An alternative explanation for the occurrence of short circuit current increases in the small intestine following challenge by bacterial enterotoxins.
Secretory diarrhoeal disease due to enterotoxins is thought to arise from the enhancement to pathologically high rates of normally occurring chloride ion and therefore fluid secretion from enterocytes. In support of this concept, many enterotoxins increase intestinal short-circuit current, regarded now as faithfully reflecting the increased chloride ion secretion. Contradicting this assumption, STa reduces absorption but does not cause secretion in vivo although short-circuit current is increased in vitro. There is therefore a mismatch between an assumed enterocyte mediated secretory event that should but does not cause net fluid secretion and an undoubtedly increased short-circuit current. It is proposed here that short-circuit current increases are not themselves secretory events but result from interrupted fluid absorption. A noteworthy feature of compounds that inhibit the increase in short-circuit current is that the majority are vasoactive, neuroactive or both. In general, vasodilator substances increase current. An alternative hypothesis for the origin of short-circuit current increases is that these result from reflex induction of electrogenic fluid absorption. This reflex enhances a compensatory response that is also present at a cellular level. An intestinal reflex is therefore proposed by which decreases in interstitial and intravascular volume or pressure within the intestine initiate an electrogenic fluid absorption mechanism that compensates for the loss of electrically neutral fluid absorption. This hypothesis would explain the apparently complex pharmacology of short-circuit current increases since many depressor substances have receptors in common with enterocytes and enteric nerves. The proposed alternative view of the origin of short-circuit current increases assumes that these do not represent chloride secretion from the enterocytes. This view may therefore aid the successful development of anti-diarrhoeal drugs to overcome a major cause of infant mortality worldwide, if short-circuit current data are being persistently misinterpreted. The putative but testable link between interstitial volume or pressure and fluid absorption also provides support for the alternative view of secretion; namely, that enhanced capillary and epithelial cell tight junctional permeability together with increased intracapillary pressure may cause secretion and not chloride exit from the enterocytes.
Laboratory of Gastrointestinal Physiology, School of Life Sciences, College of Medical, Veterinary & Life Sciences, Glasgow University, Glasgow G12 8QQ, Scotland, United Kingdom. Electronic address: M.Lucas@bio.gla.ac.uk.
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D002712:Chlorides / Q000378:metabolism*; D003967:Diarrhea / Q000139:chemically induced*; D004768:Enterotoxins / Q000633:toxicity*; D006801:Humans; D007413:Intestinal Mucosa / Q000187:drug effects / Q000502:physiology*; D007421:Intestine, Small / Q000187:drug effects / Q000502:physiology*; D015640:Ion Channel Gating / Q000502:physiology*; D008954:Models, Biological; D009994:Osmolar Concentration
D002712:Chlorides; D004768:Enterotoxins