Calcitonin mRNA activity in young obese (fa/fa) Zucker rats.

Alterations in both calcitonin (CT) secretion and plasma calcium were recently described in adult obese Zucker rats. We have investigated the CT biosynthetic activity of thyroid glands in 30-day-old obese Zucker rats (fa/fa), and their controls (Lean). Plasma calcium level was significantly increased (+0.6 mg/dl) in obese animals, but plasma phosphate was unchanged. Plasma CT levels measured by radioimmunoassay (RIA) were significantly decreased in fatty (0.50 +/- 0.03 vs 0.68 +/- 0.03 ng/ml in Leans; P less than 0.001), but thyroidal hormone content was not different between Lean and fatty rats (68.7 +/- 5.1 in Leans vs 60.5 +/- 3.6 ng/gland in fatty rats). mRNA was extracted from 10 thyroids, and translated in a rabbit reticulocyte lysate (NEN) in the presence of [35S]methionine. After polyacrylamide gel electrophoresis, specific immunoprecipitates were autoradiographed and quantified by integration. A 50% decrease in translatable CT mRNA was observed in fatty rats. In basal conditions, the biosynthetic activity of C cells in obese rats correlates with the secretion rate of the hormone in the face of unchanged thyroidal CT contents.