Regulation by Calcium of In Vivo Synthesis of 1,25-Dihydroxycholecalciferol and 21,25-Dihydroxycholecalciferol

Abstract

A metabolite of vitamin D believed to be the metabolically active form in the intestine has recently been isolated in pure form from intestine and unequivocally identified in this laboratory as 1,25-dihydroxycholecalciferol (1,25-(OH)2D3)(1, 2). Simultaneously, Lawson et al. provided evidence for this structure with a partially purified material from kidney homogenates (3). This metabolite acts more rapidly than 25-(OH)D3 to initiate intestinal calcium transport (4-6). The kidney is the site of synthesis of 1,25-(OH)2D3 from 25-(OH)D3 (7); this observation was confirmed by Gray et al. (8). Other experiments have provided strong evidence that 1,25-(0H2)D3 is the metabolically active form of vitamin D in the intestine (9-11). It, therefore, seemed possible that the concentration of 1,25-(OH)2D3 in the serum and intestinal mucosa plays an important role in the adaptation of calcium absorption to the concentration of calcium in the diet (12).This report demonstrates that dietary calcium concentration has a profound effect on the in vivo production of 1,25-(OH)2D3 and 21,25-(OH)2D3. Furthermore, these alterations in metabolite balance correlate with changes in serum calcium, but not serum phosphate concentration. An independent study has also shown that dietary strontium has an equally marked effect on the production of 1, 325 pmoles of tritiated 25-(OH)D3 was administered intrajugularly in 0.05 ml of 95% ethanol with mild ether anethesia. The rats were fasted at this time, but water continued to be available ad libitum. 12 hr later the rats were killed by decapitation and blood serum was collected. The upper 50 cm of small intestine was quickly removed, flushed with ice-cold saline, slit open, and the mucosa was scraped off with a glass slide. The kidneys and livers were removed. The fore-and hind-limbs were stripped of muscle, the long bones were split, and the marrow was discarded. All tissues were stored at -16'C until they were extracted with chloroform and methanol (14). In early experiments, the tissue from 3 to 4 animals fed the same diet was combined before extraction, while in later experiments the serum calcium concentrations of the rats were analyzed for similarity before pooling of tissues. In other cases, the tissues from each animal were analyzed individually.Radioactive metabolites were separated on a 2 X 15 cm column containing 10 g of Sephadex LH-20 equilibrated with 35% Skellysolve B (petroleum ether, b.p. 67-680C) in 65% chloroform (15). The tissue extracts were applied to the column in 1 ml of the same solvent mixture and eluted with a further 190 ml. Recovery of radioactivity from the columns varied from 90 to 105%.The elution position of 1,25-(OH)2D3 on this column has been established (2, 8). The less-polar metabolite was identified as 21,25-(OH)2D3 by cochromatography. A sample of the 2131

1,25(OH)2D3 Induced Alterations in Plasma Calcium, Inorganic Phosphate, Ultimobranhial Gland and Parathyroid Gland of the Garden Lizard, Calotes Versicolor

“…Also, increased calcium ions can inhibit this transformation by the direct action on the parathyroid and calcitonin secretion rates (BOYLE et al, 1971). In the present study the observed decrease in the plasma calcium levels on day 15 and day 30 may be explained on the basis that the elevated blood calcium has suppressed the secretion of parathyroid hormone, thus affecting the formation of 1,25(OH) 2 D 3 .…”

Growth and Mineral Metabolism in Very Low Birth Weight Infants. II. Effects of Calcium Supplementation on Growth and Divalent Cations

“…Intestinal mechanisms which influence adaptation to a calcium-deficient diet are stimulation of calcium binding protein formation (37), increase in Ca++ ATPase activity (28), and the increased formation of active vitamin D metabolites (24). The latter mechanism is influenced by calcium content of the renal cell: decreased calcium leads to increased 1,25-dihydroxycholecalciferol formation, whereas increased calcium increases the formation of the relatively inactive 24,25-dihydroxy derivative of vitamin D (5). N o information is available about the age at induction of these mechanisms or whether they are present at birth in the human.…”

“…Crowell et al ( 5 ) showed that dogs lose their purines irreversibly during hemorrhagic shock. Saugstad (12) showed that the oxypurine level in umbilical cord plasma is increased in newborn infants with clinical signs of intrauterine hypoxia.…”

Study of the Renal Tubular Interactions of Thyrocalcitonin, Cyclic Adenosine 3′, 5′ -Monophosphate, 25-Hydroxycholecalciferol, and Calcium Ion

“…An endotracheal tube was passed anfd the dogs were ventilated with a Harvard respirator. Inulin, p-aminohippurate (PAH) and aqueous vasopressin (ADH) were infused in physiological saline solution at a rate of 1 ml/min in quantities sufficient to provide plasma levels of [20][21][22][23][24][25][26][27][28][29][30] mg/100 ml and 1-3 mg/100 ml for the former two substances, and to deliver 30-60 mU/kg per h of the latter. After an equilibration period of 50-60 min, the following experimental maneuvers were performed.…”