Fig. 4. Effect of thapsigargin (Thaps) on the sulfation of apically or basolaterally internalized ricin sulf-2. MDCK II cells grown on polycarbonate filters were incubated with radioactive sulfate for 3 hours at 37°C, and thapsigargin (1 µg/ml) was added for the last 30 minutes. Then ricin sulf-2 (200 ng/ml) was added either to the apical (A) or basolateral pole (B), and the cells were further treated as described in the legend to Fig. 3.