Theophylline-induced changes in ion transport and conductance in human small intestinal mucosa

Amendments to the theory underlying Ussing chamber data of chloride ion secretion after bacterial enterotoxin exposure

Bacterial enterotoxins may cause life-threatening diarrhoeal fluid loss in part because they stimulate enterocytes to secrete fluid into the small intestine as well as preventing normal fluid uptake. Abnormal chloride ion secretion is believed to provide the osmotic driving force for the inappropriate fluid movement. Evidence for enhanced chloride secretion consists of isotopic flux measurements in Ussing chambers, the standard apparatus for permeation studies. Flux from the lumen of the intestine is assumed to be determined solely by absorptive processes and flux towards the lumen solely by secretory processes. Bacterial enterotoxin increased flux towards the lumen is taken as an evidence of enhanced secretion. Examination of the flux equation solutions shows that the existing theoretical treatment of the Ussing chamber consists of the super-imposition of two contradictory unidirectional models. In contrast, the present analysis shows that a measured 'unidirectional' flux contains information both about absorptive and secretory processes, regardless of which flux is measured. Reciprocity is predicted for the fluxes, as decreases in the absorptive processes will cause increases in apparent secretory flux. Data from the literature show that mucosal-to-serosal chloride ion flux in rabbit ileum after exposure to secretagogues correlates inversely and highly significantly (r=0.74, n=17, p<0.001) with increases in serosal-to-mucosal chloride ion flux. As a category of evidence, flux data do not provide conclusive evidence of enhanced chloride secretion after exposure to enterotoxins, since an apparently enhanced serosal-to-mucosal flux would also be noted after inhibition of the mucosal-to-serosal flux. As interruption of absorptive processes can be misinterpreted as enhanced secretion in the Ussing chamber, this is a serious deficiency in the evidence for direct enterotoxin enhancement of the intestinal chloride ion channel as a basis for diarrhoeal disease.

Electrical PD, short-circuit current and fluxes of Na and Cl across avian intestine

In vitro measurements were made of transmural potential difference (PD), short-circuit current (Isc), resistance and unidirectional fluxes of 22Na and 36Cl across the duodenum, jejunum, ileum and colon of normal sodium-replete domestic fowl (Gallus domesticus). The PD ranged from about 1 mV across the duodenum to 8 mV across the colon while the Isc was, respectively, 2.8 and 64 microA X cm-2. The jejunum and ileum exhibited values between these extremes. Unidirectional fluxes (under short-circuit conditions) of Na and Cl were lowest across the duodenum where there was no evidence of active transport of these ions. Unidirectional fluxes of Na and Cl were less across the jejunum than across the ileum or colon. A net active transport of Na (but not Cl) was observed in the ileum (= 106% of the Isc) and colon (= 50% of Isc). The possible physiological significance of these observations in the domestic fowl are discussed and are compared to that of a mammal, the rabbit.

Net sodium and glucose transport in the jejunum, ileum and colon of anaesthetized rats in response to intraluminal theophylline and anionic surfactants

Osmotically balanced solutions of sodium chloride and glucose (5-150 mmol/l) were instilled into rats prepared with two tied intestinal loops (jej-col or il-col). Luminal accumulation or disappearance of Na and glucose after 15 min. was determined, and the parameters of the linear regression lines of net Na flux (y) with initial Na concentration (x) calculated. Control cation and glucose transport were changed by dioctylsulphosuccinate and dodecylsulphate in the way described by Sund & Matheson (1978). Theophylline (10-25 mmol/l) on the other hand did not alter glucose disappearance, and had a distinctly dissimilar effect on net Na transport compared to the surfactants. This could be described as a parallel displacement of the control regression lines to the right, without improvement in correlation. This effect of theophylline was greatest in the ileum, where mean luminal Na concentration corresponding to zero net transport was raised from about 70 mmol/l under control condition to values above 200 mmol/l. The results are consistent with the view that theophylline mainly affects transcellular and the surfactants mainly paracellular sodium transport, and do not support the theory that the effect of dioctylsulphosuccinate on intestinal transport is related to an activation of the mucosal adenylate cyclase/cAMP system, at least not in short term experiments.

Failure of glucagon to influence ion transport across human jejunal and ileal mucosa in vitro

Glucagon (90 to 880 pg.ml-1) failed to influence electrical activity or fluxes of sodium and chloride across human jejunal and ileal mucosa in vitro. These results suggest that the intestinal secretion and diarrhoea produced in vivo in man during intravenous infusion of glucagon may be produced by changes in motility and blood flow and not directly by activating an ion secretory mechanism as is the case in cholera.