Effects of tetrodotoxin and ion replacements on the short-circuit current induced by Escherichiacoli heat stable enterotoxin across small intestine of the gerbil (Gerbillus cheesmani)

Abstract

The effects of mucosally added Escherichia coli heat stable enterotoxin (STa 30 ng ml -1 ) on the basal short-circuit current (Isc in µA cm -2 ) across stripped and unstripped sheets of jejuna and ilea taken from fed, starved (4 days, water ad lib) and undernourished (50% control food intake for 21 days) gerbil (Gerbillus cheesmani) were investigated. The effect of neurotoxin tetrodotoxin (TTX 10 µM) Escherichia coli heat stable enterotoxin (STa) induces secretion by rasing the concentration of cyclic GMP in the enterocyte (Field et al. 1978) which leads to electrogenic chloride secretion and inhibits elecronutral sodium chloride absorption (Guandolini et al. 1982). It has been demonstrated that the enteric nervous system is important in STa-induced secretion as shown by the inhibitory effect of neuronal blockade (Field et al. 1978, Scott et al. 1980, Eklund et al. 1986, Rolf & Levin 1994, Nzegwu & Levin 1996. Rolf and Levin (1999) showed that STa activates nitric oxide dependent myenteric plexus secretory reflex mediated by capsaicin sensitive C-fibers. Al-Majali et al. (2000) suggested that Na + /Cl -coupling may be the major mechanism for the loss of ions in the STa-induced secretory diarrhoeal response. Lucas (2001) in his review on the reconsideration of the evidence for E. coli STa enterotoxin-driven fluid secretion proposed a new model for the action of STa involving inhibition of Na + /H + exchange which explains the ability of STa to reduce absorption in vitro but its inability to cause secretion in vivo in contrast to its apparent secretory effect in vitro. The aim of the present study was to investigate whether in the desert mammals such as gerbils there is a regional differences between jejunum and ileum in their responses to STa and if there is a neural involvement in such responses. The effects of replacing chloride ions by gluconate and the effects of removing bicarbonate ions from buffer media were also investigated. MATERIALS AND METHODSAnimals and diets -Gerbils (Gerbillus cheesmani) of both sex, body weight 36-40 g, were captured from the desert in the State of Kuwait and kept in the animal house for at least three weeks before use. Three nutritional groups were used. The fed group had free access to water and food (SDS rodent diet, Essex, England) and were held in rooms maintained at 27 ± 2°C. The lights were on from 5 am until 5 pm and the humidity was 50%. For the starved groups, water was given ad lib but the food was removed 4 days before the animals were used. The chronically undernourished group was housed in individual cages and was fed 50% of the control food intake for 21 days. Animals were housed routinely in plastic cages with wired mesh bottoms to reduce coprophagy.Technique -On the day of use, animals were anaesthetised with thiopentone sodium (30 mg/kg body weight, ip). When surgical anesthesia was achieved, a mid-line incision was made along the abdomen and the entire small intestine (28-30 cm) was removed and flushed with 0.9% NaCl. Jejunal sheets were taken immediately dis...

Enterocyte chloride and water secretion into the small intestine after enterotoxin challenge: Unifying hypothesis or intellectual dead end?

“…The case against short-circuit current elevation after STa being secretion.-The second category of evidence putatively demonstrating secretion is the ability to increase short-circuit current in vitro when small and large intestinal tissue from various species is exposed to STa enterotoxin (1,36,42). While there is no doubt that such elevations can be detected, better in the fasting than in the normally fed animal (62), the unjustified and possibly debilitating premise to this category of evidence is that the inward positive current towards the serosal surface consists exclusively of luminally directed chloride ions.…”