Strain Differences in Thyrocalcitonin Content of Rat Thyroid Glands Associated with Differences in Sensitivity to Thyrocalcitonin1

Abstract

The varying sensitivities of different strains of rats to pig thyrocalcitonin (TC) were explored further. Thyroid glands from less responsive Blue Spruce and Charles River rats and more responsive Holtzman rats were bioassayed for TC content. The assays were of the crossover type in which thyroid glands from each strain were assayed in both the strain of origin and 1 or both of the other 2 strains. The previous finding that Holtzman rats were more responsive to pig TC than the other 2 strains was confirmed. The order of responsiveness of the 3 strains to rat TC (thyroid gland homog-enate) and pig TC was the same. When assayed in the strain of origin a given aliquot of thyroid gland from the 2 less responsive strains was just as effective in lowering the serum Ca as the same aliquot of Holtzman gland in Holtzman rats. Repeated crossover assays involving all 3 strains agreed in showing that thyroid glands from the 2 less responsive strains contained significantly greater amounts of intrathyroidal TC than Holtzman thyroid glands. Serum Ca levels in the 3 strains following induction of systemic hypercalcemia or cautery of the thyroid gland were not significantly different. Genetic and/or nutritional factors may be responsible for the observed differences between strains. The apparent relationship between TC "requirement" and TC concentration in the thyroid gland may reflect the demands of Ca homeostasis.