Mechanisms of control of intestinal transport: a review

Of the long list of substances which are capable of influencing intestinal transport, corticosteroids and aldosterone probably exert a prolonged, modest, controlling influence on salt and water absorption. They are likely to be important in situations where variation in colonic absorption occurs, such as in the rabbit, or in circumstances where salt depletion or salt overload are likely. There is modest evidence in favour of the possibility that the gastrointestinal hormones liberated locally after a meal limit the rate of absorption of salt and water at that time, perhaps to maintain the fluidity of luminal contents for digestion to occur. More rapid short-term regulation of salt and water absorption in localized areas of the intestine may be exerted by peptide secretions from the enteric nervous system and paracrine cells. Cholinergic and adrenergic effects on secretion and absorption respectively may be responsible for maintaining an overall balance, and there is reasonable evidence in favour of this view. There is also strong circumstantial evidence to support a role for VIP in intestinal secretion. VIP may also play a part in mediating secretory diarrhoea. A role for the large number of other neurotransmitters remains uncertain but it is likely that overall control is maintained by a combination of counter-balancing factors. Finally, under pathological circumstances, where damage to the intestine liberates such mediators of the inflammatory response as prostaglandins, histamine and bradykinin, intestinal secretion may occur and be responsible for the diarrhoea associated with these diseases.