Parathyroid cell proliferation and parathyroid hyperplasia are features of renal secondary hyperparathyroidism. Since parathyroids have recently been recognized as an important target for 1,25(OH)2D3, the effects of administration of variable doses of 1,25(OH)2D3 on ex vivo radiothymidine incorporation in the parathyroid glands, on parathyroid cell mitoses, on parathyroid weight, morphometric indices and on parathyroid protein/DNA ratio were examined in rats with uremia (subtotal nephrectomy; NX) or with calcium deficiency. 3H-thymidine incorporation (3 hr; 37 degrees C; PBS with 10 mmol glucose) was elevated in NX animals, that is, 204 +/- 51 dpm/micrograms DNA versus 96 +/- 28 in controls. In vivo pretreatment with 1,25(OH)2D3, either by intermittent i.p. injection or by osmotic minipump, dose-dependently decreased 3H-thymidine incorporation and parathyroid cell mitoses without affecting morphometric indices of parathyroid cells. Prophylactic administration (i.p.) of 1,25(OH)2D3, starting on the day of nephrectomy, prevented parathyroid hyperplasia (NX + 1,25(OH)2D3 0.84 micrograms tissue/g body wt vs. 1.25 micrograms in untreated NX and 0.54 in ad libitum fed controls), but 10 days of treatment beginning on the 21st day of uremia did not reverse existing hyperplasia (NX + 1,25(OH)2D3 1.5 micrograms/g body wt vs. 1.37 micrograms in untreated NX and 0.56 micrograms in ad libitum fed controls). The inhibitory effect was specific for 1,25(OH)2D3 and not imitated by Dexamethason. However, the effect was not specific for parathyroid hyperplasia of uremia, since similar inhibition of 3H-thymidine incorporation by 1,25(OH)2D3 was also observed in rats on low calcium diet.(ABSTRACT TRUNCATED AT 250 WORDS)

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