Isolated rat enterocytes were incubated with E. coli heat-stable enterotoxin or buffer alone and the protein kinase activity and cyclic GMP level determined on the particulate fraction or cytosol, respectively. In the control cells, particulate protein kinase activity and cyclic GMP concentration were at a maximum after 20 sec and 1 min of incubation, respectively. In heat-stable enterotoxin-treated cells the particulate protein kinase activity was significantly increased (P < 0.05) after 20 sec of incubation, but decreased (P < 0.05) after 30 sec, 1 min and 2 min, when compared to the control reaction. During this time period the concentration of intracellular cyclic GMP increased 10-fold. The effect of heat-stable enterotoxin on particulate protein kinase activity and cyclic GMP concentration was dose-dependent. Analysis of radioactive membrane phosphorylation products indicate a role for phosphoproteins with a mol. wt of 25,000 and 120,000. These results suggest that the action of heat-stable enterotoxin may involve an effect on protein kinase.
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