PubMed Health⌕ Search

Biomedical subjects

D D Bikle

Publications and source records attributed to D D Bikle.

At least 127 records · Page 7Linked to original sources
PubMed1986

Chronic administration of 1,25-dihydroxyvitamin D3: increased bone but impaired mineralization.

Male Sprague-Dawley rats were infused continuously for 13 days with vehicle or 75 pmol (31.2 ng)/day 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] by means of Alzet osmotic minipumps implanted s.c. Animals infused with 1,25-(OH)2D3 exhibited mild hypercalcemia (11.2 vs. 10.2 mg/dl in controls), a 136% increase in the serum concentration of 1,25-(OH)2D3 (187 vs. 79 pg/ml), and a 59% decrease in serum 25-hydroxyvitamin D (12 vs. 29 ng/ml). The proximal tibial metaphysis of these animals was characterized by increased trabecular bone volume (15% vs. 6.5%), osteoid accumulation (4.2% vs. 0.1%), increased osteoblast surface and number (31% and 19/mm vs. 21% and 14/mm, respectively), and decreased osteoclast surface and number (11% and 2/mm vs. 36% and 6/mm, respectively). Similar but less striking changes were seen in the lumbar vertebra. Increases in the fat-free weight and calcium content of the tibia and lumbar vertebra were consistent with the increase in trabecular bone volume after 1,25-(OH)2D3 infusion. However, tetracycline labeling and the incorporation of 45Ca in these bones were reduced by 1,25-(OH)2D3 infusion. In addition, mineralization lag time was prolonged in the lumbar vertebrae of 1,25-(OH)2D3-infused rats. Our findings indicate that chronic 1,25-(OH)2D3 administration increases bone mass, but at the cost of impaired bone mineralization.

Animals↗
PubMed1986

Assessment of the free fraction of 25-hydroxyvitamin D in serum and its regulation by albumin and the vitamin D-binding protein.

We measured the free fraction of 25-hydroxyvitamin D (25OHD) in human serum and determined that 25OHD bound to a component with an affinity constant of 7 X 10(8) M-1 and a concentration of 4.5 X 10(-6) M. This concentration was equal to that of the vitamin D-binding protein (DBP) in the same serum sample. We removed DBP from the serum using actin affinity columns and found that the affinity for 25OHD of the remaining serum components was equivalent to that of human serum albumin (6 X 10(5) M-1). We then measured the free fractions of 25OHD, DBP, and albumin in normal and cirrhotic subjects. We calculated that 88 +/- 3% (+/- SD) and 83 +/- 8% of the 25OHD were bound to DBP in the serum of normal and cirrhotic subjects, respectively. We compared previously reported data for the free fraction and the free concentration of 1,25-dihydroxyvitamin D in these subjects with the current data for the free fraction and free concentration of 25OHD. The total concentrations and free fractions of both metabolites correlated to each other and to the DBP and albumin concentrations in these subjects, but the free concentrations of these metabolites did not. We conclude that 25OHD, like 1,25-dihydroxyvitamin D, is transported in blood bound primarily to DBP and albumin. Changes in the concentrations of DBP and albumin affected the total and free fractions of 25OHD in serum, but the actual free concentration of 25OHD was independent of such changes.

Adult↗
PubMed1985

1,25-Dihydroxyvitamin D increases calmodulin binding to specific proteins in the chick duodenal brush border membrane.

In previous studies we demonstrated that the biologically active vitamin D metabolite 1,25-dihydroxyvitamin D [1,25(OH)2D] increased the calmodulin (CaM) content of chick duodenal brush border membranes (BBM) without increasing the total cellular CaM content. Therefore, we evaluated the binding of CaM to discrete proteins in the BBM and determined whether 1,25(OH)2D could influence such binding. We observed one major and several minor CaM-binding bands on autoradiograms of sodium dodecyl sulfate polyacrylamide gels incubated with [125I]CaM. The major band had a molecular weight of 102,000-105,000. It bound CaM even in the presence of EGTA, but not in the presence of trifluoperazine or excess nonradioactive CaM. The administration of 1,25(OH)2D increased the apparent binding of CaM to this protein as assessed by densitometry of the autoradiogram. This increase in CaM binding coincided with the increased ability of the same BBM vesicles to accumulate calcium. Cycloheximide in doses that markedly reduced the incorporation of [35S]methionine into BBM proteins did not reduce the ability of 1,25-dihydroxyvitamin D3 to stimulate either calcium uptake by the BBM vesicles or CaM binding to the 102,000-105,000-mol-wt protein. These results suggest that 1,25(OH)2D administration increases the CaM content of duodenal BBM by increasing the ability of a 102,000-105,000-mol-wt protein to bind CaM. This mechanism may underlie the ability of 1,25(OH)2D to stimulate calcium movement across the intestinal BBM.

Animals↗
PubMed1985

Serum protein binding of 1,25-dihydroxyvitamin D: a reevaluation by direct measurement of free metabolite levels.

Using the technique of centrifugal ultrafiltration isodialysis to measure the free concentration of 1,25-dihydroxyvitamin D [1,25-(OH)2D], we determined the affinity of serum proteins for 1,25-(OH)2D both by Scatchard analysis (increasing ligand concentration at fixed binding site concentrations) and by a novel analysis in which the binding site concentrations were varied (serial dilution) at fixed ligand concentrations. The high affinity binding constant in serum for 1,25-(OH)2D was 3.7 X 10(7) M-1 by Scatchard analysis and 4.2 X 10(7) M-1 by serial dilution analysis. Human serum albumin had a much lower affinity for 1,25-(OH)2D (5.4 X 10(4) M-1). When vitamin D-binding protein (DBP) was selectively removed from serum by an actin affinity column, the affinity of the remaining serum proteins for 1,25-(OH)2D was that of albumin. Postulating a two-site model (DBP and albumin) for transport of 1,25-(OH)2D in serum and incorporating the estimated affinity constants of DBP and albumin for this metabolite, we calculated that 85% of total circulating 1,25-(OH)2D is transported in blood bound to DBP in normal individuals (0.4% is free and 14.6% is bound to albumin). In patients with liver disease, 73% is bound to DBP (1.1% is free and 25.9% is bound to albumin). Using this same two site model, we found a reasonable correlation (r = 0.612; P less than 0.001) between the measured free 1,25-(OH)2D level and the calculated free 1,25-(OH)2D level in serum based on albumin and DBP concentrations in 16 normal subjects and 16 patients with liver disease. These results confirm the concept that although DBP is the principal protein carrier of 1,25-(OH)2D in serum, albumin is a major secondary carrier, especially in patients with low DBP levels.

Adult↗
PubMed1985

Bone disease in alcohol abuse.

We evaluated bone disease in eight white men between the ages of 49 and 61 years who had been abusing alcohol for at least 10 years. The mean density of vertebral cancellous bone was 58% of normal, whereas the mean density of appendicular cortical bone was 90% of normal. Marked reduction in active bone resorption and bone formation was seen without evidence of osteomalacia. Serum levels of calcium and magnesium were in the lower range of normal; serum levels of phosphorus, calcifediol, and calcitriol were normal; and serum levels of parathyroid hormone and nephrogenous cyclic adenosine monophosphate were in the higher range of normal. These data suggest that bone disease in these subjects is not due to inhibition of parathyroid hormone secretion or function, or abnormal vitamin D metabolism, but to an inhibition of bone remodeling by a mechanism independent of the calciotropic hormones.

Adult↗
PubMed1984

Measurement of 25,26-dihydroxyvitamin D: importance of the configuration of the C-25 hydroxyl group.

25,26-Dihydroxyvitamin D3 [25,26(OH)2D3] is chemically synthesized as two stereoisomers: 25(R),26(OH)2D3 and 25(S),26(OH)2D3. Both the R and S configurations have been claimed to be the natural form. Previous studies, however, have not considered the possibility that the stereochemical configuration of the C-25 hydroxyl group could affect the binding of 25,26(OH)2D3 to the rat serum binding protein used in assays for the measurement of this metabolite. In our study, a 50% displacement of radiolabeled 25-hydroxyvitamin D3 ([3H]25OHD3) from its initial binding is achieved with 325 fmol/mL 25(R),26(OH)2D3 and 850-1000 fmol/mL 25(S),26(OH)2D3--a difference in potency of approximately 3-fold. The potency of the R isomer in displacing [3H]25OHD3 was similar to that of 25OHD3 or 24(R),25(OH)2D3. When 25,26(OH)2D levels were measured in 12 normal subjects with both the R isomer and the S isomer as standards, the results were 625 +/- 360 fmol/mL 25(R),26(OH)2D3 equiv and 1800 +/- 1130 fmol/mL 25(S),26(OH)2D3 equiv. To determine which stereoisomer is biosynthesized, two types of experiments were performed. In the first, radiolabeled 25,26(OH)2D3 was biosynthesized from [3H]25OHD3 by using chick renal mitochondria and compared to [3H]25OHD3 as a ligand in the rat serum binding protein assay, which was used to measure 25OHD3, 25(R),26(OH)2D3, and 25(S),26(OH)2D3. Initial binding of radiolabeled 25OHD3 and 25,26(OH)2D3 to the binding protein was comparable, as was their displacement by the nonradioactive metabolites, indicating that chick renal mitochondria produce primarily 25(R),26(OH)2D3.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Calmodulin may mediate 1,25-dihydroxyvitamin D-stimulated intestinal calcium transport.

1,25-Dihydroxyvitamin D (1,25(OH)2D stimulated an increase in calmodulin content in chick duodenal brush border membranes in parallel with an increase in duodenal calcium transport in vivo and calcium uptake by brush border membrane vesicles (BBMV) in vitro. The increase in calcium uptake by BBMV was blocked by specific calmodulin antagonists. These results suggest that calmodulin mediates 1,25(OH)2D-stimulated calcium movement across the brush border membrane.

Animals↗
PubMed1984

Acute, but not chronic, metabolic acidosis disturbs 25-hydroxyvitamin D3 metabolism.

Previous studies in vitamin D deficient animals showing that acidosis may impair the production of 1,25(OH)2D3 are in conflict with studies in humans which have not shown convincing disturbances of 25OHD3 metabolism during acidosis. We have investigated the effect on renal 25OHD3 1- and 24-hydroxylase of acid loading for periods of 24 hr to 21 days. Acid loading resulted in immediate and sustained decrements in arterial pH and bicarbonate and increments in blood-ionized calcium. Acute (24-hr) acid loading decreased 1- and increased 24-hydroxylase activity. After 6 days of acid loading, no effect on 1- hydroxylase activity was observed and that on 24-hydroxylase activity was reduced. By 21 days the effect of acidosis on 24-hydroxylase activity was no longer observed. The results show that acidosis disturbs 25OHD3 metabolism in the physiological vitamin D and calcium replete state as well as in the vitamin D deficient state, but only acutely. Our data are consistent with studies in humans and suggest that, while short-term disturbances of 25OHD3 metabolism occur early in acidosis, they are transient and may not be causally related to the development of metabolic bone disease during chronic acidosis.

25-Hydroxyvitamin D3 1-alpha-Hydroxylase↗
PubMed1984

Free 1,25-dihydroxyvitamin D levels in serum from normal subjects, pregnant subjects, and subjects with liver disease.

We measured the free concentration of 1,25-dihydroxyvitamin D (1,25[OH]2D) using centrifugal ultrafiltration, and the level of vitamin D-binding protein (DBP) in 24 normal subjects, 17 pregnant subjects, and 25 alcoholic subjects with liver disease. Our objective was to determine whether the increase in total 1,25(OH)2D levels in pregnant women and the reduction in total 1,25(OH)2D levels in subjects with liver disease reflected a true difference in free 1,25(OH)2D levels or whether such differences were due solely to the variations in DBP levels (and thus, the amount of 1,25[OH]2D bound) in these groups. In subjects with liver disease the mean total 1,25(OH)2D concentration (22.6 +/- 12.5 pg/ml) and the mean DBP concentration (188 +/- 105 micrograms/dl) were nearly half the normal values (41.5 +/- 11.5 pg/ml and 404 +/- 124 micrograms/dl, respectively, P less than 0.001), whereas the mean free 1,25(OH)2D level was similar to normal values (209 +/- 91 fg/ml and 174 +/- 46 fg/ml, respectively). In contrast, in pregnant subjects the mean total 1,25(OH)2D level (82 +/- 21 pg/ml) and mean DBP level (576 +/- 128 micrograms/dl) were significantly higher than normal (P less than 0.001). Although the mean percent free 1,25(OH)2D level in pregnant subjects was below normal (0.359 +/- 0.07% vs. 0.424 +/- 0.07%, P less than 0.001), the mean free 1,25(OH)2D level was 69% higher than normal (294 +/- 98 fg/ml vs. 174 +/- 46 fg/ml, P less than 0.001). When data from all three groups were combined, there was a linear correlation between total 1,25(OH)2D and DBP levels but not between DBP and percent free 1,25(OH)2D levels; the increased DBP levels in the pregnant subjects were associated with less of an effect on percent free 1,25(OH)2D than were the reduced DBP levels in the subjects with liver disease. Our data suggest that (a) free 1,25(OH)2D levels appear to be well maintained even in subjects with liver disease and reduced DBP levels, (b) free 1,25(OH)2D levels are increased during pregnancy despite the increase in DBP levels, and (c) free 1,25(OH)2D levels cannot be inferred accurately from measurements of total 1,25(OH)2D and DBP levels alone in subjects with various physiologic and pathophysiologic conditions.

Adult↗
PubMed1984

The relationship of membrane fluidity to calcium flux in chick intestinal brush border membranes.

To evaluate whether membrane fluidity plays a role in regulating calcium flux across the intestinal brush border, we purified brush border membranes from different regions of the chick intestine and determined the relationship of their ability to transport calcium and their fluidity parameters, as determined by diphenylhexatriene fluorescence polarization. Raising the temperature from 4 to 37 C resulted in a 3-fold increase in calcium accumulation by duodenal brush border membrane vesicles (BBMV; 2.85 to 8.70 nmol/mg protein X 12 min). This rise in temperature was associated with a decrease in degree of polarization from 0.395 to 0.290. The methyl esters of cis- and trans-vaccenic acid markedly increased calcium uptake at all temperatures studied (4, 25, and 37 C); the trans isomer was more effective. Both cis- and trans-vaccenic acid lowered the degree of polarization of the BBMV from 0.340 to 0.311 and 0.293, respectively, at 25 C. cis- and trans-vaccenic acid were effective whether BBMV were prepared from vitamin D-deficient chicks or their hatchmates given 1,25-dihydroxyvitamin D [1,25-(OH)2D]. The ability to accumulate calcium correlated to the degree of polarization when calcium uptake by and degree of polarization of BBMV prepared from duodenum, jejunum, and ileum were compared at 25 C. The duodenal BBMV had the greatest calcium accumulation (9.4 nmol Ca/mg protein X 10 min) and the lowest degree of polarization (0.336); the ileal BBMV had the least calcium accumulation (3.5 nmol Ca/mg protein X 10 min) and the greatest degree of polarization (0.375); and the jejunal BBMV were intermediate (8.2 nmol Ca/mg protein X 10 min; 0.344). This rank order was the same whether the intestine was from vitamin D-deficient chicks or their hatchmates given 1,25-(OH)2D before they were killed. 1,25-(OH)2D stimulated calcium uptake by duodenal and jejunal BBMV, reaching a maximal effect at 4 h, but no changes in degree of polarization were observed during this period. A plot of the degree of polarization as a function of the reciprocal of absolute temperature showed an inflection point at approximately 25 C. Neither the slope of the plot nor the point of the inflection was altered by 1,25-(OH)2D. We conclude that chick intestinal BBMV membrane fluidity and calcium uptake correlate in relation to the effects of temperature, changes in membrane lipids, and regional differences in the intestine. However, 1,25-(OH)2D stimulates calcium uptake by BBMV without a detectable change in membrane fluidity.

Animals↗
PubMed1984

Effects of simulated weightlessness on bone mineral metabolism.

Space flight and bedrest result in a negative calcium balance and osteopenia. The mechanisms underlying these events are not well understood. In particular, it is not clear whether systemic or local factors are preeminent in mediating the effects of gravity on bone mineral content. Using a rat model that unweights only the hindlimbs, we determined whether the osteopenia induced in these rats was generalized or restricted to the unweighted limbs, and whether changes in intestinal calcium transport contributed to the alterations in bone calcium content. In this model, the hindquarters were elevated using the tail at an approximately 40 degrees angle for up to 15 days. The tibia and lumbar vertebra from experimental rats contained substantially less calcium than the same bones from pair-fed controls; at 15 days, the tibia was 86.2 +/- 2.5% of the control value (mean +/- SE), and the vertebra was 75.5 +/- 3.5% of control value (mean +/- SE). However, these differences were found only for the unweighted bones; the mandible and humerus showed no differences between experimental and pair-fed control rats. When calcium uptake by bone was evaluated after 45Ca administration, we observed an initial decrease in uptake at 5 days only in the tibia [percentage of control value, 60.5 +/- 5.5, (mean +/- SE)] and vertebra (percentage of control value, 74.3 +/- 3.7) when experimental animals were compared to pair-fed controls; there was no decrease in the humerus and mandible. However, after 10 days of unweighting , calcium uptake in the tibia and vertebra of experimental animals returned to control levels and by 15 days exceeded control levels (the tibia was 125.8 +/- 5.8% of the control value and the vertebra was 136.2 +/- 9.6% of the control value) despite the progressive decrease in total bone calcium compared to that in pair-fed controls. At no time could we demonstrate a difference in duodenal calcium transport between experimental and control animals. These data suggest that in this model, which simulates certain aspects of weightlessness, changes in local factors within the unweighted bones may have a greater impact on bone mass and turnover than changes in systemic factors that regulate overall bone mineral homeostasis.

Animals↗
PubMed1984

Differential response of duodenal epithelial cells to 1,25-dihydroxyvitamin D3 according to position on the villus: a comparison of calcium uptake, calcium-binding protein, and alkaline phosphatase activity.

To determine which region of the intestinal villus was primarily responsible for calcium uptake and whether cells from the different regions of the villus differed in their response to 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], we studied cells eluted from the duodenal villus in a sequential fashion at various times after vitamin D-deficient chicks had received 1,25-(OH)2D3. The elution scheme employed removes cells from the villus tip first and cells from the villus base last, as was documented by the distribution of alkaline phosphatase activity, sucrase activity, and cytosolic calcium-binding protein (CaBP) in the eluted fractions. Brush border membrane vesicles (BBMV) were prepared from different fractions of the villus. Calcium uptake was greatest in BBMV from cells eluted from the villus tip and least in those from the villus base. The distribution of calcium uptake and alkaline phosphatase activity in the same BBMV were parallel. After 1,25-(OH)2D3 treatment, cytosolic CaBP was observed in the cells from the villus base by 4 h and in all fractions by 8 h; at all times (from 4-24 h), cells from the villus base contained more cytosolic CaBP than did cells from the villus tip. Alkaline phosphatase activity in BBMV was stimulated in all fractions by 4 h; at all times, alkaline phosphatase activity was greatest in BBMV from cells of the villus tip. In contrast, calcium uptake by BBMV was stimulated 2 h after 1,25-(OH)2D3 administration only in cells from the villus tip and was not stimulated even by 24 h in cells from the villus base. These results indicate that the cellular response to 1,25-(OH)2D3 depends on the location of the cell on the villus and that 1,25-(OH)2D3-stimulated calcium flux across the brush border can be dissociated from 1,25-(OH)2D3-stimulated alkaline phosphatase activity and CaBP production.

Alkaline Phosphatase↗
PubMed1983

Calcium flux across chick duodenal brush border membrane vesicles: regulation by 1,25-dihydroxyvitamin D.

The vitamin D metabolite 1,25-dihydroxyvitamin D [1,25-(OH)2D] given in vivo stimulates calcium accumulation by subsequently isolated duodenal brush border membrane vesicles (BBMV). Stimulation is rapid (within 2 h), reaching a maximum between 2-4 h. This effect occurs well before stimulation of in vivo calcium transport (2-4 h), cytosolic calcium-binding protein production (4-8 h, or alkaline phosphatase activity (8 h). No cytosolic calcium-binding protein was found in the BBMV at any time. The extent of calcium accumulation by BBMV exceeds by severalfold the predicted value based on the equilibrium distribution of glucose, indicating a substantial amount of binding. The ability of the calcium ionophore A23187 to increase the rate of accumulation suggests that this binding is intravesicular. The Eadie Hofstee analysis of the rate of calcium accumulation as a function of calcium concentration is nonlinear. At submillimolar calcium concentrations, the difference in the apparent Km for calcium accumulation by BBMV from vitamin D-deficient and 1,25-(OH)2D-treated chicks is nearly 2-fold (1.9 X 10(-4) vs. 1.1 X 10(-4)M, respectively), a difference that is not observed at higher calcium concentrations. Release of calcium from preloaded BBMV with the addition of EGTA is rapid but not complete (20-30% of the initial value after 60 min). The rapidity, but not the extent, of release is increased with A23187. BBMV from vitamin D-replete and vitamin D-deficient duodena do not differ in their rate or extent of calcium release, in contrast to their different rates of calcium accumulation. We conclude that the stimulation by 1,25-(OH)2D of calcium accumulation by BBMV is one of the earliest actions of 1,25-(OH)2D on the intestine, that this process does not involve alkaline phosphatase or cytosolic calcium-binding protein, and that influx, but not efflux, of calcium is regulated.

Alkaline Phosphatase↗
PubMed1983

Calcium potentiates the cyclic nucleotide and phosphaturic response to parathyroid hormone infusion.

To determine whether calcium modulates the action of PTH, we measured the cyclic nucleotide and phosphaturic response to PTH following a 4-h infusion of glucose (day 1) and calcium (day 2). The 12 subjects were selected to provide a range of low, normal, and high endogenous PTH function. PTH stimulated nephrogenous cAMP [185 +/- 31 nmol/100 ml glomerular filtrate (GF)], cyclic guanosine monophosphate (0.44 +/- 0.09 mumol/g creatinine), and phosphate (367 +/- 59 mg P/g creatinine) excretion. Calcium infusion stimulated nephrogenous cAMP excretion in the hypoparathyroid subjects (1.42 +/- 0.35 nmol/100 ml GF) but reduced it in subjects with normal parathyroid function (-2.22 +/- 0.46 nmol/100 ml GF). Calcium infusion stimulated cGMP (0.64 +/- 0.1 mumol/g creatinine) and phosphate (113 +/- 48 P/g creatinine) excretion in all subject groups. Calcium infusion led to a 2-fold increase in the cyclic nucleotide and phosphaturic response to PTH in the normal and hypoparathyroid subjects, but had little effect on the PTH response in hyperparathyroid subjects. The extent to which calcium potentiated the ability of PTH to stimulate nephrogenous cAMP excretion correlated negatively with the basal nephrogenous cAMP excretion (r = -0.685, P less than 0.01). We conclude that calcium potentiates the acute effects of PTH on renal cyclic nucleotide and phosphate excretion. This effect is modified by the basal levels of PTH stimulation of the kidney such that it is reduced in magnitude when basal PTH stimulation is increased.

Calcium↗