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°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 μM 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 μM 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]T3binding by 10 μM verapamil or nifedipine was proportional to the reduction of cellular [125I]T3 uptake. Diltiazem (1–100 μM) 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.