Increased intestinal vitamin D receptor in genetic hypercalciuric rats. A cause of intestinal calcium hyperabsorption

In humans, familial or idiopathic hypercalciuria (IH) is a common cause of hypercalciuria and predisposes to calcium oxalate nephrolithiasis. Intestinal calcium hyperabsorption is a constant feature of IH and may be due to either a vitamin D-independent process in the intestine, a primary overproduction of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], or a defect in renal tubular calcium reabsorption. Selective breeding of spontaneously hypercalciuric male and female Sprague-Dawley rats resulted in offspring with hypercalciuria, increased intestinal calcium absorption, and normal serum 1,25(OH)2D3 levels. The role of the vitamin D receptor (VDR) in the regulation of intestinal calcium absorption was explored in 10th generation male genetic IH rats and normocalciuric controls. Urine calcium excretion was greater in IH rats than controls (2.9 +/- 0.3 vs. 0.7 +/- 0.2 mg/24 h, P < 0.001). IH rat intestine contained twice the abundance of VDR compared with normocalciuric controls (536 +/- 73 vs. 243 +/- 42 nmol/mg protein, P < 0.001), with no difference in the affinity of the receptor for its ligand. Comparable migration of IH and normal intestinal VDR on Western blots and of intestinal VDR mRNA by Northern analysis suggests that the VDR in IH rat intestine is not due to large deletion or addition mutations of the wild-type VDR. IH rat intestine contained greater concentrations of vitamin D-dependent calbindin 9-kD protein. The present studies strongly suggest that increased intestinal VDR number and normal levels of circulating 1,25(OH)2D3 result in increased functional VDR-1,25(OH)2D3 complexes, which exert biological actions in enterocytes to increase intestinal calcium transport. Intestinal calcium hyperabsorption in the IH rat may be the first example of a genetic disorder resulting from a pathologic increase in VDR.

[Vitamin A and metabolic disorders of calcium and phosphorus in patients on a long-term dialysis program]

The aim of the present work was to reveal the relationship between vitamin A serum levels and some indicators of metabolic calcium and phosphorus disorders in patients in a long-term dialyzation programme. Thirty-six patients in a long-term dialyzation programme were divided into three groups. Group A comprised 11 patients who were treated for 3-9 months but without specific treatment, group B was formed by 10 patients treated for 2-5 years who were given calcium carbonate, 3 g/24 h, by the oral route for a period of six months and group C which comprised 15 patients treated for 3-10 years who were given 1 alpha, 25-dihydroxycholecalciferol (Rocaltrol), 0.25 micrograms/24 h for a period of six months. At the end of the investigation a significant rise of total calcium and serum Ca2+ occurred in all three groups of patients an a significant decrease of increased value of phosphorus, C-PTH and vitamin A in serum in groups B and C. In group A which was without treatment there was a significant increase of serum C-PTH and vitamin A after six months. The concentration of retinyl esters in serum was within the reference range of there were undetectable values throughout the investigation in all patients. A direct relationship was found between total calcium and vitamin A, alkaline phosphatase and C-PTH in serum in all 36 patients at the onset of the dialyzation programme. Moreover there was a direct relationship between C-PTH and vitamin A in groups, A, B and C at the onset of the investigation and in groups and vitamin C at the end of the investigation.(ABSTRACT TRUNCATED AT 250 WORDS)

[Use of the equipment for hemodialysis in the diagnosis of calcium metabolism disorders in terminal chronic renal failure]

As many as 18 patients with terminal chronic renal failure were treated by program hemodialysis. The initial concentration of ionized calcium in those patients' blood plasma amounted to 1.06 +/- 0.22 mmol/l. To assess function of the systems counteracting hypocalcemia, use was made of a session of hemodialysis against calcium-free dialysate. 2/3 of the patients demonstrated satisfactory function of the homeostatic system, maintained by a high secretion of PTH and involvement into regulation processes of the extraplasmic system marked by higher buffer capacities and a large pool of metabolism-capable calcium and apparently localized in the skeleton. In 1/3 of the patients, homeostatic function turned out to be dramatically impaired, which was provoked by inhibition of the secretory capacity of the parathyroid glands and manifested clinically by low tolerance to calcium losses and osteomalacia.

Screening for hypercalciuria

Reference values for the urinary calcium/creatinine ratio (Ca/Cr ratio) in the first morning urine were established in 361 healthy children aged 5 to 15 years, on unrestricted diets. The urinary Ca/Cr ratio in the urine upon arising was independent of sex but dependent upon age. The measurement of the urinary Ca/Cr ratio in the urine upon arising while on unrestricted diets may be a reasonable screening test for elevated calcium excretion. On the basis of the urinary Ca/Cr ratios in the urine upon arising during unrestricted diets and the calciuric response to calcium restricted diets and the oral calcium loading test, idiopathic hypercalciuria (IH) was subclassified into three groups: (1) absorptive hypercalciuria; (2) renal hypercalciuria; (3) dietary hypercalciuria. The pathogenesis of IH is controversial. Our data suggest that disordered 1,25 (OH)2 vitamin D metabolism with excessive urinary phosphate excretion occurs in absorptive hypercalciuria.

Hypercalciuria with Bartter syndrome: evidence for an abnormality of vitamin D metabolism

Some children with Bartter syndrome have hypercalciuria. To determine the mechanism for this phenomenon, we studied tubular function and calcium metabolism in six such children. All patients had hypokalemic alkalosis, normotension, hyperreninemia, growth retardation, low fractional distal chloride reabsorption (4/5), and elevated urinary prostaglandin E2 excretion (5/6). In addition, all had hypercalciuria (urinary calcium 6.5 to 25.0 mg/kg/day), with evidence of nephrocalcinosis in five. None, however, had evidence of rickets or hyperparathyroidism. There was a marked elevation in the serum concentration of 1,25-dihydroxyvitamin D in all, and four patients had a response to oral calcium loading suggestive of absorptive hypercalciuria. Five children have had long-term therapy with indomethacin. They have had improvement in hypokalemia and reduced urinary prostaglandin E2 excretion as well as reductions in the serum concentration of 1,25-dihydroxyvitamin D and in urinary calcium excretion. These data suggest that hypercalciuria in some children with Bartter syndrome is associated with an excess of 1,25-dihydroxyvitamin D. The improvement in hypercalciuria with prostaglandin synthesis inhibition may result in part from correction of this vitamin D abnormality.

Immobilization osteoporosis: a review

Bone mass is not only subject to systemic hormonal homeostatic mechanisms, but also to local mechanical influences. The importance of the mechanical balance of bone has been more recently stressed by the research on the effect of weightlessness on bone, and by the introduction of the concept of "mechanostat" in the pathogenesis of osteoporotic conditions. Immobilization osteoporosis has clinical (fractures, sometimes hypercalcemia, urinary lithiasis) and radiological features. Immobilization has an effect on bone modeling and remodeling, through an increased activation of remodeling loci, and a decrease of the osteoblastic stimulus. This leads directly to a local reduction in bone mass, the increased activation multiplying the effect of the deficit in bone formation. The prevention is based on exercise if the load is applied intermittently for a daily period. It seems also that muscle weight is an important determinant of bone mass. There is a potential for recovery during the active early phase of immobilization osteoporosis that may disappear in the subsequent late (about six months) inactive phase. Permanent losses could be prevented by appropriate measures, pharmacology or exercises, applied during the first months of immobilization. No recovery has been demonstrated after the inactive phase has been reached, whatever the treatment. The cumulative effect of repeated periods of immobilization remains hypothetical.

[The development of calcium metabolic disorders in patients in the terminal stage of chronic kidney failure receiving hemodialysis treatment]

To follow calcium metabolism in patients with a terminal stage of chronic renal failure who are on hemodialysis treatment is necessary for the assessment of phosphorus/calcium metabolism in a whole, efficiency of conservative treatment, indications to surgical intervention (parathyroidectomy, renal allotransplantation). The investigation included the assessment of total and ionized calcium blood plasma levels with regard to hemodialysis duration; the correlation of clinical and roentgenological picture of secondary hyperparathyroidism. A total of 222 patients were observed, and with regard to hemodialysis duration allocated into 5 groups: in the first hemodialysis lasted for not more than 3 mos; in the second--from 3 mos to 1 yr; third group with its duration from 1 to 2 yrs; fourth, when it lasted from 2 to 3 yrs; and the fifth group--hemodialysis lasted for more than 3 yrs. It was documented that disorders in calcium metabolism were increasing in parallel with the duration of hemodialysis. Clinical and roentgenological picture of hyperparathyroidism was accompanied by an evident increment of blood plasma ionized calcium while the levels of total blood calcium were but slightly changed. As a part of combined diagnosis ionized calcium assessment is helpful in early prevention and management of calcium metabolism disorders.

[Phosphorus-calcium metabolism and dietetic correction of its disorders in chronic kidney diseases in children]

Ninety children with varying renal diseases were under observation. The investigations conducted have shown that disorders in phosphoric-calcium metabolism depend on the type, activity of chronic glomerulonephritis (CGN) and etiology of chronic renal insufficiency (CRI). Significant disorders in phosphoric-calcium metabolism were detected in patients with nephrotic and mixed types of CGN. Most manifest clinical and x-ray changes of the osseous system were observed in patients with CRI that developed as a result of the tubulointerstitial pathologic process. Low-phosphate diets with preset amounts of Ca and P were developed, composed of products with relatively low content of P, and of new dietetic products rich in Ca. The diets were used for correction of hyperphosphatemia in children with CGN and in those with CRI, simultaneously with drug therapy, to prevent or diminish disorders in phosphoric-calcium metabolism, and to reduce the risk of invalidism among children with chronic renal diseases.

Hypothesis on the regulation of cytosol calcium concentration and the aging brain

This commentary proposes that neurobiology of aging must look beyond the issue of age related changes in calcium concentrations within the cytosol. The more important question is what causes these changes. Studies of structure, function, and dynamics of membranes may begin to give a better clue about underlying mechanisms of these changes.

Eventual attenuation of hypocalciuric response to hydrochlorothiazide in absorptive hypercalciuria

The effect of long-term hydrochlorothiazide therapy on renal calcium excretion was measured in 12 well defined cases of absorptive hypercalciuria and 10 of renal hypercalciuria. Patients were studied during a control phase, at 3 to 6 months of therapy and after long-term treatment with hydrochlorothiazide (mean 61 months for absorptive hypercalciuria and 71 months for renal hypercalciuria). Evaluation comprised measurement of urinary calcium and fractional (intestinal) calcium absorption while patients were maintained on a constant metabolic diet (400 mg. calcium per day) for 3 days. In patients with absorptive hypercalciuria urinary calcium decreased significantly at 3 months of treatment (from 266 to 137 mg. per day, p less than 0.001). However, with continued treatment urinary calcium rebounded to 197 mg. per day. Of the patients with absorptive hypercalciuria 50 per cent were hypercalciuric (greater than 200 mg. per day) on long-term treatment, whereas none was hypercalciuric at 3 months. In contrast, urinary calcium in the patients with renal hypercalciuria decreased from 299 to 104 mg. per day (p less than 0.001) at 3 months of treatment and remained reduced (116 mg. per day) during long-term treatment. Intestinal calcium absorption was increased initially and remained unchanged throughout treatment in the patients with absorptive hypercalciuria. In patients with renal hypercalciuria intestinal calcium absorption decreased significantly after short-term treatment with hydrochlorothiazide and remained so after long-term therapy. The results suggest that, unlike patients with renal hypercalciuria, some with absorptive hypercalciuria lose the hypocalciuric effect of hydrochlorothiazide during long-term treatment.