Intensity of lactation modulates renal 1 alpha-hydroxylase and serum 1,25(OH)2D in rats

Maternal Mineral and Bone Metabolism During Pregnancy, Lactation, and Post-Weaning Recovery

During pregnancy and lactation, female physiology adapts to meet the added nutritional demands of fetuses and neonates. An average full-term fetus contains ∼30 g calcium, 20 g phosphorus, and 0.8 g magnesium. About 80% of mineral is accreted during the third trimester; calcium transfers at 300-350 mg/day during the final 6 wk. The neonate requires 200 mg calcium daily from milk during the first 6 mo, and 120 mg calcium from milk during the second 6 mo (additional calcium comes from solid foods). Calcium transfers can be more than double and triple these values, respectively, in women who nurse twins and triplets. About 25% of dietary calcium is normally absorbed in healthy adults. Average maternal calcium intakes in American and Canadian women are insufficient to meet the fetal and neonatal calcium requirements if normal efficiency of intestinal calcium absorption is relied upon. However, several adaptations are invoked to meet the fetal and neonatal demands for mineral without requiring increased intakes by the mother. During pregnancy the efficiency of intestinal calcium absorption doubles, whereas during lactation the maternal skeleton is resorbed to provide calcium for milk. This review addresses our current knowledge regarding maternal adaptations in mineral and skeletal homeostasis that occur during pregnancy, lactation, and post-weaning recovery. Also considered are the impacts that these adaptations have on biochemical and hormonal parameters of mineral homeostasis, the consequences for long-term skeletal health, and the presentation and management of disorders of mineral and bone metabolism.

The relationship among renal injury, changed activity of renal 1-alpha hydroxylase and bone loss in elderly rats with insulin resistance or Type 2 diabetes mellitus

BACKGROUND AND OBJECTIVE

Chronic kidney disease can lead to a decrease in active vitamin D [1,25-(OH)2D], which may be reversed by 1-alpha hydroxyvitamin D [1-alpha(OH)D]. Renal 1-alpha hydroxylase, expressed in renal tubular epithelial cells, is a key enzyme in the synthesis of 1,25-(OH)2D. 1,25- (OH)2D plays an important role in the regulation of calcium and phosphate metabolism, and its deficiency can result in osteoporosis. Type 2 diabetes mellitus (T2DM) and insulin resistance (IR) are associated with renal injury, decrease in 1,25-(OH)2D and bone loss. The study aimed to explore the relationship among renal injury, decrease in 1,25-(OH)2D and bone loss in the presence of IR or T2DM, as well as the role of renal 1-alpha hydroxylase in the process.

MATERIALS AND METHODS

Fifty 18-month-old male Wistar rats were randomized into 5 groups: normal control group (Group N), IR group (Group I), T2DM group (Group D), No.1 treatment group (Group T1), and No.2 treatment group (Group T2), 10 in each group. High-fat diet was administered to induce IR, while high-fat diet and low-dose streptozotocin were jointly applied to induce T2DM. Rats in Groups T1 and T2 were treated with vitamin D and 1-alpha(OH)D, respectively. At week 12, IR was determined by the use of euglycemic insulin clamp technique for rats in each group, and then glucose infusion rate (GIR) was calculated. Meanwhile, urinary albumin (UA), serum 25-(OH)D and 1,25-(OH)2D levels were determined by radioimmunoassay. After the rats were sacrificed, bone mineral density (BMD) in femoral bone and lumbar vertebrae was measured by the use of dual energy X-ray absorption.

RESULTS

The GIR in Group N was significantly higher than that of the other 4 groups (p<0.01). Compared with Groups N (p<0.01) or I (p<0.05), the UA levels in Groups D, T1, and T2 were obviously higher. The UA level in Group I was higher than that of Group N, but the difference was not significant (p>0.05). In Groups D and I, the UA levels showed a negative correlation with GIR. No significant difference was observed in the levels of 25-hydroxyvitamin D [25-(OH)D]. The levels of 1,25-(OH)2D in Groups D and T1 were markedly lower than that of Groups N or T2 (p<0.01). The 1,25-(OH)2D level in Group I was lower than that of Group N (p<0.05), but higher than that of Group D (p<0.01). The 1,25-(OH)2D level in Group T2 was nearly equivalent to that of Group N. In Groups D and I, the levels of 1,25-(OH)2D were negatively correlated with UA, and positively correlated with GIR. The BMD levels in lumbar vertebrae or femoral bone in Groups D and T1 were similar, but both were lower than that of Groups T2 (p<0.05) and N (p<0.01). The BMD levels were lower in Groups I and T2 compared with that of Group N (p<0.05), but higher than that of Groups D and T1 (p<0.05). The BMD levels in lumbar vertebrae or femoral bone in Groups I and D were positively correlated with GIR. The BMD level in lumbar vertebrae or femoral bone in Group D showed negative correlation with UA.

CONCLUSION

In elderly rats with T2DM or IR, renal injury may cause decreased activity of renal 1-alpha hydroxylase, which may result in bone loss and disturbance in VD metabolism, mainly manifesting as a significant reduction in the 1,25-(OH)2D level.

Postoperative tetany in Graves disease: important role of vitamin D metabolites

OBJECTIVE

To test the authors' hypothesis of the causal mechanism(s) of postoperative tetany in patients with Graves disease.

SUMMARY BACKGROUND DATA

Previous studies by the authors suggested that postoperative tetany in patients with Graves disease occurs during the period of bone restoration and resulted from continuation of a calcium flux into bone concomitant with transient hypoparathyroidism induced by surgery.

PATIENTS AND METHODS

A prospective study was carried out to investigate sequential changes in serum levels of intact parathyroid hormone (iPTH), calcium and other electrolytes, 25-hydroxyvitamin D (25OHD), 1,25-dihydroxyvitamin D (1,25(OH)2D), and bone metabolic markers in 109 consecutive patients with Graves disease who underwent subtotal thyroidectomy.

RESULTS

Preoperative serum iPTH levels negatively correlated with ionized calcium levels and positively correlated with 1,25(OH)2D or 1,25(OH)2D/25OHD. After the operation, there was a significant decline in levels of ionized calcium, magnesium, and iPTH. Serum iPTH was not detected in 15 patients after surgery. Four of these 15 patients, and 1 patient whose iPTH level was below normal, developed tetany. Preoperative serum ionized calcium levels were significantly lower, and iPTH levels were higher, in the 5 patients with tetany than in the 11 patients who did not develop tetany despite undetectable iPTH levels. The tetany group had significantly lower serum 25OHD levels and higher 1,25(OH)2D levels, and had increased 1,25(OH)2D/25OHD as an index of the renal 25OHD-1-hydroxylase activity than those in the nontetany group. These results suggest that patients with a high serum level of iPTH as a result of low serum calcium levels (secondary hyperparathyroidism) are susceptible to tetany under conditions of hypoparathyroid function after surgery.

CONCLUSIONS

Postoperative tetany occurs in patients with secondary hyperparathyroidism caused by a relative deficiency in calcium and vitamin D because of their increased demand for bone restoration after preoperative medical therapy concomitant with transient hypoparathyroidism after surgery. Calcium and vitamin D supplements may be recommended before and/or after surgery for patients in whom postoperative tetany is expected to develop.