[Principles of epithelial transport mechanisms: importance for pathophysiologic understanding, differential diagnosis and treatment of diarrheal diseases].

First, general principles of epithelial transport and the epithelial barrier function are described. The cellular mechanisms of Na-glucose cotransport, electrogenic Cl-secretion, and electrogenic Na-absorption are presented. Second, the pathophysiology of epithelial transport systems and their regulation is given as far as relevant for the onset of diarrhea. This includes congenital deficiency of transport proteins and mucosal transformation as e.g. during celiac disease, tropical sprue, and the blind loop syndrome. Finally, regulation of epithelial transport by the enteric nervous system, GI-peptides, hormones, and the intestinal immune system is discussed in respect to disturbances of this regulation as for example seen in the presence of bacterial enterotoxins.