Acute suppression of bone turnover with calcium infusion in persons with spinal cord injury

Background: Some people with chronic spinal cord injury (SCI) have low vitamin D levels and secondary hyperparathyroidism. Objective: To determine whether, and to what extent, an acute calcium infusion decreased levels of N-telopeptide (NTx), a marker of osteoclastic activity, in individuals with chronic SCI. Study Design: Case series. Subjects: Eight men with chronic SCI. A relatively low serum 25 hydroxyvitamin D concentration (25[OH]D ≤20 ng/mL) and/or a high parathyroid hormone (PTH) (>55 pg/mL) was a prerequisite for study inclusion. Methods: Calcium gluconate bolus 0.025 mmol elemental calcium/kg over 20 minutes followed by a constant infusion of 0.025 mmol/kg per hour for 6 hours was infused; blood samples were collected every 2 hours for measurement of serum total calcium, creatinine, NTx, and PTH. Results: All results are expressed as means (± SDs). Baseline serum 25-hydroxyvitamin D level was 14.5 ± 3.5 ng/mL (range: 10.2-19.6 ng/mL); PTH, 70 ± 25 pg/mL (range: 37-100 pg/mL); and NTx, 21 ± 7 nM bone collagen equivalents (BCE) (range: 14-34 nM). At 2, 4, and 6 hours after the calcium infusion, serum calcium rose from 9.3 ± 0.2 to 10.8 ± 0.9, 10.5 ± 0.8, and 10.6 ± 0.6 mg/d; PTH was suppressed from 70 ± 25 pg/mL to 18 ± 12, 16 ± 9, and 15 ± 9 pg/mL, respectively; NTx fell from 21 ± 8 nM BCE to 17 ± 5, 12 ± 4, and 12 ± 3 nM BCE, respectively. Conclusions: Serum NTx is a marker for bone collagen catabolism, and its reduction suggests that bone turnover was decreased. A relative deficiency of vitamin D associated with chronically elevated levels of PTH would be expected to increase bone turnover and to worsen the bone loss associated with immobilization.