1,25-Dihydroxyvitamin D3 stimulates osteopontin expression in rat kidney

Osteopontin (OPN) is a secreted phosphoprotein expressed constitutively in the descending thin limb (DTL) and papillary surface epithelium (PSE) of the kidney. Although its function is not fully established, a role for OPN in the regulation of calcium-mediated or calcium-dependent processes has been proposed. The aim of this study was to examine the effects of 1,25-dihydroxyvitamin D(3) (vitD), a hormone involved in the regulation of calcium homeostasis, on renal OPN expression. Four groups of rats were studied: acute vehicle (single intraperitoneal [i.p.] injection of 0.1 ml 10% ethanol-90% propylene glycol, 12 h before being killed); acute vitD (single injection of vitD, 2 ng/g i.p., 12 h before being killed); chronic vehicle (daily subcutaneous [s.c.] injection of 0.1 ml 10% ethanol-90% propylene glycol for 7 days); and chronic vitD (daily s.c. injection of vitD, 0.5 ng/g, for 7 days). Kidneys were processed for light and electron microscope immunocytochemistry, in situ hybridization, and Western blot analysis. In vehicle-treated animals, OPN mRNA and protein were expressed primarily in the DTL and PSE. In the acute vitD group, OPN mRNA and immunoreactivity appeared in the thick ascending limb (TAL) of the inner stripe of the outer medulla, and increased slightly in the DTL and PSE. The proximal tubules exhibited strong OPN immunoreactivity, but no hybridization signal. In the chronic vitD group, there was a marked increase in OPN mRNA and immunoreactivity in the distal tubule, including the TAL, as well as in the DTL and PSE. A weak hybridization signal and immunostaining were also observed in some proximal tubules. Administration of vitD causes a marked increase in OPN mRNA and protein in the rat kidney, mainly in the distal nephron, but also in the DTL, PSE, and proximal tubules. These results indicate that vitD is involved in the regulation of OPN expression in the kidney.     

1,25-Dihydroxyvitamin D3 receptors in lymphocytes from patients with rheumatoid arthritis

It has been previously found that activation of human lymphocytes in vitro causes the expression of receptors for the hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], and that 1,25(OH)2D3 has immunoregulatory properties including the ability to inhibit interleukin-2, to suppress lymphocyte proliferation, and to inhibit antibody production. In the present study we found that 13 of a group of 17 (76%) seropositive patients with rheumatoid arthritis had lymphocytes that possessed 1,25(OH)2D3 receptors (without activation in vitro) compared with only three of 17 (18%) normal individuals. The biochemical characteristics of the 1,25(OH)2D3 receptor, including affinity, sedimentation coefficient, and DNA-binding properties in the rheumatoid arthritis lymphocytes were indistinguishable from those established for this receptor in the classic target tissue of the hormone. This finding raises the possibility that 1,25(OH)2D3, acting through its receptor, might play a previously unsuspected role on lymphocytes of patients with rheumatoid arthritis.     

1,25-(OH)2D3对阿霉素肾病鼠肾小球肌营养不良蛋白聚糖表达的影响

目的:观察阿霉素(ADR)肾病鼠肾小球肌营养不良蛋白聚糖(dystroglycan,DG)的表达及1,25-(OH)2D3对其表达的影响.方法:雄性Wiser大鼠随机分为对照组和ADR肾病模型组;后者经尾静脉注射ADR诱导肾病模型.再随机分ADR组、ADR+1,25-(OH)2D3组.ADR+1,25-(OH)2D3组皮下埋置渗透性微量泵,给予1,25-(OH)2D3450 ng/(kg·min),连用6周.检测大鼠尿红细胞、尿蛋白、尿足细胞(UPC)及血清白蛋白(SA)、总胆固醇(TC)、甘油三酯(TG).肾组织电镜检测足细胞(PC)数、足突平均宽度.肾小球荧光染色定量分析检测肾小球α(β)-DG蛋白的表达.结果:与对照组相比,ADR组大鼠尿红细胞、尿蛋白、UPC、TC、TG明显升高,SA明显降低;PC数显著减少,足突宽度增加;肾小球α(β)-DG荧光染色呈节段性缺失,荧光强度显著减弱.与ADR组相比,ADR+1,25-(OH)2D3组尿红细胞、尿蛋白、UPC、TC、TG明显下降,而SA显著增加;PC数明显增加,足突宽度显著减少;肾小球α(β)-DG荧光染色恢复线型荧光,荧光强度显著增加.UPC与尿蛋白显著正相关;α-DG、β-DG光强度值与足突宽度呈负相关,与PC数呈正相关.结论:α(β)-DG在ADR肾病鼠中的表达减少.1,25-(OH)2D3,可减少ADR肾病鼠尿蛋白及UPC的排泄.上调α(β)-DG的表达,减少PC脱落,维持肾小球PC数量.     

1,25-Dihydroxyvitamin D3 induces collagen binding to the human monocyte line U937

Interactions of cells with components of the extracellular matrix can modulate cellular functions. We measured binding of a major matrix protein to U937 cells, a human promonocytic line. Radioiodinated type I or type III human collagen was bound only to U937 cells differentiated to a more mature phenotype with 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). Binding was observed at 4 degrees C and was saturable; Scatchard analysis of the binding to 1,25-(OH)2D3-pretreated U937 cells indicated a single class of high-affinity binding sites. Preincubation of U937 cells with interferon gamma did not induce collagen binding. Collagen binding did not appear to be dependent on fibronectin binding. Surface proteins of U937 cells were 125I labeled and cell membrane proteins resolved by affinity chromatography on collagen-Sepharose. Major specifically labeled bands of 180, 155, and 125 kD were identified in membrane fractions from 1,25-(OH)2D3-pretreated U937 cells only. 1,25-(OH)2D3 appears to specifically regulate collagen binding to monocyte precursors.     

Role of 1,25-Dihydroxyvitamin D3 on Intestinal Phosphate Absorption in Rats with a Normal Vitamin D Supply

In vitamin D-deficient rats, impaired intestinal phosphorus (P) absorption can be corrected by 1,25-dihydroxyvitamin D₃[1,25-(OH)₂D₃]. In the present study, it was investigated whether changes in 1,25-(OH)₂D₃ production can influence intestinal P transport also in animals with a normal supply of vitamin D. The intestinal P absorption was evaluated in rats using both the in situ duodenal loop technique and the determination of the overall gastrointestinal absorption under three conditions known to influence the production of 1,25-(OH)₂D₃: (a) variation in dietary P, (b) thyroparathyroidectomy (TPTX) with or without administration of parathyroid hormone (PTH), and (c) treatment with disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP). In all circumstances changes in duodenal absorption paralleled the changes in the overall fractional absorption. (a) Lowering dietary P stimulated P absorption. (b) TPTX decreased P absorption. This effect was corrected either by the administration of PTH or by the administration of 1,25-(OH)₂D₃. (c) EHDP, when given at a dose known to inhibit 1,25-(OH)₂D₃ formation, decreased the duodenal P absorption in both intact and TPTX animals. This effect was corrected by 1,25-(OH)₂D₃. In the TPTX-EHDP-treated animals, the administration of PTH did not rectify the low duodenal P absorption. These results support the thesis that, in rats with normal vitamin D supply, variations in the endogenous production of 1,25-(OH)₂D₃ change the rate of P absorption. However, these changes are in such magnitude that they are of relatively small importance when compared to the effect of variation in the dietary intake of P. These results also strongly suggest that the action of PTH on duodenal P transport is mediated by its effect on 1,25-(OH)₂D₃ production, inasmuch as the effect of the hormone is abolished after blocking the renal 1-hydroxylation with EHDP.     

1,25-Dihydroxyvitamin D3 increases serum and tissue accumulation of aluminum in rats

We examined the effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in both hypercalcemic and hypocalcemic rat models and the effect of exogenous 25-hydroxyvitamin D3 (25(OH)D3) on serum and tissue aluminum (Al) burdens. Rats fed a 0.2% Al diet received daily subcutaneous injections of either 1,25(OH)2D3 (80.9 ng/kg, n = 5 and 809 ng/kg, n = 8), 25 (OH)D3 (809 ng/kg, n = 4, and 8090 ng/kg, n = 8) or propylene glycol vehicle for 18 days. Rats given 809 ng/kg of 1,25(OH)2D3 were hypercalcemic and when compared with pair-fed controls had higher serum (33.1 vs. 14.3 micrograms/L, P less than 0.01), bone (21.2 vs. 13.2 micrograms/gm, P less than 0.01), and kidney (6.5 vs. 2.0 micrograms/gm, P less than 0.01) but not brain (1.2 vs. 1.5 micrograms/gm) or liver (0.9 vs. 0.8 micrograms/gm dry tissue) Al concentration. The lower dose of 1,25(OH)2D3 had no effect on serum or tissue Al. Treatment with 25(OH)D3 did not increase serum Ca and Al or tissue Al concentration. To dissociate a specific effect of exogenous 1,25(OH)2D3 from the concurrent hypercalcemia, endogenous production of 1,25(OH)2D3 was stimulated. Animals were fed a low Ca diet until hypocalcemia developed and were then divided into four groups: one given low Ca (n = 7) for 21 days, one given low Ca plus 0.2% Al (n = 7) for 21 days, one returned to a normal Ca diet (n = 4) for 30 days, and one returned to a normal Ca diet for 9 days and continued with a normal diet plus 0.2% Al (n = 5) for 21 days. Hypocalcemic rats fed the Al diet, when compared with hypocalcemic controls, had higher serum (143.6 vs. 31.8 micrograms/L, P less than 0.01), bone (16.0 vs. 2.9 micrograms/gm, P less than 0.01), and kidney (8.2 vs. 2.8 micrograms/gm, P less than 0.005) but not brain (3.4 vs. 2.3 micrograms/gm) or liver (3.8 vs. 2.3 micrograms/gm) Al concentrations. Serum, bone, and kidney Al concentration was also significantly higher than that in normocalcemic rats fed the Al diet. These results indicate that pharmacologic doses of 1,25(OH)2D3 and dietary hypocalcemia enhance gastrointestinal Al absorption and serum, kidney, and bone Al concentration.     

Effect of 1,25-Dihydroxyvitamin D3 on the Renal Handling of Pi in Thyroparathyroidectomized Rats

The kidney adapts its tubular capacity to transport inorganic phosphate (P_i) according to the dietary supply of P_i in both intact and thyropara-thyroidectomized (TPTX) rats. However, in TPTX rats the capability of the renal tubule to adapt to a high P_i diet is diminished. In TPTX rats the production of the active vitamin D₃ metabolite, 1,25-dihydroxyvitamin D₃ [1,25-(OH)₂D₃], is also reduced. 1,25-(OH)₂D₃ has been shown to have a marked effect on P_i metabolism. Therefore the question arises whether the deficient production of 1,25-(OH)₂D₃ contributes to the alteration of the tubular transport of P_i observed in chronically TPTX rats. In the present investigation, vitamin D-replete rats were sham operated (SHAM) or thyroparathyroidectomized and then pair fed diets containing either 0.2 or 1.2 g/100 g P for 7 days. During this period, groups of SHAM and TPTX rats received i.p. 2 × 13 pmol/day of 1,25-(OH)₂D₃, a dose which was shown to just normalize the decreased intestinal absorption of Ca and P_i in TPTX rats. The capacity of tubular P_i transport was then assessed by measuring the fractional excretion of P_i (FEP_i) at increasing plasma P_i concentration ([P_i]_Pl) obtained by acute infusion of P_i. The results show that in SHAM rats fed either P diet, 1,25-(OH)₂D₃ has no effect on the renal handling of P_i. In TPTX rats fed 1.2 g/100 g P diet, 1,25-(OH)₂D₃ increases FEP_i over a wide range of [P_i]_Pl. In TPTX rats fed a 0.2 g/100 g P diet, 1,25-(OH)₂D₃ does not alter FEP_i up to a [P_i]_Pl of 3.0-3.5 mM, but does increase it at higher [P_i]_Pl. In fact, on both diets TPTX rats supplemented with 1,25-(OH)₂D₃ appear to have the same renal handling of P_i as SHAM counterparts. The effect of 1,25-(OH)₂D₃ was not associated with a change in urine pH or in urinary excretion of cyclic AMP and was maintained under marked extracellular volume expansion. It was associated with a rise in plasma calcium in the TPTX rats fed the high, but not the low, P diet. In TPTX rats fed 1.2 g/100 g P diet, 25-hydroxyvitamin D₃ in doses of 2 × 130 or 2 × 1,300 pmol/day i.p. did not increase FEP_i.     

1,25-Dihydroxyvitamin D3 production by human keratinocytes. Kinetics and regulation.

Human foreskin keratinocytes in vitro metabolize 25-hydroxyvitamin D3 to a number of metabolites, including 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). This metabolite remains mostly within the cell and does not accumulate in the medium under the conditions of these experiments. With time, 1,25(OH)2D3 is catabolized, and more polar metabolites appear in both the cells and the medium. The production of 1,25(OH)2D3 has an apparent Michaelis constant (Km) for 25-hydroxyvitamin D3 of 5.4 X 10(-8) M. The levels of 1,25(OH)2D3 within the cell are increased both by increased production and decreased catabolism when parathyroid hormone(1-34) and isobutylmethylxanthine are added. Exogenously added 1,25(OH)2D3 at concentrations as low as 10(-12) M reduces endogenous 1,25(OH)2D3 production, increases 1,25(OH)2D3 catabolism, and increases 24,25-dihydroxyvitamin D3 production by an actinomycin D-sensitive process. These data indicate that the regulation of 1,25(OH)2D3 production by keratinocytes is similar to, but not identical to the regulation of 1,25(OH)2D3 by the kidney.     

Evaluation of a Role for 1,25-Dihydroxyvitamin D3 in the Pathogenesis and Treatment of X-linked Hypophosphatemic Rickets and Osteomalacia

Although a defect in renal transport of phosphate seems well established as the primary abnormality underlying the pathogenesis of X-linked hypophosphatemic rickets and osteomalacia, several observations indicate that renal phosphate wasting and hypophosphatemia cannot solely account for the spectrum of abnormalities characteristic of this disease. Thus, in the present study, we investigated the potential role of abnormal vitamin D metabolism in the pathogenesis of this disorder and the effect of 1,25-dihydroxyvitamin D(3) therapy on both the biochemical abnormalities characteristic of this disease and the osteomalacia. Four untreated patients, ages 14-30 yr, had normocalcemia (9.22+/-0.06 mg/dl); hypophosphatemia (2.25+/-0.11 mg/dl); a decreased renal tubular maximum for the reabsorption of phosphate per liter of glomerular filtrate (2.12+/-0.09 mg/dl); normal serum immunoreactive parathyroid hormone concentration; negative phosphate balance; and bone biopsy evidence of osteomalacia. The serum 25-hydroxyvitamin D(3) concentration was 33.9+/-7.2 ng/ml and, despite hypophosphatemia, the serum level of 1,25-dihydroxyvitamin D(3) was not increased, but was normal at 30.3+/-2.8 pg/ml. These data suggested that abnormal homeostasis of vitamin D metabolism might be a second defect central to the phenotypic expression of X-linked hypophosphatemic rickets/osteomalacia. This hypothesis was supported by evaluation of the long-term response to pharmacological amounts of 1,25-dihydroxyvitamin D(3) therapy in three subjects. The treatment regimen resulted in elevation of the serum 1,25-dihydroxyvitamin D levels to values in the supraphysiological range. Moreover, the serum phosphate and renal tubular maximum for the reabsorption of phosphate per liter of glomerular filtrate increased towards normal whereas the phosphate balance became markedly positive. Most importantly, however, repeat bone biopsies revealed that therapy had positively affected the osteomalacic component of the disease, resulting in normalization of the mineralization front activity. Indeed, a central role for 1,25-dihydroxyvitamin D(3) in the mineralization of the osteomalacic bone is suggested by the linear relationship between the serum level of this active vitamin D metabolite and the mineralization front activity. We, therefore, suggest that a relative deficiency of 1,25-dihydroxyvitamin D(3) is a factor in the pathogenesis of X-linked hypophosphatemic rickets and osteomalacia and may modulate the phenotypic expression of this disease.     

1,25-Dihydroxyvitamin D3 maintains adherence of human monocytes and protects them from thermal injury.

Adherence to a substratum is a characteristic feature of monocyte-macrophages which may be required for several effector functions. Human peripheral blood monocytes selected by adherence were found to readhere preferentially at 1 h to fibronectin or to a biological matrix. There was then a progressive decrease in the number of adherent cells, and by 48 h only 8-20% of monocytes remained adherent. This loss of adherence occurred while monocytes remained viable by criteria such as exclusion of trypan blue or release of lactate dehydrogenase. 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) maintained the adherence of cultured monocytes to tissue culture plastic as well as to the biological matrix. This effect was concentration- and time-dependent, and suppressed by inhibitors of protein synthesis. Cellular proteins were labeled after incubation with [35S]methionine. Analysis by two-dimensional gel electrophoresis revealed increased labeling of several distinct proteins in 1,25-(OH)2D3-treated monocytes compared with control monocytes. The increased loss of adherence and decreased overall protein synthesis observed in monocytes incubated at 45 degrees C was partially prevented by preincubation of the cells with 1,25-(OH)2D3. We further evaluated the effects of thermal stress and 1,25-(OH)2D3 on protein synthesis by monocytes, and found that 1,25-(OH)2D3 increased the synthesis of heat shock proteins, protected normal protein synthesis, and increased the rate of recovery of normal protein synthesis after the thermal stress. These observations suggest that 1,25-(OH)2D3 influences monocytes by preserving the synthesis of proteins, including those critical for the maintenance of cell adherence.     

1,25-Dihydroxyvitamin D3 stimulates rat osteoblastic cells to release a soluble factor that increases osteoclastic bone resorption.

Although 1,25-dihydroxyvitamin D3 stimulates osteoclastic bone resorption in vivo and in organ culture, the mechanism by which it effects this stimulation is unknown. We have recently found that the agent does not stimulate resorption by osteoclasts mechanically disaggregated from bone and incubated on slices of cortical bone. This suggests that the osteoclasts were removed by disaggregation from the influence of some cell type, present in intact bone, that mediates hormone responsiveness. We therefore tested the ability of osteoblastic cells derived from neonatal rat calvariae and of cloned, hormone-responsive osteosarcoma cells (UMR106) to restore hormone responsiveness to unresponsive populations of osteoclasts. We found that osteoblastic cells from both sources induced a two- to fourfold stimulation of osteoclastic bone resorption in the presence of 1,25-dihydroxyvitamin D3. Stimulation was observed at concentrations of 10(-10) M and above. Actinomycin D and cycloheximide did not affect bone resorption by osteoclasts incubated alone, but abolished the capacity of osteoblastic cells to stimulate osteoclastic resorption in the presence of 1,25-dihydroxyvitamin D3. When calvarial cells or osteoblastlike UMR cells were incubated with the hormone, they produced a factor in cell-free supernatants that stimulated bone resorption by disaggregated osteoclasts. These experiments suggest that 1,25-dihydroxyvitamin D3 stimulates bone resorption through a primary action on osteoblastic cells, that are induced by the hormone to produce a factor that stimulates osteoclastic bone resorption.     

1,25-(OH)_2D_3减轻糖尿病大鼠炎症因子的排泄及podocin表达缺失

目的:观察1,25-(OH)2D3对糖尿病大鼠尿液炎症因子单核细胞趋化蛋白- 1(MCP-1)、肿瘤坏死因子-α(TNF-α)、干扰素-λ(IFN-λ)及podocin表达水平的影响。方法:雄性Wistar大鼠随机分为对照组和糖尿病模型组;后者经链脲菌素诱导糖尿病模型成功后再随机分为糖尿病组和1,25-(OH)2D3组。给予1,25-(OH)2D3第6周测定大鼠尿红细胞、24小时尿蛋白、MCP-1、TNF-α、IFN-λ排泄及血糖水平。肾组织光镜电镜、肾小球podocin免疫组化及荧光染色定量分析检测podocin的表达。结果:与对照组相比,1,25-(OH)2D3组及糖尿病组尿红细胞、尿蛋白、MCP-1、TNF-α、IFN-λ、血糖明显升高(P<0.01),podocin蛋白表达明显下降(P<0.01)。与糖尿病组相比,1,25-(OH)2D3组尿红细胞、尿蛋白、MCP-1、TNF-α、IFN-λ、血糖明显降低(P<0.05或P<0.01),podocin蛋白表达明显增加(P<0.01)。结论:1,25-(OH)2D3可减轻糖尿病大鼠血尿、蛋白尿,减轻尿液炎症因子的排泄,恢复podocin的表达而有肾保护作...     

1,25-Dihydroxyvitamin D3 upregulates the phosphatidylinositol signaling pathway in human keratinocytes by increasing phospholipase C levels.

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) induces the differentiation of normal human keratinocytes, in part by increasing their basal intracellular calcium levels (Cai) over a period of hours. Agonists such as ATP acting through membrane receptors cause an immediate but transient increase in Cai accompanied by an increase in inositol trisphosphate (IP3). Treatment of keratinocytes for 24 h with 1 nM 1,25(OH)2D3 resulted in a two- to four-fold potentiation of the Cai response of these cells to ATP. This potentiation was inhibitable with cycloheximide, unaccompanied by a change in total intracellular calcium pools, but associated with an increase in basal IP3 levels and ATP-stimulated IP3 production. Treatment with 1,25(OH)2D3 raised the protein and mRNA levels of phospholipase C isoenzymes, particularly phospholipase C-beta 1 in a dose-dependent manner. These studies indicate that 1,25(OH)2D3 modulates the keratinocyte signal transduction pathway by induction of phospholipase isoenzymes, a previously undescribed action for this hormone.     

Serum 1,25 dihydroxy vitamin D (1,25(OH)2D3), 25 hydroxy vitamin D (25(OH)D) and parathormone levels in diabetic retinopathy

OBJECTIVES: To investigate whether there is a relationship between serum 1,25 dihydroxy vitamin D3 [1,25(OH)2D3], which is an inhibitor of angiogenesis, concentrations and severity of diabetic retinopathy (DR). DESIGN AND METHODS: Serum 1,25(OH)2D3, 25 hydroxy vitamin D [25(OH)D] and parathormone (PTH) concentrations were measured in diabetic patients (n = 66) and nondiabetic healthy subjects (n = 20). RESULTS: The mean serum 1,25(OH)2D3 concentration in diabetic patients was lower than that in nondiabetics (57.3+/-21.44 vs. 89.4+/-18.01 pmol/L, p<0.001); mean 1,25(OH)2D3 concentrations fell with increasing severity of DR [being 63.4+/-17.26 pmol/L for background DR (BDR), 47.7+/-13.27 pmol/L for preproliferative DR (pre-PDR), and 43.1+/-19.45 pmol/L for proliferative DR (PDR)]. Compared with the control group, serum 25(OH)D concentrations were found to be decreased in diabetic patients (p<0.001).There were negative correlations between 1,25(OH)2D3 and age (r = -0.331, p<0.01) and duration of diabetes (r = -0.255, p<0.05). CONCLUSION: From these findings, it was found that there was an inverse relationship between the severity of the retinopathy, i.e., neovascularization, and serum 1,25(OH)2D3 concentrations, being the lowest in PDR and the highest in diabetic patients without retinopathy (NDR) patients. The measurement of serum 1,25(OH)2D3 concentrations might be helpful to predict severity of DR in patients with diabetes mellitus.     

Differential effects of 19-nor-1,25-dihydroxyvitamin D(2) and 1,25-dihydroxyvitamin D(3) on intestinal calcium and phosphate transport

19-Nor-1,25-dihydroxyvitamin D(2) (19-norD(2)) a less calcemic and phosphatemic analog of 1,25-dihydroxyvitamin D (1,25[OH](2)D(3)), is approved for the treatment of secondary hyperparathyroidism in patients with kidney failure. We have previously demonstrated that 19-norD(2) is less active than 1,25(OH)(2)D(3) in stimulating bone resorption. In this study, we compared the potencies of 19-norD(2) and 1,25(OH)(2)D(3) in stimulating net calcium and phosphate absorption in the intestine. Mineral balance was assessed in normal rats during the last 4 days of a 14-day treatment with various daily doses of 19-norD(2) or 1,25(OH)(2)D(3). Calcium absorption increased from 16.5% +/- 7.8% in vehicle-treated rats to 27.5% +/- 7.2% in rats given 10 ng/day 1,25(OH)(2)D(3) and to 21.6% +/- 3.9%, 26.2% +/- 5.5%, and 27.4% +/- 5.1% in rats treated with 10, 50, and 100 ng/day 19-norD(2), respectively. Thus comparable stimulation of calcium transport was attained with 10 ng 1,25(OH)(2)D(3) and 100 ng 19-norD(2). Similar results were obtained for phosphate absorption, with an increase from 28.2% +/- 5.5% in vehicle-treated rats to 40.2% +/- 4.7% in rats given 10 ng/day 1,25(OH)(2)D(3) and to 32.9% +/- 2.2%, 36.2% +/- 4.5%, and 36.8% +/- 3.8% in rats given 10, 50, and 100 ng/day 19-norD(2), respectively. Vitamin D compounds are believed to increase calcium absorption by inducing a calcium channel (epithelial calcium transporter or calcium transporter-1 [CaT1]) on the luminal membrane, a calcium-binding protein (Calbindin D9k) in the cytosol, and a calcium pump (plasma membrane calcium adenosine triphosphatase-1 [PMCA1]) on the basolateral membrane. Northern-blot analysis of intestinal ribonucleic acid of vitamin D-deficient rats given seven daily injections of vehicle or 100 ng 1,25(OH)(2)D(3) or 19-norD(2) revealed that 19-norD(2) was less potent than 1,25(OH)(2)D(3) in stimulating expression of CaT1, Calbindin D9k and PMCA1. In summary, the reduced calcemic and phosphatemic activities of 19-norD(2) can be attributed to lower potency in stimulating intestinal calcium and phosphate absorption.