Direct control by calcium of 25-hydroxycholecalciferol-1-hydroxylase activity in chick kidney mitochondria

Female sex hormone replacement therapy increases serum free 1,25-dihydroxyvitamin D3: a 1-year prospective study

OBJECTIVE

Currently, hormone replacement therapy is applied successfully to reduce post-menopausal bone resorption. However, the exact mechanism by which oestrogen exerts its effect has not yet been fully elucidated. In order to determine whether changes in the biologically active 1,25-dihydroxyvitamin D3 may be of importance in this process, the concentrations of both total and free 1,25-dihydroxyvitamin D3 in serum were assessed.

DESIGN

In 36 post-menopausal women the effect of hormone replacement therapy, with a combination of 17 beta-oestradiol and norethisterone acetate, on the serum levels of total and free 1,25-dihydroxyvitamin D3 was studied after 0, 3, 6 and 12 cycles.

MEASUREMENTS

The total concentration of 1,25-dihydroxyvitamin D3 in serum was assessed using a radioreceptor assay after diethylether extraction of the samples followed by paper chromatography. The free fraction of 1,25-dihydroxyvitamin D3 was measured using symmetric dialysis. The free 1,25 dihydroxyvitamin D3 concentration was calculated by multiplying the total concentration by the free fraction.

RESULTS

During therapy, mean serum total 1,25-dihydroxyvitamin D3 concentrations (+/- SD) were 106.4 pmol/l (+/- 27.5), 155.0 pmol/l (+/- 49.5), 176.7 pmol/l (+/- 70.0) and 161.1 pmol/l (+/- 55.3) at 0, 3, 6 and 12 cycles, respectively. Serum free 1,25-dihydroxyvitamin D3 concentrations were 68 fmol/l (+/- 22), 107 fmol/l (+/- 35), 120 fmol/l (+/- 43) and 108 fmol/l (+/- 37), respectively. Baseline values of both total and free 1,25-dihydroxyvitamin D3 were significantly lower than those during therapy at all time (P < or = 0.001).

CONCLUSION

Both the serum total 1,25-dihydroxyvitamin D3 and the serum free 1,25-dihydroxyvitamin D3 concentrations are increased during combined 17 beta-oestradiol and norethisterone acetate therapy for a year. Assuming that the free concentration 1,25-dihydroxyvitamin D3 reflects the biologically active fraction, this rise may in part explain the preventive effect of hormone replacement therapy on osteoporosis.

Opposing actions of methylxanthines and dibutyryl cyclic AMP on 1,25 dihydroxyvitamin D3 production and calcium fluxes in isolated chick renal tubules

In contrast to dibutyryl cyclic AMP, the methylxanthine phosphodiesterase inhibitors theophylline and caffeine were found to inhibit the conversion of 25 hydroxyvitamin D3 to 1,25 dihydroxyvitamin D3 in isolated renal tubules from vitamin D deficient chicks. This inhibition occurred at concentrations of methylxanthines which were shown to increase renal tubule cyclic AMP levels. No effect of theophylline or caffeine on 25 hydroxyvitamin D3 metabolism in isolated chick renal mitochondria was detected. Because of a demonstrated inhibitory action of calcium (10 and 20 mumol/l) on renal mitochondrial conversion of 25 hydroxyvitamin D3 to 1,25 dihydroxyvitamin D3, the effect of theophylline and dibutyryl cyclic AMP on cellular calcium-45 efflux and total renal tubule calcium content was estimated. Theophylline 10 mmol/l was found to inhibit renal tubular calcium efflux and to increase total cellular calcium content, while dibutyryl cyclic AMP 1 mmol/l had the reverse effect on both parameters. Divergent actions of the methylxanthines and dibutyryl cyclic AMP on the formation of 1,25 dihydroxyvitamin D3 and renal tubule calcium efflux and content support the hypothesis that intracellular calcium is an important regulator of renal vitamin D metabolism. The results indicate that observed actions of methylxanthines cannot always be ascribed to cyclic AMP accumulation.

Rat renal 25-hydroxyvitamin D3 1- and 24-hydroxylases: their in vivo regulation

The effects of thyroparathyroidectomy, parathyroid hormone, 1,25-dihydroxyvitamin D3, dietary calcium, dietary phosphorus, age, and sex on the renal 25-hydroxyvitamin D3 1- and 24-hydroxylases measured in vitro in rats have been studied. Thyroparathyroidectomy of vitamin D-deficient rats abolishes 25-hydroxyvitamin D3 1-hydroxylase activity, and administration of bovine parathyroid extract to the thyroparathyroidectomized rat restores diminished 1-hydroxylase activity. Both suppression and restoration of the enzyme activities require many hours (18-24 h) independent of rapid changes in serum calcium and inorganic phosphorus levels in response to these manipulations. Administration of 1,25-dihydroxyvitamin D3 to vitamin D-deficient rats suppresses 25-hydroxyvitamin D3 1-hydroxylase activity and stimulates 25-hydroxyvitamin D3 24-hydroxylase activity within 48 h. Rats maintained on a low-calcium or a low-phosphorus diet with a daily supplement of 20 IU vitamin D3 show high 25-hydroxyvitamin D3 1-hydroxylase activity and low 24-hydroxylase activity as compared with rats similarly treated but fed a diet containing adequate calcium or adequate phosphorus. When vitamin D-sufficient rats having suppressed renal 25-hydroxyvitamin D3 1-hydroxylase activity are placed on a low-calcium vitamin D-deficient diet for 7 days, the 1-hydroxylase activity is greatly stimulated in 6-wk-old rats but much less so in rats with advancing age.

Early changes in the adaptation to a low calcium diet in the chick

Twelve hours after the diet of 3-week-old chicks was changed from a 1% to a 0.1% concentration of calcium (Ca), the growth rate and circulating levels of growth hormone had fallen while renal 25-hydroxecholecalciferol-24-hydroxylase had risen. The amount of 47Ca incorporated into bone from an injection given 18 h previously was lower than in the control birds. Over the following 2 1/2 days on the low Ca diet, the renal 1-hydroxylase activity rose and the plasma prolactin level fell, but the other parameters moved back toward the control level. It was concluded that early adjustments in hormonal and mineral metabolism counteract the acute effects of a dietary Ca shortage until longer-term adaptive changes begin to compensate for a continuing Ca deficiency. The renal hydroxylase activities were not directly influenced by the level of circulating growth hormone or prolactin.

Demonstration of vitamin D-binding protein (Gc-globulin) in the urine of Itai-itai disease patients

Vitamin D-binding protein (VDBP, known as Gc-globulin) was discovered by an immunochemical method in the urine of patients with Itai-itai disease. The urine and sera of Itai-itai disease patients produced specific precipitin lines with anti-human VDBP antisera. The electrophoretic mobility of the protein in the urine is the same as that in the serum. A significant correlation was found between VDBP and beta 2-microglobulin in the urine. Based on this result, it was concluded that Itai-itai disease is associated with disturbances of vitamin D transport and/or metabolism.

In vitro modulation of renal 25-hydroxyvitamin D3 metabolism by vitamin D3 metabolites and calcium

It has been shown that 1,25-dihydroxyvitamin D3 (1,25-(OH)2-D3) and dietary Ca modulate renal metabolism of 25-hydroxyvitamin D3 (25-OH-D3) to 1,25-(OH)2-D3 and 24,25-dihydroxyvitamin D3 (24,25-(OH)2-D3) in the rat. However, it is not known if 1,25-(OH)2-D3 and Ca act directly on the kidney to modulate 25-OH-D3 metabolism or indirectly through other mechanisms, such as the modulation of parathyroid hormone secretion. Therefore, we have used isolated renal cortical slices from the rat to study the effect of 1,25-(OH)2-D3 and Ca in vitro on renal 25-OH-D3 metabolism. Incubation of renal slices from rats fed a vitamin D-deficient, low-Ca diet with 50 nM 1,25-(OH)2-D3 for 3 h resulted in a significant decrease in 1,25-(OH)2-D3 production and a significant increase in 24,25-(OH)2-D3 production. Increasing media Ca concentration from 0.5 to 2.5 mM resulted in a significant decrease in 1,25-(OH)2-D3 production but no change in 24,25-(OH)2-D3 production. The inhibitory effect of 1,25-(OH)2-D3 was blocked by cycloheximide, but the inhibitory effect of Ca was not blocked by cycloheximide. Renal 1,25-(OH)2-D3 production was inhibited to a greater extent by incubation with 1,25-(OH)2-D3 and Ca together than by incubation with 1,25-(OH)2-D3 and Ca separately. These studies indicate that 1,25-(OH)2-D3 and Ca act directly on the kidney to modulate renal 25-OH-D3 metabolism. They also suggest that the mechanism of modulation is different for each agent.

Studies of the chick renal mitochondrial 25-hydroxyvitamin D-3 24-hydroxylase

The vitamin D metabolite, 24,25-dihydroxyvitamin D-3, is made from 25-hydroxyvitamin D-3 by an enzyme (25-hydroxyvitamin D-3 24-hydroxylase) located in the renal mitochondria of vitamin D and calcium-replete chicks. The apparent Km for 25-hydroxyvitamin D-3 for this reaction was found to be 3 x 10(-7) M, although simple Michaelis-Menten kinetics were not observed at the higher substrate concentrations. Enzymatic activity was reduced by the mitochondrial metabolic inhibitors, antimycin A and dinitrophenol, as well as the inhibitors of mixed function oxidases, 2,4-dichloro-6-phenylphenoxyethylamine, 2-diethylaminoethyl-2,2-diphenylvalerate and metyrapone. Increasing the calcium concentration from 0 to 1 x 10(-5) M or the potassium concentration from 0 to 50 mM stimulated enzymatic activity 4- to 8-fold. Increasing the phosphate or acetate concentration from 0 to 50 mM stimulated enzymatic activity 6-fold. Raising the CO2/HCO3(-) content from 0/0 to 10%/10 mM stimulated enzymatic activity 5-fold. The effects of the cations were additive to those of the anions except in the presence of 50 mM phosphate or acetate. The effect of ions on 24,25-hydroxyvitamin D-3 production by renal mitochondria from vitamin D-replete chicks resembles the effect of these ions on production of 1,25-dihydroxyvitamin D-3 by renal mitochondria from vitamin D-deficient chicks, suggesting that the same mechanisms may be utilized.

The ionic control of 1,25-dihydroxyvitamin D-3 synthesis in isolated chick renal mitochondria. The role of potassium

In previous studies it was found that change in the concentrations of Ca2+, H+, and HPO2-4 in the incubation medium altered the rates of synthesis of 1,25-dihydroxyvitamin D-3 (1,25(OH)2D-3) by isolated renal mitochondria obtained from D-deficient chicks. The present studies demonstrate that raising the medium concentration of K+ from 1 to 50 mM leads to a 6-fold increase in rate of 1,25(OH)2D-3 synthesis by isolated chick mitochondria; that the magnitnitude of this K+-dependent stimulation is enhaced by optimal concentrations of calcium (pCa = 5) and phosphate (pPi = 3) (3 mM) but not by pH (from 6.8 to 7.4); that the effect is not produced by similar changes in media Na+ concentration; and that the stimulatory effect of K+ is not blocked by ruthenium red, and inhibitor of calcium transport and of the calcium-dependent stimulation of mitochondrial 1,25(OH) 2D-3 synthesis. It was also found that valinomycin, a K+-specific ionophore, enhanced the sensitivity of the mitochondrial 1 alpha-hydroxylase activity to K+. In the presence of valinomycin, an increase of pK+ to 3 was sufficient to cause a significant stimulation of 1,25(OH)2D-3 synthesis. It was concluded that changes in the ion content of the mitochondrial matrix space regulated the activity of the 1 alpha-hydroxylase.