John G. Haddad

Abnormalities of Circulating 25-OH Vitamin D after Jejunal-Ileal Bypass for Obesity: Evidence of an Adaptive Response

Circulating levels of 25-OH vitamin D were measured in 44 patients who had undergone small intestinal bypass for obesity. Sixty-one percent had low circulating levels of the metabolite, which tended to normalize with time. This adaptive response also occurred for circulating total calcium, magnesium, albumin, and alkaline phosphatase. Serum concentrations of 25-OH vitamin D were directly related to total serum calcium and albumin. Impaired intestinal absorption of 25-OH vitamin D was seen in two patients. Following correction of total serum calcium for attendant hypoalbuminemia, 27% of patients remained hypocalcemic. The bone densities of two of 32 patients were low. In addition, skeletal biopsies of three of six patients were abnormal. It is concluded that small intestinal bypass results in at least transient deficits of circulating 25-OH vitamin D. As this operation may be associated with abnormal bone morphology, clinically significant skeletal disease may become apparent with long-term follow-up.

Vitamin D metabolite-binding proteins in human tissue.

Serum and post-microsomal supernatants of human lymphocyte, erythrocyte, skeletal muscle and parathyroid adenoma homogenates were examined for specific binding of 25-hydroxycholecalciferol (25-OHD3) and 1, 25-dihydroxycholecalciferol (1,25-(OH)2D3). Muscle, lymphocytes and parathyroid adenomata extracts contained a 6-S 25-OHD3-binding protein which was not found in erythrocyte extracts, and which was distinct from the smaller serum transport alpha-globulin. A cathodal, 1, 25-(OH)2D3-binding protein, which sedimented at 3-4 S was also detected in parathyroid tissue. These observations suggest the possibility of direct physiologic interaction between vitamin D metabolites and nucleated human tissues other than intestine and bone.

Serum concentration of 25-hydroxyvitamin D in rickets of extremely premature infants

Nine premature infants developed radiographic and biochemical rickets at a mean ±SD of 12.6±2.8 weeks of age. Serum 25-hydroxyvitamin D concentrations were all low, with a mean of

Hepatic accumulation of vitamin D3 and 25-hydroxyvitamin D3.

Concomitant intravenous administration of 25-hydroxycholecalciferol and [3H] vitamin D3 to vitamin D-depleted rats did not affect the conversion of [3H] vitamin D3 to 25-OH-[3H] vitamin D3 as indicated by a serum 25-OH-[3H] vitamin D3 to content at 3 and 24 h identical to those observed in animals receiving [3H] vitamin D3 alone. Similarly, pre-dosing with 25-OH vitamin D3 24 h earlier did not affect the conversion. Co-administration to vitamin D depleted rats of vitamin D2 or D3, at 200-fold higher doses than a control group receiving tracer [3H] vitamin D3 alone, resulted in serum 25-OH vitamin D levels that were 15-20 fold higher than the control, indicating a similar metabolic fate for synthetic and natural vitamin D in rats and the ability of increased substrate to overwhelm hepatic constraints on 25-OH vitamin D production. Following intravenous administration of 25-OH-[3H] vitamin D3 to vitamin D depleted rats, hepatic 3H content decreased in parallel with serum radioactivity. Hepatic accumulation of intravenously administered vitamin D3 ([14C] vitamin D3) alone or with 25-OH-[3H] vitamin D3, by vitamin D-depleted rats revealed a marked preference for vitamin D3; the hepatic accumulation of [14C] vitamin D3 increased to 35% of the dose by 45 min, at which time 25-OH-[3H] vitamin D3 hepatic content was 7-fold less, and decreasing. Chromatography of extracts of hepatic subcellular fractions revealed more [14C] vitamin D3 than 25-OH-[3H] vitamin D3 in the microsomes, the reported site of calciferol 25-hydroxylase. Circulating 25-OH vitamin D, therefore, has comparatively minimal potential for hepatic accumulation. Product inhibition of the calciferol 25-hydroxylase must, therefore, result from recently synthesized hepatic 25-OH vitamin D, and is not affected by exogenous 25-OH vitamin D3.

Skeletal abnormalities after jejunoileal bypass.

Jejunoileal bypass surgery is fraught with many longterm complications, among which is hypovitaminosis D. The relationship, if any, of hypovitaminosis D to the skeletal disease which may occur following this operation is, however, unknown. Consequently, we studied eight patients with low circulating levels of 25-hydroxyvitamin D who had undergone Jejunoileal bypass at least two and one-half years previously. Despite the absence of skeletal symptoms, the bone biopsies of six of these patients were abnormal. The volume of trabecular bone was diminished in the group as a whole, and half the patients had an excess of unmineralized skeletal matrix. However, no noninvasive diagnostic technique identified those patients with skeletal disease. We therefore conclude that recognition of those Jejunoileal bypass patients potentially at risk to develop clinically significant bone disease requires biopsy of the skeleton.

Monoclonal antibody with high affinity for 1,25-dihydroxycholecalciferol.

We have developed a monoclonal antibody capable of detecting 1 pg/ml of 1,25-dihydroxycholecalciferol. At a dilution of 1:80,000 of ascitic fluid this antibody has an apparent KD of 3.3 X 10(-11) ML-1. The immunogen used was a vitamin D analogue, calcitroic acid [1 alpha, 3 beta-dihydroxy-9, 10 seco-24-nor 5, 7, 10 (19) cholatriene-23-oic acid], conjugated to bovine serum albumin. Although this antibody is extremely sensitive, it also recognizes other important vitamin D3 metabolites.

LYMPHOMA, HYPERCALCEMIA, AND THE SUNSHINE VITAMIN

Under normal circumstances, parathyroid hormone and 1 ,25-dihydroxycholecalciferol (calcitriol) jointly defend the plasma calcium concentration. Parathyroid hormone increases renal distal tubular calcium reabsorption and mobilizes calcium from bone. Calcitriol, the active product of vitamin D metabolism, enhances gastrointestinal calcium absorption and also mobilizes calcium from bone. The roles of parathyroid hormone and calcitriol in calcium homeostasis are intimately interrelated. Parathyroid hormone is the primary trophic stimulator of renal 1 -hydroxylase, the enzyme that converts 25-hydroxycholecalciferol into calcitriol. Calcitriol, in turn, inhibits parathyroid hormone secretion by at least two different mechanisms: direct repression of the preproparathyroid hormone gene and inhibition of the synthesis and release of the hormone as an indirect consequence of the increase in the plasma calcium concentration. Thus, parathyroid hormone is a primary regulator of vitamin D metabolism, and calcitriol is a primary feedback inhibitor of parathyroid hormone secretion. Abnormalities in this elaborate control system underlie most cases of hypercalcemia. The many clinical and laboratory similarities between primary hyperparathyroidism and the hypercalcemia associated with various nonhematologic tumors led to the suspicion that parathyroid hormone was also responsible for so-called malignancy-associated hypercalcemia. With the identification, isolation, and cloning of parathyroid hormone-related protein [1-4], this theory has been disprovedat least its classic formulation as an ectopic parathyroid hormone syndrome. Nonetheless, it is ironic that parathyroid hormone-related protein, a protein quite distinct in structure from parathyroid hormone (save for partial homology in the 13 amino acid N-terminal region), produces hypercalcemia by masquerading as parathyroid hormone [5]. Little doubt now exists that parathyroid hormone-related protein is an important cause of malignancy-associated hypercalcemia [6-9]. What is less clear is whether other humoral factors act permissively or synergistically with parathyroid hormone-related protein in the genesis of the hypercalcemia. For example, although high circulating levels of this protein have been documented in 50% to 80% of cases of malignancy-associated hypercalcemia, there is often little correlation between the magnitude of the hypercalcemia and circulating parathyroid hormone-related protein levels. Furthermore, some patients with high levels of this protein remain normocalcemic. One wonders if already indicted factors [10]cytokines such as transforming growth factor-, tumor necrosis factor-, tumor necrosis factor- (lymphotoxin) and interleukin-1 and arachidonic acid metabolites such as prostaglandin Eor as yet unindicted humoral factors are also involved. Patients with primary hyperparathyroidism generally have either normal or elevated circulating calcitriol levels. In contrast, many patients with malignancy-associated hypercalcemia have low calcitriol levels [11]. This is usually ascribed to suppression of the parathyroid hormone-calcitriol axis by the greater degree of hypercalcemia and prevailing hypoparathyroidism. Although this is readily understandable in the case of tumors that do not secrete parathyroid hormone-related protein, tumors that do secrete this protein present a more complex situation. Although parathyroid hormone-related protein, like the hormone itself, can stimulate renal 1 -hydroxylase [12-14], most patients with parathyroid hormone-related protein-associated hypercalcemia have low calcitriol levels [11, 15-17]. The hypercalcemia and associated hypoparathyroidism apparently suppress 1 -hydroxylase activity despite high levels of circulating parathyroid hormone-related protein. It is also conceivable that solid tumors produce, along with parathyroid hormone-related protein, a substance that directly inhibits 1 -hydroxylase activity [16]. Although abnormalities in the metabolism of parathyroid hormone or parathyroid hormone-related protein account for many cases of hypercalcemia, primary disorders of vitamin D metabolism can also lead to hypercalcemia. In addition to its normal production sitethe proximal tubule of the kidneycalcitriol can be produced by many other cell types, including decidual cells, keratinocytes, bone cells, spleen cells, peripheral monocytes, and activated macrophages [18-20]. Various granulomatous disorders, including sarcoidosis and tuberculosis, have been associated with hypercalcemia; in sarcoidosis, the underlying cause of the hypercalcemiaunregulated production of calcitriol by pulmonary alveolar macrophages [21, 22]has been well delineated. Although uncommon, hypercalcemia is also a well-recognized complication of lymphoma, including both Hodgkin disease and non-Hodgkin lymphoma. Unlike most patients with malignancy-associated hypercalcemia, in whom circulating calcitriol levels are reduced (reflecting suppression of the parathyroid hormone-calcitriol axis), many hypercalcemic patients with lymphoma have calcitriol levels that are either frankly elevated or inadequately suppressed given the degree of hypercalcemia [23]. In this issue, Seymour and colleagues [24] examine calcium homeostasis in the largest series of patients with non-Hodgkin lymphoma yet reported. They first defined a reference range for circulating calcitriol levels in a control grouphypercalcemic patients with multiple myeloma. There are no intrinsic abnormalities in the parathyroid hormone-calcitriol axis in multiple myeloma, and the hypercalcemia appropriately and reversibly suppresses the parathyroid hormone and calcitriol levels [25]. Moreover, parathyroid hormone-related protein levels are generally, although not universally, very low or undetectable [6-9]. The major cause of hypercalcemia in multiple myeloma appears to be local osteolysis mediated by tumor necrosis factor- [26] and perhaps by other cytokine stimulators of bone resorption as well. Using this reference range, Seymour and colleagues classified hypercalcemic patients with lymphoma into two groups. Twelve of 22 patients (55%) had elevated circulating calcitriol levels, and the rest had levels within the established hypercalcemic reference range. That abnormalities in vitamin D metabolism might be even more pervasive than these data indicate is suggested by the observation that 18% of newly diagnosed, normocalcemic patients with lymphoma had serum calcitriol levels greater than the normocalcemic reference range. Moreover, 71% of the normocalcemic patients for whom measurements were available had an elevated fasting urinary calcium excretion, which perhaps indicates even more subtle abnormalities in the parathyroid hormone-calcitriol axis. That calcitriol is a seminal cause of hypercalcemia in at least certain patients with lymphoma is perhaps best shown by the patients response to cytotoxic chemotherapy. Seymour and colleagues studied four patients sequentially before and after the initiation of chemotherapy. During chemotherapy in each case, the plasma calcium concentration returned to normal and the circulating calcitriol level was significantly reduced. In three patients, calcitriol levels decreased to the normocalcemic reference range. The remaining patient, who also initially responded, subsequently developed progressive lymphoma and recurrent hypercalcemia, at which time calcitriol levels were again markedly elevated. Under normal circumstances, physiologic concentrations of calcitriol inhibit 1 -hydroxylase activity [19, 27]. Feedback inhibition of 1 -hydroxylase is not product inhibition but rather is mediated by a genomic effect that has yet to be fully characterized. Extrarenal 1 -hydroxylases appear to be more sensitive to feedback inhibition than the renal 1 -hydroxylase [19], which could explain why the kidney is normally the major source of circulating calcitriol. In addition to 1 -hydroxylation, all tissues with vitamin D receptors can also convert 25-hydroxycholecalciferol to 24R,25-dihydroxycholecalciferol. The latter has yet to be assigned a unique biological function, and, despite ongoing debate, most experts believe that it is nothing more than a component of the vitamin D catabolic pathway [28, 29]. Thus, calcitriol not only impedes its own synthesis (by feedback inhibition of 1 -hydroxylase) but also enhances the entry of its immediate precursor into a competing catabolic pathway (by inducing the 24R-hydroxylase). What is the meaning of the elevated circulating calcitriol levels in lymphoma-associated hypercalcemia? Calcitriol is strongly bound to a specific vitamin D-binding protein, with less than 1% circulating as the free, biologically active hormone [30]. Seymour and colleagues report only total calcitriol levels and therefore do not definitively exclude alterations in vitamin D-binding protein turnover or activity as an explanation of their findings. However, because elevated vitamin D-binding proteins have only been described in pregnancy and in patients taking estrogens, it is likely that total calcitriol levels accurately mirror free calcitriol levels in lymphoma. Calcitriol has a half-life of only about 10 hours [31], so elevated circulating levels could reflect increased synthesis, decreased clearance, or both. Metabolic clearance studies will be necessary to definitively exclude alterations in calcitriol catabolism in patients with lymphoma. However, given the usual tight control of 1 -hydroxylase activity, especially the prominent role of feedback inhibition by calcitriol itself, it is unlikely that impaired clearance alone would persistently elevate circulating calcitriol levels. Therefore, it seems reasonable to assume, as do Seymour and colleagues, that the elevated levels in patients with lymphoma reflect increased calcitriol synthesis. Is the calcitriol renal or extrarenal in origin? Renal 1 -hydroxylase activity is enhanced by parathyroid ho

Serum calcitonin concentrations in premature infants during the first 12 weeks of life

SummaryTwenty-eight premature infants of mean gestation 30.9±2.5 weeks and mean birth weight 1175±206 g had repeated serum calcitonin concentrations determined over the first 12 weeks of life. Serum calcitonin concentrations slowly fell but remained elevated even at 12 weeks of age [normal adult=71±48, 1 week (N=15)=327±167, 3 week (N=23)=270±129, 6 week (N=16)=249±154, 9 week (N=13)=214±108, 12 week (N=12)=174±11]. Throughout this period, serum total calcium was normal or low (8.4±.8–9.3±1.0). Serum phosphorus was normal or low (6.0±1.4–6.5±1.0), and serum magnesium was normal (1.7±0.24–1.8±0.34). The reason for the sustained elevation of serum calcitonin in these very small, sick, premature infants in unclear.