Andras Szabo

No decrease of 1,25(OH)2D3 receptors and duodenal calbindin-D9k in uraemic rats

Abstract. In parathyroids of uraemic patients or animals, decreased specific binding of 1,25(OH)2D3 has been observed and implicated in the genesis of secondary hyperparathyroidism of renal failure. We re‐examined binding of 1,25(OH)2D3 using chromatin preparations for receptor characterization which differed from previous studies (a) by inclusion of protease inhibitors (PMSF, aprotinin) and molybdate in the extraction buffer and (b) by omitting the K‐extraction step. With this method, the Nmax in the intestinal mucosa and parathyroids of uraemic animals was significantly higher, while the receptor sedimentation constant (S), DNA affinity and KD were all unchanged. The ratio of occupied to total receptors was not significantly altered. The regulation of 1,25(OH)2D3 receptors in response to acute injection of 1,25(OH)2D3 was abnormal. Calbindin‐D9k concentration in the intestines of uraemic and control rats was comparable both before and after administration of 1,25(OH)2D3.

Evidence for in vivo upregulation of 1,25(OH)2 vitamin D3 receptor in human monocytes.

SummaryMammalian cells increase net expression of 1,25(OH)2D3 receptors after exposure to physiological concentrations of 1,25(OH)2D3in vitro. we examined specific binding of 1,25(OH)2D3 by human monocytes before and after daily administration of 1.5–2 ug 1,25(OH)2D3 p.o. for 3 days in 5 healthy normal D-replete probands. Median specific binding (Nmax) at baseline was 793 molecules/cell and 2052 or 2828 at 24h and 72h of 1,25(OH)2D3 treatment respectively. The results suggest (a) up-regulation of 1,25(OH)2D3 receptors occurs in man and (b) monocyte preparations can be used to assess receptor regulationin vivo.

Physiological doses of calcium regulatory hormones do not normalize bone cells in uraemic rats

Low bone turnover despite normal parathyroid hormone (PTH) concentrations has been found in many patients with end‐stage renal failure. Hyporesponsiveness to the calcaemic action is also a known feature of uraemia. Hyporesponsiveness of bone surface cells involved in bone modelling has not been demonstrated to date. It was the purpose of this study using a rat model of moderate renal failure to investigate whether doses of PTH and calcitriol that reverse the effect of parathyroidectomy on calcaemia also normalize bone surface cell activity.

Hyperparathyroidism and abnormal 1,25(OH)2vitamin D3 metabolism in experimental lead intoxication

Abstract. Clinical evidence points to disturbed calcium metabolism in lead (Pb) intoxication. To further clarify the mechanisms involved, serum levels of 1,25(OH)2D3, receptors for 1,25(OH)2D3 as well as size and ultrastructure of parathyroid glands were examined in Wistar Kyoto rats exposed to 1% lead (Pb) acetate in drinking water for 10 weeks (short‐term study) or 0·001 – 1 % Pb acetate for 24 weeks (long‐term study). After administration of Pb for 10 weeks, bone Pb was significantly increased (641±66·9 (SD) vs. 0·648±0·39 mg kg‐1 ash in controls). Total serum calcium and ionized Ca2+ (1·15±0·031 vs. 1·25±0·03 mmol 1‐1) were significantly decreased. Renal function (Ccr) was unchanged, but urinary cAMP excretion and circulating 1,25(OH)2D3 (177±10·9 vs. 232±18·9 pmol 1‐1) were diminished. Specific binding of 1,25(OH)2D3 was increased in parathyroids (Bmax 128±4·7 vs. 108±0·6 fmol mg‐1 protein) and intestinal mucosa; Bmax failed to adequately rise in response to pretreatment with 1,25(OH)2D3 (2·10 ng day‐1 for 4 d) in Pb‐exposed animals. Receptor characteristics (sedimentation constant, KD, DNA affinity) were unchanged. Parathyroid weight was significantly increased (178·25 vs. 96·34 μg) with no change of estimated nuclear volume, cell volume or cell ultrastructure. After 24 weeks of Pb exposure, a dose‐dependent but non‐linear increase of parathyroid weight was noted between 0·001% and 1% Pb in drinking fluid.