Ham s f12 kaighn s modified medium

[³H]1-Methyl-4-phenylpyridinium (MPP) (specific activity (S.A.), 80 Ci/mmol) was purchased from Perkin-Elmer (Waltham, MA, USA); [³H]atenolol (S.A. 3.5 Ci/mmol) and [¹⁴C]metformin (S.A. 114 mCi/mmol) were purchased from Moravek Biochemicals (Brea, CA, USA). Unlabeled atenolol and metformin were purchased from Sigma-Aldrich (St. Louis, MO, USA) and AK Scientific (Union City, CA, USA), respectively; unlabeled MPP (purity > 99.5%) was synthesized by the Department of Chemistry and Biochemistry, University of Arizona. Ham’s F12 Kaighn’s modified medium was obtained from Sigma-Aldrich (Burlington, MA, USA). Other reagents were of analytical grade and obtained from standard sources.