Calcitriol deficiency with retained synthetic reserve in chronic renal failure

Determinants of vitamin D activation in patients with acute coronary syndromes and its correlation with inflammatory markers

BACKGROUND AND AIM

Vitamin D deficiency is a pandemic disorder affecting over 1 billion of subjects worldwide. Calcitriol (1,25(OH)2D) represents the perpetrator of the several systemic effects of vitamin D, including the anti-inflammatory, antithrombotic and anti-atherosclerotic actions, potentially preventing acute cardiovascular ischemic events. Variability in the transformation of vitamin D into 1,25(OH)2D has been suggested to modulate its cardioprotective benefits, however, the determinants of the levels of calcitriol and their impact on the cardiovascular risk have been seldom addressed and were, therefore, the aim of the present study.

METHODS AND RESULTS

A consecutive cohort of patients undergoing coronary angiography for acute coronary syndrome (ACS) were included. The levels of 25 and 1,25(OH)2 D were assessed at admission by chemiluminescence immunoassay kit LIAISON® Vitamin D assay (Diasorin Inc) and LIAISON® XL. Hypovitaminosis D was defined as 25(OH)D < 10 ng/ml, whereas calcitriol deficiency as 1,25(OH)2D < 19.9 pg/ml. We included in our study 228 patients, divided according to median values of 1,25(OH)2D (<or ≥ 41.5 pg/ml). Lower calcitriol was associated with age (p = 0.005), diabetes (p = 0.013), renal failure (p < 0.0001), use of diuretics (p = 0.007), platelets (p = 0.019), WBC (p = 0.032), 25(0H)D (p = 0,046), higher creatinine (p = 0.011), and worse glycaemic and lipid profile. A total of 53 patients (23.2%) had hypovitaminosis D, whereas 19 (9.1%) displayed calcitriol deficiency (15.1% among patients with hypovitaminosis D and 7.1% among patients with normal Vitamin D levels, p = 0.09). The independent predictors of 1,25(OH)2D above the median were renal failure (OR[95%CI] = 0.242[0.095-0.617], p = 0.003) and level of vitamin D (OR[95%CI] = 1.057[1.018-1.098], p = 0.004). Calcitriol levels, in fact, directly related with the levels of vitamin D (r = 0.175, p = 0.035), whereas an inverse linear relationship was observed with major inflammatory and metabolic markers of cardiovascular risk (C-reactive protein: r = -0.14, p = 0.076; uric acid: r = -0.18, p = 0.014; homocysteine: r = -0.19, p = 0.007; fibrinogen: r = -0.138, p = 0.05) and Lp-PLA2 (r = -0.167, p = 0.037), but not for leukocytes.

CONCLUSION

The present study shows that among ACS patients, calcitriol deficiency is frequent and can occur even among patients with adequate vitamin D levels. We identified renal failure and vitamin D levels as independent predictors of 1,25(OH)2D deficiency. Furthermore, we found a significant inverse relationship of calcitriol with inflammatory and metabolic biomarkers, suggesting a potential more relevant and accurate role of calcitriol, as compared to cholecalciferol, in the prediction of cardiovascular risk. Future trials should certainly investigate the potential role of calcitriol administration in the setting of acute coronary syndromes as much as in other inflammatory disorders, such as the SARS-CoV2 infection.

Modulation of electrolyte homeostasis by dietary nitrogen intake in growing goats

In goats, the combination of dietary N and Ca reduction caused hypocalcaemia and further changes in Ca homeostasis. The aim of the present study was to characterise the effects of dietary N reduction under normocalcaemia on mineral and bone metabolism in young goats. Young male goats of the Saanen breed were fed a diet reduced in N (8 %) for about 7 weeks (ten animals per group) and were compared with goats fed with an adequate N (14 %) diet. When N intake was reduced in young goats, plasma urea concentrations as well as renal elimination of urea were reduced. This was inversely related to creatinine in plasma and urine, which increased during a dietary N reduction as a function of reduced renal activity to save urea during N scarcity. During this decrease in renal function, associated with declined insulin-like growth factor 1 concentrations, a reduction in calcidiol and calcitriol concentrations could be observed. Meanwhile, carboxyterminal cross-linked telopeptide of type I collagen values and activity of total alkaline phosphatase were both elevated, indicating some bone remodelling processes taking place during a reduced N diet in young goats. The concentrations of inorganic phosphate (Pi) and total Ca were changed in several body fluids, indicating that Pi and Ca homeostasis was perturbed in goats fed a reduced N diet. Therefore, more research is needed to find the balance between reduction of environmental N pollution by reducing dietary N in ruminant feeding and maintaining the animal's health.

Effects of a reduced nitrogen diet on calcitriol levels and calcium metabolism in growing goats

For monogastric animals, changes in dietary protein content modulate calcium (Ca) metabolism by changing parathyroid hormone and calcitriol concentrations. However, the effects of dietary nitrogen (N) restriction on Ca metabolism are not known in ruminants. Since ruminants express endogenous recycling mechanisms very efficiently to save N, it is known that these recycling mechanisms protect ruminants against N depletion in times of dietary N restriction. Therefore, consequences on Ca metabolism induced by reduction of dietary N supply as observed in monogastric animals should not occur in ruminants. Due to this specific metabolic feature, a reduction of dietary N intake can be used to diminish environmental N pollution. The aim of the present study was to determine the consequences of a reduced N intake on Ca homeostasis and respective regulatory hormone concentrations in ruminants. Growing goats fed with a reduced N diet showed a decrease in ionised calcium (Ca2+) and total Ca concentrations while bone resorption marker carboxyterminal cross-linked telopeptide of type I collagen increased in plasma. Unexpectedly, despite hypocalcemia, concentrations of calcitriol were decreased in the animals of the N reduction group whereas calcidiol levels were not affected. From this data, it can be concluded that the Ca metabolism of growing goats can be modulated by changes of dietary N content like in monogastric animals.

Intestinal phosphate absorption in a model of chronic renal failure

Hyperphosphatemia is an important consequence of chronic renal failure (CRF). Lowering of the plasma phosphate concentration is believed to be critical in the management of patients with CRF, especially those on dialysis. Reports of the effect of CRF on the intestinal handling of phosphate in vitro have been conflicting; but what happens in vivo has not been studied. What effect a reduction in the dietary phosphate intake has on intestinal phosphate absorption in CRF in vivo is unclear. In this study, we have used the in situ intestine loop technique to determine intestinal phosphate absorption in the 5/6-nephrectomy rat model of CRF under conditions of normal and restricted dietary phosphate intake. In this model of renal disease, we found that there is no significant change in the phosphate absorption in either the duodenum or jejunum regardless of the dietary phosphate intake. There was also no change in the expression of the messenger RNA of the major intestinal phosphate carrier the sodium-dependent-IIb transporter. Furthermore, we found no change in the intestinal villus length or in the location of phosphate uptake along the villus. Our results indicate that in CRF, unlike the kidney, there is no reduction in phosphate transport across the small intestine. This makes intestinal phosphate absorption a potential target in the prevention and treatment of hyperphosphatemia.

Circulating inhibitor of gonadotropin releasing hormone secretion by hypothalamic neurons in uremia

BACKGROUND

Previous studies have suggested a neuroendocrine defect underlying uremic hypogonadism, characterized by a reduced secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH).

METHODS

We studied the GnRH-producing GT1-7 cell line and the LH-producing LbetaT-2 pituitary cell line under uremic conditions to investigate whether substances circulating in uremic plasma directly affect hypothalamic or pituitary hormone secretion. The cells were incubated with serum from 5/6-nephrectomized or sham-nephrectomized castrated rats, respectively. Furthermore, GT1 cells were incubated with delipidated sera, serum subfractions separated by molecular weight, or several peptide hormones. Cellular viability, apoptosis rate and extracellular hormone degradation were assessed separately. GnRH and LH were measured by RIA in supernatants and cell lysates. GnRH gene expression was assessed by Northern blot.

RESULTS

Uremic serum caused a reduction of extracellular GnRH concentration by 31%, whereas intracellular GnRH increased by 12%. This effect was independent of serum lipids and enzymatic GnRH degradation but was abolished by trypsin digestion. Cellular viability, apoptosis rates and GnRH gene expression did not differ between the two groups. The inhibitory activity was recovered from the high-molecular weight fraction, whereas the fraction <5 kD had stimulatory activity. In contrast, uremic serum did not affect LH secretion from LbetaT-2 cells, indicating that the hypoactivity of the hypothalamo-pituitary gonadotrope unit results from an inhibition at the hypothalamic rather than the pituitary level.

CONCLUSIONS

Our results suggest that uremic serum contains macromolecular and hydrophilic peptide(s) able to specifically suppress the neurosecretion of GnRH from GT1-7 cells.

Relative effects of PTH and dietary phosphorus on calcitriol production in normal and azotemic rats

In moderate renal failure, the serum calcitriol level is influenced by the stimulatory effect of high PTH and the inhibitory action of phosphorus retention. Our goal was to evaluate the relative effect that high PTH levels and increased dietary phosphorus had on calcitriol production in normal rats (N) and rats with moderate renal failure (Nx). Normal and Nx (3/4 nephrectomy) rats were divided into two groups: (1) rats with intact parathyroid glands (IPTG) and (2) parathyroidectomized rats in which PTH was replaced (PTHR) by the continuous infusion of rat 1-34 PTH, 0.022 microgram/hr/100 g body wt, using a miniosmotic Alzet pump. To test the effect of dietary phosphorus, rats received either a moderate (MPD, 0.6% P) or a high phosphorus (HPD, 1.2%) diet for 14 days. The experimental design included pair-fed N and Nx rats with either IPTG or PTHR. Serum calcitriol and PTH levels in N rats fed a MPD were 69 +/- 3 and 40 +/- 5 pg/ml, respectively. In Nx rats on a MPD, serum calcitriol levels decreased only if hyperparathyroidism was not allowed to occur (76 +/- 4 vs. 62 +/- 4 pg/ml in Nx-IPTG-MPD and Nx-PTHR-MPD groups respectively, P < 0.05). Even in N rats on a HPD, high PTH levels (67 +/- 8 pg/ml in the N-IPTG-HPD group) were required to maintain normal serum calcitriol levels (69 +/- 4 vs. 56 +/- 6 pg/ml in Nx-IPTG-HPD and Nx-PTHR-HPD groups, respectively; P < 0.05). In Nx rats on a HPD, the development of secondary hyperparathyroidism (286 +/- 19 pg/ml in the Nx-IPTG-HPD group) prevented a decrease in serum calcitriol levels (68 +/- 7 pg/ml). In contrast, serum calcitriol levels were low in the Nx-PTHR-HPD group (52 +/- 4 pg/ml, P < 0.05), which were deprived of the adaptative increase in endogenous PTH production. In conclusion, our results in rats indicate that in moderate renal failure, an elevated PTH level maintains calcitriol production and overcomes the inhibitory action of phosphorus retention.

The effects of menopause and age on calcitropic hormones: a cross-sectional study of 655 healthy women aged 35 to 90

Although women lose 30% of their skeletal mass after the menopause, the mechanism of this loss is uncertain. Clearly estrogen deficiency is important but whether this works only through direct effects on the skeleton is uncertain. To examine these mechanisms further we have evaluated calcium-related metabolic factors in 655 healthy women. Fasting blood samples were collected from all subjects who were up to 35 years past the menopause, and fasting urine and 24-h urine samples were collected in 365 women who were up to 25 years past the menopause. In the first 15 years postmenopause, there was a rise in total plasma calcium due to a rise in albumin. Bone resorption (hydroxyproline creatinine ratio), bone formation (alkaline phosphatase), and the urine calcium creatinine ratio all rose at menopause and remained elevated for the next 25 years. There was a transient further rise in bone resorption for the 10 years following menopause. Neither PTH nor the free calcitriol index changed for the first 10 years following menopause. Ten years past the menopause, although total calcitriol rose, the free calcitriol index fell due to a rise in vitamin D binding protein. PTH began to rise at 15 years past menopause. GFR fell gradually over the 25 years following menopause. Thus following menopause there is an increase in bone turnover and increased urine calcium loss independent of any effect of PTH or calcitriol, suggesting a direct effect of estrogen deficiency on bone and kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

The regulation of calcitriol by parathyroid hormone and absorbed dietary phosphorus in subjects with moderate chronic renal failure

The two best-recognized stimuli for calcitriol production are parathyroid hormone (PTH) and dietary phosphorus deprivation. We studied the relative importance of these two stimuli in subjects with moderate chronic renal failure (MCRF). We recruited 10 subjects with MCRF aged 49 +/- 13 years (mean +/- SD) and having a creatinine clearance rate of 31 +/- 24 mL/min. After an overnight fast, they received 400 IU human PTH 1-34 subcutaneously, and blood and urine samples were collected over the subsequent 24 hours. They then took aluminum hydroxide 30 mmol/d for 2 months to reduce gut phosphorus absorption, at the end of which the PTH stimulation test was repeated. Responses were compared with those obtained in 11 normal subjects aged 44 +/- 16 years and having a creatinine clearance rate of 102 +/- 37 mL/min who were only studied while on their normal diet. Following PTH stimulation in the normal subjects but not the MCRF subjects, there was a significant increase in plasma calcium and calcitriol levels, with a significant decrease in renal phosphorus threshold. In both groups there was a similar and significant increase in urine cyclic adenosine monophosphate (cAMP) levels. Following restriction of absorption of dietary phosphorus in MCRF subjects, plasma calcitriol levels increased compared with baseline values. This study shows that these MCRF subjects were unable to respond with an increase in calcitriol to a PTH stimulus that produced a similar urine cAMP response in normal subjects. However, they were capable of responding to a reduction in phosphorus absorption with an increase in calcitriol.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of calcium carbonate and low-dose vitamin D/1,25(OH)2D3 in reducing the risk of developing renal osteodystrophy in children on continuous ambulatory peritoneal dialysis

Eight children with terminal renal insufficiency on continuous ambulatory peritoneal dialysis were followed for 12 months to evaluate laboratory parameters of mineral ion and bone metabolism. Calcium carbonate (range 47-295 mg/kg body weight per day) was given in combination with low doses of either vitamin D or 1,25(OH2D3. Blood urea nitrogen and serum phosphate concentrations remained well controlled throughout the observation period. A significant increase in serum calcium levels from 2.35 +/- 0.18 to 2.61 +/- 0.22 mmol/l (mean +/- SD) was observed during the first 6 months. Alkaline phosphatase activity and mid-C-regional parathyroid hormone, both indirect parameters of bone metabolism, revealed no evidence of severe secondary hyperparathyroidism. Our data indicate that calcium carbonate may be sufficient to induce relative hypercalcaemia in uraemic children, and thus reduce the risk of developing renal osteodystrophy. Unwanted side-effects of vitamin D preparations, i.e. increased intestinal phosphate absorption and hypercalcaemia after successful renal transplantation, may thus be avoided.

Recent advances in pediatric metabolic bone disease: the consequences of altered phosphate homeostasis in renal insufficiency and hypophosphatemic vitamin D-resistant rickets

Over the past decade our understanding of the pathogenesis of altered mineral homeostasis in chronic renal failure (CRF) and X-linked hypophosphatemic vitamin D-resistant rickets (XLH) has increased, and has provided a rational approach for the use of the 1 alpha-hydroxylated analogues of vitamin D in their therapy. Recent evidence suggests that intracellular phosphate (Pi) retention in CRF plays a major role in decreasing serum 1,25-dihydroxyvitamin D (1,25(OH)2D) levels, which are responsible for the progressive rise in serum parathyroid hormone (PTH) concentrations through the direct action of 1,25(OH)2D on the parathyroid gland. 1,25(OH)2D levels affect the number of intracellular 1,25(OH)2D receptors, preproPTH mRNA levels and the set point for calcium suppression of PTH release. Further in experimental CRF, the maintenance of normal 1,25(OH)2D levels prevents parathyroid gland hyperplasia. These studies indicate that depressed renal 1 alpha-hydroxylase activity due to Pi retention is a major factor in directly increasing PTH secretion, which in turn contributes significantly to the severity of renal osteodystrophy. Thus the aim of therapy in early CRF should be to maintain normal levels of 1,25(OH)2D which can be achieved by either dietary Pi restriction and oral Pi binders or by administering small doses of 1 alpha-hydroxylated metabolites. The long term consequences of these two different therapeutic regimens still need to be assessed. In XLH, evidence is rapidly accumulating that alterations in 1 alpha-hydroxylase activity secondary to impaired Pi handling by the proximal renal tubule, results in decreased serum 1,25(OH)2D levels, which might be responsible for a number of the associated abnormalities documented in both treated and untreated XLH patients. These abnormalities include decreased calcium and Pi absorption by the intestine and low normal serum calcium values. In vitamin D- and Pi-treated patients 1,25(OH)2D levels are further depressed, with a resultant increase in PTH values, and the development of tertiary hyperparathyroidism in a small number of patients. The use of 1 alpha-hydroxylated analogues rather than vitamin D together with Pi supplements decreases the severity of hyperparathyroidism, improves Pi absorption from the intestine and markedly ameliorates the degree of osteomalacia. Whether long-term therapy with these analogues will prevent the development of tertiary hyperparathyroidism in patients with XLH is unclear.