Nerve growth factor and norepinephrine concentrations in weight-bearing and non-weight-bearing bones of euthyroid and hyperthyroid rats.

UNLABELLED Thyroid hormone, nerve growth factor (NGF) and norepinephrine (NE) and weight-bearing affect bone metabolism, yet interactions between these factors and osseous tissue have not been investigated. Therefore, the aims of the study were to measure NGF and NE concentrations in weight-bearing and non-weight-bearing bones from euthyroid (control) and hyperthyroid (HT) rats. Hyperthyroidism was induced by oral intake of triiodothyronine (90 mg/kg/day) for 21 days. Histomorphometry on distal femurs verified significant trabecular bone loss in HT rats compared to euthyroid animals. NGF concentrations were assayed via ELISA, whilst NE concentrations were measured via HPLC and ECD. In euthyroid rats: (i) the concentration of NGF in ribs (914 ng/g) was almost 3-fold greater than in femurs (326 ng/g wet weight of tissue) (ii) the concentrations of NE in ribs (74.7 ng/g) and calvaria (87.4 ng/g) were 2.5-3.5-fold greater than either femurs (24.0 ng/g) or tibiae (30.5 ng/g) and (iii) NE concentrations were comparable between ribs (74.7 ng/g) and calvaria (87.4 ng/g) and similar between tibiae (30.5 ng/g) and femurs (24.0 ng/g). In HT rats: (i) the concentration of NGF in ribs (1802 ng/g) was 4-fold greater than in femurs (402 ng/g) (ii) NE concentrations in ribs (23.3 ng/g) and calvaria (13.6 ng/g) were 4.5-fold and 2.6-fold greater respectively than in tibiae (5.2 ng/g), while ribs had almost a 2-fold higher concentration of NE than calvaria. In HT rats compared to euthyroid animals: (i) NGF concentrations almost doubled in ribs but there was little change in the NGF concentration in femurs (ii) there was a reduction in NE concentrations in calvaria by 84%, in ribs by 69% in tibiae by 83% and 55% in femur (NS). CONCLUSIONS (i) Non-weight-bearing is associated with higher concentrations of NGF and NE than weight-bearing in bones in euthyroid and HT rats; (ii) Hyperthyroidism exerts opposite effects on NGF and NE in bone and (iii) Hyperthyroidism interacts with weight-bearing to determine NGF and NE concentrations in bone. Therefore, the influence of thyroid hormone on NGF and NE in bone may need to be taken into account when considering the action of thyroid hormone on bone in either euthyroid or hyperthyroid states.