Effects of cholera toxin on cellular and paracellular sodium fluxes in rabbit ileum

Additive effects of ileal secretagogues in the rat

Although clinical diarrhea is often caused by more than one enteropathogen, it is not known whether secretagogues have additive effects on secretion. This question was examined in anesthetized, ventilated rats by exposing the ileum to secretory agents with different mechanisms of action. Four hours after inoculation of ileal loops with either cholera toxin or saline, transport was measured during perfusion with Ringer's solution, with Ringer's solution containing Escherichia coli heat-stable enterotoxin or mannitol, or with Ringer's solution containing both heat-stable enterotoxin and mannitol. We found that heat-stable enterotoxin caused similar decrements in water absorption in loops exposed to Ringer's solution, hypertonic mannitol, cholera toxin, and cholera toxin plus mannitol. By contrast, the decrement in water transport caused by hypertonic mannitol was inversely related to the level of ongoing water transport. In addition, the electrolyte transport changes caused by each enterotoxin were preserved despite the presence of other secretory agents. These data suggest that the transport effects of secretagogues with different mechanisms of action are additive in the ileum. However, the absolute decrements in water absorption may be independent of (heat-stable enterotoxin and cholera toxin) or dependent on (hypertonic mannitol) the ongoing level of water transport.

Rapid decrease in electrical conductance of mammalian duodenal mucosa in vitro. Combined effects of prostaglandin E2 and bicarbonate

The effects of HCO3- on transepithelial conductance of guinea pig (and, in some experiments, rabbit) duodenum have been investigated using stripped preparations in Ussing-type chambers. Initial conductance amounted to approximately 27 mS/cm2. In the presence of serosal or bilateral HCO3- (20 mM), it fell to approximately 15 mS/cm2 within 60 min. With mucosal HCO3- or HCO3- -free solutions, conductance decreased to only approximately 23 mS/cm2. In the absence of HCO3-, serosal but not mucosal addition of HCO3- prompted a steep conductance drop that was 50% complete within 6 min. These effects were the same in proximal and distal segments, whereas conductance levels in the latter were higher. The effects of HCO3- required serosal Na+; they were partly prevented or reversed by serosal ouabain (3 x 10(-5) M), furosemide (10(-3) M), and other loop diuretics, and indomethacin (10(-5) M). Serosal addition of prostaglandin E2 (10(-7) M, followed by 10(-6) M) reduced conductance from approximately 22 to approximately 17 mS/cm2. This effect required serosal HCO3- and the presence of indomethacin. 8-Br-cyclic adenosine monophosphate (10(-3)M, serosal side) mimicked the effects of prostaglandin E2. Indomethacin and prostaglandin E2 did not influence the secretory flux of HCO3- (approximately 0.5 mumol/cm2.h, pH-stat method). Our results assign a critical role to serosal HCO3- in the maintenance of a low tissue permeability, dependent on the availability of prostaglandins (cyclic adenosine monophosphate) and involving HCO3- access to the cell rather than secretion.

Comparative study of the effect of cholera toxin and sodium deoxycholate on the paracellular permeability and on net fluid and electrolyte transfer in the rat colon

1. The effect of deoxycholate and cholera toxin on the transfer of water, sodium, potassium and chloride and on mucosal permeability was studied in perfusion experiments on rat colon in vivo. The influence of both secretagogues on surface morphology was assessed by scanning electron microscopy. 2. Deoxycholate turned the absorption of water, sodium and chloride to secretion and enhanced potassium secretion. Cholera toxin induced water and sodium secretion, inhibited chloride absorption and enhanced potassium secretion. 3. Deoxycholate increased reversibly the mucosal permeability as measured by the colonic clearance of 51CrEDTA and glucose, whereas cholera toxin decreased the colonic 51CrEDTA clearance. 4. Deoxycholate caused protrusion of the luminal cell surface and an increase of exfoliation of epithelial cells. The epithelial continuity was preserved. The only change induced by cholera toxin was an enhanced mucus extrusion. 5. Our results are consistent with the view that deoxycholate causes fluid secretion by filtration whereas cholera toxin enhances the secretory activity of the epithelium.

Decreased cyclic GMP levels in rat cecum mucosa during adaptive stimulation of Na-K-ATPase

In rat cecal mucosa, Na-K-ATPase specific activity and sodium and fluid absorption were increased by giving polyethylene glycol administration with the drinking water. Whereas cyclic AMP levels were unchanged, cyclic GMP was reduced by about 50%. This finding suggests a regulatory role of cyclic GMP in intestinal sodium and fluid absorption.