Fig. 4.
A–D: kinetics of TCA, dehydroepiandrosterone sulfate (DHEAS), estrone sulfate (E1S), and digoxin (DIG) uptake in cells transfected with human OSTα/β (OSTab) and empty vector (Mock). E: estradiol-17β-d-glucuronide (E217G) served as a negative control for OSTα/β-mediated uptake. OSTab and Mock cells were preincubated with ECF Na− buffer (pH 7.4, 10 min at 37°C), and uptake was initiated by addition of TCA (0.03–1,000 µM), DHEAS (0.003–1,000 µM), E1S (0.003–1,000 µM), DIG (0.01–100 µM), or E217G (0.04, 10, and 100 µM) (3H-labeled substrate concentration was ~300 nCi/ml) for 30 s or 2 min at 37°C. Uptake rate measurements were normalized to total cell protein. Values are means ± SD from 3 independent experiments. Vmax and Km were not calculated, because uptake rate appeared to be linear. ns, nonsignificant.