Inhibitory effects of calcium channel blockers on thyroid hormone uptake in neonatal rat cardiomyocytes

The effects of the Ca2+ channel blockers verapamil, nifedipine, and diltiaz em on triiodothyronine (T-3) and thyroxine (T-4) uptake were tested in cult ured cardiomyocytes from 2-day-old rats. Experiments were performed at 37 d egreesC in medium with 0.5% BSA for [I-125] T-3 (100 pM) or 0.1% BSA for [I -125] T-4 (350 pM). The 15-min uptake of [I-125] T-3 was 0.124 +/- 0.013 fm ol/pM free T-3 (n = 6); [125I] T-4 uptake was 0.032 +/- 0.003 fmol/pM free T-4 (n = 12). Neither T-3 nor T-4 uptake was affected by 1% DMSO (diluent f or nifedipine and verapamil). Uptake of [I-125] T-3 but not of [I-125] T-4 was dose dependently reduced by incubation with 1-100 muM verapamil (49-87% , P < 0.05) or nifedipine (53-81%, P < 0.05). The relative decline in [I-12 5] T-3 uptake after 4 h of incubation with 10 muM verapamil or nifedipine w as less than after 15 min or 1 h, indicating that the major inhibitory effe ct of the Ca2+ channel blockers occurred at the level of the plasma membran e. The reduction of nuclear [I-125] T-3 binding by 10 muM verapamil or nife dipine was proportional to the reduction of cellular [I-125] T-3 uptake. Di ltiazem (1-100 muM) had no dose-dependent effect on [I-125] T-3 uptake but reduced [I-125] T-4 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 [I-125] T-3 uptake. In conclusion, the inhibitory effects of Ca2 channel blockers on T-3 uptake in cardiomyocytes are not secondary to thei r effects on Ca2+ influx but, rather, reflect interference with the putativ e T-3 carrier in the plasma membrane.