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

Abstract

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.

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.