Effect of 1,25-dihydroxyvitamin D3 on TSH secretion from rat pituitary cells in culture.

The effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on TSH secretion from rat pituitary cells was studied. When incubating cells with 1,25(OH)2D3 even at 100 X the physiological concentrations (10(-8)), no effect on basal TSH secretion was observed. The TRH-induced TSH secretion increased after a 24-h incubation with 10(-8) mol/l 1,25(OH)2D3 (2.9 +/- 0.2 ng/well vs 4.3 +/- 0.5 ng/well, mean +/- SD; P less than 0.05). When serum was omitted from the incubation medium, the potentiating effect of 1,25(OH)2D3 on the TRH-induced TSH release was blunted. No effect on cellular protein content was observed after incubating the cells with 10(-8) mol/l 1,25(OH)2D3. The results indicate that at unphysiological concentrations, 1,25(OH)2D3 affects the TRH-induced TSH secretion from pituitary cells. The physiological significance remains unclear.