Blood levels of calcitonin in microphthalmic (mi/mi) osteopetrotic mouse cannot account for the resistance of bone to this hormone.

Basal and calcium-stimulated levels of calcitonin were measured in blood from microphthalmic-osteopetrotic and normal mice. The basal levels in the osteopetrotic mice were not different from those in normal mice (119.8 +/- 8.1 and 93.8 +/- 8.6 pg/ml, mean +/- s.e.m., respectively). Similarly, the calcium-stimulated increase in calcitonin following injection of calcium was the same in both groups of mice. Previously it has been demonstrated that calvaria from osteopetrotic mice have a blunted response to the biochemical effects of calcitonin compared to calvaria from normal mice. The present data indicates that this decreased response to calcitonin is not due to excessive blood levels of calcitonin or to altered sensitivity of the thyroid gland to blood calcium concentration.