Plasma calcium, plasma and thyroidal calcitonin, and histomorphometric bone changes in parathyroidectomized rats.

Abstract This study examined physiological and bone morphological changes resulting from parathyroidectomy (PTX) in male rats. Two diets were provided: one contained 1.2% calcium, 1% phosphorus; the other 0.4-0.6% calcium, 0.6% phosphorus. However, at 51/2 months post-PTX all rats were transferred to a diet containing 0.3% calcium. The daily ration for each rat was 16 g. The parameters examined were: (1) Plasma calcium concentrations; (2) plasma and thyroidal calcitonin levels; and (3) histomorphometry of metaphyseal trabecular bone. When maintained on the lower calcium diet, fasted plasma calcium concentrations of PTX rats stabilized between 5 and 6 mg/dl. In contrast, in rats fed the higher calcium diet, these values gradually rose to between 7 and 8 mg/dl (5 months post-PTX). After transfer to the 0.3% calcium diet, plasma calcium values fell to >6 mg/dl. Thyroidal calcitonin content following PTX rose to values three times those of age-matched controls regardless of the daily calcium intake. The volume of trabecular bone in the tibial metaphysis increased threefold by 61/2 months after PTX; however, there was a decrease in osteoid on these bone surfaces. The static parameters of bone resorption and formation in PTX rats were not statistically different from controls; however, the ratio of osteoblasts to lining cells on trabecular surfaces increased following PTX. The cause of the increase in thyroidal calcitonin following PTX is as yet unknown, but appears to be unrelated to the changes in plasma calcium in rats fed a high calcium diet. This rise in plasma calcium is attributed to accumulation of calcium in the bone surface exchangeable compartment which is reversible. The increase in volume of trabecular bone may be due to slight changes in rates of bone turnover which are not detectable in analysis of static parameters. There was no evidence that the epiphyseal growth plate, or the rate of enchondral bone formation was affected by PTX. The effect of high dietary calcium on post-PTX plasma calcium values points out the need for close control of calcium content of rat chow if the rat is to be used as a model for studying calcium homeostasis or hormonal effects on bone remodeling.