Fig. 4. Measurements of intracellular cAMP levels after treatment with PT, CT or forskolin. BCEC monolayers were incubated with PT (low: 200 ng/ml, high: 1 µg/ml), cholera toxin (CT; 1 µg/ml) or forskolin (50 µM) for 2 hours. Intracellular cAMP levels were determined using a cAMP enzyme immunoassay system (Amersham Pharmacia Biotech, Braunschweig, Germany). The values shown represent the means of analysis performed in triplicate. The standard deviations (error bars, n=3) are indicated.