Abstract
We have already indicated the striking difference in the response of young and adult guinea pigs to relatively large single doses of parathormone. 1 , 2 For the purposes of this study we consider a guinea pig fully adult when the major epiphyseal cartilage plates of the humerus, tibia and femur are closed.
We produced the essential lesions of ostitis fibrosa in young guinea pigs by daily administration of 10 to 20 units of parathormone for 10 to 34 days. It is usually assumed that a more prolonged period of treatment would produce a greater effect. However, the animals' increasing age during the period of treatment had to be considered. Four guinea pigs were treated daily, beginning at the age of 2 to 7 days, with daily doses increasing to 20 units during the last 2 to 3 months. The total length of the treatment was about 110 days, at the end of which time the epiphyses were not closed. In the treated animals the bones showed no marked changes when compared with their normal litter-mate controls, and the changes were insignificant when compared with those previously reported in younger animals. While the number of animals was small, the data were consistent enough to warrant the conclusion that the serum calcium is lowered toward the end of a long course of treatment of guinea pigs with parathormone.
We believe that for a time the severity of the lesions increased in these animals. However, when the animals reached a certain age the effect of parathormone on the bone was decreased, and healing actually occurred after that time in spite of the continued administration of parathormone. This is not to be interpreted as evidence of immunity developed in the course of the treatment.
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