Calcitonin mRNA activity in genetically obese rats.

The etiology of the alterations in calcitonin (CT) secretion and plasma calcium of genetically obese (fa/fa) rats is unknown. In this study, we tested the postulate that there is an early occurrence of abnormalities in CT biosynthesis by thyroid glands of these rodents. Male genetically obese Zucker (fa/fa) rats and their lean littermates were therefore studied from 30 days to 10 months of age. Obese animals were characterized by hypercalcemia (delta approximately equal to 1 mg/dl), already present at 30 days of age. Increased thyroidal CT stores began at 6 weeks of age in fatty rats. Plasma CT levels were decreased in obese animals from 30 days to 10 weeks of age and were not different in leans and fatties 2 weeks later, but were higher at 10 months in fatty rats. Poly A + RNA were extracted from thyroid glands and subjected to translation assays. After SDS-PAGE, specific immunoprecipitates were autoradiographed and quantified by integration. A similar translation product with an apparent mol wt of 15,000 was specifically immunoprecipitated with CT antisera in obese (fa/fa) and lean Zucker rats at different ages. In 30-day-old fatty rats, a 50% decrease in translatable CT mRNA was observed in association with decreased plasma CT levels. In 12-week-old fatty rats, the translatable CT mRNA activity was unchanged or higher when compared to lean littermates, and clearly higher in 10-month-old fatty rats. The CT mRNA levels measured by dot-blot or northern blot hybridization paralleled at each stage studied the CT mRNA activity, determined by translation. It was concluded that in basal conditions, plasma CT level variations during development reflect the biosynthetic activity of C cells in genetically obese rats. The data presented in this study strengthen the point of an early occurrence of abnormalities in CT mRNA activity and in plasma calcium of fa/fa rats.