The effect of cyclic nucleotides and cholera toxin on in vivo and in vitro phosphorylation of small intestinal brush border membranes.

The effect of cyclic nucleotides and cholera toxin on the phosphorylation of the brush border membrane proteins of the rat jejunum was studied. Phosphorylation was analyzed by autoradiography of brush border membrane proteins separated by SDS-polyacrylamide gel electrophoresis. Phosphorylation was performed either in vivo by perfusion of the jejunum with [32P]orthophosphate followed by an analysis of the isolated membranes or in vitro by phosphorylation of isolated brush border membranes by [gamma-32P]ATP in the presence of saponin. The addition of cholera toxin (10 micrograms/ml) or dibutyryl-cAMP (5 mmol/l) to the perfusate was unable to produce significant changes in the phosphoprotein pattern. On the other hand, cAMP (at 5 mumol/l) induced an increase of the phosphorylation of a 86 kDa protein when freshly isolated brush border membranes were phosphorylated by [gamma-32P]ATP. However, the same effect could also be induced by low concentrations of cGMP (0.1 mumol/l). It is concluded that brush border membranes from rat jejunum do not contain cAMP-dependent protein kinase activity and that cAMP-dependent protein phosphorylation of this membrane does probably not represent the final event of cholera toxin-induced secretion.