Mechanistic basis of alterations in mucosal water and electrolyte transport

Diarrhoea can, in principle, result from the stimulation of secretion, the inhibition of absorption, or both. In practice, it is the simultaneous stimulation of secretion and inhibition of absorption that has most frequently been observed. The majority of substances affecting the intestine fall into two categories, those stimulating secretion and inhibiting absorption and those having the opposite effects. It is therefore important to understand both the mechanisms of secretion and the mechanisms of absorption. In this chapter, the basic cellular mechanisms by which water and electrolytes are actively absorbed and secreted in the small and large intestine have been presented. The steps at which these mechanisms are thought to be regulated by one or all of the three potential intracellular mediators, cAMP, cGMP and Ca++, have been indicated. It is hoped that an understanding of these basic cellular mechanisms will aid in understanding the pathophysiological conditions of diarrhoea described in the following chapters.