Influence of rhein on rat colonic Na+,K+-ATPase and permeability in vitro.

Rhein, an aglucone of a laxative-acting anthraquinone, which induces net secretion in the human jejunum and colon, does not induce net secretion under open circuit current in in vitro conditions but reduces net absorption of electrolytes and water. In the stripped rat descending colon at a concentration of 2.10(-4) mol/l, the Na+, K+-ATPase is almost completely inhibited resulting in a decreased net absorption of electrolytes and water. There are no changes of the diffusional resistance of the paracellular pathway. Results point to a cellular mechanism of rhein by which absorption is inhibited so that physiological cellular secretion dominates. This does not rule out an additional activation of cellular secretion but net secretion can be explained solely by inhibition of absorption without the assumption of any additional secretory mechanism.