title: Inhibitory effects of calcium channel blockers on thyroid hormone uptake in neonatal rat cardiomyocytes.
abstract: The effects of the Ca2+ channel blockers verapamil, nifedipine, and diltiazem on triiodothyronine (T3) and thyroxine (T4) uptake were tested in cultured cardiomyocytes from 2-day-old rats. Experiments were performed at 37 degrees C in medium with 0.5% BSA for [125I]T3 (100 pM) or 0.1% BSA for [125I]T4 (350 pM). The 15-min uptake of [125I]T3 was 0.124 +/- 0.013 fmol/pM free T3 (n = 6); [125I]T4 uptake was 0.032 +/- 0.003 fmol/pM free T4 (n = 12). Neither T3 nor T4 uptake was affected by 1% DMSO (diluent for nifedipine and verapamil). Uptake of [125I]T3 but not of [125I]T4 was dose dependently reduced by incubation with 1-100 microM verapamil (49-87%, P < 0.05) or nifedipine (53-81%, P < 0.05). The relative decline in [125I]T3 uptake after 4 h of incubation with 10 microM verapamil or nifedipine was less than after 15 min or 1 h, indicating that the major inhibitory effect of the Ca2+ channel blockers occurred at the level of the plasma membrane. The reduction of nuclear [125I]T3 binding by 10 microM verapamil or nifedipine was proportional to the reduction of cellular [125I]T3 uptake. Diltiazem (1-100 microM) had no dose-dependent effect on [125I]T3 uptake but reduced [125I]T4 uptake by 45% (P < 0.05) at each concentration tested. Neither the presence of 20 mM K+ nor the presence of low Ca2+ in the medium affected [125I]T3 uptake. In conclusion, the inhibitory effects of Ca2+ channel blockers on T3 uptake in cardiomyocytes are not secondary to their effects on Ca2+ influx but, rather, reflect interference with the putative T3 carrier in the plasma membrane.
authors.affiliation: Department of Internal Medicine III, Erasmus University Medical School, 3000 DR Rotterdam, The Netherlands.
authors.forename: F A
authors.initials: FA
authors.lastname: Verhoeven
authors.forename: E P
authors.initials: EP
authors.lastname: Moerings
authors.forename: J M
authors.initials: JM
authors.lastname: Lamers
authors.forename: G
authors.initials: G
authors.lastname: Hennemann
authors.forename: T J
authors.initials: TJ
authors.lastname: Visser
authors.forename: M E
authors.initials: ME
authors.lastname: Everts
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000831:Animals, Newborn; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D004110:Diltiazem / Q000494:pharmacology; D007457:Iodine Radioisotopes; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D009206:Myocardium / Q000166:cytology*; D009543:Nifedipine / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D013974:Thyroxine / Q000493:pharmacokinetics*; D014284:Triiodothyronine / Q000493:pharmacokinetics*; D014700:Verapamil / Q000494:pharmacology*
chemical_list: D002121:Calcium Channel Blockers; D007457:Iodine Radioisotopes; D014284:Triiodothyronine; D000255:Adenosine Triphosphate; D014700:Verapamil; D004110:Diltiazem; D009543:Nifedipine; D013974:Thyroxine; D011188:Potassium; D002118:Calcium