Kyoji Ikeda

Increased Chloride Reabsorption as an Inherited Renal Tubular Defect in Familial Type II Pseudohypoaldosteronism

HYPERKALEMIA is a common electrolyte disorder. Among its common causes are renal insufficiency; adrenal insufficiency; hyporeninemic hypoaldosteronism, usually associated with diabetes mellitus, mi...

The Advantage of Alfacalcidol Over Vitamin D in the Treatment of Osteoporosis

Abstract. Although alfacalcidol is widely used in the treatment of osteoporosis, its mechanism of action in bone is not fully understood. Alfacalcidol stimulates intestinal calcium (Ca) absorption, increases urinary Ca excretion and serum Ca levels, and suppresses parathyroid hormone (PTH) secretion. It remains to be clarified, especially under vitamin D-replete conditions, whether alfacalcidol exerts skeletal effects solely via these Ca-related effects, whether the resultant suppression of PTH is a prerequisite for the skeletal actions of alfacalcidol, and, by inference, whether alfacalcidol has an advantage over vitamin D in the treatment of osteoporosis. To address these issues, we (1) compared the effects of alfacalcidol p.o. (0.025–0.1 μg/kg BW) vis-à-vis vitamin D3 (50–400 μg/kg BW) on bone loss in 8-month-old, ovariectomized (OVX) rats as a function of their Ca-related effects, and (2) examined whether the skeletal effects of alfacalcidol occur independently of suppression of PTH, using parathyroidectomized (PTX) rats continuously infused with hPTH(1–34). The results indicate that (1) in OVX rats, alfacalcidol increases BMD and bone strength more effectively than vitamin D3 at given urinary and serum Ca levels: larger doses of vitamin D3 are required to produce a similar BMD-increasing effect, in the face of hypercalcemia and compromised bone quality; (2) at doses that maintain serum Ca below 10 mg/dl, alfacalcidol suppresses urinary deoxypyridinoline excretion more effectively than vitamin D3; and (3) alfacalcidol is capable of increasing bone mass in PTX rats with continuous infusion of PTH, and therefore acts independently of PTH levels. It is suggested that alfacalcidol exerts bone-protective effects independently of its Ca-related effects, and is in this respect superior to vitamin D3, and that the skeletal actions of alfacalcidol take place, at least in part, independently of suppression of PTH. Together, these results provide a rationale for the clinical utility of alfacalcidol and its advantage over vitamin D3 in the treatment of osteoporosis.

A hypercalcemic nude rat model that completely mimics human syndrome of humoral hypercalcemia of malignancy.

SummaryTumors causing humoral hypercalcemia of malignancy (HHM) were implanted to athymic nude rats. In one of these rat models transplanted with uterine cancer (UCC), a complete reproduction of human HHM syndrome was achieved: hypercalcemia, hypophosphatemia with increased urinary phosphate and cyclic AMP excretion, and suppressed serum 1,25-dihydroxy-vitamin D (1,25(OH)2D) level. In another hypercalcemic nude rat model implanted with oral cavity cancer (OCC), all the features were similar except for markedly elevated serum 1,25(OH)2D. Hypercalcemia disappeared by surgical removal of the tumors in both models, confirming the humoral mechanisms for causing these features. Furthermore, in UCC tumor-bearing rats, hypophosphatemia, increased renal phosphate excretion, and reduced serum 1,25(OH)2D concentration were already present when these rats were only marginally hypercalcemic. These results raise the possibility that the changes in renal tubular phosphate handling and vitamin D metabolism in HHM are not secondary to hypercalcemia but are due to direct effects of the humoral factor(s) that cause this syndrome. Extracts of both tumors exhibited stimulation of cyclic AMP production in osteoblastlike cells, UMR 106, which could be almost completely inhibited by parathyroid hormone (PTH) antagonist, human PTH(3–34). By comparing the nature and characteristics of humoral factor(s) from UCC and OCC models, mechanisms responsible for causing these abnormalities can be explored. Thus, these nude rat models can be useful for elucidating the underlying mechanism of the development of HHM.

Effect of Combination Treatment with a Vitamin D Analog (OCT) and a Bisphosphonate (AHPrBP) in a Nude Mouse Model of Cancer-Associated Hypercalcemia

Hypercalcemia represents one of the important paraneoplastic syndromes affecting morbidity and mortality of cancer patients. We and others have demonstrated that vitamin D analogs with little calcemic activities suppress the transcription of the parathyroid hormone‐related peptide (PTHrP) gene, a major humor responsible for cancer hypercalcemia, and thereby prevent the development of hypercalcemic syndrome. The present study was undertaken: to compare the therapeutic efficacy of a vitamin D analog, 22‐oxa‐1,25‐dihydroxyvitamin D3 (OCT), and a bisphosphonate (disodium 3‐amino‐1‐hydroxypropylidene‐1,1‐bisphosphonate pentahydrate [AHPrBP]), an inhibitor of osteoclastic bone resorption, on cancer‐induced hypercalcemia; and to see if the effect could be enhanced by combination treatment, using a nude mouse model implanted with a human pancreas carcinoma (FA‐6). After a single intravenous administration, OCT (5 μg/kg of body weight [BW]) was as effective as AHPrBP (10 mg/kg of BW) in lowering blood ionized calcium levels in tumor‐bearing nude mice, and their combination further enhanced the therapeutic effect. Although AHPrBP lost its efficacy after repeated injections, OCT was still effective after the third administration. The therapeutic effect of OCT in cancer hypercalcemia was observed in four other human tumors, including another pancreas carcinoma (PAN‐7), two squamous cell carcinomas of the lung (KCC‐C1 and LC‐6), and a squamous carcinoma of the pharynx (PHA‐1), all of which elaborated PTHrP into the circulation. Treatment with OCT resulted in a decrease in circulating PTHrP levels by approximately 50% in two representative models. However, the mechanism underlying the antihypercalcemic effect of OCT seemed complex, involving inhibition of PTHrP production, suppression of excessive bone resorption, and an antitumor activity. OCT also markedly inhibited the body weight loss with tumor growth, while AHPrBP, which exhibited a similar antihypercalcemic effect, was less effective than OCT in preventing cachexia. The anticachectic activity of their combination did not exceed that of OCT alone, suggesting a hypercalcemia‐dependent as well as an independent mechanism of cancer cachexia. It is concluded that OCT may be useful, either as a single agent or in combination with bisphosphonates, for the treatment of cancer‐associated hypercalcemia and cachexia.

Inhibition ofin vitro mineralization by aluminum in a clonal osteoblastlike cell line, MC3T3-E1

SummaryThe direct effect of aluminum on mineralization was examined using an osteoblastlike cell line, MC3T3-E1. The mineralization process was quantitated by measuring45Ca accumulation into the cell and matrix layer of MC3T3-E1 cells in culture. The accumulation of45Ca into the cell and matrix layer increased dramatically after 13 days of culture without a parallel change in the DNA content of these cells. Because nodular clusters of cells appear around the same period in which a massive mineralization occurs, the marked increase in45Ca accumulation after the 13th day of culture appears to represent deposition of45Ca into the extracellular matrix. Thus, this culture system offers a useful model for making a quantitative estimation of osteoblast-mediated mineralizationin vitro. When aluminum was added to this system, the accumulation of45Ca into the cell matrix layer was inhibited in a dose-dependent manner: 10−6 M aluminum reduced45Ca accumulation to 40.8±2.7% of that in nontreated cells without affecting alkaline phosphatase activity or the DNA content of these cells. Because the concentration of aluminum used in this study is well within the range of serum aluminum levels seen in chronic dialysis patients, the direct effects of aluminum on osteoblast-mediated mineralization shown in the present study may underlie the development of so-called aluminum-induced “osteomalacia” in certain dialysis patients.

Role of insulin in the stimulation of renal 25-hydroxyvitamin D3-1α-hydroxylase by phosphorus deprivation in rats

The increase in serum 1,25(OH)2D concentration in response to dietary phosphorus (P) depreviation is dependent on the presence of insulin in rats. The present study was undertaken to clarify whether insulin exerts its effects by affecting the renal production of 1,25(OH)2D. The 25(OH)D-1 alpha-hydroxylase activity in kidney homogenates was markedly stimulated by P deprivation in control rats (0.20 +/- 0.06 pmol/g tissue/min in the rats on a normal P diet v 1.3 +/- 0.15 pmol/g/min in the rats on a low P diet; 6.5-fold increase). In contrast, in streptozotocin-diabetic rats, the increase in the renal 1 alpha-hydroxylase activity in response to P deprivation (0.25 +/- 0.01 pmol/g/min; 3.6-fold increase) as well as the enzyme activity in the rats on a normal P diet (0.07 +/- 0.01 pmol/g/min) was markedly suppressed. Furthermore, all the changes in the renal 1 alpha-hydroxylase activity in insulin-deficient rats disappeared by insulin replacement (0.16 +/- 0.01 pmol/g/min in the rats on a normal P diet v 1.3 +/- 0.01 pmol/g/min in the rats on a low P diet; eightfold increase). These results demonstrate that the stimulation of 1 alpha-hydroxylase in response to dietary P deprivation is blunted by insulin deficiency and is fully restored by insulin replacement. It is suggested that insulin, in addition to its direct stimulatory effect on 1 alpha-hydroxylase, alters the responsiveness of renal 1 alpha-hydroxylase to P deprivation. These effects of insulin on 1 alpha-hydroxylase may be responsible for the change in serum 1,25(OH)2D concentration in response to dietary P deprivation, although the possibility cannot be ruled out that insulin also affects the metabolic clearance of 1,25(OH)2D.