Ultrastructure of the platypus and echidna mandibular glands.

The secretory units of the platypus and echidna mandibular glands consist of a single serous cell type. Secretory granules within the cells of the platypus mandibular gland stained intensely with the periodic acid-Schiff staining procedure but failed to stain with Alcian Blue, suggesting the granules contained neutral glycoproteins. Secretory granules within the mandibular glands of the echidna failed to stain with the methods used indicating little if any glycoprotein was associated with the secretory granules. Ultrastructurally, secretory granules of the platypus mandibular gland were electron dense with a central core of less electron-dense material and were membrane bound. In contrast, those of the echidna presented a lamellated appearance and also were limited by a membrane. These secretory granules appeared to form as a result of concentric layering of lamellae within cisternae of the Golgi membranes. The intralobular ductal system of the platypus was more extensively developed than that of the echidna. The striated ducts of both species were characterized by elaborate infoldings of the basolateral plasmalemma and an abundance of associated mitochondria.

Microscopy of the koala mandibular (submandibular) glands.

Koala mandibular (submandibular) glands are compound tubuloacinar glands, the secretory units of which consist only of serous cells. Intercellular canaliculi occur between the serous cells, which are continuous with a minute lumen that courses through the centre of each secretory unit. Intercalated ducts are abundant and join striated ducts, the latter being characterized by elaborate basolateral infoldings of the plasmalemma. Secretory granules within the serous cells fail to stain with either the PAS or Alcian Blue (pH 2.5) staining procedures. Ultrastructurally, the secretory granules are membrane bound, and consist of a homogeneous electron lucent material with a fine filamentous texture. The granules tend to coalesce into irregular shaped complexes of secretory material. Discharge of secretory material into the canalicular lumen is a common observation.

Comparison of prostaglandin and cimetidine in protection of isolated gastric glands against indomethacin injury.

Prostaglandins can protect the in vivo gastric mucosa against necrosis produced by a variety noxious agents. Cimetidine has also been shown to have protective properties in humans and in some models of experimental injury. Whether prostaglandins or cimetidine may protect gastric mucosal cells directly in the absence of systemic factors remains controversial. In the present study, the potential protective actions of prostaglandin and cimetidine against indomethacin injury were assessed in isolated rat gastric glands. Gastric glands were pre-incubated in oxygenated medium with either placebo, 16,16 dimethyl prostaglandin E(2) (dm PGE(2)) or cimetidine and incubated at 37 degrees C in medium containing 0.5 mg/ml of indomethacin for 2, 4 and 6 hrs. Cell injury and protection was assessed by the Fast Green exclusion test (viability test), leakage of lactate dehydrogenase (LDH) into the medium, and by scanning and transmission electron microscopy. In addition, the generation of PGE(2) by the gland cells was determined using RIA assay. Indomethacin by itself significantly reduced the viability of gastric glands, increased LDH release into the medium and produced prominent ultrastructural damage. In contrast to cimetidine, co-incubation of gastric glands with dm PGE(2) added to indomethacin, significantly reduced indomethacin-induced injury, increased the number of viable cells, reduced LDH leakage and diminished the extent of ultrastructural damage. The dose of indomethacin (5 microg/ml) which significantly inhibited the generation of PGE(2) (up to 90% inhibition) had no effect on cell viability nor LDH release. We conclude that 1) exogenous PGE2 exerts a potent protective activity in vitro which is independent on neural, vascular and hormonal factors; 2) inhibition of endogenous PGs may not the primary mechanism in the deleterious action of indomethacin against damage to gastric glandular cells and 3) indomethacin can exert a direct cytotoxic effect on the mucosal cells in gastric glands.

Immunohistochemical localization of gastrin-releasing peptide, neuronal nitric oxide synthase and neurone-specific enolase in the uterus of the North American opossum, Didelphis virginiana.

The present study has demonstrated the immunohistochemical localization of gastrin-releasing peptide (GRP), neuronal nitric oxide synthase (nNOS) and neurone-specific enolase (NSE) in the uterus of the North American opossum. Although the presence of GRP, nNOS and NSE has been reported recently in the uterus of eutherian species this is the first description of these peptides in a metatherian species. Metatherian mammals are of interest because in these species it is the prolonged lactation phase of development that is the period of primary reproductive investment rather than intrauterine development as is true of eutherian mammals. The opossum, like other marsupial species, has a very abbreviated gestation period which in Didelphis lasts only 12.5 days. GRP was localized in the cytoplasm of cells forming the surface lining epithelium and the glandular epithelium of the opossum endometrium late in pregnancy, at 11.5 days of gestation. Likewise, immunoreactivities of nNOS and NSE were found primarily within the epithelial cells of the endometrium at 11.5 days of gestation. As these peptides and enzymes appear primarily at the time of establishment of the yolk sac placenta (between day 10 and day 12.5 gestation), the present results strongly suggest that these factors may play a fundamental role in the placentation of the opossum.

Mechanisms of guanylin action via cyclic GMP in the kidney.

Guanylin, uroguanylin, and lymphoguanylin are small peptides that activate cell-surface guanylate cyclase receptors and influence cellular function via intracellular cGMP. Guanylins activate two receptors, GC-C and OK-GC, which are expressed in intestine and/or kidney. Elevation of cGMP in the intestine elicits an increase in electrolyte and water secretion. Activation of renal receptors by uroguanylin stimulates urine flow and excretion of sodium, chloride, and potassium. Intracellular cGMP pathways for guanylins include activation of PKG-II and/or indirect stimulation of PKA-II. The result is activation of CFTR and/or C1C-2 channel proteins to enhance the electrogenic secretion of chloride and bicarbonate. Similar cellular mechanisms may be involved in the renal responses to guanylin peptides. Uroguanylin serves as an intestinal natriuretic hormone in postprandial states, thus linking the digestive and renal organ systems in a novel endocrine axis. Therefore, uroguanylin participates in the complex physiological processes underlying the saliuresis that is elicited by a salty meal.

Guanylin peptides: renal actions mediated by cyclic GMP.

The guanylin family of cGMP-regulating peptides has three subclasses of peptides containing either three intramolecular disulfides found in bacterial heat-stable enterotoxins (ST), or two disulfides observed in guanylin and uroguanylin, or a single disulfide exemplified by lymphoguanylin. These small, heat-stable peptides bind to and activate cell-surface receptors that have intrinsic guanylate cyclase (GC) activity. Two receptor GC signaling molecules have been identified that are highly expressed in the intestine (GC-C) and/or the kidney (OK-GC) and are selectively activated by the guanylin peptides. Stimulation of cGMP production in renal target cells by guanylin peptides in vivo or ex vivo elicits a long-lived diuresis, natriuresis, and kaliuresis. Activation of GC-C receptors in target cells of intestinal mucosa markedly stimulates the transepithelial secretion of Cl(-) and HCO(-)/(3), causing enhanced secretion of fluid and electrolytes into the intestinal lumen. Bacterial ST peptides act as mimics of guanylin and uroguanylin in the intestine, which provide a cellular mechanism underlying the diarrhea caused by ST-secreting strains of Escherichia coli. Uroguanylin and guanylin may participate in a novel endocrine axis linking the digestive system and kidney as a physiological mechanism that influences Na(+) homeostasis. Guanylin, uroguanylin, and/or lymphoguanylin may also serve within intrarenal signaling pathways controlling cGMP production in renal target cells. Thus we propose that guanylin regulatory peptides participate in a complex multifactorial biological process that evolved to regulate the urinary excretion of NaCl when dietary salt levels exceed the body's physiological requirements. This highly integrated and redundant mechanism allows the organism to maintain sodium balance by eliminating excess NaCl in the urine. Uroguanylin, in particular, may be a prototypical "intestinal natriuretic hormone."

Brunner's glands: a structural, histochemical and pathological profile.

Brunner's glands are unique to mammalian species and in eutherians are confined primarily to the submucosa of the proximal duodenum. In the majority of species examined, they begin at the gastrointestinal junction and extend for variable distances distally in the wall of the proximal small intestine. Ducts of individual glands empty either directly into the intestinal lumen or unite with overlying intestinal glands (crypts of Lieberkühn) dependent on the species. Secretory units of Brunner's glands consist of epithelial tubules that show frequent distal branchings. The secretory units, with the exception of those found in rabbits and horses, consist primarily of a mucin producing cell type. However, other cell types normally associated with the overlying intestinal epithelium may be encountered scattered within the secretory units reflecting the developmental origin of these glands. Secretion from Brunner's glands contributes to a layer of mucus that forms a slippery, viscoelastic gel that lubricates the mucosal lining of the proximal intestinal tract. The unique capacity of this mucus layer to protect delicate underlying epithelial surfaces is due primarily to the gel-forming properties of its glycoprotein molecules. Mucin glycoproteins produced by Brunner's glands consist primarily but not exclusively of O-linked oligosaccharides attached to the central protein core of the glycoprotein molecule. Human Brunner's glands produce class III mucin glycoproteins and are thought to be the product of mucin gene MUC6 which is assigned to chromosome 11 (11p15-11p15.5 chromosome region). In addition to mucin glycoproteins and a limited amount of bicarbonate, numerous additional factors (epidermal growth factor, trefoil peptides, bactericidal factors, proteinase inhibitors, and surface-active lipids) have been identified within the secretory product of Brunner's glands. These factors, incorporated into the mucus layer, guard against the degradation of this protective barrier and underlying mucosa by gastric acid, pancreatic enzymes, and other surface active agents associated with this region. Yet other factors produced by Brunner's glands function to provide active and passive immunological defense mechanisms, promote cellular proliferation and differentiation, as well as contribute factors that elevate the pH of luminal contents of this region by promoting secretion of the intestinal mucosa, pancreatic secretion and gall bladder contraction. Additional insights concerning the role of Brunner's glands in the mammalian gastrointestinal tract as well as their possible evolution in this class of vertebrates have been gained from a basic understanding of their pathobiology.

Relationship between the degree of endometrial periglandular fibrosis and the presence of angiotensin-converting enzyme in the equine endometrium.

Endometrial periglandular fibrosis (EPF) has been proposed as a possible aetiology for equine embryonic and fetal loss. However, the pathophysiology of EPF is not well understood. Angiotensin-converting enzyme (ACE) is found in macrophages, endothelium (during angiogenesis) and myofibroblasts at sites of fibrosis in the heart, kidneys, liver and skin in several species. An increase in local tissue ACE-binding activity appears to be a critical event in the initiation and progression of fibrosis in these tissues. The aim of this study was to investigate the correlation between ACE activity in the equine endometrium and the degree of EPF, as determined by histological evaluation and morphometry based on a collagen-specific stain. ACE-binding activity values were significantly higher in the endometrial samples with moderate EPF (modified Kenney EPF category IIB) compared with endometria in all other categories. Ultrastructurally, the fibroblasts surrounding the glandular basal laminae in modified Kenney EPF category IIB and III endometria were undergoing myofibroblastic transformation-like changes. These observations indicate a possible link between ACE activity and the onset of EPF in mares.

Guanylin peptides: cyclic GMP signaling mechanisms.

Guanylate cyclases (GC) serve in two different signaling pathways involving cytosolic and membrane enzymes. Membrane GCs are receptors for guanylin and atriopeptin peptides, two families of cGMP-regulating peptides. Three subclasses of guanylin peptides contain one intramolecular disulfide (lymphoguanylin), two disulfides (guanylin and uroguanylin) and three disulfides (E. coli stable toxin, ST). The peptides activate membrane receptor-GCs and regulate intestinal Cl- and HCO3- secretion via cGMP in target enterocytes. Uroguanylin and ST also elicit diuretic and natriuretic responses in the kidney. GC-C is an intestinal receptor-GC for guanylin and uroguanylin, but GC-C may not be involved in renal cGMP pathways. A novel receptor-GC expressed in the opossum kidney (OK-GC) has been identified by molecular cloning. OK-GC cDNAs encode receptor-GCs in renal tubules that are activated by guanylins. Lymphoguanylin is highly expressed in the kidney and heart where it may influence cGMP pathways. Guanylin and uroguanylin are highly expressed in intestinal mucosa to regulate intestinal salt and water transport via paracrine actions on GC-C. Uroguanylin and guanylin are also secreted from intestinal mucosa into plasma where uroguanylin serves as an intestinal natriuretic hormone to influence body Na+ homeostasis by endocrine mechanisms. Thus, guanylin peptides control salt and water transport in the kidney and intestine mediated by cGMP via membrane receptors with intrinsic guanylate cyclase activity.

Guanylin peptides: cyclic GMP signaling mechanisms

Guanylate cyclases (GC) serve in two different signaling pathways involving cytosolic and membrane enzymes. Membrane GCs are receptors for guanylin and atriopeptin peptides, two families of cGMP-regulating peptides. Three subclasses of guanylin peptides contain one intramolecular disulfide (lymphoguanylin), two disulfides (guanylin and uroguanylin) and three disulfides (E. coli stable toxin, ST). The peptides activate membrane receptor-GCs and regulate intestinal Cl- and HCO3- secretion via cGMP in target enterocytes. Uroguanylin and ST also elicit diuretic and natriuretic responses in the kidney. GC-C is an intestinal receptor-GC for guanylin and uroguanylin, but GC-C may not be involved in renal cGMP pathways. A novel receptor-GC expressed in the opossum kidney (OK-GC) has been identified by molecular cloning. OK-GC cDNAs encode receptor-GCs in renal tubules that are activated by guanylins. Lymphoguanylin is highly expressed in the kidney and heart where it may influence cGMP pathways. Guanylin and uroguanylin are highly expressed in intestinal mucosa to regulate intestinal salt and water transport via paracrine actions on GC-C. Uroguanylin and guanylin are also secreted from intestinal mucosa into plasma where uroguanylin serves as an intestinal natriuretic hormone to influence body Na+ homeostasis by endocrine mechanisms. Thus, guanylin peptides control salt and water transport in the kidney and intestine mediated by cGMP via membrane receptors with intrinsic guanylate cyclase activity.

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.

Lymphoguanylin: cloning and characterization of a unique member of the guanylin peptide family.

Guanylin and uroguanylin are small peptides containing two disulfide bonds that activate membrane guanylate cyclase-receptors in the intestine, kidney and other epithelia. Hybridization assays with a uroguanylin complementary DNA (cDNA) detected uroguanylin-like messenger RNAs (mRNAs) in the opossum spleen and testis, but these transcripts are larger than uroguanylin mRNAs. RT of RNA from spleen to produce cDNAs for amplification in the PCR followed by cloning and sequencing revealed a novel lymphoid-derived cDNA containing an open reading frame encoding a 109-amino acid polypeptide. This protein shares 84% and 40% of its residues with preprouroguanylin and preproguanylin, respectively. A 15-amino acid, uroguanylin-like peptide occurs at the COOH-terminus of the precursor polypeptide. However, this peptide is unique in having only three cysteine residues. We named the gene and its peptide product lymphoguanylin because the source of the first cDNA isolated was spleen and its mRNA is expressed in all of the lymphoid tissues tested. A 15-amino acid form of lymphoguanylin containing a single disulfide bond was synthesized that activates the guanylate cyclase receptors of human T84 intestinal and opossum kidney (OK) cells, although with less potency than uroguanylin and guanylin. Northern and/or RT-PCR assays detected lymphoguanylin mRNA transcripts in many tissues and organs of opossums, including those within the lymphoid/immune, cardiovascular/renal, reproductive, and central nervous organ systems. Lymphoguanylin joins guanylin and uroguanylin in a growing family of peptide agonists that activate transmembrane guanylate cyclase receptors, thus influencing target cell function via the intracellular second messenger, cGMP.

Morphometric analysis of endometrial periglandular fibrosis in mares.

OBJECTIVES: To develop an objective, quantifiable assay for endometrial periglandular fibrosis (EPF) and correlate assay results with histologic and ultrastructural changes in equine endometrial biopsy specimens. SAMPLE POPULATION: Endometrial biopsy specimens from 70 mares from 3 to 27 years old in estrus. PROCEDURE: In a double-blinded study design, endometrial biopsy specimens were graded histologically (modified Kenney classification) for EPF and inflammation. Endometrial periglandular collagen volume fraction (%EPCVF) was determined by light microscopic image analysis of picrosirius red-stained sections. Specimens from selected mares were examined by transmission electron microscopy. RESULTS: %EPCVF values varied significantly among the 4 modified Kenney EPF categories (I, IIA, IIB, and III) and increased with increasing age of mares. Morphologically, EPF consisted of concentric layers of transformed fibroblasts with myofibroblastic features and deposition of fibrillar collagen around unaltered glandular basal laminae. CONCLUSIONS AND CLINICAL RELEVANCE: %EPCVF correlates well with morphologic changes in endometrial biopsy specimens. Determination of %EPCVF could be useful in evaluation and clinical management of subfertile mares and in investigations of the pathogenesis of EPF.

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.

Guanylyl cyclase receptors and guanylin-like peptides in reptilian intestine.

Receptors for guanylin and uroguanylin were identified on the mucosal surface of enterocytes lining the intestine of the bobtail skink (Tiliqua rugosa), king's skink (Egernia kingii), and knight anole (Anolis equestris) by receptor autoradiography using 125I-ST (Escherichia coli heat-stable enterotoxin) as the radioligand. Specific, high-affinity binding of 125I-ST to receptors was found on the microvillus border of enterocytes and little or no specific binding of 125I-ST was observed in other strata comprising the gut wall. The American alligator (Alligator mississippensis) also exhibited receptor binding, but unlike the other three species had relatively high levels of apparent nonspecific binding. A comparison of intestinal cGMP accumulation responses between the American alligator and the knight anole demonstrated a greater magnitude of cGMP responses to ST and guanylin in vitro in the knight anole relative to the tissue cGMP accumulation responses of alligators. Treatment with ST resulted in markedly greater tissue cGMP accumulation responses in both species compared to treatment with guanylin. To complete a paracrine signaling pathway in reptilian intestine, guanylin-like peptides that stimulated cGMP accumulation in human T84 intestinal cells were isolated from the intestinal mucosa of alligators. We conclude that functional receptor-guanylyl cyclases and one or more endogenous guanylin/uroguanylin-like peptides occur in the intestinal tract of reptiles as well as in the intestines of mammals and birds. Thus, higher vertebrates have a conserved signaling pathway that regulates intestinal function through the first-messenger peptides, guanylin and/or uroguanylin, and the intracellular second messenger, cGMP.

Signal transduction pathways via guanylin and uroguanylin in stomach and intestine.

Guanylin and uroguanylin are peptides that activate receptor guanylate cyclases (GCs) and elicit increased intestinal secretion. Bacteria that cause traveler's diarrhea produce heat-stable toxins (STs) that mimic this action. Investigation of the distribution and identity of receptor GCs in the gastrointestinal tract of rats revealed that receptors were localized to epithelial cells in stomach and intestine. Clusters of cells in gastric mucosa and enterocytes lining the intestine exhibited specific binding of 125I-labeled ST. Ligated loops of stomach and intestine treated with intraluminal ST had significant increases in guanosine 3',5'-cyclic monophosphate (cGMP), with duodenum exhibiting the greatest response. Expression of guanylate cyclase C (GCC) mRNA and a truncated, GCC-like mRNA was found in both stomach and intestine. Both mRNAs were isolated as cDNAs encoding the GC catalytic domain. The 0.9-kilobase (kb) cDNA is 99.8% identical to GCC, whereas the truncated, 0.75-kb GCC-like cDNA has a 159-nucleotide deletion and is 96.6% identical to GCC at the protein level. Uroguanylin and guanylin mRNAs were detected in stomach and intestine. Uroguanylin mRNA was most abundant in small intestine, whereas guanylin mRNA was highest in large intestine. Thus the stomach and intestine are targets for regulation of transport by guanylin and uroguanylin via cGMP.

Comparison of effects of uroguanylin, guanylin, and Escherichia coli heat-stable enterotoxin STa in mouse intestine and kidney: evidence that uroguanylin is an intestinal natriuretic hormone.

BACKGROUND: Uroguanylin and guanylin are intestinal peptides that activate a receptor-guanylate cyclase, which is also a receptor for Escherichia coli heat-stable enterotoxin (STa). These peptides may have a role in the body's regulation of fluid and electrolytes. METHODS: STa, bioactive guanylin, and bioactive uroguanylin were evaluated for effects in: 1) the suckling mouse intestinal fluid secretion assay; 2) an in vitro suckling mouse intestinal loop assay; 3) an intestinal receptor autoradiography assay; 4) a control or agonist-stimulated assay for cGMP response in T84 cells; and 5) an in vivo renal function assay in mice. RESULTS: In vivo, orally administered uroguanylin and STa but not guanylin, stimulated intestinal fluid secretion. All three peptides activated intestinal guanylate cyclase and had common intestinal receptors. In vitro, after pretreatment with chymotrypsin, only uroguanylin and STa retained agoinst activity. Chymostatin preserved guanylin activity. STa and uroguanylin induced diuresis, natriuresis, and kaliuresis. Guanylin was less potent than uroguanylin and STa. CONCLUSIONS: The results suggest that the endogenous intestinal peptides, uroguanylin and guanylin, regulate water and electrolyte homeostasis both through local effects on intestinal epithelia and endocrine effects on the kidney.