Nine-month-old female rats were double-labeled with bone markers and subjected to right hindlimb immobilization or served as control for 0, 2, 10, 18, or 26 weeks. The right limb was immobilized against the abdomen, thus unloading it, while the left limb was overloaded during ambulation. Single photon absorptiometry and cancellous bone histomorphometry were performed on dissected intact femur and 20-microns-thick undecalcified specimens of the proximal tibial metaphysis. In the unloaded limb, immobilization-induced muscle and cancellous bone loss occurred rapidly before 10 weeks and stabilized at 50% less bone mass after 18 weeks. Unloading caused a negative bone balance from a combination of elevated bone resorption and depressed bone formation. At 2, 10, and 18 weeks of immobilization, the ratios of bone resorption to bone formation surfaces were 1.6, 1.5, and 1.3, respectively; at 26 weeks, the ratio was 1. The bone loss was accompanied by poorer trabecular architecture (trabecular number decreased and trabecular separation increased), reaching the maximum at 18 weeks and stabilizing thereafter. These observations are in general agreement with Frost's postulate for mechanical effects on lamellar bone remodeling, and the findings on disuse osteoporosis in man. Therefore, the one-legged immobilization model can be useful in studies of the mechanisms of structural adaptation to mechanical usage.

SUMMARY The thyroidal uptake of 36ClO4− and the effect of stable perchlorate ion and functional state of the thyroid, such as after long-term administration of propylthiouracil (PTU) or after hypophysectomy, on the uptake of 36ClO4− were studied in rats and guinea-pigs. Based on the analysis of uptake curves, radioactive inulin and sulphate spaces, and histological measurements of compartmental sizes of the thyroid, the relative concentrations of 36ClO4− in the various compartments were calculated and compared with those of 36Cl-. The passage of 36ClO4− and 36Cl− across both the basal and the apical membranes of the follicular cells in the different conditions were analysed. The results suggest that 36ClO4− is concentrated in the lumen of the thyroid by a two-step process. It is actively transported across the basal cell membrane from the interstitial fluid into follicular cells, and is further concentrated in passage across the apical membrane into the lumen. The evidence suggests that the latter process is also an active one.

SUMMARY Previous studies indicated that 36Cl-labelled perchlorate is concentrated by rat and rabbit thyroid gland. However, the extent of concentration of radioactive perchlorate in the gland was much less than that of iodide. Since perchlorate itself has a marked effect on anion transport in the thyroid and the specific activity of available [36C]perchlorate is very low, the stable anion as a carrier present in the injected radioactive perchlorate solution may affect the uptake of this radioactive compound by the gland. In this study, radioactive solutions of perchlorate and iodide containing different amounts of stable perchlorate or iodide (dosages ranged from 0·005 to 5 m-equiv./kg. body weight) were injected into groups of rats and guineapigs, and the thyroid: plasma concentration ratios of radioactive perchlorate and iodide were calculated and compared. These experiments were also repeated in animals pretreated with thyroid-stimulating hormone (TSH), after chronic administration of propylthiouracil (PTU), as well as in hypophysectomized animals. At the same dose levels of perchlorate, there was no difference in thyroid: plasma concentration ratios of radioactive perchlorate and iodide in control rats and guinea-pigs or in treated ones.

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