxin (STa) in the intestine. GC-C-deficient mice show resistance to STa in intestine but saluretic and diuretic effects of uroguanylin and STa are not disturbed. Here we describe the cellular effects of these peptides using immortalized human kidney epithelial (IHKE-1) cells with properties of the proximal tubule, analyzed with the slow-whole-cell patch clamp technique. Uroguanylin (10 or 100 nm) either hyperpolarized or depolarized membrane voltages (V(m)). Guanylin and STa (both 10 or 100 nm), as well as 8-Br-cGMP (100 microm), depolarized V(m). All peptide effects were absent in the presence of 1 mm Ba(2+). Uroguanylin and guanylin changed V(m) pH dependently. Pertussis toxin (1 microg/ml, 24 h) inhibited hyperpolarizations caused by uroguanylin. Depolarizations caused by guanylin and uroguanylin were blocked by the tyrosine kinase inhibitor, genistein (10 microm). All three peptides increased cellular cGMP. mRNA for GC-C was detected in IHKE-1 cells and in isolated human proximal tubules. In IHKE-1 cells GC-C was also detected by immunostaining. These findings suggest that GC-C is probably the receptor for guanylin and STa. For uroguanylin two distinct signaling pathways exist in IHKE-1 cells, one involves GC-C and cGMP as second messenger, the other is cGMP-independent and connected to a pertussis toxin-sensitive G protein.

H/dipeptide absorption across the human intestinal epithelium is controlled indirectly via a functional Na/H exchanger

BACKGROUND & AIMS

For optimal nutrient absorption to occur, the enterocyte must express a range of specialist ion-driven carrier proteins that function cooperatively in a linked and mutually dependent fashion. Thus, absorption via the human intestinal H(+)-coupled di/tripeptide transporter (hPepT1) is dependent on maintenance of the trans-apical driving force (the H(+)-electrochemical gradient) established, in part, by brush-border Na(+)/H(+) exchanger (NHE3) activity. This study aimed to examine whether physiologic regulation of NHE3 activity can limit hPepT1 capacity and, therefore, protein absorption after a meal.

METHODS

hPepT1 and NHE3 activities were determined in intact human intestinal epithelial Caco-2 cell monolayers by measurements of [(14)C]glycylsarcosine transport and uptake, (22)Na(+)-influx, H(+)-influx, and H(+)-efflux. Expression of NHE regulatory factors was determined by reverse-transcriptase polymerase chain reaction.

RESULTS

Optimal dipeptide transport was observed in the presence of a transapical pH gradient and extracellular Na(+). At apical pH 6.5, and only in Na(+)-containing media, protein kinase A activation (by forskolin or vasoactive intestinal peptide) or selective NHE3 inhibition (by S1611) reduced transepithelial dipeptide transport and cellular accumulation by a reduction in the capacity (without effect on affinity) of dipeptide uptake.

CONCLUSIONS

Protein kinase A-mediated modulation of intestinal dipeptide absorption is indirect via effects on the apical Na(+)/H(+) exchanger.

Structure and function of the heat-stable enterotoxin receptor/guanylyl cyclase C

Guanylyl cyclase C (GC-C) was found to function as the principal receptor for heat-stable enterotoxins (STa), major causative factors in E. coli-induced secretory diarrhea. GC-C is enriched in intestinal epithelium, but was also detected in other epithelial tissues. The enzyme belongs to the family of receptor guanylyl cyclases, and consists of an extracellular receptor domain, a single transmembrane domain, a kinase homology domain, and a catalytic domain. GC-C is modified by N-linked glycosylation and, at least in the small intestine, by proteolysis, resulting in a STa receptor that is coupled non-covalently to the intracellular domain. So far two endogenous ligands of mammalian GC-C have been identified i.e. the small cysteine-rich peptides guanylin and uroguanylin. The guanylins are released in an auto- or paracrine fashion into the intestinal lumen but may also function as endocrine hormones in gut-kidney communication and as regulators of ion transport in extra-intestinal epithelia. They are thought to activate GC-C by inducing a conformational change in the extracellular portion of the homotrimeric GC-C complex, which allows two of the three intracellular catalytic domains to dimerize and form two active catalytic clefts. In the intestine, activation of GC-C results in a dual action: stimulation of Cl and HCO3 secretion, through the opening of apical CFTR Cl channels; and inhibition of Na absorption, through blockade of an apical Na/H exchanger. The principal effector of the GC-C effect on ion transport is cGMP dependent protein kinase type II, which together with GC-C and the ion transporters, may form a supramolecular complex at the apical border of epithelial cells.

In vivo evaluation of an 111In-labeled ST-peptide analog for specific-targeting of human colon cancers

In vitro competitive binding studies of In-DOTA-NCS-6-Ahx-Phe(19)-ST[1-19] vs. 125I-Tyr(5)-6-Ahx-Phe(19)-ST[1-19] with guanylate cyclase -C (GC-C) receptors on human colon cancer LS-180 cells revealed an IC(50) value of 7.7 +/- 0.1.6 nM. The in vitro cellular residualization studies of the 111In-DOTA-NCS-ST peptide and GC-C receptor mediated stimulated cGMP production with LS-180 cells demonstrates that this peptide selectively binds to LS-180 cells in an agonistic fashion. In vivo biodistribution studies in LS-180 tumor bearing SCID mice demonstrates that the 111In-DOTA-NCS-ST peptide targets the tumor with a specific uptake of 0.94 +/- 0.31%ID/g at 1 hr p.i. and approximately 23% was retained by the tumor at 4 hrs p.i. The radioactivity cleared rapidly from the blood stream with 84.5 +/- 3.4%ID at 1h p.i. found in the urine. High activity in urine and kidney, and minimal activity in liver and intestines, demonstrates preferential clearance of the radioactivity through the renal/urinary pathway. The specific in vitro and in vivo accumulation of the radioactivity by LS-180 human colonic cancer cells highlights the potential of radiometallated-DOTA-ST analogs as diagnostic/therapeutic radiopharmaceuticals.

Guanylyl cyclase C agonists regulate progression through the cell cycle of human colon carcinoma cells

The effects of Escherichia coli heat-stable enterotoxin (ST) and uroguanylin were examined on the proliferation of T84 and Caco2 human colon carcinoma cells that express guanylyl cyclase C (GC-C) and SW480 human colon carcinoma cells that do not express this receptor. ST or uroguanylin inhibited proliferation of T84 and Caco2 cells, but not SW480 cells, in a concentration-dependent fashion, assessed by quantifying cell number, cell protein, and [(3)H]thymidine incorporation into DNA. These agonists did not inhibit proliferation by induction of apoptosis, assessed by TUNEL (terminal deoxynucleotidyl transferase-mediated dNTP-biotin nick end labeling of DNA fragments) assay and DNA laddering, or necrosis, assessed by trypan blue exclusion and lactate dehydrogenase release. Rather, ST prolonged the cell cycle, assessed by flow cytometry and [(3)H]thymidine incorporation into DNA. The cytostatic effects of GC-C agonists were associated with accumulation of intracellular cGMP, mimicked by the cell-permeant analog 8-Br-cGMP, and reproduced and potentiated by the cGMP-specific phosphodiesterase inhibitor zaprinast but not the inactive ST analog TJU 1-103. Thus, GC-C agonists regulate the proliferation of intestinal cells through cGMP-dependent mechanisms by delaying progression of the cell cycle. These data suggest that endogenous agonists of GC-C, such as uroguanylin, may play a role in regulating the balance between epithelial proliferation and differentiation in normal intestinal physiology. Therefore, GC-C ligands may be novel therapeutic agents for the treatment of patients with colorectal cancer.

Radiometallated receptor-avid peptide conjugates for specific in vivo targeting of cancer cells

New receptor-avid radiotracers are being developed for site-specific in vivo targeting of a myriad of receptors expressed on cancer cells. This review exemplifies strategies being used to design radiometallated peptide conjugates that maximize uptake in tumors and optimize their in vivo pharmacokinetic properties. Efforts to produce synthetic peptide analogues that target the following three receptor systems are highlighted: Gastrin releasing peptide (GRP), alpha-melanocyte stimulating hormone (alpha-MSH), and guanylate cyclase-C (GC-C) receptors.

Atrial natriuretic peptide and guanylin-activated guanylate cyclase isoforms in human sweat glands

The ultracytochemical localization of membrane-bound guanylate cyclases A and C, stimulated by atrial natriuretic peptide and guanylin respectively, has been studied in human sweat glands. The results showed that the peptides stimulated guanylate cyclases A and C in both eccrine and apocrine glands. In the secretory cells, enzymatic activity was present on the plasma membranes and on intracellular membranes involved in the secretory mechanism. In eccrine glands, the cells of the excretory duct also presented enzymatic activity on the plasma membranes. In both glands, myoepithelial cells, surrounding the secretory cells, exhibited only guanylate cyclase A activity. These localizations of enzymatic activity suggest a role for both atrial natriuretic peptide and guanylin in regulating glandular secretion.

Guanylyl cyclase-C receptor mRNA distribution along the rat nephron

Guanylin (GN) and uroguanylin (UGN) are two recently identified peptides that have been shown to affect water and electrolyte transport in both the intestine and the kidney. Mechanistically, the effects of both peptides are thought to be mediated by intracellular cGMP which results from ligand binding to a plasma membrane guanylyl cyclase-C (GC-C) receptor. To date, the specific intrarenal site(s) of GN and UGN action have not been established. To begin to address this issue, the present studies utilized semi-quantitative RT-PCR to assess the distribution of GC-C mRNA in specific microdissected segments of the rat nephron. GC-C mRNA expression was highest in the cortical collecting tubule, followed by the proximal convoluted tubule, medullary thick ascending limb and collecting tubule, and thin limbs of Henle's loop. Expression levels were significantly lower in all other segments tested, including the glomerulus. The renal tubular expression pattern for cGMP-dependent protein kinase II (cGK-II) mRNA, which is activated in response to GN/UGN-dependent cGMP accumulation, was similar to that for GC-C. Notably, both GN and UGN mRNAs were also expressed along the nephron. The highest levels of expression for both peptides were detected in the medullary collecting tubule. Lower, but comparable levels of GN and UGN expression also occurred in the cortical collecting tubule, cortical and medullary thick ascending limb, and thin limbs of Henles loop. In the proximal convoluted tubule, GN mRNA expression was also quite high, while UGN mRNA was almost undetectable. The presence of renal GC-C and cGK-II in the kidney are consistent with a proposed endocrine function for GN and UGN. In addition however, the present data suggest that intrarenally synthesized GN and UGN may also contribute to the regulation of renal tubular transport.

Prouroguanylin overproduction and localization in the intestine of zinc-deficient rats

Identification of the upregulation of preprouroguanylin mRNA in the rat small intestine during zinc deficiency provides a potential mechanistic link between production of the intestinal hormone uroguanylin and the diarrhea that may accompany zinc deficiency. In the current study, in situ hybridization demonstrated that the number of preprouroguanylin mRNA-expressing cells was significantly higher in zinc-deficient rats than in zinc-adequate rats. Immunohistochemical studies, with a uroguanylin peptide affinity-purified antibody, demonstrated that immunoreactivity was localized to the tips of villi of the duodenum and jejunum in zinc-adequate rats. However, positive cells were scattered throughout the villus of zinc-deficient rats. A subset of cells, perhaps enterochromaffin cells, exhibited the predominant staining, whereas no specific staining was found in goblet cells or lymphocytes of the lamina propria. Western blotting demonstrated that the expression of prouroguanylin in both duodenum and jejunum was elevated by dietary zinc depletion. These results show that dietary zinc deficiency upregulates prouroguanylin in intestinal cells, which is consistent with a role for uroguanylin in the etiology of diarrhea observed in human zinc deficiency.

Salt intake and sensitivity of intestinal and renal Na+-K+ atpase to inhibition by dopamine in spontaneous hypertensive and Wistar-Kyoto rats

The present study evaluated the activity of jejunal Na+-K+-ATPase and its sensitivity to inhibition by dopamine in spontaneous hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats during low (LS), normal (NS) and high (HS) salt intake. Basal jejunal Na+-K+-ATPase activity in SHR on LS intake was higher than in WKY rats. Jejunal Na+-K+-ATPase activity in WKY rats, but not in SHR, on LS intake was significantly reduced (20% decrease) by dopamine (1 microM) and SKF 38393 (10 nM), but not quinerolane (10 nM), this being antagonized the D1 receptor antagonist (SKF 83566). Changing from LS to NS or HS intake in WKY rats increased basal jejunal Na+-K+-ATPase activity and attenuated the inhibitory effect of dopamine. In SHR, changing from LS to NS or HS intake increased basal jejunal Na+-K+-ATPase activity. Basal renal Na+-K+-ATPase activity in SHR on LS intake was similar to that in WKY rats and was insensitive to inhibition by dopamine. Changing from LS to NS or HS intake in WKY rats increased basal renal Na+-K+-ATPase activity without affecting the inhibitory effect of dopamine. In SHR, changing from LS to NS or HS intake failed to alter basal renal Na+-K+-ATPase activity. It is concluded that inhibition of jejunal Na+-K+ ATPase activity by D1 dopamine receptor activation is dependent on salt intake in WKY rats, and SHR animals fail to respond to dopamine, irrespective of their salt intake.

Regulation of intestinal gene expression by dietary zinc: induction of uroguanylin mRNA by zinc deficiency

The regulation of gene expression by nutrients plays an important role in the overall manifestations of nutritional deficiencies. Insufficient intakes of dietary micronutrients, such as zinc, produce profound effects in multiple organs and tissues. One of the major challenges, however, is to identify genes affected by changes in nutritional status. Differential display of mRNA has proved to be a valuable technique in meeting this challenge. In our ongoing search for genes responsive to dietary zinc, we compared small intestinal mRNA from rats that were fed zinc-deficient or -adequate diets using differential display to generate 3' anchored expressed sequence tags (EST). EST for intestinal mRNAs with altered expression due to zinc deficiency include two peptide hormones, intestinal fatty acid binding protein, intestinal alkaline phosphatase II, a proteasomal ATPase, cis-Golgi p28 and two subunits of the ubiquinone oxidoreductase. The EST for one of the hormones yielded the sequence for the 3' end of an mRNA encoding preprouroguanylin and was used to clone the remaining portion of the rat cDNA via 5' rapid amplification of cDNA ends. Northern blot analysis of RNA from rat intestine demonstrated that preprouroguanylin mRNA was 2.5-fold more abundant during zinc deficiency. Uroguanylin, a natriuretic peptide hormone, is an endogenous ligand for the same guanylate cyclase C that the Escherichia coli heat-stable enterotoxin (STa) binds when it causes secretory diarrhea by activating the cystic fibrosis transmembrane conductance regulator, thus altering fluid balance in the intestine. This suggests a mechanism whereby zinc deficiency could induce uroguanylin levels in the intestine and cause or potentiate diarrhea.

Ultracytochemical detection of guanylate cyclase C activity in alimentary tract and associated glands of the rat. Influence of pH, ATP and the ions Mg2+ and Mn2+

Intestinal guanylate cyclase C is activated by guanylin, an endogenous peptide. This activity seems to be modulated by adenine nucleotides, the ions Mg2+ and Mn2+, and pH. In this study, we report an ultracytochemical method for the localization of guanylate cyclase C activity at the electron microscope level. We studied the enzymatic activity in the presence or absence of guanylin and/or ATP, in the presence of the ions Mg2+ or Mn2+, and at different pH levels. The greatest distribution of enzymatic activity was detected in samples incubated at pH 8 and 7.4 in the presence of guanylin, Mg2+ and ATP. Guanylate cyclase C activity was detected at the surface epithelium of stomach and intestine, and in liver, exocrine pancreas and parotid gland. In the intestine, enzymatic activity was more widely distributed in the duodenum than in the jejunum-ileum and colon. In the small intestine, activity was more evident in the upper portion than in the basal portion of the villus. In samples incubated at pH 8 and 7.4 in the absence of ATP, enzymatic activity was detected only in small intestine, liver and exocrine pancreas. Enzymatic activity was present in duodenum incubated at pH 8 and 7.4 in the presence of Mn2+ and in the presence or absence of ATP. No samples incubated in all these experimental conditions but at pH 5 or samples incubated in the presence of guanylin only or in the absence of guanylin, displayed guanylate cyclase C activity. Our results suggest that a complete ultracytochemical detection of guanylate cyclase C activity requires guanylin as stimulator, and incubation in the presence of Mg2+ and ATP at pH 8 and 7.4.

Increased urinary excretion of uroguanylin in patients with congestive heart failure

Uroguanylin is a small-molecular-weight peptide that activates membrane-bound receptor-guanylate cyclases in the intestine, kidney, and other epithelia. Uroguanylin has been shown to participate in the regulation of salt and water homeostasis in mammals via cGMP-mediated processes, bearing a distinct similarity to the action of the atriopeptins, which play a defined role in natriuresis and act as prognostic indicators of severe congestive heart failure (CHF). The objectives of this study were to measure the urinary levels of uroguanylin and the circulating plasma levels of atrial natriuretic peptide (ANP) in healthy individuals (n = 53) and patients with CHF (n = 16). Urinary excretion of uroguanylin was assessed by a cGMP accumulation bioassay employing human T84 intestinal cells. In individuals without CHF, the concentration of uroguanylin bioactivity was 1.31 +/- 0.27 nmol cGMP/ml urine and 1.73 +/- 0.25 micromol cGMP/24-h urine collection. The urinary bioactivity of uroguanylin in males (1.74 +/- 0.55 nmol cGMP/ml urine; n = 27) tended to be higher than the excretion levels in females (0.94 +/- 0.16 nmol cGMP/ml urine; n = 26) over a 24-h period but did not achieve statistical significance. Both male and female groups showed 24-h temporal diurnal variations with the highest uroguanylin levels observed between the hours of 8:00 AM and 2:00 PM. The circulating level of ANP was 12.1 +/- 1.6 pg/ml plasma and did not significantly vary with respect to male/female population or diurnal variation. In patients with CHF, the concentration of plasma ANP and urinary uroguanylin bioactivity increased substantially (7.5-fold and 70-fold, respectively, both P </= 0.001) compared with healthy levels. Uroguanylin was purified from the urine of CHF patients and shown to be the bioactive, COOH-terminal, 16 amino acid portion of the human prouroguanylin protein. The increased urinary uroguanylin excretion observed during CHF may be an adaptive response to this cardiovascular pathophysiology.

Renal effects of uroguanylin and guanylin in vivo

Uroguanylin and guanylin are newly discovered endogenous heat-stable peptides that bind to and activate a membrane bound guanylyl cyclase signaling receptor (termed guanylyl cyclase C; GC-C). These peptides are not only found in blood but are secreted into the lumen of the intestine and effect a net secretion of electrolytes (Na+, K+, Cl-, HCO3-) and fluid into the intestine via a cyclic guanosine-3', 5'-monophosphate (cGMP) mechanism. GC-C is also the receptor for Escherichia coli heat-stable enterotoxin (STa) and activation by STa results in a diarrheal illness. Employing mouse renal in vivo models, we have demonstrated that uroguanylin, guanylin, and STa elicit natriuretic, kaliuretic, and diuretic effects. These biological responses are time- and dose-dependent. Maximum natriuretic and kaliuretic effects are observed within 30-40 min following infusion with pharmacological doses of the peptides in a sealed-urethra mouse model. Our mouse renal clearance model confirms these results and shows significant natriuresis following a constant infusion of uroguanylin for 30 min, while the glomerular filtration rate, plasma creatinine, urine osmolality, heart rate, and blood pressure remain constant. These data suggest the peptides act through tubular transport mechanisms. Consistent with a tubular mechanism, messenger RNA-differential display PCR of kidney RNA extracted from vehicle- and uroguanylin-treated mice show the message for the Na+/K+ ATPase gamma-subunit is down-regulated. Interestingly, GC-C knockout mice (Gucy2c -/-) also exhibit significant uroguanylin-induced natriuresis and kaliuresis in vivo, suggesting the presence of an alternate receptor signaling mechanism in the kidney. Thus, uroguanylin and guanylin seem to serve as intestinal and renal natriuretic peptide-hormones influencing salt and water transport in the kidney through GC-C dependent and independent pathways. Furthermore, our recent clinical probe study has revealed a 70-fold increase in levels of urinary uroguanylin in patients with congestive heart failure. In conclusion, our studies support the concept that uroguanylin and guanylin are endogenous effector peptides involved in regulating body salt and water homeostasis.

Zinc and intestinal function

Zinc is an abundant trace element in the human body that is essential for growth and development and immune function. It is important for the formation of biomembranes and zinc finger motifs found in DNA transcription factors and has catalytic function in metalloenzymes. The intestine is the site of zinc absorption and the major route of zinc excretion. Dietary inadequacy or conditions that decrease zinc absorption or increase its losses from the gastrointestinal tract, urine, or skin may quickly cause zinc deficiency due to the limited availability of rapidly exchangeable zinc pools in the body. Diarrhea is both a sign and a cause of zinc deficiency. The mechanism by which zinc deficiency causes diarrhea is not known. At this time, there is no readily available sensitive test for the detection of zinc deficiency, and therefore clinical suspicion remains the main mode of detection. In some individuals with diarrheal disease, zinc supplementation lessens diarrhea. Those receiving prolonged supplemental zinc therapy need to be monitored for copper deficiency.

Role of the prosequence of guanylin

Guanylin is a guanylyl cyclase (GC)-activating peptide that is mainly secreted as the corresponding prohormone of 94 amino acid residues. In this study, we show that the originally isolated 15-residue guanylin, representing the COOH-terminal part of the prohormone, is released from the prohormone by cleavage of an Asp-Pro amide bond under conditions applied during the isolation procedures. Thus, the 15-residue guanylin is probably a non-native, chemically induced GC-activating peptide. This guanylin molecule contains two disulfide bonds that are absolutely necessary for receptor activation. We demonstrate that the folding of the reduced 15-residue guanylin results almost completely in the formation of the two inactive disulfide isomers. In contrast, the reduced form of proguanylin containing the entire prosequence folds to a product with the native cysteine connectivity. Because proguanylin lacking the 31 NH2-terminal residues of the prosequence folds only to a minor extent to guanylin with the native disulfide bonds, it is evident that this NH2-terminal region contributes significantly to the correct disulfide-coupled folding. Structural studies using CD and NMR spectroscopy show that native proguanylin contains a considerable amount of alpha-helical and, to a lesser extent, beta-sheet structural elements. In addition, a close proximity of the NH2- and the COOH-terminal regions was found by NOESY. It appears that this interaction is important for the constitution of the correct conformation and provides an explanation of the minor guanylyl cyclase activity of proguanylin by shielding the bioactive COOH-terminal domain from the receptor.

Structure and activity of OK-GC: a kidney receptor guanylate cyclase activated by guanylin peptides

Uroguanylin, guanylin, and lymphoguanylin are small peptides that activate renal and intestinal receptor guanylate cyclases (GC). They are structurally similar to bacterial heat-stable enterotoxins (ST) that cause secretory diarrhea. Uroguanylin, guanylin, and ST elicit natriuresis, kaliuresis, and diuresis by direct actions on kidney GC receptors. A 3,762-bp cDNA characterizing a uroguanylin/guanylin/ST receptor was isolated from opossum kidney (OK) cell RNA/cDNA. This kidney cDNA (OK-GC) encodes a mature protein containing 1,049 residues sharing 72.4-75.8% identity with rat, human, and porcine forms of intestinal GC-C receptors. COS or HEK-293 cells expressing OK-GC receptor protein were activated by uroguanylin, guanylin, or ST13 peptides. The 3.8-kb OK-GC mRNA transcript is most abundant in the kidney cortex and intestinal mucosa, with lower mRNA levels observed in urinary bladder, adrenal gland, and myocardium and with no detectable transcripts in skin or stomach mucosa. We propose that OK-GC receptor GC participates in a renal mechanism of action for uroguanylin and/or guanylin in the physiological regulation of urinary sodium, potassium, and water excretion. This renal tubular receptor GC may be a target for circulating uroguanylin in an endocrine link between the intestine and kidney and/or participate in an intrarenal paracrine mechanism for regulation of kidney function via the intracellular second messenger, cGMP.

Guanylin regulatory peptides: structures, biological activities mediated by cyclic GMP and pathobiology

The guanylin family of bioactive peptides consists of three endogenous peptides, including guanylin, uroguanylin and lymphoguanylin, and one exogenous peptide toxin produced by enteric bacteria. These small cysteine-rich peptides activate cell-surface receptors, which have intrinsic guanylate cyclase activity, thus modulating cellular function via the intracellular second messenger, cyclic GMP. Membrane guanylate cyclase-C is an intestinal receptor for guanylin and uroguanylin that is responsible for stimulation of Cl- and HCO3- secretion into the intestinal lumen. Guanylin and uroguanylin are produced within the intestinal mucosa to serve in a paracrine mechanism for regulation of intestinal fluid and electrolyte secretion. Enteric bacteria secrete peptide toxin mimics of uroguanylin and guanylin that activate the intestinal receptors in an uncontrolled fashion to produce secretory diarrhea. Opossum kidney guanylate cyclase is a key receptor in the kidney that may be responsible for the diuretic and natriuretic actions of uroguanylin in vivo. Uroguanylin serves in an endocrine axis linking the intestine and kidney where its natriuretic and diuretic actions contribute to the maintenance of Na+ balance following oral ingestion of NaCl. Lymphoguanylin is highly expressed in the kidney and myocardium where this unique peptide may act locally to regulate cyclic GMP levels in target cells. Lymphoguanylin is also produced in cells of the lymphoid-immune system where other physiological functions may be influenced by intracellular cyclic GMP. Observations of nature are providing insights into cellular mechanisms involving guanylin peptides in intestinal diseases such as colon cancer and diarrhea and in chronic renal diseases or cardiac disorders such as congestive heart failure where guanylin and/or uroguanylin levels in the circulation and/or urine are pathologically elevated. Guanylin peptides are clearly involved in the regulation of salt and water homeostasis, but new findings indicate that these novel peptides have diverse physiological roles in addition to those previously documented for control of intestinal and renal function.

Enterochromaffin-like cells, a cellular source of uroguanylin in rat stomach

Uroguanylin is an endogenous peptide ligand for guanylyl cyclase-C, an apical membrane receptor predominantly located in the gastrointestinal epithelium. It regulates intestinal and renal fluid and electrolyte transport through the second messenger, cyclic GMP. Uroguanylin messenger RNA and the peptide are present in rat stomach, but the cellular source has not been identified. We separated gastric mucosal cells by size into seven fractions (F1-F7) and enriched endocrine cells into F1-F3 using counterflow elutriation. Uroguanylin messenger RNA and peptide were found in F1-F3 by Northern blot analysis and an RIA specific for rat uroguanylin. Uroguanylin-producing cells were identified as endocrine cells by immunocytochemical methods using antisera for uroguanylin, prouroguanylin, and chromogranin A, as well as by in situ hybridization cytochemistry. Double-staining showed that uroguanylin and histamine are colocalized in enterochromaffin-like (ECL) cells that release histamine, leading to the stimulation of gastric acid secretion from parietal cells. Uroguanylin is synthesized in ECL cells. These findings should contribute to elucidating the physiological functions of ECL cells and the cyclic GMP-mediated gastric ion transport mechanism.

Effects of guanylin and uroguanylin on rat jejunal fluid and electrolyte transport: comparison with heat-stable enterotoxin

The effects of rat guanylin, human guanylin, human uroguanylin and STa on net fluid and electrolyte transport in the closed jejunal loop were compared in anesthetized rats. STa administered into the lumen caused a concentration-dependent (10(-8) to 10(-6) M) inhibition of net fluid and NaCl absorption in the jejunal loop. Uroguanylin had a similar but weaker effect than STa. Both rat and human guanylin inhibited fluid and NaCl absorption only at 10(-6) M. Their order of potency was STa > human uroguanylin > rat guanylin = human guanylin. Changing the luminal pH from 5 to 8 failed to affect the action of guanylin on fluid absorption. Both STa and uroguanylin, but not guanylin, increased the luminal pH by stimulating bicarbonate secretion. Pretreatment of the jejunal loop with guanylin (10(-6) M) 5 min before the instillation of STa (10(-7) M) significantly reduced the inhibitory effect of STa on fluid absorption. It is concluded that guanylin and uroguanylin administered into the rat jejunal lumen have an STa-like action on fluid and electrolyte transport. Guanylin may act as an endogenous antagonist of STa in the rat jejunum and prevent excessive fluid loss by STa.

Compartmentalization of extracellular cGMP determines absorptive or secretory responses in the rat jejunum

We examined potential mechanisms by which angiotensin subtype-2 (AT2) receptor stimulation induces net fluid absorption and serosal guanosine cyclic 3',5'-monophosphate (cGMP) formation in the rat jejunum. L-arginine (L-ARG) given intravenously or interstitially enhanced net fluid absorption and cGMP formation, which were completely blocked by the nitric oxide (NO) synthase inhibitor, N-nitro-L-arginine methylester (L-NAME), but not by the specific AT2 receptor antagonist, PD-123319 (PD). Dietary sodium restriction also increased jejunal interstitial fluid cGMP and fluid absorption. Both could be blocked by PD or L-NAME, suggesting that the effects of sodium restriction occur via ANG II at the AT2 receptor. L-ARG-stimulated fluid absorption was blocked by the soluble guanylyl cyclase inhibitor 1-H-[1,2,4]oxadiazolo[4, 2-alpha]quinoxalin-1-one (ODQ). Cyclic GMP-specific phosphodiesterase in the interstitial space decreased extracellular cGMP content and prevented the absorptive effects of L-ARG. Angiotensin II (ANG II) caused an increase in net Na+ and Cl- ion absorption and 22Na+ unidirectional efflux (absorption) from the jejunal loop. In contrast, intraluminal heat-stable enterotoxin of Escherichia coli (STa) increased loop cGMP and fluid secretion that were not blocked by either L-NAME or ODQ. These findings suggest that ANG II acts at the serosal side via AT2 receptors to stimulate cGMP production via soluble guanylyl cyclase activation and absorption through the generation of NO, but that mucosal STa activation of particulate guanylyl cyclase causes secretion independently of NO, thus demonstrating the opposite effects of cGMP in the mucosal and serosal compartments of the jejunum.

Carboxy-terminal extension stabilizes the topological stereoisomers of guanylin

The peptide hormone guanylin constitutes two topological stereoisomers, which are connected through an equilibrium of interconversion. To investigate the importance of amino acid residues in the central region between the inner cysteines and at the carboxy terminus for this isomerism, synthetic derivatives of guanylin were compared by HPLC, 2D1H NMR spectroscopy and by their guanylyl cyclase-C (GC-C)-activating potency. An increase in the central sterical bulk by introduction of diiodo-Tyr9 had virtually no effect on the isomerization kinetics. Compared to guanylin, carboxy-terminal amidation did not affect the equilibrium between the two isoforms either. In contrast, two significantly stabilized isomers were obtained by extending the carboxy terminus of guanylin with one additional leucine resembling the characteristic of human uroguanylin isomers. This effect was intensified by a further Lys-Lys extension, thus revealing that the conformational exchange between the guanylin isomers is dependent on the extent of the sterical hindrance in the carboxy-terminal region of this peptide. Demonstrated by 2D NMR spectroscopy, the separated isomers of the carboxy-terminally extended derivatives of guanylin exhibit unambiguously closely related structures as found originally for guanylin isomers, which are only detectable as a mixture. Because only one of the stabilized guanylin isomers activates guanylyl cyclase-C, the three-dimensional structure of the GC-C-activating guanylin isomer is now defined. The stabilized isoforms of guanylin described in this study represent suitable tools for the separate functional investigation of the GC-C-agonistic isomer of guanylin as well as of its isomeric counterpart.

One peptide, two topologies: structure and interconversion dynamics of human uroguanylin isomers

The peptide hormone uroguanylin stimulates chloride secretion via activation of intestinal guanylyl cyclase C (GC-C). It is characterized by two disulfide bonds in a 1-3/2-4 pattern that causes the existence of two topological stereoisomers of which only one induces intracellular cGMP elevation. To obtain an unambiguous structure-function relationship of the isomers, we determined the solution structure of the separated uroguanylin isoforms using NMR spectroscopy. Both isomers adopt well-defined structures that correspond to those of the isomers of the related peptide guanylin. Furthermore, the structure of the GC-C-activating uroguanylin isomer A closely resembles the structure of the agonistic Escherichia coli heat-stable enterotoxin. Compared with guanylin isomers, the conformational interconversion of uroguanylin isomers is retarded significantly. As judged from chromatography and NMR spectroscopy, both uroguanylin isoforms are stable at low temperatures, but are subject to a slow pH-dependent mutual isomerization at 37 degrees C with an equilibrium isomer ratio of approximately 1:1. The conformational exchange is most likely under the sterical control of the carboxy-terminal leucine. These results imply that GC-C is activated by ligands exhibiting the molecular framework corresponding to the structure of uroguanylin isomer A.

Regulation of intestinal Cl- and HCO3-secretion by uroguanylin

Uroguanylin is an intestinal peptide hormone that may regulate epithelial ion transport by activating a receptor guanylyl cyclase on the luminal surface of the intestine. In this study, we examined the action of uroguanylin on anion transport in different segments of freshly excised mouse intestine, using voltage-clamped Ussing chambers. Uroguanylin induced larger increases in short-circuit current (Isc) in proximal duodenum and cecum compared with jejunum, ileum, and distal colon. The acidification of the lumen of the proximal duodenum (pH 5.0-5.5) enhanced the stimulatory action of uroguanylin. In physiological Ringer solution, a significant fraction of the Isc stimulated by uroguanylin was insensitive to bumetanide and dependent on HCO3- in the bathing medium. Experiments using pH-stat titration revealed that uroguanylin stimulates serosal-to-luminal HCO3- secretion (Js-->lHCO3-) together with a larger increase in Isc. Both Js-->lHCO3- and Isc were significantly augmented when luminal pH was reduced to pH 5.15. Uroguanylin also stimulated the Js-->lHCO3- and Isc across the cecum, but luminal acidity caused a generalized decrease in the bioelectric responsiveness to agonist stimulation. In cystic fibrosis transmembrane conductance regulator (CFTR) knockout mice, the duodenal Isc response to uroguanylin was markedly reduced, but not eliminated, despite having a similar density of functional receptors. It was concluded that uroguanylin is most effective in acidic regions of the small intestine, where it stimulates both HCO3- and Cl-secretion primarily via a CFTR-dependent mechanisms.

Structure and activity of uroguanylin and guanylin from the intestine and urine of rats

Uroguanylin and guanylin are related peptides that activate common guanylate cyclase signaling molecules in the intestine and kidney. Uroguanylin was isolated from urine and duodenum but was not detected in extracts from the colon of rats. Guanylin was identified in extracts from small and large intestine but was not detected in urine. Uroguanylin and guanylin have distinct biochemical and chromatographic properties that facilitated the separation, purification, and identification of these peptides. Northern assays revealed that mRNA transcripts for uroguanylin were more abundant in small intestine compared with large intestine, whereas guanylin mRNA levels were greater in large intestine relative to small intestine. Synthetic rat uroguanylin and guanylin had similar potencies in the activation of receptors in T84 intestinal cells. Production of uroguanylin and guanylin in the mucosa of duodenum is consistent with the postulate that both peptides influence the activity of an intracellular guanosine 3',5'-cyclic monophosphate signaling pathway that regulates the transepithelial secretion of chloride and bicarbonate in the intestinal epithelium.

Signaling pathways for guanylin and uroguanylin in the digestive, renal, central nervous, reproductive, and lymphoid systems

Guanylin and uroguanylin are peptides that stimulate membrane guanylate cyclases (GC) and regulate intestinal and renal function via cGMP. Complementary DNAs were isolated encoding opossum preproguanylin and a 279-amino acid portion of a receptor-guanylate cyclase expressed in opossum kidney (OK) cells (GC-OK). The tissue expression of messenger RNA transcripts for these signaling molecules were then compared. Northern and/or reverse transcription-PCR assays revealed that guanylin, uroguanylin, and GC-OK messenger RNAs are expressed in tissues within the digestive, renal, central nervous, reproductive, and lymphoid organ systems. Receptor autoradiography localized the receptors for uroguanylin and guanylin to renal proximal tubules and seminiferous tubules of testis. Synthetic guanylin and uroguanylin peptides activated the receptor-GCs in opossum kidney cortex and in cultured OK cells eliciting increased intracellular cGMP. Expression of agonist and receptor-GC signaling molecules provides a pathway for paracrine and/or autocrine regulation of cellular functions via cGMP in the digestive, renal, central nervous, reproductive, and lymphoid/immune organ systems. Uroguanylin also links the intestine and kidney in a potential endocrine axis that activates tubular receptor-GCs and influences renal function.

The uroguanylin gene (Guca1b) is linked to guanylin (Guca2) on mouse chromosome 4

Uroguanylin is an endogenous ligand of the intestinal receptor guanylate cyclase-C (GC-C). Both uroguanylin and the related peptide ligand guanylin bind to GC-C and stimulate an increase in cyclic GMP, inducing chloride secretion via the cystic fibrosis transmembrane conductance regulator. We describe the cloning of the complete mouse uroguanylin gene (Guca1b) and show that Guca1b is tightly linked to the mouse guanylin gene on chromosome 4. The two genes are structurally similar, being composed of three short exons; the uroguanylin gene spans 2.4 kb and the guanylin gene spans 1.7 kb. Uroguanylin mRNA is most prominent in proximal small intestine, whereas guanylin mRNA is predominantly expressed in distal small intestine and colon. The upstream promoter sequence of the mouse uroguanylin gene contains a canonical TATA element at the site of transcription initiation and consensus binding sites for several known transcription factors, including HNF-1 and Sp1 within the first 1 kb. Although the gene structure and coding sequences of uroguanylin and guanylin are similar, the 5' flanking sequences and patterns of expression of these two genes in the intestine are different. It is likely that uroguanylin and guanylin represent gene duplications that have evolved to allow overlapping and complementary patterns of expression in the intestine.