Effects of guanylin and uroguanylin on rat jejunal fluid and electrolyte transport: comparison with heat-stable enterotoxin
Introduction
The enterotoxigenic Escherichia coli causes diarrhea by the elaboration of several enterotoxins that alter the intestinal transport of fluid and electrolyte. Heat-labile enterotoxins have structures highly homologous to cholera toxins and activate adenylate cyclase in epithelial cells. An elevation of intracellular concentration of cAMP increases Cl− secretion and results in diarrhea. The heat-stable enterotoxin (STa), on the other hand, stimulates guanylate cyclase and increases cGMP, which, like cAMP, evokes Cl− secretion and diarrhea. Heat-stable enterotoxins are 18 or 19 amino acid residue peptides containing three disulfide bonds 1, 2, 3. Their receptor appears to be guanylate cyclase C present in plasma membranes [4]. Receptor radioautography using 125I-STa indicates that STa receptors are distributed in the epithelial lining of the lumen of the small intestine and colon and duodenal glands in human and experimental animals 5, 6, 7, 8.
The physiological implication of the presence of STa receptors in human intestine and kidney remained unknown until the discovery of guanylin from the rat small intestine [9]and uroguanylin from urine and intestine of the opossum [10]. These peptides have structural homology to STa. Human guanylin and uroguanylin consist of 15 and 16 amino acid residues containing two disulfide bonds. Rat and human guanylin differ only in one amino acid residue (Gly2 and Asn2). Guanylin and uroguanylin displace the bindings of 125I-STa to human intestinal tumor cell line (T84 cells) and rat enterocytes 9, 10, 11, increase intracellular cGMP levels and the short-circuit current (Isc) in T84 cells as well as the human intestine [12]and mouse colon [13]presumably via the activation of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel [13]. More recent studies using CFTR knockout mice 14, 15confirm this view though the duodenal Isc response to uroguanylin was not completely eliminated in the absence of CFTR. The evidence in vitro supports a hypothesis that guanylin and uroguanylin are endogenous ligands of STa receptors in the intestine and are physiological regulators of intestinal fluid and electrolyte transport 16, 17. However, their release mechanisms and actions in vivo have not been fully evaluated yet 11, 18. In an attempt to understand a possible function of luminally released guanylin and uroguanylin in vivo, we have studied their effects on fluid and electrolyte transport in anesthetized rats and compared them with those of STa.
Section snippets
Closed jejunal loop
Male Wistar rats (Japan SLC Inc., Hamamatsu) weighing 230±30 g were used. They were kept for 1 week under standardized environmental conditions: 23°C, 50–55% humidity and artificial lighting between 09:00 and 21:00 h. They were deprived of food but not water for 24 h before the experiments. Anesthesia was induced by an intraperitoneal injection of pentobarbital (60 mg/kg) (Abbott Laboratories, Chicago). All the rats were placed on an electrically heated pad under an overhead lamp and their
Time course
Fig. 1 shows the time course of net fluid transport after the instillation of 2 ml saline into the jejunal loop with or without STa (10−7 M), human guanylin (10−6 M), and human uroguanylin (10−6 M). The injected saline was absorbed in a time-dependent manner and ∼38% of the injected solution was absorbed in 30 min. Luminal STa and human uroguanylin significantly (P<0.05) inhibited the fluid absorption and net fluid transport was nearly unchanged for 30 min. Human guanylin also decreased net
Discussion
The closed jejunal loop technique provides quantitative information on simultaneous net movements of fluid and electrolyte in vivo, though it does not provide information on unidirectional flux of each electrolyte and details of the mechanisms involved. When saline was instilled into the loop, it was absorbed at a constant rate (25 μl/min/g). In the present study, all guanylin related peptides inhibited the absorption of saline. The action of guanylin or uroguanylin, like STa, was rapid; the
Acknowledgements
We wish to thank Dr. V. Wray for his advice. This work was supported by a grant and Monbusho International Scientific Research Program from the Ministry of Education, Science, and Culture, Japan.
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Guanylin and uroguanylin mRNA expression is increased following Roux-en-Y gastric bypass, but guanylins do not play a significant role in body weight regulation and glycemic control
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