gms | German Medical Science

Objectives: The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a common electrolyte disorder in clinical practice, which causes hyponatremia (HypoNa). HypoNa is related to osteoporosis; however, the studies on its effect on bone tissue are rare. This study investigated the effect of SIADH induced HypoNa on bone tissue in a rat model.

Methods: Seven-month old female Sprague Dawley rats were divided into 3 groups: (1) normonatremic rats (NormoNa, n=11), (2) hyponatremic rats (HypoNa, n=11)), (3) HypoNa-corrected rats (HypoNa-cor, n=8). HypoNa-rats were infused with desmopressin at a rate 5 ng/h via subcutaneous implanted osmotic pumps (Alzet, Durect Co., USA) to induce hyponatremia. NormoNa groups received 0.9% saline solution in pumps. The pumps were implanted at week 0 and either replaced in the HypoNa group or removed in HypoNa-cor group at week 6. All animals received liquid rodent diet (BioServ, USA) and demineralized water without restrictions throughout the experiment. Three rats of the NormoNa and HypoNa groups were analyzed at week 6. The remaining rats were examined at week 12. The fourth lumbar vertebral body (L4) and both femora were analyzed by biomechanical and micro-computed tomographical analyses. A one-way analysis of variance followed by Tukey test were applied to reveal the differences between the groups at week 12 and t-test at week 6 (p < 0.05).

Results and conclusion: In serum, hyponatremia was confirmed in HypoNa rats by sodium concentration of 110±5 mmol/L, whereas in NormoNa, it averaged to 140±2 mmol/L throughout the experiment. In the HypoNa-corr group, it was 117±11 mmol/L after 6 weeks of treatments. After sodium correction, it returned to the NormoNa levels (142±1 mmol/L) at week 12. A similar pattern was observed in calcium and magnesium concentrations. After 6 weeks of treatments, biomechanical properties and trabecular structural parameters were impaired in the HypoNa group compared to the NormoNa group in both L4 and femur. In femur, bone volume fraction and bone mineral density were also reduced. After 12 weeks, the effect of HypoNa on bone parameters further increased. Correction of HypoNa significantly ameliorated bone parameters to the level observed in NormoNa group in femur, whereas in L4 the degree of changes did not reach a significant level.

Concluding, SIADH induced hyponatremia impaired bone tissue, and its correction recovered bone and serum parameters to the level observed in NormoNa rats. The stronger effect in femur than in L4 can be explained by a known various response of different skeletal sites to the treatments. This study demonstrated the importance of correction of HypoNa to prevent and treat osteoporosis.