1,25-DIHYDROXYVITAMIN D-3 REDUCES THE NUMBER OF ALPHA(1)-ANDRENERGIC RECEPTORS IN FRTL-5 RAT-THYROID CELLS

Noradrenaline and ATP evokes a transient increase in the intracellular calcium concentration ([Ca2+](i)) in FRTL-5 cells. In a previous stud y, we showed that 1,25-dihydroxyvitamin-D-3 (1,25(OH)(2)-D-3) increase s the ATP evoked changes in [Ca2+](i). In the present paper, we found that pre-incubating the cells with 10 nM 1,25(OH)(2)-D-3 for 48 h did not affect the noradrenaline-evolved increase in [Ca2+](i). We subsequ ently examined if this could be due to an effect of 1,25(OH)(2)-D-3 on al-adrenergic receptor number, or receptor affinity. Pretreatment wit h 10 nM 1,25(OH)(2)-D-3 for 48 h decreased the binding of the alpha(1) -adrenergic specific antagonist [H-3]prazosin by 55% (B-max for 1,25(O H)(2)-D-3 treated =: 27.6 +/- 5.0 fmol/mg protein, untreated = 61.7 +/ - 5.4 fmol/mg protein). No effect of 1,25(OH)(2)-D-3 on the affinity f or [H-3]prazosin was observed. The effect of 1,25(OH)(2)-D-3 on the [H -3]prazosin binding was both time and dose-dependent and could first b e seen after 8-12h of 1,25(OB)(2)-D-3 treatment, indicating a genomic effect. The effect of 1,25(OH)(2)-D-3 could be abolished with the prot ein synthesis inhibitor cycloheximide. No effect on the [H-3]prazosin binding could be seen after a 48 h preincubation with 100 nM of either 24,25-dihydroxyvitamin D-3 and 25-dihydroxyvitamin D-3, indicating th at the effect of 1,25(OH)(2)-D-3 was specific. The cellular cAMP conce ntration was decreased after 48 h treatment with 10 nM 1,25(OH)(2)-D-3 . When TSH was replaced with dibutyryl cAMP or forskolin the [H-3]praz osin binding increased. 1,25(OH)(2)-D-3 also reduced the dibutyryl cAM P and forskolin stimulated [H-3]prazosin binding. In addition, 1,25(OH )(2)-D-3 reduced the [H-3]prazosin binding in TSH deficient media. Our results suggest that 1,25(OH)(2)-D-3 reduces the number of alpha(1)-a drenergic receptors via a dual mechanism: by reducing cAMP production and by affecting some mechanism(s) downstream from the production of c AMP.