Secretion of Calcitonin in the Genetically Obese Zucker Rat (fa/fa)

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Abstract
Adult Zucker fatty rats have C-cell hyperplasia and increased thyroidal calcitonin (CT) compared to lean controls. Both secretion of CT and responsiveness to CT were evaluated in order to see whether they, too, were altered. Fat rats and lean littermates, 13-15 mo. old, were used. CT secretion was provoked by feeding for 2 h after an 18-h fast, giving pentagastrin i.v. and injecting CaCl2 i.v. CT was measured by radioimmunoassay. Responsiveness to CT was examined by giving porcine or salmon CT i.v. and measuring serum Ca 1-3 h later. For CT secretion, compared to leans the fat rats showed higher fasting serum Ca 1-3 h later. For CT secretion, compared to leans the fat rats showed higher fasting serum Ca and CT and a greater rise in CT after feeding, a similar 5- to 10-fold increase in CT after i.v. pentagastrin, and a greater rise in both serum Ca and CT at various times between 5 min and 3 h after i.v. CaCl2. For CT responsiveness, fat and lean rats were equally responsive to i.v CT in terms of the fall in plasma Ca 1-3 h later. Fat rats can secrete as much or more CT in response to provocative stimuli as lean rats and they appear normally responsive to injected CT. Inability to release CT and insensitivity to CT do not underly the C-cell hyperplasia, increased thyroidal CT and increased circulating CT in the fat rat.